U.S. patent application number 10/482462 was filed with the patent office on 2004-12-09 for two-dimensional photonic crystal surface-emitting laser.
Invention is credited to Hatano, Takuji, Noda, Susumu, Yokoyama, Mitsuru.
Application Number | 20040247009 10/482462 |
Document ID | / |
Family ID | 19040807 |
Filed Date | 2004-12-09 |
United States Patent
Application |
20040247009 |
Kind Code |
A1 |
Noda, Susumu ; et
al. |
December 9, 2004 |
Two-dimensional photonic crystal surface-emitting laser
Abstract
Two-dimensional photonic crystal surface-emitting laser
comprising a two-dimensional photonic crystal, having media
different in refractive index arrayed in a two-dimensional cycle,
disposed in the vicinity of an active layer that emits light by the
injection of carriers, wherein the two-dimensional photonic crystal
consists of square lattices having equal lattice constants in
perpendicular directions, and a basic lattice consisting of a
square with one medium as a vertex has an asymmetric refractive
index distribution with respect to either one of the two diagonals
of the basic lattice to thereby emit light in a constant polarizing
direction.
Inventors: |
Noda, Susumu; (Kyoto-shi,
JP) ; Yokoyama, Mitsuru; (Takatsuki-shi, JP) ;
Hatano, Takuji; (Suita-shi, JP) |
Correspondence
Address: |
SIDLEY AUSTIN BROWN & WOOD LLP
717 NORTH HARWOOD
SUITE 3400
DALLAS
TX
75201
US
|
Family ID: |
19040807 |
Appl. No.: |
10/482462 |
Filed: |
July 21, 2004 |
PCT Filed: |
July 3, 2002 |
PCT NO: |
PCT/JP02/06764 |
Current U.S.
Class: |
372/99 |
Current CPC
Class: |
H01S 5/183 20130101;
H01S 5/11 20210101 |
Class at
Publication: |
372/099 |
International
Class: |
H01S 003/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 5, 2001 |
JP |
2001-204315 |
Claims
1. A two-dimensional photonic crystal surface-emitting laser having
a two-dimensional photonic crystal placed near an active layer that
emits light when carriers are injected thereinto, the
two-dimensional photonic crystal having media having different
refractive indices arrayed with a two-dimensional period, wherein
the two-dimensional photonic crystal is formed as a square lattice
having patches of one medium arrayed periodically at equal
intervals in two mutually perpendicular directions, and at least
part of fundamental lattices, of which each has a shape of a square
that has vertices thereof at patches of the one medium and of which
a length of each side equals a minimum period of equally sized
patches of the one medium, has an asymmetric refractive index
distribution with respect to one of two diagonal lines of those
fundamental lattices.
2. A two-dimensional photonic crystal surface-emitting laser having
a two-dimensional photonic crystal placed near an active layer that
emits light when carriers are injected thereinto, the
two-dimensional photonic crystal having media having different
refractive indices arrayed with a two-dimensional period, wherein
mode degeneration at a .GAMMA. point of the two-dimensional
photonic crystal is resolved.
3. A two-dimensional photonic crystal surface-emitting laser as
claimed in claim 1 or 2, wherein a frequency at which the active
layer exhibits a maximum gain is coincident with a frequency at
which the two-dimensional photonic crystal resonates.
4. A two-dimensional photonic crystal surface-emitting laser as
claimed in claim 1 or 2, wherein the two-dimensional photonic
crystal has a first medium that has a predetermined refractive
index and a second medium that has a refractive index different
from the refractive index of the first medium and of which equally
sized patches are arrayed at equal intervals in mutually
perpendicular first and second directions within the first medium,
and at least part of fundamental lattices that have vertices
thereof at patches of the second medium have a third medium of
which patches are arrayed asymmetrically with respect to one of two
diagonal lines of those fundamental lattices.
5. A two-dimensional photonic crystal surface-emitting laser as
claimed in claim 4, wherein, assuming that a length of each side of
the fundamental lattices is "a," the patches of the third medium
are arrayed within a width of "0.1a" of one side of the fundamental
lattices or within a width of "0.1a" of a bisecting normal to one
side of the fundamental lattices.
