U.S. patent application number 09/796474 was filed with the patent office on 2001-09-06 for electron-beam excitation laser.
Invention is credited to Den, Tohru, Iwasaki, Tatsuya.
Application Number | 20010019565 09/796474 |
Document ID | / |
Family ID | 18579698 |
Filed Date | 2001-09-06 |
United States Patent
Application |
20010019565 |
Kind Code |
A1 |
Iwasaki, Tatsuya ; et
al. |
September 6, 2001 |
Electron-beam excitation laser
Abstract
An electron-beam excitation laser has a laser structure with a
light emitter and reflectors on one hand and an electron source on
the other hand, wherein at least part of the light emitter or
reflectors has a multidimensional photonic crystal structure. An
electron-beam excitation laser includes an electron source emitting
electrons and a laser structure consisting of a light emitter and
reflectors, accelerates electrons from the electron source, and
irradiates the electrons to the laser structure to emit a laser
beam from the laser structure, wherein the reflectors and/or the
light emitter in the laser structure are formed with
multidimensional photonic crystals in which dielectrics with
different dielectric constants are arrayed in a plurality of
directions at periodic intervals, and one of the dielectrics with
different dielectric constants may be formed with a light-emitting
material.
Inventors: |
Iwasaki, Tatsuya; (Tokyo,
JP) ; Den, Tohru; (Tokyo, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Family ID: |
18579698 |
Appl. No.: |
09/796474 |
Filed: |
March 2, 2001 |
Current U.S.
Class: |
372/39 ; 372/41;
372/43.01 |
Current CPC
Class: |
H01S 5/0222 20130101;
H01S 5/183 20130101; H01S 5/423 20130101; H01S 5/3018 20130101;
H01S 5/11 20210101; H01S 5/04 20130101; H01S 5/02208 20130101; H01S
3/0959 20130101 |
Class at
Publication: |
372/39 ; 372/43;
372/41 |
International
Class: |
H01S 003/14; H01S
005/00; H01S 003/16 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 3, 2000 |
JP |
2000-059209 |
Claims
What is claimed is:
1. An electron-beam excitation laser which has a laser structure
with a light emitter and reflectors on one hand and an electron
source on the other hand, wherein at least part of the light
emitter or reflectors has a multidimensional photonic crystal
structure.
2. An electron-beam excitation laser which includes an electron
source emitting electrons and a laser structure consisting of a
light emitter and reflectors, accelerates electrons from the
electron source, and irradiates the electrons to the laser
structure to emit a laser beam from the laser structure, wherein
the reflectors in the laser structure are formed with
multidimensional photonic crystals in which dielectrics with
different dielectric constants are arrayed in a plurality of
directions at periodic intervals.
3. The electron-beam excitation laser according to claim 2, wherein
one of the dielectrics with different dielectic constants in the
multidimensional photonic crystal is vacuum.
4. The electron-beam excitation laser according to claim 2, wherein
said multidimensional photonic crystal is formed by regularly and
two-dimensionally arraying in a first dielectric cylindrical second
dielectrics with a brachyaxis shorter than the wavelength of light
emitted at intervals shorter than the wavelength of light
emitted.
5. The electron-beam excitation laser according to claim 2, wherein
a part of said multidimensional photonic crystal has defects.
6. The electron-beam excitation laser according to claim 5, wherein
said defects are dielectrics which differ in size from other
dielectrics.
7. The electron-beam excitation laser according to claim 2, wherein
said multidimensional photonic crystal, which is formed by
anodizing aluminum, has such an anodized alumina nanohole structure
that cylindrical nanoholes are regularly and two-dimensionally
arrayed in an alumina layer.
8. An electron-beam excitation laser which includes an electron
source emitting electrons and a laser structure consisting of a
light emitter and reflectors, accelerates electrons from the
electron source, and irradiates the electrons to the laser
structure to emit a laser beam from the laser structure, wherein
the light emitter in the laser structure is formed with a
multidimensional photonic crystal in which dielectrics with
different dielectric constants are arrayed in a plurality of
directions at periodic intervals, and one of the dielectrics with
different dielectric constants is formed with a light-emitting
material.
9. The electron-beam excitation laser according to claim 8, wherein
said multidimensional photonic crystal is formed by regularly and
two-dimensionally arraying in a first dielectric cylindrical second
dielectrics with a brachyaxis shorter than the wavelength of light
emitted at intervals shorter than the wavelength of light
emitted.
10. The electron-beam excitation laser according to claim 9,
wherein said second dielectrics are formed with a light-emitting
material.
11. The electron-beam excitation laser according to claim 10,
wherein said light-emitting material is made of a II-VI
semiconductor or zinc oxide.
12. The electron-beam excitation laser according to claim 9,
wherein said first dielectric is made of a light-emitting material,
and said second dielectrics arrayed are voids.
13. The electron-beam excitation laser according to claim 12,
wherein said light-emitting material is made of a II-VI
semiconductor or zinc oxide.
14. The electron-beam excitation laser according to claim 8,
wherein a part of the multidimensional photonic crystal has
defects.
15. The electron-beam excitation laser according to claim 14,
wherein said defects are dielectrics which differ in size from
other dielectrics.
16. The electron-beam excitation laser according to claim 8,
wherein said multidimensional photonic crystal, which is formed by
anodizing aluminum, has such an anodized alumina nanohole structure
that cylindrical nanoholes are regularly and two-dimensionally
arrayed in an alumina layer.
17. The electron-beam excitation laser according to claim 8,
wherein said light-emitting material is made of a II-VI
semiconductor or zinc oxide.
18. An electron-beam excitation laser which includes an electron
source emitting electrons and a laser structure consisting of a
light emitter and reflectors, accelerates electrons from the
electron source, and irradiates the electrons to the laser
structure to emit a laser beam from the laser structure, wherein
the light emitter and reflectors in the laser structure are formed
with multidimensional photonic crystals in which dielectrics with
different dielectric constants are arrayed in a plurality of
directions at periodic intervals, and one of the dielectrics with
different dielectric constants in the multidimensional photonic
crystal forming the light emitter is formed with a light-emitting
material.
19. The electron-beam excitation laser according to claim 18,
wherein one of the dielectrics with different dielectric constants
forming said light reflector in the multidimensional photonic
crystal is vacuum.
20. The electron-beam excitation laser according to claim 18,
wherein said multidimensional photonic crystal is formed by
regularly and two-dimensionally arraying in a first dielectric
cylindrical second dielectrics with a brachyaxis shorter than the
wavelength of light emitted at intervals shorter than the
wavelength of light emitted.
21. The electron-beam excitation laser according to claim 20,
wherein said second dielectrics in the multidimensional photonic
crystal forming the light emitter is made of a light-emitting
material.
22. The electron-beam excitation laser according to claim 21,
wherein said light-emitting material is made of a II-VI
semiconductor or zinc oxide.
23. The electron-beam excitation laser according to claim 20,
wherein said first dielectric in the multidimensional photonic
crystal forming the light emitter is made of a light-emitting
material, and said second dielectrics arrayed are voids.
24. The electron-beam excitation laser according to claim 23,
wherein said light-emitting material is made of a II-VI
semiconductor or zinc oxide.
25. The electron-beam excitation laser according to claim 18,
wherein a part of the multidimensional photonic crystal has
defects.
26. The electron-beam excitation laser according to claim 25,
wherein said defects are dielectrics which differ in size from
other dielectrics.
