U.S. patent application number 10/572493 was filed with the patent office on 2007-11-01 for optical device.
Invention is credited to Genichi Hatakoshi, Katsutaro Ichihara, Urara Ichihara, Kunihiko Ishihara, Keishi Ohashi.
Application Number | 20070253051 10/572493 |
Document ID | / |
Family ID | 34381789 |
Filed Date | 2007-11-01 |
United States Patent
Application |
20070253051 |
Kind Code |
A1 |
Ishihara; Kunihiko ; et
al. |
November 1, 2007 |
Optical Device
Abstract
A conductive thin film has first and second surfaces and at
least one opening extending through from the first surface to the
second surface. At least on one of the first and second surfaces,
first and second periodic surface patterns having different period
lengths are provided. The period length of the second periodic
surface pattern is substantially equal to an odd integral multiple
of a half of the period length of the first periodic surface
pattern. With this, surface plasmon polaritons excited by the first
periodic surface pattern undergo odd-order Bragg reflection by the
second periodic surface pattern. As a result, the intensity of the
light falling on the first surface and transmitted to the second
surface through the opening is increased with high efficiency.
Inventors: |
Ishihara; Kunihiko; (Tokyo,
JP) ; Hatakoshi; Genichi; (Tokyo, JP) ;
Ohashi; Keishi; (Tokyo, JP) ; Ichihara;
Katsutaro; (Kanagawa, JP) ; Ichihara; Urara;
(Kanagawa, JP) |
Correspondence
Address: |
SCULLY SCOTT MURPHY & PRESSER, PC
400 GARDEN CITY PLAZA
SUITE 300
GARDEN CITY
NY
11530
US
|
Family ID: |
34381789 |
Appl. No.: |
10/572493 |
Filed: |
September 17, 2004 |
PCT Filed: |
September 17, 2004 |
PCT NO: |
PCT/JP04/14306 |
371 Date: |
February 27, 2007 |
Current U.S.
Class: |
428/644 |
Current CPC
Class: |
Y10T 428/12694 20150115;
G03B 21/56 20130101 |
Class at
Publication: |
359/212 |
International
Class: |
G02B 26/08 20060101
G02B026/08 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 29, 2003 |
JP |
2003-338627 |
Sep 29, 2003 |
JP |
2003-338636 |
Sep 30, 2003 |
JP |
2003-341971 |
Claims
1. An optical device comprising a conductive thin film having:
first and second surfaces; at least one opening extending through
the film from the first surface to the second surface; and a
periodic surface pattern disposed on at least one of the first and
second surfaces, an intensity of light falling on the first surface
and transmitted to the second surface through the opening being
increased as compared with a case where any periodic surface
pattern is not disposed, wherein the periodic surface pattern
includes first and second periodic surface patterns having
different period lengths, and the period length of the second
periodic surface pattern is substantially equal to odd time(s) as
much as 1/2 of the period length of the first periodic surface
pattern.
2. An optical device comprising a conductive thin film having:
first and second surfaces; at least one opening extending through
the film from the first surface to the second surface; and a
periodic surface pattern disposed on at least one of the first and
second surfaces, an intensity of light falling on the first surface
and transmitted to the second surface through the opening being
increased as compared with a case where any periodic surface
pattern is not disposed, wherein the periodic surface pattern
includes first and second periodic surface patterns having
different period lengths, and surface plasmon polaritons excited by
the first periodic surface pattern undergo odd-order Bragg
reflection by the second periodic surface pattern.
3. The optical device according to claim 1, wherein a ratio
P.sub.2/P.sub.1 of a period length P.sub.2 of the second periodic
surface pattern with respect to a period length P.sub.1 of the
first periodic surface pattern is within a range of .+-.0.2 of odd
time(s) as much as 0.5.
4. The optical device according to claim 1, wherein a ratio
P.sub.2/P.sub.1 of a period length P.sub.2 of the second periodic
surface pattern with respect to a period length P.sub.1 of the
first periodic surface pattern is within a range of 0.5.+-.0.2.
5. The optical device according to claim 1, wherein a ratio
P.sub.2/P.sub.1 of a period length P.sub.2 of the second periodic
surface pattern with respect to a period length P.sub.1 of the
first periodic surface pattern is within a range of 1.5.+-.0.2.
6. The optical device according to claim 1, wherein the intensity
of the light transmitted through the opening increases by 20% or
more as compared with that of light transmitted through an opening
of a device which is not provided with the second periodic surface
pattern.
7. The optical device according to claim 2, wherein the intensity
of the light transmitted through the opening increases by 20% or
more as compared with that of light transmitted through an opening
of a device which is not provided with the second periodic surface
pattern.
8. The optical device according to claim 1, wherein the first and
second periodic surface patterns are disposed on the same plane as
the first or second surface.
9. The optical device according to claim 2, wherein the first and
second periodic surface patterns are disposed on the same plane as
the first or second surface.
10. The optical device according to claim 1, wherein the first
periodic surface pattern is disposed so as to correspond to a
region on which the light falls, and the second periodic surface
pattern is disposed externally from the first periodic surface
pattern.
11. The optical device according to claim 2, wherein the first
periodic surface pattern is disposed so as to correspond to a
region on which the light falls, and the second periodic surface
pattern is disposed externally from the first periodic surface
pattern.
12. The optical device according to claim 10, wherein the region
provided with the first periodic surface pattern is broader than
the region on which the light falls.
13. The optical device according to claim 11, wherein the region
provided with the first periodic surface pattern is broader than
the region on which the light falls.
14. An optical device comprising a conductive thin film having:
first and second surfaces; and a plurality of periodically arranged
openings extending through the film from the first surface to the
second surface, an intensity of light falling on the first surface
and transmitted to the second surface through the plurality of
openings being increased as compared with a case where the
plurality of openings are not periodically arranged, wherein the
plurality of periodically arranged openings are regarded as first
periodic openings, the conductive thin film includes one of second
periodic openings and periodic surface patterns formed with a
period length which is different from that of the first periodic
openings, the second periodic openings including a plurality of
openings which are different from the plurality of openings
extending through the film from the first surface to the second
surface, the periodic surface patterns being formed on one of the
first and second surfaces, and the period length of the second
periodic openings or the periodic surface patterns is substantially
equal to odd time(s) as much as 1/2 of the period length of the
first periodic openings.
15. An optical device comprising a conductive thin film having:
first and second surfaces; and a plurality of periodically arranged
openings extending through the film from the first surface to the
second surface, an intensity of light falling on the first surface
and transmitted to the second surface through the plurality of
openings being increased as compared with a case where the
plurality of openings are not periodically arranged, wherein the
plurality of periodically arranged openings are regarded as first
periodic openings, the conductive thin film includes one of second
periodic openings and periodic surface patterns formed with a
period length which is different from that of the first periodic
openings, the second periodic openings including a plurality of
openings which are different from the plurality of openings
extending through the film from the first surface to the second
surface, the periodic surface patterns being formed on one of the
first and second surfaces, and surface plasmon polaritons excited
by the first periodic openings undergo odd-order Bragg reflection
by the second periodic openings or the periodic surface
patterns.
16. The optical device according to claim 14, wherein a ratio
P.sub.2/P.sub.1 of a period length P.sub.2 of the second periodic
openings or the periodic surface patterns with respect to a period
length P.sub.1 of the first periodic openings is within a range of
.+-.0.2 of odd time(s) as much as 0.5.
17. The optical device according to claim 14, wherein a ratio
P.sub.2/P.sub.1 of a period length P.sub.2 of the second periodic
openings or the periodic surface patterns with respect to a period
length P.sub.1 of the first periodic openings is within a range of
0.5.+-.0.2.
18. The optical device according to claim 14, wherein a ratio
P.sub.2/P.sub.1 of a period length P.sub.2 of the second periodic
openings or the periodic surface patterns with respect to a period
length P.sub.1 of the first periodic openings is within a range of
1.5.+-.0.2.
19. The optical device according to claim 14, wherein the intensity
of the light transmitted through the first periodic openings
increases by 20% or more as compared with that of the light
transmitted through periodic openings of a device which is not
provided with the second periodic openings or the periodic surface
patterns.
20. The optical device according to claim 15, wherein the intensity
of the light transmitted through the first periodic openings
increases by 20% or more as compared with that of light transmitted
through periodic openings of a device which is not provided with
the second periodic openings or the periodic surface patterns.
21. The optical device according to claim 14, wherein the first
periodic openings are disposed so as to correspond to a region on
which light falls, and the second periodic openings or the periodic
surface patterns are disposed externally from the first periodic
openings.
22. The optical device according to claim 15, wherein the first
periodic openings are disposed so as to correspond to a region on
which light falls, and the second periodic openings or the periodic
surface patterns are disposed externally from the first periodic
openings.
23. The optical device according to claim 14, wherein a region
provided with the first periodic openings is broader than the
region on which light falls.
24. The optical device according to claim 15, wherein a region
provided with the first periodic openings is broader than the
region on which light falls.
25. An optical device comprising a conductive thin film having:
first and second surfaces; at least one opening extending through
the film from the first surface to the second surface; and a
periodic surface pattern disposed on at least one of the first and
second surfaces, an intensity of light falling on the first surface
and transmitted to the second surface through the opening being
increased as compared with a case where any periodic surface
pattern is not disposed, wherein the periodic surface pattern is
disposed on the second surface, and a period length of the periodic
surface pattern is substantially equal to odd time(s) as much as
1/2 of a value .lamda./n.sub.d obtained by dividing a wavelength
.lamda. of the transmitted light whose intensity is increased by an
effective refractive index n.sub.d of a medium substantially
adjacent to the second surface.
26. An optical device comprising a conductive thin film having:
first and second surfaces; at least one opening extending through
the film from the first surface to the second surface; and a
periodic surface pattern disposed on at least one of the first and
second surfaces, an intensity of light falling on the first surface
and transmitted to the second surface through the opening being
increased as compared with a case where any periodic surface
pattern is not disposed, wherein the periodic surface pattern is
disposed on the second surface, and assuming that a wavelength of
the transmitted light whose intensity is increased is .lamda., a
permittivity of the conductive thin film is .epsilon..sub.m, and a
permittivity of a medium substantially adjacent to the second
surface is .epsilon..sub.d, the period length of the periodic
surface pattern is substantially equal to odd time(s) as much as
1/2 of
.lamda./(.epsilon..sub.m.epsilon..sub.d/(.epsilon..sub.m+.epsilon..sub.d)-
).sup.1/2.
27. An optical device comprising a conductive thin film having:
first and second surfaces; at least one opening extending through
the film from the first surface to the second surface; and a
periodic surface pattern disposed on at least one of the first and
second surfaces, an intensity of light falling on the first surface
and transmitted to the second surface through the opening being
increased as compared with a case where any periodic surface
pattern is not disposed, wherein the periodic surface pattern is
disposed on the second surface, and surface plasmon polaritons in
the periodic surface pattern undergo odd-order Bragg reflection in
the periodic surface pattern.
28. The optical device according to claim 25, wherein a ratio
P.sub.3/(.lamda./n.sub.d) between a value .lamda./n.sub.d obtained
by dividing the wavelength .lamda. of the transmitted light whose
intensity is increased by the effective refractive index n.sub.d of
the medium substantially adjacent to the second surface and a
period length P.sub.3 of the periodic surface pattern is within a
range of .+-.0.2 of odd time(s) as much as 0.5.
29. The optical device according to claim 26, wherein assuming that
the wavelength of the transmitted light whose intensity is
increased is .lamda., the permittivity of the conductive thin film
is .epsilon..sub.m, and the permittivity of the medium
substantially adjacent to the second surface is .epsilon..sub.d, a
relation formula of the period length P.sub.3 of the periodic
surface pattern
P.sub.3/(.lamda./(.epsilon..sub.m.epsilon..sub.d/(.epsilon..sub.m+.epsilo-
n..sub.d)).sup.1/2) is within a range of .+-.0.2 of odd time(s) as
much as 0.5.