6. A two-dimensional photonic crystal surface-emitting laser as
claimed in claim 4, wherein, assuming that a length of each side of
the fundamental lattices is "a," and that the first and second
directions are, with an origin set at patches of the second medium,
X- and Y-axes, the patches of the third medium are arrayed within a
radius of "0.1a" of a point having X- and Y-coordinates of (na/4,
ma/4), where n=0, 2, and 4, and m=1 and 3 or (na/4, ma/4), where
n=1 and 3, and m=0, 2, and 4.
7. A two-dimensional photonic crystal surface-emitting laser as
claimed in claim 4, wherein the patches of the second medium and
the patches of the third medium are differently sized.
8. A two-dimensional photonic crystal surface-emitting laser as
claimed in claim 4, wherein the second medium and the third medium
are formed of a same material.
9. A two-dimensional photonic crystal surface-emitting laser as
claimed in claim 1 or 2, wherein the two-dimensional photonic
crystal has a first medium that has a predetermined refractive
index and a second medium that has a refractive index different
from the refractive index of the first medium and of which patches
are arrayed at equal intervals in mutually perpendicular first and
second directions within the first medium, and at least in part of
fundamental lattices that have vertices thereof at patches of the
second medium, the patches of the second medium have, as seen in a
plan view, an asymmetrical shape with respect to one of two
diagonal lines of those fundamental lattices.
10. A two-dimensional photonic crystal surface-emitting laser as
claimed in claim 1 or 2, wherein the two-dimensional photonic
crystal has patches of different media, each rectangular in shape
as seen in a plan view, arrayed in close contact with one another,
and, with respect to the patches of at least one medium, the
patches of two other media adjacent thereto in two mutually
perpendicular directions have different refractive indices.
11. A two-dimensional photonic crystal surface-emitting laser as
claimed in claim 10, wherein the two-dimensional photonic crystal
is formed by cementing together two one-dimensional diffraction
gratings having equal periods in such a way that directions of the
periods thereof are perpendicular to each other.
Description
TECHNICAL FIELD
[0001] The present invention relates to a two-dimensional photonic
crystal surface-emitting laser that has a two-dimensional photonic
crystal so as to be capable of surface light emission.
BACKGROUND ART
[0002] Japanese Patent Application Laid-Open No. 2000-332351
discloses a two-dimensional photonic crystal surface-emitting laser
having a two-dimensional photonic crystal placed near an active
layer so as to achieve surface light emission by exploiting the
resonance of the two-dimensional photonic crystal. The
two-dimensional photonic crystal surface-emitting laser disclosed
in this publication has a lower clad layer, an active layer, and an
upper clad layer laid on a substrate. The lower clad layer
incorporates a two-dimensional photonic crystal near the active
layer.
[0003] The two-dimensional photonic crystal is produced by forming
hollow holes in a semiconductor layer of, for example, n-type InP,
and is formed as a triangular or square lattice having media having
different refractive indices arrayed with a predetermined
two-dimensional period. The hollow holes may be filled with SiN or
the like. The active layer is formed as a multiple quantum well
structure using, for example, an InGaAs/InGaAsP-based semiconductor
material, and emits light when carriers are injected into it.
[0004] The lower clad layer is formed of, for example, an n-type
InP semiconductor as described above, and the upper clad layer is
formed of, for example, a p-type InP semiconductor. The active
layer is sandwiched between the lower and upper clad layers to form
a double hetero junction and thereby confine carriers so that the
carriers that contribute to light emission concentrate in the
active layer.
[0005] On the top surface of the upper clad layer and on the bottom
surface of the substrate, there are formed electrodes of gold or
the like. When a voltage is applied between the electrodes, the
active layer emits light, and an evanescent component that leaks
out of the active layer enters the two-dimensional photonic
crystal. Light having a wavelength coincident with the lattice
constant of the two-dimensional photonic crystal resonates with it,
and is thereby amplified. As a result, the two-dimensional photonic
crystal achieves surface light emission, emitting coherent
light.
[0006] For example, in a two-dimensional photonic crystal formed as
a square lattice as shown in FIG. 35, resonance occurs in the
following manner. The two-dimensional photonic crystal 40 is formed
as a square lattice having a second medium 12 in the form of hollow
holes or the like formed with equal periods in two mutually
perpendicular directions within a first medium 11. The square
lattice has representative directions called the .GAMMA.-X and
.GAMMA.-M directions, respectively. Let the interval between two
patches of the second medium 12 that are mutually adjacent in the
.GAMMA.-X direction (hereinafter, this interval will be referred to
as the "lattice constant") be "a," then there exist a plurality of
square lattice sections E1 having lattice points at patches of the
second medium 12 and measuring "a" on each side (hereinafter, such
a section will be referred to representatively as the "fundamental
lattice").