27. The electron-beam excitation laser according to claim 18,
wherein said multidimensional photonic crystal, which is formed by
anodizing aluminum, has such an anodized alumina nanohole structure
that cylindrical nanoholes are regularly and two-dimensionally
arrayed in an alumina layer.
28. The electron-beam excitation laser according to claim 18,
wherein said second dielectrics in the multidimensional photonic
crystal forming the light emitter is made of a light-emitting
material.
29. The electron-beam excitation laser according to claim 18,
wherein said first dielectric in the multidimensional photonic
crystal forming the light emitter is made of a light-emitting
material, and said second dielectrics arrayed are voids.
30. The electron-beam excitation laser according to claim 18,
wherein said light-emitting material is made of a II-VI
semiconductor or zinc oxide.
31. The electron-beam excitation laser according to claim 28,
wherein said light-emitting material is made of a II-VI
semiconductor or zinc oxide.
32. The electron-beam excitation laser according to claim 29,
wherein said light-emitting material is made of a II-VI
semiconductor or zinc oxide.
Description
BACKGROUNG OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an electron-beam excitation
laser, and more particularly, to an electron-beam excitation laser
emitting light distributed from the visible region to the
ultraviolet region. An electron-beam excitation laser according to
the present invention can be used as a light source for a laser
beam printer, an optical recorder, optical communications, a
pointer, a display, and so on.
[0003] 2. Related Background Art
[0004] A laser consists commonly of a laser medium, a resonator
including a reflector, and an excitation source. A laser medium
obtains energy from an excitation source to emit light with a
specific wavelength, so that a laser beam is generated in a state
that allows amplification with gain. An optical resonator reflects
light emitted by the laser medium to return it to the medium. The
light is caused to reciprocate many times to be progressively
amplified thereby cause laser oscillation. Lasers come in various
types according to what they use as a laser medium: a gas laser
which uses a He--Ne mixture, argon gas or the like; a solid laser
which uses Nd:YAG, Ti:sapphire or the like; a dye laser which uses
a dye; and a semiconductor laser which uses a semiconductor made of
GaAlAs or the like. An optical excitation laser which uses light, a
current injection laser which uses current, an electron-beam
excitation laser which uses an electron beam. As excitation source,
such lasers are known as: a light excitation laser which uses
light; a current injection laser which uses current; and an
electron beam excitation laser which uses electron beam. Resonators
include a Fabry-Perot resonator, which uses a reflector, a prism,
and a diffraction grating, and a ring resonator. Semiconductor
lasers include those which use a cleavage plane or a multilayer
film as a reflecting surface, distribution feedback (DFB) lasers,
and distribution Bragg reflection (DBR) lasers.
[0005] A semiconductor laser is small and light, consumes a small
amount of electric power, and offers high electricity-light
conversion efficiency. To provide a higher-density optical recorder
and a color display (refer to Japanese Patent Application Laid-Open
No. 6-89075), a small laser is hoped for which emits light with a
short wavelength, especially light distributed from the blue region
to the ultraviolet region. For a reason of the exposure performance
of a laser beam printer also, such a small laser is demanded.
However, it is difficult to cause laser oscillation by current
injection, using a wide-band-gap semiconductor because of its
electrical characteristics, such as the fact that such a
semiconductor cannot freely be doped. On the other hand, all types
of direct-gap semiconductors, especially direct-gap semiconductors
which are difficult to use for current injection lasers can be used
for electron-beam excitation lasers. Because of this, direct-gap
semiconductors are expected to be used for laser oscillation from
the ultraviolet region to the infrared region.
[0006] A common electron-beam excitation laser will be described
below. FIG. 17 shows the structure of a conventional electron-beam
excitation laser. In the figure, a reference numeral 101 indicates
a substrate; 102, a light emitter (active layer); and 103 and 104,
reflectors. When electrons are emitted from an electron source, not
shown, the light emitter 102 is excited, and the reflectors 103 and
104 serves as resonators, so that laser oscillation occurs, thus
emitting a laser beam. In FIG. 17, a reference numeral 200
indicates a direction of electron emission, and a reference numeral
300 indicates a direction of laser beam emission. Such is also the
case with other figures. Semiconductors containing a group II-VI
group compound, such as ZnS, CdTe, or ZnSe, are mainly used for the
active layer. As a light reflector, are used a substrate cleavage
plane, a metal reflector made of Al or the like, dielectric
multilayer film made of SiO.sub.2, TiO.sub.2, or the like, and
multilayer reflector, that is, a combination of two compound
semiconductors with different refractive indexes are used for the
reflectors constituting a resonator.
[0007] A small electron-beam excitation laser has been reported
which was designed using a spint type electron emitting diode and a
CdTe/CdMnTe-based laser structure (Applied Physics Letters, Vol.
62, p. 796, 1993).
[0008] The following problems have prevented conventional
electron-beam excitation lasers as described above from being
brought into practical use:
[0009] (1) Conventional electron-beam excitation lasers are low in
light emission efficiency and high in oscillation threshold value.
Laser oscillation occurs over a wide wavelength region in various
modes.
[0010] (2) When a thick reflecting layer is provided so that its
side to which an electron beam is irradiated has sufficient light
reflecting capability, an electron beam attenuates by the time it
reaches the active layer, thus lowering light emission efficiency.
To prevent such a reduction in light emission efficiency,
electron-beam accelerating voltage must be increased.
[0011] (3) Because a laser structure (a semiconductor layer) is
made by vapor-phase growth methods, such as MBE and CBE, the
structure is costly, so that lasers using such a structure are
difficult to offer at low cost.
SUMMARY OF THE INVENTION
[0012] In light of the foregoing, it is an object of the present
invention to provide a high-performance electron-beam excitation
laser which features improved light emission efficiency, a reduced
oscillation threshold value, a narrow laser oscillation wavelength
range, a reduced number of laser oscillation modes, and the like
and is easy to produce at low cost.
[0013] An electron-beam oscillation laser according to the present
invention which has a laser structure with a light emitter and
reflectors on one hand and an electron source on the other hand,
wherein at least either the reflectors or light emitter has a
multidimensional photonic structure.
[0014] Specifically, an electron-beam excitation laser according to
the present invention which includes an electron source emitting
electrons and a laser structure consisting of a light emitter and
reflectors, accelerates electrons from the electron source, and
irradiates the electrons to the laser structure to emit a laser
beam from the laser structure, wherein the reflectors in the laser
structure are formed with multidimensional photonic crystals in
which dielectrics with different dielectric constants are arrayed
in a plurality of directions at periodic intervals.
[0015] An electron-beam excitation laser according to the present
invention which includes an electron source emitting electrons and
a laser structure consisting of a light emitter and reflectors,
accelerates electrons from the electron source, and irradiates the
electrons to the laser structure to emit a laser beam from the
laser structure, wherein the light emitter in the laser structure
is formed with a multidimensional photonic crystal in which
dielectrics with different dielectric constants are arrayed in a
plurality of directions at periodic intervals, and one of the
dielectrics with different dielectric constants is formed with a
light-emitting material.
[0016] An electron-beam excitation laser according to the present
invention which includes an electron source emitting electrons and
a laser structure consisting of a light emitter and reflectors,
accelerates electrons from the electron source, and irradiates the
electrons to the laser structure to emit a laser beam from the
laser structure, wherein the light emitter and reflectors in the
laser structure are formed with multidimensional photonic crystals
in which dielectrics with different dielectric constants are
arrayed in a plurality of directions at periodic intervals, and one
of the dielectrics with different dielectric constants in the
multidimensional photonic crystal forming the light emitter is
formed with a light-emitting material.