30. The optical device according to claim 25, wherein a ratio
P.sub.3/(.lamda./n.sub.d) between a value .lamda./n.sub.d obtained
by dividing the wavelength .lamda. of the transmitted light whose
intensity is increased by the effective refractive index n.sub.d of
the medium substantially adjacent to the second surface and a
period length P.sub.3 of the periodic surface pattern is within a
range of 0.5.+-.0.2.
31. The optical device according to claim 26, wherein assuming that
the wavelength of the transmitted light whose intensity is
increased is .lamda., the permittivity of the conductive thin film
is .epsilon..sub.m, and the permittivity of the medium
substantially adjacent to the second surface is .epsilon..sub.d, a
relation formula of the period length P.sub.3 of the periodic
surface pattern
P.sub.3/(.lamda./(.epsilon..sub.m.epsilon..sub.d/(.epsilon..sub.m+.epsilo-
n..sub.d)).sup.1/2) is within a range of 0.5.+-.0.2.
32. The optical device according to claim 25, wherein a ratio
P.sub.3/(.lamda./n.sub.d) between a value .lamda./n.sub.d obtained
by dividing the wavelength .lamda. of the transmitted light whose
intensity is increased by the effective refractive index n.sub.d of
the medium substantially adjacent to the second surface and a
period length P.sub.3 of the periodic surface pattern is within a
range of 1.5.+-.0.2.
33. The optical device according to claim 26, wherein assuming that
the wavelength of the transmitted light whose intensity is
increased is .lamda., the permittivity of the conductive thin film
is .epsilon..sub.m, and the permittivity of the medium
substantially adjacent to the second surface is .epsilon..sub.d, a
relation formula of the period length P.sub.3 of the periodic
surface pattern
P.sub.3/(.lamda./(.epsilon..sub.m.epsilon..sub.d(.epsilon..sub.m+.epsilon-
..sub.d)).sup.1/2) is within a range of 1.5.+-.0.2.
34. An optical device comprising a conductive thin film having:
first and second surfaces; at least one opening extending through
the film from the first surface to the second surface; and a
plurality of periodic surface patterns disposed on the first and
second surfaces, an intensity of light falling on the first surface
and transmitted to the second surface through the opening being
increased as compared with a case where any periodic surface
pattern is not disposed, wherein the plurality of periodic surface
patterns include first and second periodic surface patterns
disposed on the first surface and having mutually different period
lengths, and a third periodic surface pattern disposed on the
second surface, the period length of the second periodic surface
pattern is substantially equal to odd time(s) as much as 1/2 of the
period length of the first periodic surface pattern, and the period
length of the third periodic surface pattern is substantially equal
to odd time(s) as much as 1/2 of a value .lamda./n.sub.d obtained
by dividing a wavelength .lamda. of the transmitted light whose
intensity is increased by an effective refractive index n.sub.d of
a medium substantially adjacent to the second surface.
35. An optical device comprising a conductive thin film having:
first and second surfaces; at least one opening extending through
the film from the first surface to the second surface; and a
plurality of periodic surface patterns disposed on the first and
second surfaces, an intensity of light falling on the first surface
and transmitted to the second surface through the opening being
increased as compared with a case where any periodic surface
pattern is not disposed, wherein the plurality of periodic surface
patterns include first and second periodic surface patterns
disposed on the first surface and having mutually different period
lengths, and a third periodic surface pattern disposed on the
second surface, the period length of the second periodic surface
pattern is substantially equal to odd time(s) as much as 1/2 of the
period length of the first periodic surface pattern, and assuming
that a wavelength of the transmitted light whose intensity is
increased is .lamda., a permittivity of the conductive thin film is
.epsilon..sub.m, and a permittivity of a medium substantially
adjacent to the second surface is .epsilon..sub.d, the period
length of the third periodic surface pattern is substantially equal
to odd time(s) as much as 1/2 of
.lamda./(.epsilon..sub.m.epsilon..sub.d(.epsilon..sub.m+.epsilon..sub.d))-
.sup.1/2.
36. An optical device comprising a conductive thin film having:
first and second surfaces; at least one opening extending through
the film from the first surface to the second surface; and a
plurality of periodic surface patterns disposed on the first and
second surfaces, an intensity of light falling on the first surface
and transmitted to the second surface through the opening being
increased as compared with a case where any periodic surface
pattern is not disposed, wherein the plurality of periodic surface
patterns include first and second periodic surface patterns
disposed on the first surface and having mutually different period
lengths, and a third periodic surface pattern disposed on the
second surface, surface plasmon polaritons excited by the first
periodic surface pattern undergo odd-order Bragg reflection by the
second periodic surface pattern, and surface plasmon polaritons in
the third periodic surface pattern undergo odd-order Bragg
reflection in the third periodic surface pattern.
37. The optical device according to claim 34, wherein a ratio
P.sub.2/P.sub.1 of a period length P.sub.2 of the second periodic
surface pattern with respect to a period length P.sub.1 of the
first periodic surface pattern is within a range of .+-.0.2 of odd
time(s) as much as 0.5, and a ratio P.sub.3/(.lamda./n.sub.d)
between a value .lamda./n.sub.d obtained by dividing the wavelength
.lamda. of the transmitted light whose intensity is increased by
the effective refractive index n.sub.d of the medium substantially
adjacent to the second surface and a period length P.sub.3 of the
third periodic surface pattern is within a range of .+-.0.2 of odd
time(s) as much as 0.5.
38. The optical device according to claim 35, wherein a ratio
P.sub.2/P.sub.1 of a period length P.sub.2 of the second periodic
surface pattern with respect to a period length P.sub.1 of the
first periodic surface pattern is within a range of .+-.0.2 of odd
time(s) as much as 0.5, and assuming that the wavelength of the
transmitted light whose intensity is increased is .lamda., the
permittivity of the conductive thin film is .epsilon..sub.m, and
the permittivity of the medium substantially adjacent to the second
surface is .epsilon..sub.d, a relation formula of the period length
P.sub.3 of the third periodic surface pattern
P.sub.3/(.lamda./(.epsilon..sub.m.epsilon..sub.d/(.epsilon..sub.m+.epsilo-
n..sub.d)).sup.1/2) may be within the range of .+-.0.2 of odd
time(s) as much as 0.5.
39. The optical device according to claim 34, wherein a ratio
P.sub.2/P.sub.1 of a period length P.sub.2 of the second periodic
surface pattern with respect to a period length P.sub.1 of the
first periodic surface pattern is within a range of 0.5.+-.0.2, and
a ratio P.sub.3/(.lamda./n.sub.d) between a value .lamda./n.sub.d
obtained by dividing the wavelength .lamda. of the transmitted
light whose intensity is increased by the effective refractive
index n.sub.d of the medium substantially adjacent to the second
surface and a period length P.sub.3 of the third periodic surface
pattern is within a range of 0.5.+-.0.2.
40. The optical device according to claim 35, wherein a ratio
P.sub.2/P.sub.1 of a period length P.sub.2 of the second periodic
surface pattern with respect to a period length P.sub.1 of the
first periodic surface pattern is within a range of 0.5.+-.0.2, and
assuming that the wavelength of the transmitted light whose
intensity is increased is .lamda., the permittivity of the
conductive thin film is .epsilon..sub.m, and the permittivity of
the medium substantially adjacent to the second surface is
.epsilon..sub.d, a relation formula of the period length P.sub.3 of
the third periodic surface pattern
P.sub.3/(.lamda./(.epsilon..sub.m.epsilon..sub.d/(.epsilon..sub.m+.epsilo-
n..sub.d)).sup.1/2) may be within a range of 0.5.+-.0.2.
41. The optical device according to claim 34, wherein a ratio
P.sub.2/P.sub.1 of a period length P.sub.2 of the second periodic
surface pattern with respect to a period length P.sub.1 of the
first periodic surface pattern is within a range of 1.5.+-.0.2, and
a ratio P.sub.3/(.lamda./n.sub.d) between a value .lamda./n.sub.d
obtained by dividing the wavelength .lamda. of the transmitted
light whose intensity is increased by the effective refractive
index n.sub.d of the medium substantially adjacent to the second
surface and a period length P.sub.3 of the third periodic surface
pattern is within a range of 1.5.+-.0.2.
42. The optical device according to claim 35, wherein a ratio
P.sub.2/P.sub.1 of a period length P.sub.2 of the second periodic
surface pattern with respect to a period length P.sub.1 of the
first periodic surface pattern is within a range of 1.5.+-.0.2, and
assuming that the wavelength of the transmitted light whose
intensity is increased is .lamda., the permittivity of the
conductive thin film is .epsilon..sub.m, and the permittivity of
the medium substantially adjacent to the second surface is
.epsilon..sub.d, a relation formula of the period length P.sub.3 of
the third periodic surface pattern
P.sub.3/.lamda./(.epsilon..sub.m.epsilon..sub.d/(.epsilon..sub.m+.epsilon-
..sub.d)).sup.1/2) may be within a range of 1.5.+-.0.2.
43. The optical device according to claim 2, wherein an opening
diameter of the opening is smaller than a wavelength of the
incident light.
44. The optical device according to claim 15, wherein an opening
diameter of the opening is smaller than a wavelength of the
incident light.
45. The optical device according to claim 27, wherein an opening
diameter of the opening is smaller than a wavelength of the
incident light.
46. The optical device according to claim 36, wherein an opening
diameter of the opening is smaller than a wavelength of the
incident light.
47. The optical device according to claim 2, wherein the periodic
surface pattern is formed into a concentrically circular shape.
48. The optical device according to claim 15, wherein the periodic
surface pattern is formed into a concentrically circular shape.
49. The optical device according to claim 27, wherein the periodic
surface pattern is formed into a concentrically circular shape.
50. The optical device according to claim 36, wherein the periodic
surface pattern is formed into a concentrically circular shape.
51. The optical device according to claim 2, wherein the periodic
surface pattern is periodically disposed centering on the
opening.
52. The optical device according to claim 15, wherein the periodic
surface pattern is periodically disposed centering on the
opening.
53. The optical device according to claim 27, wherein the periodic
surface pattern is periodically disposed centering on the
opening.
54. The optical device according to claim 36, wherein the periodic
surface pattern is periodically disposed centering on the
opening.
55. An optical head which records information in an optical
recording medium by light from a light source, the optical head
comprising: waveguide means for guiding the output light of the
light source; condenser means for condensing the light guided by
the waveguide means; and an optical device which irradiates the
optical recording medium with a part of the light condensed by the
condenser means, the optical device comprising a conductive thin
film having: first and second surfaces; at least one opening
extending through the film from the first surface to the second
surface; and a periodic surface pattern disposed on at least one of
the first and second surfaces, an intensity of light falling on the
first surface and transmitted to the second surface through the
opening being increased as compared with a case where any periodic
surface pattern is not disposed, wherein the periodic surface
pattern includes first and second periodic surface patterns having
different period lengths, and surface plasmon polaritons excited by
the first periodic surface pattern undergo odd-order Bragg
reflection by the second periodic surface pattern.
56. An optical head which records information in an optical
recording medium by light from a light source, the optical head
comprising: waveguide means for guiding the output light of the
light source; condenser means for condensing the light guided by
the waveguide means; and an optical device which irradiates the
optical recording medium with a part of the light condensed by the
condenser means, the optical device comprising a conductive thin
film having: first and second surfaces; and a plurality of
periodically arranged openings extending through the film from the
first surface to the second surface, an intensity of light falling
on the first surface and transmitted to the second surface through
the plurality of openings being increased as compared with a case
where the plurality of openings are not periodically arranged,
wherein the plurality of periodically arranged openings are
regarded as first periodic openings, the conductive thin film
includes one of second periodic openings and periodic surface
patterns formed with a period length which is different from that
of the first periodic openings, the second periodic openings
including a plurality of openings which are different from the
plurality of openings extending through the film from the first
surface to the second surface, the periodic surface patterns being
formed on one of the first and second surfaces, and surface plasmon
polaritons excited by the first periodic openings undergo odd-order
Bragg reflection by the second periodic openings or the periodic
surface patterns.