[0007] When light L having a wavelength ".lambda." coincident with
the lattice constant "a" of the fundamental lattice E1 propagates
in the .GAMMA.-X direction, the light L is diffracted at lattice
points. Of the different components of the light, only those
diffracted in the directions of 0.degree., .+-.90.degree., and
180.degree. with respect to the direction of propagation of the
light fulfill the Bragg condition. Lattice points exist also in the
directions of propagation of the light that has been diffracted in
the directions of 0.degree., .+-.90.degree., and 180.degree., and
thus the diffracted light is diffracted again in the directions of
0.degree., .+-.90.degree., and 180.degree. with respect to its
direction of propagation.
[0008] When light L from one lattice point is refracted once or
more than once, the diffracted light returns to the original
lattice point. This causes resonance. On the other hand, the light
diffracted in the direction perpendicular to the plane of the
figure also fulfills the Bragg condition. As a result, light
amplified through resonance is emitted through the upper clad
layer, achieving surface light emission. This phenomenon occurs at
every lattice point, permitting coherent laser emission all over
the surface area.
[0009] FIG. 36 is a band diagram of the two-dimensional photonic
crystal 40 structured as shown in FIG. 35. Along the vertical axis
is taken the normalized frequency, i.e., the frequency of light
normalized by being multiplied by "a/c," where "c" represents the
speed of light (in m/sec) and "a" represents the lattice constant
(in m). Along the horizontal axis is taken the wave-number vector
of light.
[0010] In this figure, the plotted lines indicate the dispersion
relation of light. The figure shows that there are a few places in
it where the gradient is zero. This means that there are a few
points where the group velocity of light is zero and thus resonance
occurs. In particular, at the point .GAMMA., as described above,
not only light diffracted in different directions within the plane
but also light diffracted in the direction perpendicular to the
plane fulfils the Bragg condition, and accordingly it is possible
to extract, in the direction perpendicular to the plane, coherent
light produced through resonance in different directions within the
plane.
[0011] Incidentally, the point .GAMMA. is defined in the following
manner. Let the unit vectors in a rectangular coordinate system be
"x" and "y," then the primitive translational vectors "a.sub.1" and
"a.sub.2" with respect to a square lattice with a lattice constant
"a" are given by
a.sub.1=ax
a.sub.2=ay
[0012] For the translational vectors "a.sub.1" and "a.sub.2," the
primitive reciprocal lattice vectors "b.sub.1" and "b.sub.2" are
given by
b.sub.1=(2.pi./a)y
b.sub.2=(2.pi./a)x
[0013] On the basis of the primitive reciprocal lattice vectors
"b.sub.1" and "b.sub.2," the point at which the wave-number vector
"k" of light has the value given by formula (1) below is called the
.GAMMA. point.
k=nb.sub.1+mb.sub.2 (1)
[0014] where "n" and "m" are arbitrary integers.
[0015] Accordingly, at the .GAMMA. point, where the wave-number
vector of light fulfills formula (1), the aforementioned phenomenon
occurs in any band. In a two-dimensional photonic crystal
surface-emitting laser, as indicated by the part S in the figure,
it is typical to use the second order band, which corresponds to
the case in which the lattice constant "a" is equal to the
wavelength ".lambda.."
[0016] FIG. 37 shows the details of the part S. The two-dimensional
photonic crystal has four frequencies, namely A, B, C, and D in
order of increasing frequency, at which the group velocity is zero;
that is, it has four resonant frequencies. Hereinafter, the
resonant states at the resonant frequencies A, B, C, and D will be
referred to as the modes A, B, C, and D, respectively.
[0017] FIGS. 38 and 39 show the electric field distributions
observed when the two-dimensional photonic crystal is in the mode-A
and mode-B resonant states, respectively. These images are the near
field pattern images at the time of laser oscillation. Arrows
indicate the directions and magnitudes of electric fields. As shown
in these figures, in the modes A and B, the directions of electric
fields are not uniform. That is, the polarization direction is not
uniform. As a result, as shown in FIGS. 40 and 41, which show the
electric field distributions in the far field pattern in the modes
A and B, the polarization direction in the mode A is such as to run
around an electrode 7, and the polarization direction in the mode B
is such as to radiate to and from the electrode 7.