[0017] According to the present invention, using multidimensional
photonic crystals for reflectors in a laser structure allows light
confinement to be effective, laser efficiency to increase, the
laser oscillation wavelength range to be narrow, and the
probability of laser oscillation in a single mode to increase.
Especially, making part of the reflector on the side of
electron-beam emission vacuum provides the reflector with both
satisfactory reflection performance and a sufficient electron-beam
transmittance, thus allowing laser oscillation to occur at a low
threshold value (i.e., at a low acceleration voltage).
[0018] According to the present invention, using a multidimensional
photonic crystal for a light emitter in a laser structure allows
laser efficiency to increase, the laser oscillation wavelength
range to be narrow, and the probability of laser oscillation in a
single mode to increase in a mode in which photonic band group
velocity is low and a local mode accompanying defects. Using
multidimensional photonic crystals for both a light emitter and
reflectors in a laser structure offers more advantages and higher
performance, compared with using a multidimensional photonic
crystal for either the light emitter or reflectors. Giving a
multidimensional photonic crystal an anodized alumina nanohole
structure allows an electron-beam excitation laser to be easily
made at low cost.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 shows the structure of an electron-beam excitation
laser of the present invention;
[0020] FIG. 2 is a perspective view of the laser structure in FIG.
1;
[0021] FIG. 3 is a perspective view of another laser structure;
[0022] FIG. 4 is a perspective view of still another laser
structure;
[0023] FIG. 5 is a perspective view of a further laser
structure;
[0024] FIG. 6 shows a direction of laser beam emission;
[0025] FIG. 7 shows a multi-electron-beam laser;
[0026] FIGS. 8A, 8B and 8C show a laser structure whose reflector
only is made of a photonic crystal, a laser structure whose light
emitters only are made of a photonic crystal, and a laser structure
whose reflector and light emitters are made of a photonic
crystal;
[0027] FIGS. 9A, 9B and 9C show a two-dimensional photonic
crystal;
[0028] FIGS. 10A, 10B and 10C show a three-dimensional photonic
crystal;
[0029] FIG. 11 shows a photonic crystal containing a defect;
[0030] FIGS. 12A and 12B show a two-dimensional photonic crystal
which is used for a reflector;
[0031] FIGS. 13A, 13B and 13C show two-dimensional photonic
crystals which are used for light emitters;
[0032] FIGS. 14A, 14B and 14C show two-dimensional photonic
crystals which are used for a reflector and light emitters;
[0033] FIG. 15 shows a two-dimensional photonic crystal made of
anodized alumina;
[0034] FIG. 16 shows an apparatus which produces two-dimensional
photonic crystals with anodized aluminum; and
[0035] FIG. 17 shows a conventional electron-beam excitation
laser.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0036] Referring now to the drawings, embodiments of the present
invention will be described in detail below. FIG. 1 shows an
electron-beam excitation laser of the present invention. In the
figure, a reference numeral 1 indicates a vacuum container. An
electron source 2 which emits electrons is disposed at the bottom
of the vacuum container 1, and a laser structure 3 which produces a
laser beam is disposed opposite to the electron source 2 under the
top of the vacuum container 1. The laser structure 3 consists of a
light emitter (a light-emitting layer) 4 and reflectors (reflecting
layers) 5 and 6 which sandwich the light emitter. The reflectors 5
and 6 serve as a resonator which repeatedly reflect light from the
light emitter 4. Electron accelerating means 7 is a power supply
which applies a predetermined voltage between the electron source 2
and laser structure 3 to accelerate electrons from the electron
source 2.
[0037] Electron emission devices, such as a thermoelectron emission
device, an electric-field emission device, an MIM type electron
emission device, surface conduction type electron emission device,
can be used as the electron source 2. To reduce a laser in size,
electron emission devices are preferably used which are small and
highly efficient and can be formed on a substrate, such as a spint
type electric-field emission device, an MIM type electron emission
device, and a surface conduction type electron emission device. As
the electron source 2, electron emission materials which are good
at emitting electrons, such as diamond and carbon nanotubes, may be
disposed opposite to the laser structure on a substrate. A
photoelectron emission device can be used as the electron source 2.
When a photoelectron emission device is used in such a manner,
electrons emitted from the photoelectron emission device due to
input light are accelerated using electron accelerating means and
irradiated to the laser structure, so that output light is emitted
from the laser structure. This means that an optical amplifier and
a light-light converter are provided. The electron accelerating
means can be controlled to modulate the amplifier.
[0038] A power supply or a pulse power supply which range in
voltage from 10 V to 100 kV can be used as the electron
accelerating means 7. A DC power supply and a pulse power supply
can provide continuous laser-beam oscillation and pulsed laser-beam
oscillation, respectively. An electron-beam focusing electrode can
be disposed in the vacuum container 1 to control the electron-beam
diameter.
[0039] As described above, the laser structure 3 consists of the
light emitter 4 and reflectors 5 and 6, which serve as a resonator.
In the embodiment in FIG. 1, the laser structure 3 is structured by
interposing the light emitter 4 between the reflectors 5 and 6 as
shown in FIG. 2. Variations of the laser structure 3 are available.
For example, the light emitter 4 and reflectors 5 and 6 are
disposed side by side, with the emitter in between, as shown in
FIG. 3; the light emitter 4 is embedded in the reflector 6 so that
the reflector is in contact with the top, bottom, and sides of the
emitter 4, as shown in FIG. 4; and the entire light emitter 4 is
embedded in the reflector 5, as shown in FIG. 5.
[0040] When a predetermined voltage is applied to the electron
source 2, using the electron accelerating means 7, electrons from
the electron source 2 are accelerated and irradiated to the laser
structure 3. Irradiating electrons to the laser structure excites
the light emitter 4 in the structure 3 and makes the reflectors 5
and 6 serve as a resonator. Thus laser oscillation occurs, so that
a laser beam is emitted. Appropriately setting the reflectance of
the reflectors in each of the laser structures in FIGS. 2 through 5
allows any of the following laser beam emission directions to be
chosen at will: (1) the direction in parallel with the direction of
electron incidence (the direction of an arrow A), as shown in FIG.
6, (2) the direction opposite to the direction in parallel with the
direction of electron incidence (the direction of an arrow B), and
(3) the direction at right angles to the direction of electron
incidence (the direction of an arrow C).
[0041] When a plurality of electron sources 2 are disposed in the
vacuum container 1 as shown in FIG. 7, a plurality of laser
structures 3 can be disposed opposite to each of the plurality of
electron sources. Disposing a plurality of electron sources 2 and
laser structures 3 in such a manner provides a multi-electron-beam
excitation laser which emit a plurality of laser beams at a time.
In FIG. 7, a reference numeral 4 indicates a light emitter; 5, 6,
and 11, reflectors; 7, electron accelerating means; 13, a control
electrode; and 14, electron number controlling means. In the
following description, parts with the same function are provided
with the same reference numeral. Applying a predetermined voltage
between the electron sources 2 and the control electrode 13, using
the electron quantity controlling means 14 allows the number of
electrons from the electron sources 2 to be controlled. Such an
arrangement can be applied to the apparatus in FIG. 1.