57. An optical head which records information in an optical
recording medium by light from a light source, the optical head
comprising: waveguide means for guiding the output light of the
light source; condenser means for condensing the light guided by
the waveguide means; and an optical device which irradiates the
optical recording medium with a part of the light condensed by the
condenser means, the optical device comprising a conductive thin
film having: first and second surfaces; at least one opening
extending through the film from the first surface to the second
surface; and a periodic surface pattern disposed on at least one of
the first and second surfaces, an intensity of light falling on the
first surface and transmitted to the second surface through the
opening being increased as compared with a case where any periodic
surface pattern is not disposed, wherein the periodic surface
pattern is disposed on the second surface, and surface plasmon
polaritons in the periodic surface pattern undergo odd-order Bragg
reflection in the periodic surface pattern.
58. An optical head which records information in an optical
recording medium by light from a light source, the optical head
comprising: waveguide means for guiding the output light of the
light source; condenser means for condensing the light guided by
the waveguide means; and an optical device which irradiates the
optical recording medium with a part of the light condensed by the
condenser means, the optical device comprising a conductive thin
film having: first and second surfaces; at least one opening
extending through the film from the first surface to the second
surface; and a plurality of periodic surface patterns disposed on
the first and second surfaces, an intensity of light falling on the
first surface and transmitted to the second surface through the
openings being increased as compared with a case where there is not
any periodic surface pattern, wherein the plurality of periodic
surface patterns include first and second periodic surface patterns
disposed on the first surface and having mutually different period
lengths, and a third periodic surface pattern disposed on the
second surface, surface plasmon polaritons excited by the first
periodic surface pattern undergo odd-order Bragg reflection by the
second periodic surface pattern, and surface plasmon polaritons in
the third periodic surface pattern undergo odd-order Bragg
reflection in the third periodic surface pattern.
59. An optical recording device comprising: an optical head which
records information in an optical recording medium by light from a
light source, the optical head comprising: waveguide means for
guiding the output light of the light source; condenser means for
condensing the light guided by the waveguide means; and an optical
device which irradiates the optical recording medium with a part of
the light condensed by the condenser means, the optical device
comprising a conductive thin film having: first and second
surfaces; at least one opening extending through the film from the
first surface to the second surface; and a periodic surface pattern
disposed on at least one of the first and second surfaces, an
intensity of light falling on the first surface and transmitted to
the second surface through the opening being increased as compared
with a case where any periodic surface pattern is not disposed,
wherein the periodic surface pattern includes first and second
periodic surface patterns having different period lengths, and
surface plasmon polaritons excited by the first periodic surface
pattern undergo odd-order Bragg reflection by the second periodic
surface pattern.
60. An optical recording device comprising: an optical head which
records information in an optical recording medium by light from a
light source, the optical head comprising: waveguide means for
guiding the output light of the light source; condenser means for
condensing the light guided by the waveguide means; and an optical
device which irradiates the optical recording medium with a part of
the light condensed by the condenser means, the optical device
comprising a conductive thin film having: first and second
surfaces; and a plurality of periodically arranged openings
extending through the film from the first surface to the second
surface, an intensity of light falling on the first surface and
transmitted to the second surface through the plurality of openings
being increased as compared with a case where the plurality of
openings are not periodically arranged, wherein the plurality of
periodically arranged openings are regarded as first periodic
openings, the conductive thin film includes one of second periodic
openings and periodic surface patterns formed with a period length
which is different from that of the first periodic openings, the
second periodic openings including a plurality of openings which
are different from the plurality of openings extending through the
film from the first surface to the second surface, the periodic
surface patterns being formed on one of the first and second
surfaces, and surface plasmon polaritons excited by the first
periodic openings undergo odd-order Bragg reflection by the second
periodic openings or the periodic surface patterns.
61. An optical recording device comprising: an optical head which
records information in an optical recording medium by light from a
light source, the optical head comprising: waveguide means for
guiding the output light of the light source; condenser means for
condensing the light guided by the waveguide means; and an optical
device which irradiates the optical recording medium with a part of
the light condensed by the condenser means, the optical device
comprising a conductive thin film having: first and second
surfaces; at least one opening extending through the film from the
first surface to the second surface; and a periodic surface pattern
disposed on at least one of the first and second surfaces, an
intensity of light falling on the first surface and transmitted to
the second surface through the opening being increased as compared
with a case where any periodic surface pattern is not disposed,
wherein the periodic surface pattern is disposed on the second
surface, and surface plasmon polaritons in the periodic surface
pattern undergo odd-order Bragg reflection in the periodic surface
pattern.
62. An optical recording device comprising: an optical head which
records information in an optical recording medium by light from a
light source, the optical head comprising: waveguide means for
guiding the output light of the light source; condenser means for
condensing the light guided by the waveguide means; and an optical
device which irradiates the optical recording medium with a part of
the light condensed by the condenser means, the optical device
comprising a conductive thin film having: first and second
surfaces; at least one opening extending through the film from the
first surface to the second surface; and a plurality of periodic
surface patterns disposed on the first and second surfaces, an
intensity of light falling on the first surface and transmitted to
the second surface through the openings being increased as compared
with a case where there is not any periodic surface pattern,
wherein the plurality of periodic surface patterns include first
and second periodic surface patterns disposed on the first surface
and having mutually different period lengths, and a third periodic
surface pattern disposed on the second surface, surface plasmon
polaritons excited by the first periodic surface pattern undergo
odd-order Bragg reflection by the second periodic surface pattern,
and surface plasmon polaritons in the third periodic surface
pattern undergo odd-order Bragg reflection in the third periodic
surface pattern.
63. A condensing device which condenses only light having a
specific wavelength from light from a light source, comprising: an
optical device comprising a conductive thin film having: first and
second surfaces; at least one opening extending through the film
from the first surface to the second surface; and a periodic
surface pattern disposed on at least one of the first and second
surfaces, an intensity of light falling on the first surface and
transmitted to the second surface through the opening being
increased as compared with a case where any periodic surface
pattern is not disposed, wherein the periodic surface pattern
includes first and second periodic surface patterns having
different period lengths, and surface plasmon polaritons excited by
the first periodic surface pattern undergo odd-order Bragg
reflection by the second periodic surface pattern.
64. A condensing device which condenses only light having a
specific wavelength from light from a light source, comprising: an
optical device comprising: a conductive thin film having: first and
second surfaces; and a plurality of periodically arranged openings
extending through the film from the first surface to the second
surface, an intensity of light falling on the first surface and
transmitted to the second surface through the plurality of openings
being increased as compared with a case where the plurality of
openings are not periodically arranged, wherein the plurality of
periodically arranged openings are regarded as first periodic
openings, the conductive thin film includes one of second periodic
openings and periodic surface patterns formed with a period length
which is different from that of the first periodic openings, the
second periodic openings including a plurality of openings which
are different from the plurality of openings extending through the
film from the first surface to the second surface, the periodic
surface patterns being formed on one of the first and second
surfaces, and surface plasmon polaritons excited by the first
periodic openings undergo odd-order Bragg reflection by the second
periodic openings or the periodic surface patterns.
65. A condensing device which condenses only light having a
specific wavelength from light from a light source, comprising: an
optical device comprising a conductive thin film having: first and
second surfaces; at least one opening extending through the film
from the first surface to the second surface; and a periodic
surface pattern disposed on at least one of the first and second
surfaces, an intensity of light falling on the first surface and
transmitted to the second surface through the opening being
increased as compared with a case where any periodic surface
pattern is not disposed, wherein the periodic surface pattern is
disposed on the second surface, and surface plasmon polaritons in
the periodic surface pattern undergo odd-order Bragg reflection in
the periodic surface pattern.
66. A condensing device which condenses only light having a
specific wavelength from light from a light source, comprising: an
optical device comprising a conductive thin film having: first and
second surfaces; at least one opening extending through the film
from the first surface to the second surface; and a plurality of
periodic surface patterns disposed on the first and second
surfaces, an intensity of light falling on the first surface and
transmitted to the second surface through the openings being
increased as compared with a case where there is not any periodic
surface pattern, wherein the plurality of periodic surface patterns
include first and second periodic surface patterns disposed on the
first surface and having mutually different period lengths, and a
third periodic surface pattern disposed on the second surface,
surface plasmon polaritons excited by the first periodic surface
pattern undergo odd-order Bragg reflection by the second periodic
surface pattern, and surface plasmon polaritons in the third
periodic surface pattern undergo odd-order Bragg reflection in the
third periodic surface pattern.
67. A near-field optical microscope comprising: an optical device
which condenses only light having a specific wavelength from light
from a light source; and detection means for receiving the light
emitted by the optical device, the optical device comprising a
conductive thin film having: first and second surfaces; at least
one opening extending through the film from the first surface to
the second surface; and a periodic surface pattern disposed on at
least one of the first and second surfaces, an intensity of light
falling on the first surface and transmitted to the second surface
through the opening being increased as compared with a case where
any periodic surface pattern is not disposed, wherein the periodic
surface pattern includes first and second periodic surface patterns
having different period lengths, and surface plasmon polaritons
excited by the first periodic surface pattern undergo odd-order
Bragg reflection by the second periodic surface patter.
68. A near-field optical microscope comprising: an optical device
which condenses only light having a specific wavelength from light
from a light source; and detection means for receiving the light
emitted by the optical device, the optical device comprising: a
conductive thin film having: first and second surfaces; and a
plurality of periodically arranged openings extending through the
film from the first surface to the second surface, an intensity of
light falling on the first surface and transmitted to the second
surface through the plurality of openings being increased as
compared with a case where the plurality of openings are not
periodically arranged, wherein the plurality of periodically
arranged openings are regarded as first periodic openings, the
conductive thin film includes one of second periodic openings and
periodic surface patterns formed with a period length which is
different from that of the first periodic openings, the second
periodic openings including a plurality of openings which are
different from the plurality of openings extending through the film
from the first surface to the second surface, the periodic surface
patterns being formed on one of the first and second surfaces, and
surface plasmon polaritons excited by the first periodic openings
undergo odd-order Bragg reflection by the second periodic openings
or the periodic surface patterns.
69. A near-field optical microscope comprising: an optical device
which condenses only light having a specific wavelength from light
from a light source; and detection means for receiving the light
emitted by the optical device, the optical device comprising a
conductive thin film having: first and second surfaces; at least
one opening extending through the film from the first surface to
the second surface; and a periodic surface pattern disposed on at
least one of the first and second surfaces, an intensity of light
falling on the first surface and transmitted to the second surface
through the opening being increased as compared with a case where
any periodic surface pattern is not disposed, wherein the periodic
surface pattern is disposed on the second surface, and surface
plasmon polaritons in the periodic surface pattern undergo
odd-order Bragg reflection in the periodic surface pattern.
70. A near-field optical microscope comprising: an optical device
which condenses only light having a specific wavelength from light
from a light source; and detection means for receiving the light
emitted by the optical device, the optical device comprising a
conductive thin film having: first and second surfaces; at least
one opening extending through the film from the first surface to
the second surface; and a plurality of periodic surface patterns
disposed on the first and second surfaces, an intensity of light
falling on the first surface and transmitted to the second surface
through the openings being increased as compared with a case where
there is not any periodic surface pattern, wherein the plurality of
periodic surface patterns include first and second periodic surface
patterns disposed on the first surface and having mutually
different period lengths, and a third periodic surface pattern
disposed on the second surface, surface plasmon polaritons excited
by the first periodic surface pattern undergo odd-order Bragg
reflection by the second periodic surface pattern, and surface
plasmon polaritons in the third periodic surface pattern undergo
odd-order Bragg reflection in the third periodic surface pattern.