[0018] On the other hand, the modes C and D are degenerated so that
those resonant states occur at the same frequency. Thus, at the
point .GAMMA., how polarization occurs is determined by the linear
sum of the electric field distributions ascribable to the modes C
and D. Thus, the polarization direction is not uniquely determined
but remains unstable.
[0019] As described above, with a conventional two-dimensional
photonic crystal surface-emitting laser, no matter in which of the
four resonant modes (practically three, since the modes C and D are
degenerated) of the two-dimensional photonic crystal it is formed
to resonate, the polarization direction of the light emitted from
it is not uniform. This makes the conventional two-dimensional
photonic crystal surface-emitting laser unusable in devices that
use uniformly polarized light.
DISCLOSURE OF THE INVENTION
[0020] An object of the present invention is to provide a
two-dimensional photonic crystal surface-emitting laser that can
emit uniformly polarized light.
[0021] To achieve the above object, according to one aspect of the
present invention, in a two-dimensional photonic crystal
surface-emitting laser having a two-dimensional photonic crystal
that has media having different refractive indices arrayed with a
two-dimensional period and that is placed near an active layer that
emits light when carriers are injected thereinto, the
two-dimensional photonic crystal is formed as a square lattice
having patches of one medium arrayed periodically at equal
intervals in two mutually perpendicular directions, and at least
part of fundamental lattices, of which each has the shape of a
square that has its vertices at patches of the one medium and of
which the length of each side equals the minimum period of equally
sized patches of the one medium, has an asymmetric refractive index
distribution with respect to one of the two diagonal lines of those
fundamental lattices.
[0022] According to another aspect of the present invention, in a
two-dimensional photonic crystal surface-emitting laser having a
two-dimensional photonic crystal that has media having different
refractive indices arrayed with a two-dimensional period and that
is placed near an active layer that emits light when carriers are
injected thereinto, the degeneration of modes at the .GAMMA. point
of the two-dimensional photonic crystal is resolved.
[0023] With these structures, it is possible to resolve mode
degeneration by breaking the crystal symmetry of the
two-dimensional photonic crystal or by another means. This makes it
possible to make the two-dimensional photonic crystal resonate at
four different frequencies in the second order .GAMMA. point of its
band diagram through appropriate setting of the refractive indices
and sizes of the media and other parameters.
[0024] According to the present invention, the two-dimensional
photonic crystal surface-emitting lasers described above may be so
structured that the frequency at which the active layer exhibits
the maximum gain is coincident with the frequency at which the
two-dimensional photonic crystal resonates.
[0025] According to the present invention, the two-dimensional
photonic crystal surface-emitting lasers described above may be so
structured that the two-dimensional photonic crystal has a first
medium that has a predetermined refractive index and a second
medium that has a refractive index different from the refractive
index of the first medium and of which equally sized patches are
arrayed at equal intervals in mutually perpendicular first and
second directions within the first medium, and that at least part
of fundamental lattices that have their vertices at patches of the
second medium have a third medium of which patches are arrayed
asymmetrically with respect to one of the two diagonal lines of
those fundamental lattices. With this structure, the patches of the
third medium are arrayed, for example, at random positions within
the first medium.
[0026] According to the present invention, the two-dimensional
photonic crystal surface-emitting lasers described above may be so
structured that, assuming that the length of each side of the
fundamental lattices is "a," the patches of the third medium are
arrayed within a width of "0.1a" of one side of the fundamental
lattices or within a width of "0.1a" of the bisecting normal to one
side of the fundamental lattices.
[0027] According to the present invention, the two-dimensional
photonic crystal surface-emitting lasers described above may be so
structured that, assuming that the length of each side of the
fundamental lattices is "a" and that the first and second
directions are, with an origin set at patches of the second medium,
the X- and Y-axes, the patches of the third medium are arrayed
within a radius of "0.1a" of points of which the X- and
Y-coordinates are
(na/4, ma/4), where n=0, 2, and 4, and m=1 and 3
or
(na/4, ma/4), where n=1 and 3, and m=0, 2, and 4.