[0042] Material for the laser structure 3 will be described below.
Any light-emitting material which emits light distributed over the
ultraviolet, visible, and infrared regions may be used as a laser
medium forming the light emitter 4 in the laser structure 3. Such
material includes a semiconductor, a fluorescent substance, dye,
and light-emitting glass. A direct-transition semiconductor can
preferably be used which is made from a group II-group VII compound
such as ZnO, ZnS, or CdS; a group IIIb-group V compound such as
AlAs or GaP; a group III-group V compound such as GaN or AlN; a
chalcogenide compound, such as MgS or MnS; or a mixed crystal of
these compounds.
[0043] Available fluorescent substances include red fluorescent
substances for CRTs, such as Zn.sub.3(PO.sub.4).sub.2:Mn.sup.2+;
(Zn, Cd)S:Ag, YVO.sub.4:Eu.sup.+3, Y.sub.2O.sub.3:Eu.sup.3+ and
Y.sub.2O.sub.2S:Eu.sup.- 3+; green fluorescent substances such as
Y.sub.3Al.sub.5O.sub.12:Tb.sup.3+- ; blue fluorescent substances,
such as ZnS:Ag; and (La, Y) OBr:Ce.sup.3+, (La, Gd)OBr:Ce.sup.3+
and so forth. Fluorescent substances used for light-emitting
displays excited by an electron beam of a low voltage ranging from
10 to 100 V may also be used, including ZnO:Zn,
SnO.sub.2:Eu.sup.+3, and
Y.sub.2O.sub.3:Eu.sup.3++In.sub.2O.sub.3.
[0044] The reflectors may be made of multidimensional photonic
crystal, described later; a substrate cleavage plane; a metal, such
as Al, Ag, or the like; a multi layer of SiO.sub.2 and TiO.sub.2 or
a multi layer with a combination of two compound semiconductors or
the like with different refractive indexes. The embodiment uses a
multidimensional photonic crystal especially for the laser
structure 3. Specifically, a multidimensional photonic crystal is
used for the reflectors 5 and 6 in the laser structure 3 as shown
in FIG. 8A, for the light emitter 4 in the laser structure 3 as
shown in FIG. 8B, or for the reflectors 5 and 6 and light emitter 4
as shown in FIG. 8C. In FIG. 8A, either of the reflectors, which
sandwich the light emitter 4, may be a multidimensional photonic
crystal.
[0045] The photonic crystal, which is detailed in "Photonic
Crystals," J. D. Joannnopoulous et al., Princeton University Press,
1995, pp. 94-104, will be briefly described below. The photonic
crystal has an artificial multidimensional periodic structure,
which is formed by periodically arranging two types or more of
dielectrics with different refractive indexes (dielectric
constants). In such a medium, periodicity of the refractive index
in the order of wave length generates the photonic bands in an
analogy to the band generation theory semiconductors in which an
electron wave is Bragg-reflected so that the dispersion relation
between Energy E and wave number k generates bands. A wavelength
region where no light exists, that is, a photonic band gap is
formed, depending on the periodic structure. To control such a
photonic band, its structural interval needs to be about the light
wavelength to a fraction of the light wavelength.
[0046] Photonic crystals are classified according to their periodic
dimensional number into the following: (1) one-dimensional (1D),
(2) two-dimensional (2D), and (3) three-dimensional (3D). The 1D
photonic crystal has a structure which is periodic
one-dimensionally. For example, laminated film and the DFB
structure are made of a 1D crystal. The 2D photonic crystal has a
structure which is periodic two-dimensionally (that is, in the x
and y directions). For example, as shown in FIGS. 9A, 9B and 9C,
the structure is formed by regularly arraying in a first dielectric
21 cylindrical second dielectrics 22 which differ in dielectric
constant from the first dielectric. The second dielectrics 22,
which have a brachyaxis shorter than the wavelength of light
emitted, are regularly and two-dimensionally arrayed in the first
dielectric 21 at intervals shorter than the wavelength of light
emitted. FIG. 9A is a schematic perspective view of the structure
of a 2D photonic crystal, and FIG. 9B is its plan view. The second
dielectrics 22 are arrayed squarely. As shown in FIG. 9C, the
second dielectrics 22 may be arrayed trianglerly.
[0047] The 3D photonic crystal has a structure which is periodic
three-dimensionally. For example, as shown in FIG. 10A, the 3D
photonic crystal can be provided by making the 2D photonic crystal
(FIG. 9A) periodic in the z direction. To make the 2D photonic
crystal periodic in the z direction, third dielectrics 23 are
provided between cylindrical second dielectrics. The first through
third dielectrics 21 to 23 differ in dielectric constant. As shown
in FIG. 10B, a structure is available which is formed by
periodically arraying dielectric spheres 24 alternately in the x
and y directions to stack them in the z direction. As shown in FIG.
10C, dielectric spheres 25 can be stacked to provide the 3D
photonic crystal. In FIG. 10B the dielectric rods 24 provide first
dielectrics, and air between the dielectric rods 24 provides second
dielectrics. Such is also the case with FIG. 10C.
[0048] By way of example, the multidimensional photonic crystals
have been specifically described above. When these photonic
crystals are used for a reflector or a light emitter in a laser
structure, any materials (including air and a vacuum) can be used
as dielectrics with different dielectric constants. For example, in
FIG. 9A, glass and silicon may be used as the first and second
dielectrics 21 and 22, respectively, or glass and air may be used
as the first and second dielectrics 21 and 22, respectively.
Because making a photonic crystal mult-dimensional increases
controllability of its band structure, thus providing an especially
effective photonic band gap, a multidimensional photonic crystal,
more specifically, a photonic crystal with a structure having two
dimensions or more is preferably used.
[0049] For the photonic crystal, a photonic band due to its
periodic structure increases light emission efficiency because of a
reduction in the state density of wavelength of light emitted and
anisotropic dispersion. A wider photonic band gap is preferable. It
is preferable that the 2D photonic crystal be formed by regularly
arraying cylindrical second dielectrics 22 in honeycomb formation
in a first dielectric 21 so that they are symmetrical about six
directions as shown in FIG. 9C, in that the photonic band gap is
open.
[0050] Defects can be disposed in part of a photonic crystal. For
example, as shown in FIG. 11, defects 26 with a small diameter are
disposed in some of the cylindrical second dielectrics 22 which are
regularly arrayed in the first dielectrics 21. These defects cause
local disorder in the photonic crystal, so that the oscillation
threshold value becomes smaller, thus easily providing laser
oscillation in a single mode. The defects 26 have only to differ in
diameter from the second dielectrics 22. Defects which are larger
in diameter than the second dielectrics 22 may be used.
[0051] FIGS. 12A and 12B are sectional views showing a 2D photonic
crystal used for a reflector in the laser structure 3. In the
figures, the two-dimensionally formed structure is drawn
periodically in only one direction, and black arrows 80 indicate
directions in which the structure is not periodic. Such is also the
case with other figures. FIG. 12A shows that the direction in which
the photonic crystal is not periodic is in parallel with an
electron incidence direction 200. FIG. 12B shows that the direction
in which the photonic crystal is not periodic is at right angles to
the electron incidence direction. Especially, when a photonic
crystal is used for the reflector 6 on the side of electron
incidence, its structure is desirably vacuum in part to increase
distance traveled by electrons. This provides a reflecting layer
which has both sufficient reflection performance and electron-bean
transmittance, thus making laser oscillation possible at a low
threshold value (a low acceleration voltage). If for a part of the
dielectric is used vacuum to form a photonic crystal as described
above, for example, the second dielectrics 22 in the 2D photonic
crystal in FIG. 9A are made vacuum. When second dielectrics in the
2D photonic crystal in FIG. 12A are made vacuum, electrons are
preferably irradiated to the crystal at right angles to the
direction of its period to reduce electron energy loss and
effectively excite the crystal.