Description
TECHNICAL FIELD
[0001] The present invention relates to an optical device, more
particularly to an optical device provided with a conductive thin
film including an opening having a diameter which is not more than
a wavelength of incident light and a periodic surface pattern, the
optical device being capable of achieving a very high
throughput.
BACKGROUND ART
[0002] An optical recording medium such as a compact disk read only
memory (CD-ROM), a digital video disk, or a digital versatile disk
(DVD) has features such as a high recording density, a compact
design, portability, and tenacity. Additionally, not only a price
of the medium itself but also a price of a recording and
reproducing device are decreasing. Therefore, the medium
increasingly becomes an attractive data storage medium. Moreover,
in this type of optical recording medium, there is a demand for a
higher recording density in order to make possible the recording of
video data for a longer time.
[0003] To increase the recording density of the optical recording
medium in excess of the present value, it is necessary to reduce a
size of a light beam which writes or reads data. Mainly a
wavelength of the light beam and a numerical aperture of the
condenser lens determine the size of the light spot in a focal
point at a time when a usual optical system, that is, a condenser
lens is used. In general, the size of the light spot can be reduced
by use of a light source having a short wavelength and a lens
having a high numerical aperture. However, in this method, there
exists a spot size limitation by a so-called diffractive
limitation, and the size of the spot is approximately a half of the
wavelength of the light source.
[0004] In recent years, a near-field optical technology has
attracted attention as a technology which is not bound by this
diffractive limitation. For example, when the light beam is emitted
and transmitted (passed) through a small hole whose size is not
more than the wavelength, there is formed a micro light spot having
approximately the same size as the hole size. In a case where this
is utilized, when the hole is brought close to the recording
medium, it is expected to realize writing or reading of a micro pit
by the micro light spot which is not limited by the wavelength of
the light source.
[0005] On the other hand, there has been a problem to be solved in
an optical head utilizing such near-field optical technology. The
problem is that light use efficiency is low, and it is difficult to
sufficiently transmit the light via the hole. A power of the light
transmitted through the hole (hole diameter d) which is disposed in
a metal film and whose size is not more than wavelength .lamda. is
proportional to a fourth power of (d/.lamda.), and remarkably
decays as described in "Theory of Diffraction by Small Hole"
authored by H. A. Bethe, Physical Review, vol. 66, pages 163 to 182
(1944). Therefore, the light transmission via the small hole has
latent problems such as a signal-to-noise ratio which is
excessively low for the reading and a light intensity which is
excessively low for the writing. As a result, there has not been
obtained so far a practical optical head using the near-field
optical technology.
[0006] To conquer such situation, there have been already proposed
several optical transmission technologies in which a transmittance
of light transmitted through a hole array is remarkably improved by
using a metal thin film provided with the hole array having a
diameter that is less than the light wavelength. For example, refer
to "Extraordinary Optical Transmission through Sub-wavelength Hole
Arrays" authored by Ebbesen et. al., Nature, vol. 391, pages 667 to
669 (Feb. 12, 1988), and Japanese Patent Application Laid-Open Nos.
11-72607 (or U.S. Pat. No. 5,973,316), 2000-111851 (or U.S. Pat.
No. 6,040,936), 2000-171763 (or U.S. Pat. No. 6,236,033), and
2001-133618 (or U.S. Pat. No. 6,285,020).
[0007] According to these proposals, when the holes are arranged in
a periodic array, or a periodic surface pattern is disposed in the
metal thin film together with the holes, the light intensity
largely increases as compared with a case where there is not any
periodic hole or surface pattern, the light intensity being an
intensity of light emitted to the metal thin film and transmitted
through one or more holes made in the metal thin film and each
having a diameter that is not more than the wavelength. According
to an experimental verification, an increase ratio sometimes
reaches 1,000-folds. It is said that this increase is caused at a
time when the light falling on the metal thin film interacts
resonantly with a surface plasmon mode excited by the metal thin
film.
[0008] Moreover, to increase a quantity of transmitted light in the
near-field optical recording head, there has been already proposed
a read/write head for an optical recording device, whose metal film
is provided with holes each having a size that is not more than the
wavelength, and the periodic surface pattern. In the head, there is
utilized a surface plasmon-enhancement effect produced by the
periodic surface pattern, and the head has a very high transmitted
light power density and resolution. Refer to, for example, Japanese
Patent Application Laid-Open No. 2001-291265. Since this proposed
optical recording head has the holes formed in the metal film and
the periodic surface pattern disposed in at least one of metal film
surfaces, the light falling on one of the metal film surfaces
interacts with the surface plasmon mode in at least one of the
metal film surfaces. This increases the light transmitted through
the holes extending through the metal film.
[0009] However, even in the above-described high-efficiency optical
transmission technology utilizing the conventional surface
plasmons, a sufficient light transmission efficiency has not been
obtained yet, and there has not been realized up to now the device
whose hole diameter is less than the wavelength and which exhibits
the sufficient transmission efficiency.
[0010] On the other hand, the surface plasmons excited by the
surface of the metal film can pass as surface plasmon polaritons on
the surface of the metal film. Furthermore, it is reported that the
light transmitted by the surface plasmon polaritons is
Bragg-reflected by a periodic structure in the same manner as in
usual light. It is also reported that the surface plasmon
polaritons passing on the metal film surface are efficiently
reflected, because the periodic surface pattern functions as a
mirror in a case where the period of the periodic surface pattern
disposed on the metal film surface has a period length which is
very close to that of the Bragg reflection conditions. Refer to,
for example, "Two-dimensional Optics with Surface Plasmon
Polaritons" authored by H. Ditlbacher, et al., Applied Physics
Letters, 81, p1762 (Sep. 2, 2002).
[0011] However, H. Ditibacher, et al. only report the mirror
function on the metal film surface.
[0012] As described above, it is very difficult to transmit the
light via the holes which are not larger than the wavelength. To
solve this problem, there are several proposals utilizing the
transmitted light amplified by the surface plasmon effect, but a
utilization efficiency of the resultant transmitted light with
respect to the incident light is not sufficient yet.
DISCLOSURE OF THE INVENTION
[0013] Therefore, an object of the present invention is to make
possible transmission of light with a high efficiency in an optical
device provided with a conductive thin film having an opening and a
periodic surface pattern.
[0014] In a case where transmitted light amplified by a surface
plasmon effect is utilized, when a utilization efficiency of the
resultant transmitted light with respect to incident light is
improved, it is important to efficiently utilize energy converted
into surface plasmons by a periodic structure and inhibit the
energy from being scattered. That is, when the energy scattered
externally from the periodic structure is efficiently returned by
the Bragg reflection, it is possible to realize high-efficiency
light transmission.
[0015] An optical device in a first aspect of the present invention
is an optical device comprising a conductive thin film having:
first and second surfaces; at least one opening extending through
the film from the first surface to the second surface; and a
periodic surface pattern disposed on at least one of the first and
second surfaces, an intensity of light falling on the first surface
and transmitted to the second surface through the opening being
increased as compared with a case where any periodic surface
pattern is not disposed. The periodic surface pattern includes
first and second periodic surface patterns having different period
lengths, and the period length of the second periodic surface
pattern is substantially equal to odd time(s) as much as 1/2 of the
period length of the first periodic surface pattern.
[0016] Here, the period length of the second periodic surface
pattern may be a period length in a case where surface plasmon
polaritons excited by the first periodic surface pattern undergo
odd-order Bragg reflection by the second periodic surface pattern.
To obtain an effect of the odd-order Bragg reflection, assuming
that the period length of the first periodic surface pattern is
P.sub.1, and the period length of the second periodic surface
pattern is P.sub.2, P.sub.2/P.sub.1 needs to be within a range of
.+-.0.2 of odd time(s) as much as 0.5, and a remarkable increase of
light transmittance can be obtained in this range.
[0017] Furthermore, the first and second periodic surface patterns
may be disposed on the same plane as the first or second surface.
The first periodic surface pattern is disposed so as to correspond
to a region on which light falls on, so that a region provided with
the first periodic surface pattern is irradiated with the light
falling on one surface. The second periodic surface pattern is
disposed externally from the first periodic surface pattern.
Furthermore, the region provided with the first periodic surface
pattern is set to be broader than the region on which the light
falls to thereby set a luminous flux diameter of the light with
which the first periodic surface pattern is irradiated to be
smaller than the region provided with the first periodic surface
pattern. Consequently, a higher effect is obtained. Here, the
luminous flux diameter of the light is defined by a width with
which light intensity becomes 1/e.sup.2 of a peak intensity.
[0018] Moreover, an optical device in a second aspect of the
present invention is an optical device in which there are
periodically arranged a plurality of openings extending through the
film from a first surface to a second surface and in which an
intensity of light transmitted through the plurality of openings is
increased as compared with a case where the plurality of openings
are not periodically arranged. The plurality of periodically
arranged openings are regarded as first periodic openings. There
are disposed second periodic openings or periodic surface patterns
formed with a period length which is different from that of the
first periodic openings, the second periodic openings including a
plurality of openings which are different from the plurality of
openings extending through the film from the first surface to the
second surface, the periodic surface patterns being formed on one
of the first and second surfaces. The period length of the second
periodic openings or the periodic surface patterns is substantially
equal to odd time(s) as much as 1/2 of the period length of the
first periodic openings.
[0019] Here, the period length of the second periodic openings or
the periodic surface patterns may be a period length in a case
where surface plasmon polaritons excited by the first periodic
openings undergo odd-order Bragg reflection by the second periodic
openings or the periodic surface patterns. To obtain an effect of
the odd-order Bragg reflection, assuming that the period length of
the first periodic openings is P.sub.1, and the period length of
the second periodic openings or the periodic surface patterns is
P.sub.2, P.sub.2/P.sub.1 needs to be within a range of .+-.0.2 of
odd time(s) as much as 0.5, and a remarkable increase of light
transmittance can be obtained in this range.
[0020] Furthermore, the first periodic openings are disposed so as
to correspond to the region on which light falls, so that a region
provided with the first periodic openings is irradiated with the
light falling on one surface. The second periodic openings or the
periodic surface patterns are disposed externally from the first
periodic openings. The region provided with the first periodic
surface patterns is set to be larger than the region on which the
light falls to thereby set a luminous flux diameter of the light
with which the first periodic openings are irradiated to be smaller
than the region provided with the first periodic openings.
Consequently, a higher effect is obtained.
[0021] Moreover, an optical device in a third aspect of the present
invention is an optical device comprising a conductive thin film
having: first and second surfaces; at least one opening extending
through the film from the first surface to the second surface; and
a periodic surface pattern disposed on at least one of the first
and second surfaces, an intensity of light falling on the first
surface and transmitted to the second surface through the opening
being increased as compared with a case where any periodic surface
pattern is not disposed. The periodic surface pattern is disposed
on the second surface, and a period length of the periodic surface
pattern is substantially equal to odd time(s) as much as 1/2 of a
value .lamda./n.sub.d obtained by dividing a wavelength .lamda. of
the transmitted light whose intensity is increased by an effective
refractive index n.sub.d of a medium substantially adjacent to the
second surface. Alternatively, assuming that a wavelength of the
transmitted light whose intensity is increased is .lamda., a
permittivity of the conductive thin film is .epsilon..sub.m, and a
permittivity of a medium substantially adjacent to the second
surface is .epsilon..sub.d, the period length of the periodic
surface pattern is substantially equal to odd time(s) as much as
1/2 of
.lamda./(.epsilon..sub.m.epsilon..sub.d/(.epsilon..sub.m+.epsilon..sub.d)-
).sup.1/2.
[0022] Here, surface plasmon polaritons in the periodic surface
pattern disposed on the second surface may undergo odd-order Bragg
reflection in the periodic surface pattern. To obtain an effect of
the odd-order Bragg reflection, a ratio P.sub.3/(.lamda./n.sub.d)
between a value .lamda./n.sub.d obtained by dividing the wavelength
.lamda. of the transmitted light whose intensity is increased by
the effective refractive index n.sub.d of the medium substantially
adjacent to the second surface and a period length P.sub.3 of the
periodic surface pattern may be within a range of .+-.0.2 of odd
time(s) as much as 0.5. Alternatively, assuming that the wavelength
of the transmitted light whose intensity is increased is .lamda.,
the permittivity of the conductive thin film is .epsilon..sub.m,
and the permittivity of the medium substantially adjacent to the
second surface is .epsilon..sub.d, a relation formula of the period
length P.sub.3 of the periodic surface pattern
P.sub.3/(.lamda./(.epsilon..sub.m.epsilon..sub.d/(.epsilon..sub.m-
+.epsilon..sub.d)).sup.1/2) may be within the range of .+-.0.2 of
odd time(s) as much as 0.5. It is possible to obtain a remarkable
increase of a light transmittance in this range. On the other hand,
a further effect in this structure lies in that the intensity of
the light transmitted through the opening can be strongly
concentrated in the vicinity of the opening. That is, when the
period length P.sub.3 of the periodic surface pattern is within the
above-described range, it is possible to remarkably increase the
intensity of the light in the vicinity of the opening on an
emission side.
[0023] Furthermore, an optical device in a fourth aspect of the
present invention is characterized in that the structure of the
first aspect is combined with that of the third aspect. That is, an
optical device of the fourth aspect is an optical device comprising
a conductive thin film having: first and second surfaces; at least
one opening extending through the film from the first surface to
the second surface; and a plurality of periodic surface patterns
disposed on the first and second surfaces, an intensity of light
falling on the first surface and transmitted to the second surface
through the opening being increased as compared with a case where
any periodic surface pattern is not disposed. The plurality of
periodic surface patterns include first and second periodic surface
patterns disposed on the first surface and having mutually
different period lengths, and a third periodic surface pattern
disposed on the second surface. The period length of the second
periodic surface pattern is substantially equal to odd time(s) as
much as 1/2 of the period length of the first periodic surface
pattern. The period length of the third periodic surface pattern is
substantially equal to odd time(s) as much as 1/2 of a value
.lamda./n.sub.d obtained by dividing a wavelength .lamda. of the
transmitted light whose intensity is increased by an effective
refractive index n.sub.d of a medium substantially adjacent to the
second surface. Alternatively, assuming that a wavelength of the
transmitted light whose intensity is increased is .lamda., a
permittivity of the conductive thin film is .epsilon..sub.m, and a
permittivity of a medium substantially adjacent to the second
surface is .epsilon..sub.d, the period length of the third periodic
surface pattern is substantially equal to odd time(s) as much as
1/2 of
.lamda./(.epsilon..sub.m.epsilon..sub.d/(.epsilon..sub.m+.epsilon..sub.d)-
).sup.1/2.
[0024] Here, the period length of the second periodic surface
pattern may be a period length in a case where surface plasmon
polaritons excited by the first periodic surface pattern undergo
odd-order Bragg reflection by the second periodic surface pattern.
To obtain an effect of the odd-order Bragg reflection, assuming
that the period length of the first periodic surface pattern is
P.sub.1, and the period length of the second periodic surface
pattern is P.sub.2, P.sub.2/P.sub.1 needs to be within a range of
.+-.0.2 of odd time(s) as much as 0.5, and a remarkable increase of
light transmittance can be obtained in this range. Furthermore, the
third periodic surface pattern disposed on the second surface is
disposed so that the surface plasmon polaritons undergo odd-order
Bragg reflection by the third periodic surface pattern disposed on
the second surface. To obtain the effect of the odd-order Bragg
reflection, a ratio P.sub.3/(.lamda./n.sub.d) between a value
.lamda./n.sub.d obtained by dividing the wavelength .lamda. of the
transmitted light whose intensity is increased by the effective
refractive index n.sub.d of the medium substantially adjacent to
the second surface and a period length P.sub.3 of the third
periodic surface pattern may be within a range of .+-.0.2 of odd
time(s) as much as 0.5. Alternatively, assuming that the wavelength
of the transmitted light whose intensity is increased is .lamda.,
the permittivity of the conductive thin film is .epsilon..sub.m,
and the permittivity of the medium substantially adjacent to the
second surface is .epsilon..sub.d, a relation formula of the period
length P.sub.3 of the third periodic surface pattern
P.sub.3/(.lamda./(.epsilon..sub.m.epsilon..sub.d/(.epsilon..sub.m+.epsilo-
n..sub.d)).sup.1/2) may be within the range of .+-.0.2 of odd
time(s) as much as 0.5. It is possible to obtain a remarkable
increase of a light transmittance in such range. On the other hand,
a further effect in this structure lies in that the intensity of
the light transmitted through the opening can be strongly
concentrated in the vicinity of the opening. That is, when the
period length P.sub.3 of the third periodic surface pattern is
within the above-described range, it is possible to remarkably
increase the intensity of the light in the vicinity of the opening
on an emission side.
[0025] Moreover, in the present invention, there is provided an
optical head for an optical recording medium. This optical head is
an optical head which records at least information in the optical
recording medium by light from a light source, and the optical head
comprises: waveguide means for guiding the output light of the
light source; condenser means for condensing the light guided by
the waveguide means; and any one of the optical devices in the
above-described first to fourth aspects of the present invention.
The optical device is disposed so as to bring an opening of the
optical device close to the optical recording medium, and the
optical recording medium is irradiated with a part of the condensed
light.
[0026] According to the above-described constitution, there is
obtained an optical head in which the intensity of the light
emitted from the small opening is increased to a value that is not
less than a value required for the recording in the optical
recording medium, and the information can be recorded with a
density that is higher than ever.
[0027] Furthermore, in the present invention, there is provided an
optical recording device. The optical recording device comprises
the optical head in order to record the information in the optical
recording medium by the light from the light source.
[0028] According to the above constitution, the optical recording
device is obtained in which the information can be recorded with
the density that is higher than ever.
[0029] Additionally, there is provided a condensing device which
condenses only light having a specific wavelength from the light
from the light source by use of the optical device of the present
invention.
[0030] Moreover, there is provided a near-field optical microscope
which condenses only light having a specific wavelength from the
light from the light source to irradiate a sample with the light by
use of the optical device of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a sectional view of an optical device in a first
embodiment of the present invention;
[0032] FIG. 2(A) is a plan view showing a first surface of a
conductive thin film in the optical device shown in FIG. 1;
[0033] FIG. 2(B) is a plan view (bottom plan view) showing a second
surface of the conductive thin film in the optical device shown in
FIG. 1;
[0034] FIG. 3 is a sectional view showing a first modification of
the optical device of FIG. 1;
[0035] FIG. 4(A) is a plan view showing a first surface of a
conductive thin film in a second modification of the optical device
of FIG. 1;
[0036] FIG. 4(B) is a plan view (bottom plan view) showing a second
surface of the conductive thin film in the second modification;
[0037] FIG. 5(A) is a plan view showing a first surface of a
conductive thin film in a third modification of the optical device
of FIG. 1;
[0038] FIG. 5(B) is a plan view (bottom plan view) showing a second
surface of the conductive thin film in the third modification;
[0039] FIG. 6(A) is a plan view showing a first surface of a
conductive thin film in a fourth modification of the optical device
of FIG. 1;
[0040] FIG. 6(B) is a plan view (bottom plan view) showing a second
surface of the conductive thin film in the fourth modification;
[0041] FIG. 7(A) is a plan view showing a first surface of a
conductive thin film in a fifth modification of the optical device
of FIG.1;
[0042] FIG. 7(B) is a plan view (bottom plan view) showing a second
surface of the conductive thin film in the fifth modification;
[0043] FIG. 8(A) is an explanatory view of a function of an optical
device having a conventional structure;
[0044] FIG. 8(B) is a conceptual diagram showing a function of the
optical device of FIG. 1;
[0045] FIG. 9 is a graph showing a correlation between a period
length of a periodic surface pattern and an enhancement factor of
transmitted light in the optical device of FIG. 1;
[0046] FIG. 10(A) shows an electric field distribution in the
optical device having a conventional structure;
[0047] FIG. 10(B) shows an electric field distribution in the
optical device of FIG. 1;
[0048] FIG. 11 is a sectional view of an optical device in a second
embodiment of the present invention;
[0049] FIG. 12 is a diagram showing a first modification of the
optical device of FIG. 11;
[0050] FIG. 13 is a diagram showing a second modification of the
optical device of FIG. 11;
[0051] FIG. 14(A) is a conceptual diagram showing the function of
the optical device having the conventional structure;
[0052] FIG. 14(B) is a conceptual diagram showing the function of
the optical device of FIG. 11;
[0053] FIG. 15 is a graph showing a correlation between a period
length of a periodic surface pattern and an enhancement factor of
light intensity in the optical device of FIG. 11;
[0054] FIG. 16(A) shows an electric field distribution in the
optical device including the conventional structure;
[0055] FIG. 16(B) shows an electric field distribution in the
optical device of FIG. 12;
[0056] FIG. 17 is a diagram showing one embodiment of an optical
head of the present invention;
[0057] FIG. 18 is a diagram showing one embodiment of an optical
recording and reproducing device of the present invention;
[0058] FIG. 19(A) is a diagram showing one embodiment of a
condensing device of the present invention;
[0059] FIG. 19(B) is a diagram showing another embodiment of the
condensing device of the present invention; and
[0060] FIG. 20 is a diagram showing one embodiment of a near-field
microscope of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiment 1
[0061] Next, an embodiment of the present invention will be
described with reference to the drawings.
[0062] FIGS. 1, and 2(A) and (B) are diagrams showing an optical
device 10 in a first embodiment of the present invention. The
optical device 10 has a conductive thin film 20 including a first
surface 20a and a second surface 20b. The first surface 20a of the
conductive thin film 20 is irradiated with incident light. The
conductive thin film 20 has at least one hole whose diameter is d,
that is, an opening 30. The conductive thin film 20 is made of a
metal or a doped semiconductor material, and aluminum, silver,
gold, chromium or the like is preferable. In a case where light of
a visible light region is used as incident light, silver is
preferable from a viewpoint of optical loss, but the material is
not necessarily limited to silver.
[0063] The conductive thin film 20 is provided with a first
periodic surface pattern 40a in at least one of the first surface
20a and the second surface 20b (the first surface 20a only in the
present embodiment, but the patterns may be formed on the opposite
surfaces), and the film is provided with a second periodic surface
pattern 40b whose period length is different from that of the first
periodic surface pattern.
[0064] The first periodic surface pattern 40a is disposed so as to
correspond to incident light. To be more specific, the first
periodic surface pattern 40a is disposed in the same region as a
region irradiated with the incident light, or a slightly larger
region. In other words, the first periodic surface pattern 40a is
disposed so that the region provided with the first periodic
surface pattern 40a is irradiated with the incident light. In a
case where these regions are circular, the first periodic surface
pattern 40a is formed so that a diameter of the region provided
with the first periodic surface pattern 40a becomes larger than a
luminous flux diameter of the incident light. It is to be noted
that the luminous flux diameter of the light is defined by a width
with which the light intensity becomes 1/e.sup.2 of a peak
intensity.
[0065] The second periodic surface pattern 40b is disposed in a
periphery of a region provided with the first periodic surface
pattern 40a. A period length P.sub.2 of the second periodic surface
pattern 40b is odd time(s) (value assumed to be substantially equal
to odd time(s) as much as 1/2) as much as about 1/2 of the period
length P.sub.1 of the first periodic surface pattern 40a. FIGS. 1
and 2(A) show a case where the period length P.sub.2 of the second
periodic surface pattern 40b is about 1/2 of the period length
P.sub.1 of the first periodic surface pattern 40a, but, as shown in
FIG. 3, the period length P.sub.2 of the second periodic surface
pattern 40b may be about 3/2 of the period length P.sub.1 of the
first periodic surface pattern 40a.