[0052] FIGS. 13A to 13C are sectional views of a 2D photonic
crystal used for the light emitter 4 in the laser structure 3.
FIGS. 13A and 13B show that the direction in which the 2D photonic
crystal is not periodic is in parallel with an electron incidence
direction, and FIG. 13C shows that the direction in which the 2D
photonic crystal is not periodic is at right angles to the electron
incidence direction. In FIG. 13A, a light-emitting material is used
for the second dielectrics 22 in FIGS. 9A to 9C, and rods made of
the light-emitting material are periodically arrayed between first
dielectrics 21. In FIG. 13B, a light-emitting material is used for
the first dielectrics 21 in FIGS. 9A to 9C, and the second
dielectrics 22, which are voids, are periodically arrayed. In FIG.
13C, a light-emitting material is used for second dielectrics 22 as
shown in FIG. 13A, and these dielectrics are periodically
arrayed.
[0053] FIGS. 14A to 14C are sectional views of 2D photonic crystals
used for a light emitter and a reflector. In FIG. 14A, the
direction in which the 2D photonic crystal is not periodic is in
parallel with an electron incidence direction, and a light-emitting
material is used for a part of the 2D photonic crystal for second
dielectrics 22 as described using FIG. 13A to form the light
emitter 4 in the 2D photonic crystal. If the light emitter 4 is
formed on the side of a laser structure center, reflectors 11 are
formed with photonic crystals on both sides of the light emitter
4.
[0054] In FIG. 14B, the direction in which the 2D photonic crystal
is not periodic is at right angles to an electron incidence
direction, and a light-emitting material is used for a part of the
2D photonic crystal as in FIG. 14A to form the light emitter 4. A
photonic crystal on top of the light emitter 4 is a reflector 5,
and a photonic crystal at the bottom of the light emitter is a
reflector 6. In FIG. 14C, a two-dimensional photonic crystal which
contains periodic voids 50 is used for the reflectors 5 and 6, and
a two-dimensional photonic crystal whose light-emitting material
contains periodic voids 50 is used for the light emitter 4. The
direction in which the two-dimensional photonic crystals as the
reflectors and light emitter are not periodic is in parallel with
an electron incidence direction.
[0055] If a 3D photonic crystal is used for the light emitter 4 in
the laser structure 3, at least one of the first, second, and third
dielectrics 21, 22, and 23 is made of a light-emitting material in
FIG. 10A. In FIG. 10B, for example, all dielectric rods 24 or some
of them are made of a light-emitting material. In FIG. 10C,
dielectric spheres 25 need to be made of a light-emitting material,
or gaps between the dielectric spheres 25 need to be filled with a
light-emitting material.
[0056] Thes, using photonic crystals for the reflectors, light
emitter, or both of these in the laser structure 3 offers the
following advantages:
[0057] (1) Using a photonic band gap in a multidimensional photonic
crystal for a reflector makes high-level light reflection and
containment possible. This allows oscillation to easily occur in a
single mode within a narrow frequency range and light emission
anisotropy to be controlled using photonic-crystal
multidimensionality.
[0058] (2) Periodically disposing a light-emitting material in a
multidimensional photonic crystal increases light emission
efficiency and reduces the oscillation threshold value. This is
reasoned as follows. A photonic crystal allows an optical mode in
which group velocity is low to be entered. Time of interaction
between a material system and a radiation field is inversely
proportional to group velocity. Thus a group velocity reduction is
used to increase the amplification factor (O Plus E, Vol. 21, p.
1533, 1999).
[0059] (3) Introducing local disorder, that is, defects into a
photonic crystal allows a mode in which light exists locally in a
photonic band gap to be entered. If a light-emitting frequency is
in the photonic band gap, spontaneous light emission and inductive
light emission are prohibited because no optical mode in which
light is emitted is available at frequencies other than frequencies
in the above-described mode. Using such a photonic crystal with
defects for a light emitter or a reflector causes the
above-described mode to be entered, the frequency range for light
emission and laser oscillation to be narrowed, and time and space
coherence to be increased. This makes it easy to cause laser
oscillation in a single mode at a low threshold value.
[0060] The multidimensional photonic crystal has the
above-described advantages. However, the crystal is difficult to
produce, it does not find wide application. Because a periodic
structure several hundreds of nanometers in size needs to be formed
to make a photonic crystal which emits light in the visible region,
the crystal is difficult to make. To make a photonic crystal,
techniques are used which include electron-beam exposure, dry
etching, and selective growth. However, because these techniques
pose problems of a poor yield, high cost, and so on, a
spontaneously formed regular nanostructure is preferably used.
[0061] Spontaneously formed nanostructures include an array of fine
spheres of polystyrene or the like, a bound of fine fibers, and
anodized alumina film. Of these, anodized alumina film is the best,
because anodizing, a simple technique, provides a two-dimensional
periodic structure, that is, a 2D photonic crystal with a large
area and a high aspect ratio. The interval can be adjusted to
within a range of several tens of nanometers to five hundred
nanometers, so that a photonic crystal can be made which emits
light distributed from the visible region to the ultraviolet
region. The anodized alumina nanohole will be described below.
[0062] Anodized alumina nanoholes can be made by anodizing aluminum
film, aluminum foil, an aluminum sheet, or the like in a certain
oxidizing solution (refer to R. C. Furneaux, W. R. Rigby, & A.
P. Davidson, NATURE, Vol. 337, p. 147, (1989). FIG. 15
schematically shows anodized alumina nanoholes. Anodized alumina
52, which consists mainly of aluminum, contains many cylindrical
nanoholes 53. These nanoholes 53 are formed almost at right angles
to the surface of a substrate. The nanoholes 53 are almost equally
spaced so that they are parallel with each other. A reference
numeral 51 indicates an aluminum sheet or aluminum film.
[0063] That is, the nanoholes 53 (which correspond to the
cylindrical second dielectrics 22 in FIGS. 9A to 9C) are arrayed
regularly in honeycomb formation in the anodized aluminum 52 (which
corresponds to the first dielectrics 21 in FIGS. 9A to 9C). The
diameter 2r of an alumina nanohole, which is indicated by a
reference numeral 54, ranges from several nanometers to hundreds of
nanometers. The interval 2R between the alumina nanoholes, which is
indicated by a reference numeral 55, ranges from several tens of
nanometers to several hundreds of nanometers. The diameter and
interval can be controlled according to anodization conditions. By
adjusting anodization time or the like, the thickness of the
anodized alumina 52 and the depth of the nanoholes 53 can be
controlled to within a range of 10 to 500 .mu.m, for example. The
diameter 2r can be increased by etching, A phosphoric acid solution
or the like can be used for etching.