[0066] The periodic surface pattern refers to a pattern in which
regions raised from a certain reference face and/or depressed
regions are periodically or regularly repeatedly arranged (in the
form of, for example, a regular two-dimensional grid or concentric
circle). Here, the term "periodic surface pattern" should be
distinguished from the opening 30 extending through the whole
thickness of the conductive thin film 20. That is, the term
"periodic surface pattern" is used for describing a periodic
protrusion or dent which does not extend through the whole
thickness of the conductive thin film 20 and therefore which is not
an opening. The periodic surface pattern may be formed into any
arbitrary pattern. In a case where the periodic surface patterns
are disposed around one opening, a plurality of dents or
protrusions are preferably concentrically arranged centering on the
opening, or a plurality of concentric grooves or protrusions may be
arranged. However, the intention of the present invention is not
restricted even by any specific dimension of the periodic surface
pattern.
[0067] The incident light having an intensity denoted with symbol
I.sub.incident and shown by an arrow in an upper part of FIG. 1
(similarly in FIG. 3) is directed to the first surface 20a of the
conductive thin film 20. The light is transmitted as output light
having an increased intensity denoted with symbol I.sub.output and
shown by an arrow in a lower part of FIG. 1 from the opening 30 in
the second surface 20b of the conductive thin film 20. It is to be
noted that a transmission intensity increases even in a case where
the light passes through this structure and moves in an opposite
direction, that is, if the light falls on the second surface 20b
and is transmitted as the output light from the first surface.
[0068] A diameter d of the opening 30 is preferably smaller than a
wavelength of the incident light falling on the opening 30 in order
to increase and maximize the transmission intensity and obtain a
maximum resolution. That is, the opening on an emission side
preferably has a diameter which is less than the wavelength.
[0069] A thickness of the conductive thin film 20 is denoted with
t, and the conductive thin film 20 has to be sufficiently thickened
so as to be optically opaque. That is, the thickness t has to be
larger than a penetration thickness of the incident light.
[0070] FIGS. 1 to 3 show a thin conductive thin film 20 which is
not supported. That is, the conductive thin film 20 is not adjacent
or fixed to a support structure (substrate). However, in the
present invention, the conductive thin film is deposited on glass
or quartz, and the thin conductive thin film 20 may be fixed to the
substrate.
[0071] When the substrate is used, the periodic surface pattern may
be disposed on either of an exposed surface of the conductive thin
film 20 and the surface of the film on the side of an interface
between the film and the substrate. When the periodic surface
pattern is disposed on the surface of the conductive thin film on
the side of the interface between the film and the substrate, for
example, a negative pattern is produced on the surface of the
substrate, and the conductive thin film is deposited on the
substrate provided with the negative pattern, so that the periodic
surface pattern can be disposed on the conductive thin film.
[0072] In FIGS. 1 and 3, the opening 30 is drawn so as to extend
through the film from the first surface to the second surface. In
other words, the opening 30 is shown so as to extend through the
whole thickness of the conductive thin film 20 in FIGS. 1 and 3.
However, unless the opening 30 extends through the conductive thin
film 20, there may be left a conductive thin film which is
optically sufficiently thin, that is, whose thickness is
approximately smaller than the penetration thickness of the
incident light. For example, in a case where the conductive thin
film is fixed to the substrate, and an opening is to be formed from
a side opposite to a substrate side, to prevent the substrate
surface from being damaged at a time when the opening is formed so
as to extend through the whole thickness of the conductive thin
film, the conductive thin film may be left which is approximately
optically sufficiently thin without being extended through the
opening.
[0073] Furthermore, the opening 30 shown in FIGS. 1 to 3 is
circular, but the shape may be another shape such as a slit shape,
a rectangle, a polygon, or an ellipse without departing from the
scope of the present invention. As described above, even from the
scope of the present invention, the opening on the emission side
preferably has a diameter which is smaller than the wavelength in
order to obtain a high resolution property that is not more than
the wavelength. In a case where the opening has the slit shape, the
rectangular shape, or the elliptic shape, at least a length in a
short-axis direction is preferably smaller than the wavelength.
[0074] Moreover, in FIGS. 1 to 3, the first and second periodic
surface patterns 40a, 40b are drawn as concentric circles, but they
are not limited to the periodic surface patterns formed into the
concentric circles. For example, as shown in FIGS. 4(A) and (B), a
plurality of dents may be arranged at a first interval (period
length) around the opening 30 in the form of grids to form a first
periodic surface pattern, and a plurality of dents may be arranged
at a second interval (period length) which is different from the
first interval externally from the first periodic surface pattern
in the form of grids to form a second periodic surface pattern. As
shown in FIGS. 5(A) and (B), a plurality of grooves (of a
one-dimensional periodic arrangement) may be arranged in parallel
at a first interval (period length) along a predetermined direction
to form a first periodic surface pattern, and a plurality of
grooves may be arranged in parallel with one another at a second
interval which is different from the first interval to form a
second periodic surface pattern.
[0075] Furthermore, each of FIGS. 1 to 3 shows only one opening 30.
As shown in FIGS. 6(A) and (B) or FIGS. 7(A) and (B), a plurality
of openings 30 may be periodically formed with a first period
length to form first periodic openings. In this case, around the
first periodic openings, there are formed second periodic openings
31 (FIG. 6) or periodic surface patterns 40b (FIG. 7) which are
periodically arranged with a second period length which is
different from the first period length. Here, the second periodic
openings 31 are a plurality of holes extending through the film
from the first surface 20a to the second surface 20b, and the
periodic surface patterns 40b are a plurality of dents formed in
the first surface 20a and/or the second surface 20b.
[0076] Moreover, although not shown, the first periodic openings
(corresponding to 30 in FIG. 6) may be constituted of a plurality
of openings arranged into the concentric circles. Similarly, the
second periodic openings (corresponding to 31 in FIG. 6) may be
constituted of a plurality of openings arranged into concentric
circles. A plurality of dents may be arranged into concentric
circles as periodic surface patterns which replace the second
periodic openings (corresponding to 40b in FIG. 7).
[0077] Here, a function of the optical device of the present
invention will be described with reference to conceptual diagrams
shown in FIGS. 8(A) and (B).
[0078] FIG. 8(A) is for describing a function of a conventional
structure. The incident light (luminous flux diameter D.sub.L)
having the intensity I.sub.incident interacts with surface plasmon
polaritons by the first periodic surface pattern 40a formed in the
conductive thin film 20. The light is transmitted from the opening
30 to the second surface as output light having the increased
intensity I.sub.output. However, a part of energy converted from
light into surface plasmons by the first periodic surface pattern
40a passes as the surface plasmon polaritons on the surface of the
conductive thin film 20, that is, an interface between the
conductive thin film 20 and a dielectric. The energy is scattered
externally from the first periodic surface pattern as shown by
white arrows. On the other hand, in the optical device of the
present invention shown in FIG. 8(B), since the surface plasmon
polaritons passing externally from the first periodic surface
pattern are Bragg-reflected by the second periodic surface pattern,
it is possible to collect even the energy which has been heretofore
scattered into the opening 30 with good efficiency, and
high-efficiency light transmission via the openings is
obtained.
[0079] Next, the period length P.sub.1 of the first periodic
surface pattern or the first periodic opening will be described in
accordance with preferable dimensions in consideration of a surface
plasmon mode.
[0080] In general, conditions for exciting the surface plasmons in
the conductive thin film provided with the periodic structure
having the period length P.sub.1 are represented as follows from a
law of conservation of momentum: k.sub.sp=k.sub.x+iG (Equation 1),
wherein k.sub.sp denotes a wave number vector of the surface
plasmons generated in the interface between the conductive thin
film and the dielectric, k.sub.x denotes a wave number vector
component of the incident light in a direction of a plane of the
conductive thin film, that is, in a tangent direction, G denotes a
reciprocal lattice vector (G=2.pi./P.sub.1) of the periodic
structure, and i denotes an arbitrary integer.
[0081] Moreover, an absolute value of the wave number vector
k.sub.sp of the surface plasmons is given by the following
dispersion relation formula:
|k.sub.sp|=(.omega./c)(.epsilon..sub.m.epsilon..sub.d/(.epsilon-
..sub.m+.epsilon..sub.d)).sup.1/2 (Equation 2), wherein .omega.
denotes an angular frequency of the incident light, c denotes
velocity of light, .epsilon..sub.m denotes permittivity of the
conductive thin film, .epsilon..sub.d denotes permittivity of a
dielectric medium adjacent to the conductive thin film, and they
have a relation of .epsilon..sub.m<0,
.epsilon..sub.m|>.epsilon..sub.d.
[0082] In a case where the light is emitted perpendicularly into
the conductive thin film, the surface plasmon mode is effectively
excited, and the wavelength .lamda. with which the intensity of the
light transmitted on the emission side is most increased is
represented by the following equation obtained by combining the
above equations 1 and 2:
.lamda.=P.sub.1(.epsilon..sub.m.epsilon..sub.d/(.epsilon..sub.m+.epsilon.-
.sub.d)).sup.1/2 (Equation 3)
[0083] On the other hand, the energy converted into the surface
plasmons by the periodic structure having the period length P.sub.1
passes as the surface plasmon polaritons in the interface between
the conductive thin film and the dielectric. At this time, the
following equation represents conditions for Bragg-reflecting the
surface plasmon polaritons having the wave number vector k.sub.sp
by the periodic structure having the period length P.sub.2:
2P.sub.2=m(2.pi./k.sub.sp) (Equation 4), wherein m is generally
established by an arbitrary integer, but m is usually limited to an
odd number in order to obtain a strong Bragg reflection effect.
Furthermore, in a case where it is considered that the surface
plasmon polaritons decay depending on a propagation distance, when
m is smaller, the efficiency is high, and m=1 is preferable from
the respect of the efficiency. On the other hand, when it is
difficult to select m=1 from difficulty in forming a micro
structure during manufacturing, m=3 is preferably selected.
[0084] There is derived from the above equations 1 and 4 the
following relation between P.sub.1 and P.sub.2 for effectively
exciting the surface plasmon mode by the periodic structure having
the period length P.sub.1, and efficiently Bragg-reflecting the
propagating surface plasmon polaritons by the periodic structure
having the period length P.sub.2 in a case where the light is
allowed to fall perpendicularly on the conductive thin film:
P.sub.2=(m/2)P.sub.1 (Equation 5). It is seen from this equation
that the incident light is transmitted through the opening with
good efficiency in the relation of P.sub.2/P.sub.1=m/2.
[0085] In actual, there were prepared a plurality of optical
devices having different period lengths P.sub.2, and an enhancement
factor of the transmitted light was measured.
[0086] Each optical device was prepared by first forming silver
into a film having a thickness of 300 nm on a quartz substrate by a
DC sputtering process. Next, by use of a focused ion beam (FIB), a
concentric circular groove constituting the first periodic surface
pattern 40a was formed into a period length P.sub.1=600 nm, a depth
of 100 nm, and a frequency of 10. Subsequently, externally from the
groove, a concentric circular groove constituting the second
periodic surface pattern 40b was formed into a period length
P.sub.2=150 to 1650 nm, a depth of 100 nm, and a frequency of 10.
At this time, an outer diameter of the outermost concentric
circular groove in the first periodic surface pattern 40a is about
12 .mu.m. A width of the groove was set to a half of one period.
Thereafter, a micro circular opening having a diameter of 100 nm
was formed in the center of the concentrically circular periodic
surface pattern to thereby form the optical device 10 by FIB
working. A device which did not have the second periodic surface
pattern was simultaneously prepared separately from the optical
devices, and was used as a sample for confirming an increased
effect of the transmitted light.