[0064] Two-stage anodizing or a method which forms honeycomb-like
texture (concaves of nanohole start points) on an aluminum surface
and then anodizing the surface can be used to regularly array holes
(Masuda, OPTRONICS, No. 8, p. 221, 1998). Filling anodized-alumina
nanoholes with dielectrics or a light-emitting material provides a
highly functional 2D photonic crystal. Electrodeposition is an easy
and highly controllable method for filling nanoholes. However, film
forming methods including electrophoresis, application,
penetration, CVD, or the like can also be used for that purpose.
Anodized alumina can be used to easily make a 2D photonic crystal
as described using FIGS. 9A to 9C. Using anodized alumina for the
reflectors 5 and 6 and light emitter 4 in the laser structure 3 as
described with reference to FIGS. 12A through 14C allows an
inexpensive electron-beam excitation laser to be easily
produced.
[0065] Embodiments of an electron-beam excitation laser of the
present invention will be described in detail below. The inventor
made a reflector for a laser structure made from anodized alumina
(2D photonic crystal) as described with reference to FIG. 15 and
tested the reflector to evaluate it. Embodiments 1 through 6 will
be described below. In these embodiments, the laser structure 3 has
a structure shown in FIGS. 12A and 12B. A CdS single crystal was
used for the light emitter 4 in the laser structure 3. The CdS
single crystal was polished until it was 15 .mu.m thick and
annealed in an Ar atmosphere at 550.degree. C. for one hour. Then
reflectors which consisted of 2D photonic crystals made from
anodized alumina were formed on both sides of the CdS single
crystal to make a laser structure. The embodiments 1 through 6 are
six types of reflectors which consisted of photonic crystals. An
aluminum film reflector was made as a comparative example.
[0066] A method for making a reflector using anodized alumina will
be described below. Aluminum film 1 .mu.m thick is formed on a CdS
single crystal by deposition. Any method, such as sputtering, CVD,
or vacuum metallization, can be used to form such film. Before
anodization, concaves are formed on the surface of the aluminum
film to provide nanohole start points for anodization. This
operation allows nanoholes to be regularly arrayed in alumina. To
make nanoholes with a high aspect ratio, it is preferable that the
concaves be formed in honeycomb formation opposite to an array of
nanoholes in anodized alumina. To form nanohole start points
(concave), methods can be used, including a method which emits a
focused ion beam (FIB), a method which makes hollows by such press
patterning as disclosed in Japanese Patent Application Laid-Open
No. 10-121292, a method which uses SPM including AFM, a method
which makes hollows by etching after resist patterns are formed,
and the like.
[0067] Of these methods, the method which uses a focused ion beam
is the best for the following reasons. That is, the method does not
need troublesome steps, such as resist application, electron-beam
exposure, and resist removal. The method also allows nanohole start
points to be formed by direct drawing at desired positions a short
time and eliminates the need for a workpiece to be pressurized.
Thus the method can be used for a workpiece which is not
mechanically strong. By emitting a focused Ga ion beam, dots of
concaves were formed at 190 nm intervals in honeycomb formation.
Here, Ga was used as an ion species for focused ion beam
processing, the acceleration voltage was 30 kV, the ion beam
diameter was 100 nm, and the ion current was 300 pA. A focused ion
beam was irradiated to each dot for 10 msec.
[0068] The above-described alumina film was anodized to make
nanoholes. FIG. 16 shows an apparatus which make anodized-alumina
nanoholes. In the figure, a reference numeral 40 indicates a
thermostatic bath; 41, a sample; 42, a Pt cathode (a Pt electrode);
43, an electrolyte; 44, a reaction bath; 45, a power supply which
applies anodization voltage; 46, an ammeter which measures
anodization current. In addition, the apparatus incorporates a
computer and the like to automatically control and measure the
voltage and current, which are not shown in FIG. 16. The sample 41
and cathode 42 are immersed in the electrolyte whose temperature is
kept constant in the thermostatic bath. The power supply 45 applies
a voltage between the sample 41 and cathode 42 to anodize the
sample. The electrolyte used for anodizution is, for example, a
solution of oxalic acid, phosphoric acid, sulfuric acid, or chromic
acid.
[0069] Because the interval between nanoholes, that is, the
structural interval relates with anodization voltage as expressed
by the following equation, it is desirable that the anodization
voltage be set according to a start point array (a structural
interval).
2R=2.5.times.Va
[0070] 2R (nm): nanohole interval
[0071] Va (V): anodization voltage
[0072] Alumina nanohole depth can be controlled by adjusting
aluminum film thickness or anodization time. For example, nanoholes
can be made to penetrate through the entire film thickness, or
aluminum film with a desired thickness can be left. By immersing an
alumina nanohole layer in an acid solution (for example, a
phosphoric solution), (pore wide treatment), nanoholes can be
enlarged as appropriate. In addition, by controlling acid
concentration, treatment time, and temperature, alumina nanoholes
with a desired diameter can be formed. In the embodiment, an
alumina nanohole layer was anodized in a 0.3M phosphoric bath at
75V. The layer was also subjected to the pore wide treatment, that
is, immersed in a 5 wt % phosphoric acid solution at 25.degree. C.
for 70 minutes to enlarge nanoholes until their diameter reached
150 nm.
[0073] Laser structures in Embodiments 1 through 6 will be
described below. In Embodiment 1, almost the entire aluminum film
was converted into anodized alumina film to make such a laser
structure that the light emitter 4 is interposed between alumina
films (2D photonic crystals) as shown in FIG. 12A. In Embodiment 2,
anodization was completed when aluminum film 100 nm thick was left.
That is, the light emitter was sandwiched by aluminum films, which,
in turn, were sandwiched by alumina nanoholes (2D photonic
crystals).
[0074] In Embodiment 3, Ag film 100 nm thick, which serves as a
reflecting film and antistatic film, was formed on the anodized
alumina film in Embodiment 1. As is the case with Embodiment 2, in
Embodiment 4, aluminum film on one side only was anodized to obtain
anodized alumina film, and an electron-beam was irradiated to the
film from the side of the anodized alumina film. As is the case
with Embodiment 2, in Embodiment 5, aluminum film on one side only
was anodized to obtain anodized alumina film, and an electron-beam
was irradiated to the film from the side of aluminum film. In
Embodiment 6, niobium film was formed on aluminum film and anodized
to form anodized alumina film in which nanoholes were made
horizontally (i.e., in parallel with the film surface) as shown in
FIG. 12B. That is, in Embodiment 6, electrons were incident in the
direction in which the 2D photonic crystal is periodic. In
Comparatibe Example 1, anodization was not performed, so that
aluminum film 100 nm thick was directly used as reflectors.
[0075] The thus produced laser structures in Embodiments 1 through
6 were evaluated using a laser structure in Comparative Example 1.
These laser structures which use a light emitter consisting of a
CdS single crystal and reflectors consisting of 2D photonic
crystals were placed in a vacuumizer, and vacuumization was
performed until a pressure of 10.sup.-6 Pa was reached. Next, the
laser structures were cooled to a liquid nitrogen temperature, and
electrons emitted from an opposite electron gun made of LaB.sub.6
were accelerated to an acceleration voltage of 10 to 50 keV and
irradiated to the structures. As a result, green light with a
wavelength around 520 nm was obtained by laser oscillation.
Specifically, the laser oscillation threshold value was 15 to 20
A/cm.sup.2 for the laser structure in Embodiment 1. On the other
hand, the value was 20 to 50 A/cm.sup.2 for the laser structure in
Comparative Example 1.