[0087] First, a light transmission spectrum was measured with
respect to the device which did not have any second periodic
surface pattern, and it was confirmed in the device including this
constitution that a transmission peak appeared in the vicinity of
about 650 nm. Next, semiconductor laser having a wavelength of 650
nm was used as a light source, and a light transmission intensity
from the opening was checked with respect to the device which did
not have any second periodic surface pattern, and the devices
(optical devices 10) having the second periodic surface patterns.
The light transmission intensity from the opening was measured in a
position right above the opening by use of a microspectroscopic
device. The center of the concentrically circular region as the
first periodic surface pattern was perpendicularly irradiated with
laser light, and the luminous flux diameter in the irradiated face
was set to about 12 .mu.m.
[0088] FIG. 9 shows a measurement result, that is, an enhancement
factor of the transmitted light with respect to the period length
P.sub.2 of the second periodic surface pattern. Here, it is assumed
that the abscissa of the drawing indicates a ratio of
P.sub.2/P.sub.1 of the period length P.sub.2 of the second periodic
surface pattern with respect to the period length P.sub.1 of the
first periodic surface pattern. The enhancement factor of the
transmitted light was calculated by the following equation:
Gain=Enhancement factor of transmitted light=(light transmission
intensity from opening of device having second periodic surface
pattern)/(light transmission intensity from opening of device which
does not have second periodic surface pattern) (Equation 6).
[0089] As shown in FIG. 9, the enhancement factor of the
transmitted light exhibits a remarkable increase, when a value of
P.sub.2/P.sub.1 corresponds to a value close to a value of 0.5,
1.5, or 2.5, that is, a value close to odd time(s) as much as 1/2
(regarded as a value which is substantially equal to odd time(s) as
much as 1/2). When a deviation from the odd time(s) as much as 1/2
is large, an effect of enlargement rapidly decreases. However, when
the value of P.sub.2/P.sub.1 is within a range of .+-.0.2 of odd
time(s) as much as 0.5, a remarkable increase of the transmitted
light is obtained. A range of P.sub.2/P.sub.1 in which the
enhancement factor of the transmitted light is 1.2 or more is 0.32
to 0.57, 1.42 to 1.62, and 2.43 to 2.64. The enhancement factors of
the transmitted light at a time when the value of P.sub.2/P.sub.1
is 0.5, 1.5, and 2.5 are 1.34, 1.35, and 1.31, respectively. A
reason why the value of P.sub.2/P.sub.1 giving the maximum value of
the enhancement factor of the transmitted light slightly deviates
from the odd time(s) as much as 1/2 is supposedly that the wave
number vector k.sub.sp of the surface plasmon polaritons coupled or
Bragg-reflected by the periodic surface pattern with good
efficiency slightly depends on not only the period length but also
the groove depth or groove shape.
[0090] On the other hand, the effect of the Bragg reflection by the
second periodic surface pattern was confirmed by electromagnetic
calculation. FIGS. 10(A) and (B) show distributions of absolute
values of electric fields in the conventional structure and optical
device of the present invention. As an electric field distribution,
there is shown an only one region (region R (calculation region)
shown by a broken line in an upper part of FIG. 10(A)) by use of
the opening 30 as a symmetric face. A periphery of silver
corresponding to the conductive thin film 20 is all regarded as air
without considering any substrate. The period length of the
periodic surface pattern in the conventional structure and the
period length P.sub.1 of the first periodic surface pattern are 600
nm, the period length P.sub.2 of the second periodic surface
pattern is 300 nm, and the wavelength .lamda. of the incident light
is 650 nm. It can be visually confirmed from FIG. 10(A) that the
energy is scattered externally from the first periodic surface
pattern in the conventional structure. However, it can be visually
confirmed that the energy is inhibited from being scattered by the
second periodic surface pattern in the optical device of the
present invention of FIG. 10(B).
[0091] As described above, according to the present embodiment, the
period length of the second periodic surface pattern is set to be
substantially equal to odd time(s) as much as 1/2 of the period
length of the first periodic surface pattern in the optical device
provided with the conductive thin film including the opening having
the diameter which is not more than the wavelength and two periodic
surface patterns having different period lengths. Consequently, the
energy scattered externally from the first periodic surface pattern
is Bragg-reflected by the second periodic surface pattern with good
efficiency to thereby raise the efficiency, and high-efficiency
light transmission can be realized.
[0092] Moreover, according to the present embodiment, the second
period length is set to be substantially equal to odd time(s) as
much as 1/2 of the first period length in the optical device
provided with the conductive thin film including the first periodic
openings arranged with the first period length, and the second
periodic openings arranged with the second period length or the
periodic surface pattern. Consequently, there is an effect that the
energy scattered externally from the first periodic opening is
Bragg-reflected by the second periodic opening or the periodic
surface pattern with good efficiency to thereby raise the
efficiency, and the high-efficiency light transmission is
realized.
Embodiment 2
[0093] Next, a second embodiment of the present invention will be
described with reference to FIGS. 11 to 13.
[0094] FIG. 11 is a diagram showing an optical device 10 of the
present embodiment. The optical device 10 has a conductive thin
film 20 including a first surface 20a and a second surface 20b. The
first surface 20a of the conductive thin film 20 is irradiated with
incident light. The conductive thin film 20 has at least one hole
having a diameter d, that is, an opening 30.
[0095] The conductive thin film 20 is provided with a third
periodic surface pattern 40c on the second surface 20b of the
conductive thin film 20. Here, a reason why the periodic surface
pattern 40c formed on the second surface 20b is referred to as the
third periodic surface pattern is that the pattern can be combined
with the first and second periodic surface patterns 40a and 40b
shown in FIG. 1 or 3 as shown in FIG. 12 or 13. A period length
P.sub.3 of the third periodic surface pattern 40c is the odd
time(s) (value assumed to be substantially equal to 1/2) as much as
about 1/2 of a value .lamda./n.sub.d obtained by dividing a
wavelength .lamda. of a light source by an effective refractive
index n.sub.d of a medium substantially adjacent to the second
surface 20b. Alternatively, assuming that a permittivity of silver
in the wavelength .lamda. of the light source is .epsilon..sub.m,
and a permittivity of the medium substantially adjacent to the
second surface is .epsilon..sub.d, the period length P.sub.3 of the
third periodic surface pattern 40c is the odd time(s) (value
assumed to be substantially equal to 1/2) of
.lamda./(.epsilon..sub.m.epsilon..sub.d/(.epsilon..sub.m+.epsilon..sub.d)-
).sup.1/2.
[0096] The first surface of the conductive thin film may or may not
be provided with the periodic surface pattern, or may be provided
with a plurality of periodic surface patterns having different
period lengths. In a case where the first surface is provided with
a plurality of periodic surface patterns, the period length of the
periodic surface pattern may include the structure described above
in Embodiment 1. FIGS. 12 and 13 show a structure obtained by
combining the first and second periodic surface patterns shown in
FIGS. 1 and 3, respectively, as described above. In each of FIGS.
12 and 13, the first surface is provided with the first periodic
surface pattern 40a, and the second periodic surface pattern 40b
whose period length is different from that of the first periodic
surface pattern. The period length P.sub.2 of the second periodic
surface pattern 40b is the odd time(s) (regarded as a value which
is substantially equal to odd time(s) as much as 1/2) as much as
about 1/2 of the period length P.sub.1 of the first periodic
surface pattern 40a. FIG. 12 shows a case where the period length
P.sub.2 of the second periodic surface pattern 40b is about 1/2 of
the period length P.sub.1 of the first periodic surface pattern
40a, and the period length P.sub.3 of the third periodic surface
pattern 40c is about 1/2 of .lamda./n.sub.d or about 1/2 of
.lamda./(.epsilon..sub.m.epsilon..sub.d/(.epsilon..sub.m+.epsilon..sub.d)-
).sup.1/2. FIG. 13 shows a case where each period length is about
3/2.
[0097] The incident light having an intensity denoted with symbol
I.sub.incident and shown by an arrow in an upper part of FIG. 11 to
13 travels to the first surface 20a of the conductive thin film 20,
and the light is transmitted as output light having an increased
intensity denoted with symbol I.sub.output and shown by an arrow in
a lower part of FIG. 11 to 13 from the opening 30 in the second
surface 20b of the conductive thin film 20.
[0098] FIGS. 11 to 13 show a thin conductive thin film 20 which is
not supported. That is, the conductive thin film 20 is not adjacent
or fixed to a support structure (substrate). However, in the
present invention, the conductive thin film is deposited on glass
or quartz, and the thin conductive thin film 20 may be fixed to the
substrate.
[0099] Here, a function of the optical device of the present
invention will be described with reference to conceptual diagrams
shown in FIGS. 14(A) and (B).
[0100] FIG. 14(A) is for describing a function of a conventional
structure. The first surface 20a of the conductive thin film 20 is
irradiated with the incident light having the intensity
I.sub.incident. The light passed through the opening 30 and emitted
on a second surface side interacts with a surface plasmon mode by
the periodic surface pattern formed on the conductive thin film,
and is transmitted as the output light having an increased
intensity I.sub.output. Here, the periodic surface pattern is
formed on the second surface 20b, that is, the surface on an
emission side which is a side opposite to the surface irradiated
with the incident light. Even in such structure, the incident light
interacts with the surface plasmon mode, and the intensified light
is emitted. At this time, a relation between the period length of
the periodic surface pattern for exciting the surface plasmons and
the wavelength is given by the above equation 3. However, a part of
energy converted by the periodic surface pattern propagates as
surface plasmon polaritons on the surface of the conductive thin
film, that is, an interface between the conductive thin film and a
dielectric, and the part is scattered externally from the periodic
surface pattern. Furthermore, a part of the energy converted by the
periodic surface pattern is emitted in a perpendicular direction of
the periodic surface pattern depending on diffraction conditions.
That is, the light is also emitted from a position distant from the
vicinity of the opening, and the efficiency is low for a structure
to obtain intense light from the vicinity of the opening.
[0101] On the other hand, in the optical device of the present
invention shown in FIG. 14(B), the light emitted from the opening
interacts with the surface plasmon mode depending on the third
periodic surface pattern, and propagates as surface plasmon
polaritons. Here, the third periodic surface pattern also satisfies
odd-order Bragg reflection conditions of the surface plasmon
polaritons. Consequently, the energy scattered externally from the
third periodic surface pattern is suppressed. Furthermore, since
the energy diffracted and emitted in the perpendicular direction of
the periodic surface pattern is also suppressed, the energy can be
utilized with good efficiency, and it is possible to obtain
high-efficiency light transmission and efficient concentration of
the energy in the vicinity of the opening.
[0102] Next, the period length P.sub.3 of the periodic surface
pattern will be described in accordance with a preferable
dimension.
[0103] The light allowed to fall on the surface 20a is transmitted
through the opening 30, and the light emitted from the opening 30
to the second surface 20b interacts with the surface plasmon mode
of the third periodic surface pattern, and propagates as surface
plasmon polaritons. The conditions for Bragg-reflecting the surface
plasmon polaritons by the periodic structure having the period
length P.sub.3 are described by replacing P.sub.2 of the above
equation 4 with P.sub.3. Furthermore, when the above equation 2 is
combined, the relation between P.sub.3 and wavelength .lamda. is
derived as follows:
P.sub.3=(m/2)(.lamda./(.epsilon..sub.m.epsilon..sub.d/(.epsilon..sub.m+.e-
psilon..sub.d)).sup.1/2) (Equation 7), wherein m is generally
established by an arbitrary integer, but m is limited to an odd
number in order to suppress the diffraction of the third periodic
surface pattern in the perpendicular direction. Furthermore, in a
case where it is considered that the surface plasmon polaritons
decay depending on a propagation distance, when m is small, the
efficiency is high, and m=1 is preferable from the respect of the
efficiency. On the other hand, when it is difficult to select m=1
because of difficulty in forming the micro structure during the
manufacturing, m=3 is preferably selected.
[0104] Moreover, if |.epsilon..sub.m|>>.epsilon..sub.d, the
equation 7 can be approximated by the following equation:
P.sub.3.apprxeq.(m/2)(.lamda./n.sub.d) (Equation 8), wherein
n.sub.d is .epsilon..sub.d.sup.1/2.
[0105] It can be understood from this equation, the optical device
of FIG. 11 can transmit light with good efficiency, when P.sub.3 is
equal to odd time(s) as much as 1/2 of a value obtained by dividing
the wavelength .lamda. of the light source by a permittivity
n.sub.d of a dielectric medium adjacent to the conductive thin
film.
[0106] In actual, there were prepared a plurality of optical
devices of the present embodiment, and transmission characteristics
of the devices were measured.
[0107] Each optical device was prepared by first forming silver
into a film having a thickness of 300 nm on a quartz substrate by a
DC sputtering process. Next, by use of a focused ion beam (FIB), a
concentric circular groove constituting the third periodic surface
pattern 40c was formed into a period length P.sub.3=150 to 1050 nm,
a depth of 100 nm, and a frequency of 10. A width of the groove was
set to a half of one period. Thereafter, a micro circular opening
having a diameter of 200 nm was formed in the center of the
concentrically circular periodic surface pattern to thereby form
the optical device 10 by FIB working.
[0108] To measure the transmission characteristics, first a light
transmission spectrum was measured with respect to the device
having a period length P.sub.3=600 nm, and it was confirmed in the
device including this constitution that a transmission peak
appeared in the vicinity of about 650 nm. Next, semiconductor laser
having a wavelength of 650 nm was used as a light source, and a
light intensity in the vicinity of the opening was checked with
respect to the optical device 10 whose period length P.sub.3 was
changed. The light intensity in the vicinity of the opening was
measured in a position distant by about 20 nm from the opening by
use of a near-field optical microscope. The center of the opening
in the first surface was perpendicularly irradiated with laser
light, and the luminous flux diameter in the irradiated face was
set to about 12 .mu.m.
[0109] FIG. 15 shows a measurement result which is an enhancement
factor of the light intensity with respect to the period length
P.sub.3 of the periodic surface pattern. Here, it is assumed that
the abscissa of the drawing indicates a ratio of P.sub.3/.lamda. of
the period length P.sub.3 of the periodic surface pattern with
respect to the wavelength .lamda. (=650 nm) of the incident light.
The enhancement factor of the light intensity was calculated by the
following equation: Gain=Enhancement factor of light
intensity=(light intensity in position distant by about 20 nm from
opening on emission side of device having periodic surface
pattern)/(light intensity in position distant by about 20 nm from
opening on emission side of device having periodic length
P.sub.3=600 nm of periodic surface pattern) (Equation 9).
[0110] As shown in FIG. 15, the enhancement factor of the light
intensity exhibits a remarkable increase, when a value of
P.sub.3/.lamda. corresponds to a value close to a value of 0.5 or
1.5, that is, a value close to the odd time(s) as much as 1/2
(regarded as a value which is substantially equal to odd time(s) as
much as 1/2). When a deviation from the odd time(s) as much as 1/2
is large, an effect of enlargement rapidly decreases. However, when
the value of P.sub.3/.lamda. is within a range of .+-.0.2 of odd
time(s) as much as 0.5, a remarkable increase of the light
intensity is obtained. A range of P.sub.3/.lamda. in which the
enhancement factor of the light intensity is 1.15 or more is 0.42
to 0.65 and 1.43 to 1.62. The enhancement factors of the light
intensity at a time when the values of P.sub.3/.lamda. is 0.5 and
1.5 are 1.26 and 1.20, respectively. However, the wave number
vector k.sub.sp of the surface plasmon polaritons coupled or
Bragg-reflected by the periodic surface pattern with good
efficiency slightly depends on not only the period length but also
the groove depth or groove shape. Therefore, it is presumed that
the value of P.sub.3/.lamda. giving the maximum value of the
enhancement factor of the light intensity might deviate slightly
from the odd time(s) as much as 0.5.
[0111] The optical device of FIG. 11 is provided with the periodic
surface pattern on the second surface only, and is not provided
with the periodic surface pattern on the first surface, but as
described above with reference to FIGS. 12 and 13, the first
surface may be provided with the first and second periodic surface
structures shown in FIG. 1 or 3. In FIGS. 12 and 13, the conductive
thin film is not fixed to the substrate, but the conductive thin
film can be deposited and formed on quartz. In this case, the
period length P.sub.1 of the first periodic surface pattern is
described by the above equation 2 by use of a permittivity
.epsilon..sub.d (refractive index n.sub.d is
n.sub.d=.epsilon..sub.d.sup.1/2) of the quartz substrate. FIG. 12
shows a case where the period length P.sub.2 of the second periodic
surface pattern is about P.sub.1/2, but from a viewpoint of ease of
preparing, the pattern may be formed so that P.sub.2 is about
P.sub.1 3/2 as shown in FIG. 13. To prepare the patterns, first the
concentrically circular groove forming the first periodic surface
pattern 40a is formed on the quartz substrate, and the
concentrically circular groove forming the second periodic surface
pattern 40b is formed externally from the first periodic surface
pattern by a patterning process using a focused ion beam (FIB) or
electron beam exposure. On the grooves, silver forming the
conductive thin film 20 is formed into the film by a DC sputtering
process. Furthermore, the concentrically circular groove forming
the third periodic surface pattern 40c is formed on the surface of
the film made of silver by the FIB. Finally, a micro opening may be
formed in the center of the concentrically circular periodic
surface pattern by use of the FIB.
[0112] On the other hand, the effect of the Bragg reflection by the
third periodic surface pattern was confirmed by electromagnetic
calculation. FIG. 16 shows distributions of absolute values of
electric fields in the conventional structure of the third periodic
surface pattern and the optical device of the present invention.
However, it is assumed that the first surface is provided with the
first and second periodic surface patterns of FIG. 1. Moreover, any
substrate is not considered, and it is assumed that silver
corresponding to the conductive thin film 20 is all surrounded with
air. The period length P.sub.1 of the first periodic surface
pattern is 600 nm, the period length P.sub.2 of the second periodic
surface pattern is 300 nm, the period length of the third periodic
surface pattern in the conventional structure is 600 nm, the period
length P.sub.3 of the third periodic surface pattern is 300 nm in
the optical device of the present invention, and the wavelength
.lamda. of the incident light is 650 nm. As an electric field
distribution, there is shown an only one region (region R shown by
a broken line in an upper part of FIG. 16(A)) by use of the opening
30 as a symmetric face. It can be visually confirmed from FIG.
16(A) that the energy is scattered externally from the third
periodic surface pattern, and further diffracted light is emitted
in a perpendicular direction of the third periodic surface pattern
in a case where the third periodic surface pattern has the
conventional structure. However, it can be visually confirmed that
the energy is inhibited from being scattered and the periodic
surface pattern is inhibited from being emitted in the
perpendicular direction by the third periodic surface pattern in
the optical device of the present invention of FIG. 16(B).
[0113] In the present embodiment, there has been described an
example in which the opening is circular, but the present invention
is not limited to this embodiment. For example, the opening can be
formed into a slit shape, a rectangular shape, an elliptical shape
or the like.
[0114] As described above, according to the present embodiment, the
periodic surface pattern is disposed on an emission face side, and
the period length is set to be substantially equal to odd time(s)
as much as 1/2 of the value .lamda./n.sub.d obtained by dividing
the wavelength .lamda. of the transmitted light by the effective
refractive index n.sub.d of the medium adjacent to the emission
face in the optical device provided with the conductive thin film
including at least one opening and the periodic surface pattern
disposed on at least one surface. Consequently, the energy scatted
externally from the opening on the emission face side can be
Bragg-reflected by the periodic surface pattern with good
efficiency to thereby realize high-efficiency light
transmission.
[0115] Moreover, according to the present embodiment, assuming that
the wavelength of the light to be transmitted is .lamda., the
permittivity of the conductive thin film is .epsilon..sub.m, and
the permittivity of the medium substantially adjacent to the
emission face is .epsilon..sub.d, the period length of the periodic
surface pattern is set to be substantially equal to odd time(s) as
much as 1/2 of
.lamda./(.epsilon..sub.m.epsilon..sub.d/(.epsilon..sub.m+.epsilon..sub.d)-
).sup.1/2. In consequence, a similar effect is obtained.
Embodiment 3
[0116] FIG. 17 shows one embodiment of an optical head constituted
using an optical device of the present invention.
[0117] An "optical recording medium" used in description of the
present embodiment means an arbitrary medium with respect to which
data is written or read using light, but the present invention is
not limited to a phase change medium, a magnetic optical medium,
and a dyestuff medium. When the medium is a magnetic optical
medium, writing is optically performed, and reading is performed
magnetically, not optically in some case.
[0118] An optical head 200 in FIG. 17 is formed into a slider shape
for floating the optical head at a predetermined height by rotation
of an optical recording medium 150. Laser light emitted from laser
80 is introduced via an optical fiber 100, and collimated by
disposing a collimator lens 60 constituted of a micro lens.
Furthermore, the collimated light changes its optical path at right
angles by a total reflection mirror 70, and the light is further
guided to the optical device 10 of the present embodiment by a
focus lens 50 disposed under the mirror.
[0119] Next, there will be described one embodiment of an optical
recording and reconstructing device using an optical head of the
present invention.
[0120] FIG. 18 shows the optical recording and reconstructing
device. The optical recording and reconstructing device has: an
optical recording medium 340 which rotates by a rotation shaft 310;
a suspension 320 which rotatably supports the optical head 200 (the
optical fiber 100 and the laser 80 are not shown) centering on the
rotation shaft; and a head actuator 330 which rotates the
suspension 320. When the optical recording medium 340 is rotated at
a high speed, the optical head 200 positioned in a tip of the
suspension 320 floats, the head runs while a distance between the
surface of the optical device 10 and the optical recording medium
340 is kept at 100 nm or less, and it is possible to record the
information with a density which is higher than ever.
[0121] To reproduce the information recorded in the optical
recording medium, when a photo detector is formed on the surface of
the optical device 10 on the side of the optical recording medium,
reflected light can be read from the medium in the optical head of
FIG. 17.
[0122] Moreover, there is also possible a method in which the
optical head of the present invention is constituted as a
write-only head, a reproduction head is disposed facing this
optical recording head with the medium being sandwiched between the
heads, and the transmitted light of the medium is detected. A leak
magnetic flux from the medium can be reproduced by the head using a
magnetoresistive effect by use of a magnetic optical recording
medium as an optical recording medium.
[0123] Furthermore, when the optical device of the present
invention is used, it is possible to apply the device broadly to
nano photonics such as a condensing device for condensing only
light having a specific wavelength, and a near-field optical
microscope.
[0124] FIGS. 19(A) and (B) show embodiments of the condensing
device for condensing the only light having the specific wavelength
by use of the optical device of the present invention. The light
allowed to fall on the optical device 10 of the present invention
passes through one or a plurality of openings, and is introduced
into one or a plurality of optical fibers 410 disposed so as to
communicate with the opening. That is, the optical device 10 of the
present invention functions as a condenser, and the light is
coupled to the optical fibers 410 with good efficiency.
[0125] FIG. 20 shows one embodiment of a near-field optical
microscope of the present invention. The optical device 10 for use
in the shown near-field optical microscope is disposed in a tip of
a probe 510 for the near-field optical microscope. The light
allowed to fall on the probe 510 passes through the opening of the
10, a sample 520 is irradiated with the light, and the light is
detected by a photo detector 530. That is, when the sample is
irradiated with the light intensified by the optical device 10 of
the present invention, high-resolution high-sensitivity detection
is possible.
[0126] There have been described and shown above various types of
optical devices in a certain type of application, but alterations
and modifications are possible as long as the devices do not depart
from spirits and broad contents of the present invention which are
limited only by claims of the present description.
* * * * *