[0076] Embodiments 2 and 3 needed a little higher acceleration
voltage than Embodiment 1. However, Embodiment 1 allowed
oscillation by low-current excitation, so that oscillation was
highly stable over time. Although the laser oscillation wavelength
range was a little wider in Embodiment 4, compared with Embodiment
2 Embodiment 4 allowed oscillation at a low oscillation threshold
value. In Embodiment 5, the oscillation threshold value was high as
in Comparative Example 1. However, in Embodiment 5, the laser
oscillation wavelength range was slightly narrower than in
Comparative Example 1. For the laser structure in Embodiment 6, the
laser oscillation threshold value was low, at 10 to 15 A/cm.sup.2.
In Embodiments 1, 2, 3, and 6, especially Embodiment 6, the laser
oscillation wavelength range was narrow, and a reduced number of
laser oscillation modes were available. The results obtained from
Embodiments 1 through 6 show that using a 2D photonic crystal made
from anodized alumina for a reflector in a laser structure reduces
laser oscillation threshold current.
[0077] By filling nanoholes in anodized alumina film (a 2D photonic
crystal) with a light-emitting material, four types of laser
structure with a light emitter was made to evaluate them. The
structures, which are as shown in FIGS. 13A and 14A, correspond to
Embodiments 7 through 10. Anodized alumina film whose nanoholes
were filled with ZnO, that is, a photonic alumina crystal in which
ZnO rods were arrayed two-dimensionally was used for the light
emitter 4.
[0078] Specifically, Nb film 100 nm thick was formed on a quartz
substrate, then aluminum film 1 .mu.m thick was formed on the Nb
film by DC sputtering. Next, as is the case with Embodiment 1, the
aluminum film was anodized to make alumina nanoholes. These
nanoholes were formed at 140 nm intervals in honeycomb formation.
Then the aluminum film was anodized at 56 V, using a 0.3M
phosphoric acid bath. Finally, the film was subjected to pore wide
treatment, that is, immersed in a 5 wt % phosphoric acid solution
at 25.degree. C. for 50 minutes to enlarge nanoholes until their
diameter reached about 110 nm.
[0079] By electrodeposition, the nanoholes were filled with ZnO to
make a light emitter. The substrate was immersed in a 0.1M zinc
nitrate solution at 60.degree. C. together with an opposite Pt
electrode. Next, a voltage of about -5 V was applied to the
substrate to form ZnO crystals In the nanoholes. In Embodiment 7,
ZnO crystals were let to grow until they protruded from the
nanoholes, and then the substrate surface was polished. In
Embodiment 8, ZnO was deposited in the nanoholes until a nanohole
depth of about 300 nm was reached (Embodiments 7 and 8 correspond
to the laser structure in FIG. 13A). The substrate was heat-treated
in a He atmosphere at 400.degree. C. for one hour and overlaid with
Ag film 100 nm thick by vapor deposition to make a light
emitter.
[0080] In Embodiment 9 (corresponding to FIG. 14A), Nb film strips
5 .mu.m wide were made, then aluminum film was formed over these
strips and anodized to fill nanoholes with ZnO. ZnO was deposited
on those parts of the aluminum film having an underlying Nb film (5
.mu.m wide) were present. That is, some of the nanoholes in the
anodized alumina film (the 2D photonic crystal) were filled with
the light-emitting material. In Embodiment 10, a beam of more ions
were locally irradiated to the substrate during anodized alumina
production to form shallow start points. As a result, anodized
alumina nanoholes which locally had a smaller diameter were formed
as shown in FIG. 11 (the structure in FIG. 13A was provided with
defects in embodiment 10).
[0081] The thus produced laser structures in Embodiments 7 through
10 were placed in a vacuumizer, and vacuumization was performed
until a pressure of 10.sup.-6 Pa was reached. Next, the laser
structures were cooled to a liquid nitrogen temperature, and
electrons were emitted from an opposite electron gun made of
LaB.sub.6 to irradiate a beam of electrons accelerated to an
acceleration voltage of 10 to 50 keV to the structures. As a
result, laser oscillation could be caused near a ultraviolet
wavelength of 390 nm. The laser oscillation threshold value was 15
to 20 A/cm.sup.2 for the laser structures. On the other hand, the
value was 20 to 50 A/cm.sup.2 for a laser structure in Comparative
Example 2, which was formed by depositing ZnO and Ag on Nb
film.
[0082] As described above, the results show that using a 2D
photonic crystal made from anodized alumina for a reflector in a
laser structure reduces laser oscillation threshold current. In
Embodiments 7 through 10, especially Embodiments 8 and 9, the laser
oscillation wavelength range was narrow, and a reduced number of
laser oscillation modes were available. In Embodiment 10, a mode
due to defects was found. In Embodiments 7 through 10, introducing
a light emitter into a photonic crystal probably caused the
oscillation threshold value to decrease with decreasing group
velocity.
[0083] In Embodiment 8, parts which contained almina nanoholes
about 700 nm high unfilled with ZnO possibly served as photonic
crystals (reflectors). In Embodiment 9, that anodize alumina film
on the sides of the light emitter consisting of a photonic crystal
whose nanoholes were not filled probably served as a reflector in
the photonic crystal to effectively contain light. In Embodiment
10, defects in the 2D photonic crystal (anodized alumina film) may
have contributed to laser oscillation.
[0084] In Embodiment 11, a laser structure which uses a GRINSH
(graded index separate confinement) type ZnCdSe/ZnSe
heterostructure produced by MBE for a light emitter was made and
evaluated. The laser structure corresponds to FIG. 12A. The light
emitter has a heterostructure. The heterostructure consists of a 1
.mu.m thick ZnSe buffer layer on an InGaAs (100) substrate and a
quantum well which is made from Zn.sub.0.75Cd.sub.0.25Se interposed
between refractive-index change layers of Zn1-xCdxSe (x=0 to 0.05)
and is disposed on top of the buffer layer. The refractive-index
change layer is 500 nm thick.
[0085] A reflector made from anodized alumina was formed on the
heterostructure in the same was as in Embodiment 2 to make the
laser structure. In the embodiment, start points were arrayed at
170-nm intervals in honeycomb formation, and anodization was
performed at 68 V in a 0.3M phosphoric acid bath. Finally, the
laser structure was subjected to pore wide treatment, that is,
immersed in a 5 wt % phosphoric acid solution at 25.degree. C. for
70 minutes to enlarge nanoholes until their diameter reached about
140 nm.
[0086] A spint type electron source was provided which has 10.sup.4
to 10.sup.5 Mo chips per square millimeter. The thus produced laser
structure was placed opposite to the spint type electron source in
a glass container. After the container was evacuated, it was
hermetically sealed. As electron accelerating means, a high-voltage
power supply was connected to the electron source and laser
structure. When a beam of electrons accelerated to an acceleration
voltage of 10 to 50 keV was irradiated to the structure, blue light
with a wavelength around 480 nm was obtained by laser oscillation.
For the embodiment, the laser oscillation threshold value was 0.3
to 0.5 mA/cm.sup.2. On the other hand, for a comparative example
where no anodized alumina film was disposed, the value was 0.5 to 1
mA/cm.sup.2. The laser oscillation wavelength range was found to be
narrow, and a reduced number of laser oscillation modes were found
to be available.
[0087] In Embodiment 12, by electron-beam lithography, a wafer with
an InGaAsP/InP multiplex quantum well active layer was given a 2D
photonic crystal structure to make a laser structure. The laser
structure was evaluated. It is as shown in FIG. 14C. A wafer was
provided by letting a 200 nm thick InGaAs etching stop layer, a 100
nm thick InP layer, an SCH (separate confinement heterostructure)
multiplex quantum well active layer, and 100 nm thick InP layer 15
grow on an InP substrate. The active layer consists of an SCH layer
50 nm thick which is made from InGaAsP (band gap energy wavelength
.lambda.g=1.1 .mu.m), an InGaAsP well 7 nm thick (.lambda.g=1.36
.mu.m), and an InGaAsP barrier layer 15 nm thick (.lambda.g=1.1
.mu.m).
[0088] SiO.sub.2 film was formed on the wafer, and photoresist was
applied to the wafer to form such a resist mask that circular
openings 250 nm in radius are arrayed at 460-nm intervals in
hexagonal lattice formation. Using the resist mask, the
circular-opening pattern was transferred onto SiO.sub.2 film by
reactive ion etching. Using the SiO.sub.2 mask, two-dimensional
hole rows were formed on the wafer. Finally, the SiO.sub.2 mask was
removed to make a laser structure.
[0089] An electron source was made by forming nitrogen-doped
diamond film on a silicone substrate by hot-filament CVD under the
following conditions: (1) tungsten filament
temperature=2300.degree. C., (2) substrate temperature=800.degree.
C., (3) reaction pressure=100 torr, and (4) ratio of reactive gas
to hydrogen=0.6%. The reactive gas was provided by saturating
methanol with (NH.sub.2).sub.2CO, diluting the saturated solution
ten times with acetone, and vaporizing the dilution.
[0090] In this way, a laser structure was made whose light emitter
and reflectors consist of 2D photonic crystals, with the emitter
interposed vertically between the reflectors. In a vaccumizer, the
laser structure was placed opposite to the electron source with a 2
mm separation in between. When electrons were emitted from the
electron source to irradiate a beam of electrons accelerated to an
acceleration voltage of 10 to 50 keV to the structure, laser
oscillation could be caused near a wavelength of 1.3 .mu.m. The
laser oscillation threshold value was 0.2 to 0.5 mA/cm.sup.2. On
the other hand, the value was 0.5 to 0.8 mA/cm.sup.2 for a
comparative example in which no two-dimensional rows were provided.
The embodiment shows that using a laser structure whose reflectors
consist of 2D photonic crystals reduces the oscillation threshold
value. For the embodiment, the laser oscillation wavelength range
was found to be narrow, and a reduced number of laser oscillation
modes were found to be available.
[0091] In Embodiment 13, a laser structure whose reflectors consist
of 3D photonic crystals of dielectric spheres was made and
evaluated. The 3D photonic crystal in FIG. 10C, metal reflecting
film, and ZnO film were used for the reflector 6, reflector 5, and
light emitter 4, respectively. ZnO film was formed on a sapphire
(0001) substrate by laser MBE to make the ZnO light emitter. To
form the film, a KrF excimer laser beam was irradiated to a ZnO
sintered target at an oxygen partial pressure of 1.times.10.sup.-6
torr and a substrate temperature of 550.degree. C. to evaporate
ZnO. The resulting film was about 60 nm thick.
[0092] Drops of a water solution (4 wt %) in which particles 170 nm
in diameter (standard deviation of 3%) made from polystyrene were
dispersed were let to fall to evaporate water. As a result a
reflector was formed in which, polystyrene particles structured
themselves three-dimensionally and arranged. Ag film 100 nm thick
was formed as antistatic and reflecting film on the face of the
substrate. Ag film 200 nm thick was formed as reflecting film on
the back of the substrate.
[0093] In this way, a laser structure was made which has reflectors
consisting of such 3D photonic crystals that dielectric spheres are
arrayed on top of a ZnO light emitter. The laser structure was
placed in a vacuumizer. When electrons were emitted from an
LaB.sub.6 electron gun opposite to the laser structure to irradiate
a beam of electrons accelerated to an acceleration voltage of 10 to
50 keV to the structure, ultraviolet light with a wavelength near
390 nm could be caused by laser oscillation. The laser oscillation
threshold value was 0.1 to 0.4 A/cm.sup.2. On the other hand, the
value was 0.5 to 1 A/cm.sup.2 for Comparative Example 3 in which no
polystyrene particles were disposed. The embodiment shows that
using a laser structure whose reflectors consist of 3D photonic
crystals formed with arrayed polystyrene particles reduces the
oscillation threshold value. For the embodiment, the laser
oscillation wavelength range was found to be narrow, and a reduced
number of laser oscillation modes were found to be available.
[0094] In Embodiment 14, a laser structure was made whose light
emitter has a 3D photonic crystal structure and subjected to
evaluation test. The light emitter 4 has a structure as shown in
FIG. 10C. The structure is formed by placing (CdS) illuminants
between dielectric spheres 25. Metal reflecting film is used for
the reflectors 5 and 6. Specifically, the following process was
repeated to make such a 3D photonic crystal structure that
illuminants are placed between dielectric spheres 25. As is the
case with Embodiment 13, particles 380 nm in diameter made from
polystyrene are dispersed and arrayed on a quartz substrate. Then
the substrate is first immersed in a 0.025M CdSO.sub.4 solution and
then in an S.dbd.C (NH.sub.2).sub.2 solution. This operation is
repeated. During operation, both solutions are kept at 60.degree.
C. Ammonia is dissolved as a catalyst in both solutions. Ag
reflecting film 100 nm thick was formed on the face and back of the
substrate.
[0095] In this way, a laser structure was made whose light emitter
has such a 3D photonic crystal structure that illuminants are
placed between dielectric spheres 25. The laser structure was place
in a vacuumizer. When electrons were emitted from an LaB.sub.6
electron gun opposite to the laser structure to irradiate a beam of
electrons accelerated to an acceleration voltage of 30 to 80 keV to
the structure, green light with a wavelength near 520 nm could be
caused by laser oscillation. For the embodiment, the laser
oscillation threshold value was lower, compared with Comparative
Example 4 where no polystyrene particles were disposed. That is, in
the embodiment, using the laser structure whose light emitter has
such a 3D photonic crystal structure that illuminants are placed
between dielectric spheres reduced threshold current density and
the number of laser oscillation modes and narrowed the laser
oscillation wavelength range.
[0096] As described above, the present invention has the following
advantages:
[0097] (1) Using a laser structure with reflectors consisting of
multidimensional photonic crystals improves resonator performance
and laser emission efficiency. Using such a laser structure also
narrows the laser oscillation wavelength range and provides an
electron-beam excitation laser which operates in a reduced number
of laser oscillation modes. Especially, making some of the
dielectrics constituting a photonic crystal vacuum causes the
substantial intensity of an electron beam reaching the light
emitter to increase. Thus the laser oscillation threshold current
density and threshold voltage can be reduced.
[0098] (2) Using a multidimensional photonic crystal for a light
emitter in a laser structure provides an electron-beam excitation
laser which features increased laser emission efficiency, a narrow
laser oscillation wavelength range, and a reduced number of laser
oscillation modes available.
[0099] (3) Using multidimensional photonic crystals for reflectors
and a light emitter in a laser structure provides higher
performance, compared with using a multidimensional photonic
crystal for either the reflectors or light emitter.
[0100] (4) Making a multidimensional photonic crystal from anodized
alumina allows an electron-beam excitation laser to be easily
produced at low cost.
* * * * *