[0028] With this structure, patches of the third medium are
arrayed, for example, at positions (0, a/4), (0, 3a/4), and (a/4,
a/2).
[0029] According to the present invention, the two-dimensional
photonic crystal surface-emitting lasers described above may be so
structured that the patches of the second medium and the patches of
the third medium are differently sized.
[0030] According to the present invention, the two-dimensional
photonic crystal surface-emitting lasers described above may be so
structured that the second medium and the third medium are formed
of the same material.
[0031] According to the present invention, the two-dimensional
photonic crystal surface-emitting lasers described above may be so
structured that the two-dimensional photonic crystal has a first
medium that has a predetermined refractive index and a second
medium that has a refractive index different from the refractive
index of the first medium and of which patches are arrayed at equal
intervals in mutually perpendicular first and second directions
within the first medium, and that, at least in part of fundamental
lattices that have their vertices at patches of the second medium,
the patches of the second medium have, as seen in a plan view, an
asymmetrical shape with respect to one of the two diagonal lines of
those fundamental lattices. With this structure, the patches of the
second medium are arrayed periodically in the first and second
directions, and the patches of the second medium are formed to
have, for example, the shape of an ellipse of which the major axis
is coincident with the first direction.
[0032] According to the present invention, the two-dimensional
photonic crystal surface-emitting lasers described above may be so
structured that the two-dimensional photonic crystal has patches of
different media, each rectangular in shape as seen in a plan view,
arrayed in close contact with one another, and, with respect to the
patches of at least one medium, the patches of two other media
adjacent thereto in two mutually perpendicular directions have
different refractive indices.
[0033] With this structure, the two-dimensional photonic crystal is
formed to have a checkered pattern with patches of media having
different refractive indices arrayed adjacent to one another so
that, for example, as seen in a plan view, patches of the second
medium arrayed above patches of the first medium have a different
refractive index from patches of the third medium arrayed on the
right and left of patches of the first medium. At positions
diagonal to patches of the first medium may be arrayed patches of
one of the first to third media or patches of another medium.
Moreover, patches of the first medium may be arrayed with an
interval equal to the wavelength or equal to a period that fulfils
the conditions for the wave-number vector at the .GAMMA. point
defined earlier. Even though the fundamental lattice is square, the
patches of the individual media do not have to be square.
[0034] According to the present invention, the two-dimensional
photonic crystal surface-emitting laser described above may be so
structured that the two-dimensional photonic crystal is formed by
cementing together two one-dimensional diffraction gratings having
equal periods in such a way that the directions of the periods
thereof are perpendicular to each other.
[0035] With this structure, for example, by cementing together a
first and a second diffraction grating that each have grooves
formed with a predetermined one-dimensional period and that are
formed of a first and a second medium, respectively, having
different refractive indices; it is possible to form a
two-dimensional photonic crystal in which the refractive index at
places where the grooves of the first and second diffraction
gratings overlap, the average refractive index at places where the
grooves of the first diffraction grating overlap the second medium,
the average refractive index at places where the grooves of the
second diffraction grating overlap the first medium, and the
average refractive index at places where the first and second media
overlap are different from one another.
BRIEF DESCRIPTION OF DRAWINGS
[0036] FIG. 1 is a partially cutaway perspective view of the
two-dimensional photonic crystal surface-emitting laser of a first
embodiment of the invention.
[0037] FIG. 2 is a plan view of the two-dimensional photonic
crystal of the first embodiment of the invention.
[0038] FIG. 3 is a band diagram of the second order .GAMMA. point
of the two-dimensional photonic crystal of the first embodiment of
the invention.
[0039] FIG. 4 is a diagram showing the electric field distribution
in the near field pattern in the mode A of the two-dimensional
photonic crystal of the first embodiment of the invention.
[0040] FIG. 5 is a diagram showing the electric field distribution
in the near field pattern in the mode B of the two-dimensional
photonic crystal of the first embodiment of the invention.
[0041] FIG. 6 is a diagram showing the electric field distribution
in the near field pattern in the mode C of the two-dimensional
photonic crystal of the first embodiment of the invention.
[0042] FIG. 7 is a diagram showing the electric field distribution
in the near field pattern in the mode D of the two-dimensional
photonic crystal of the first embodiment of the invention.
[0043] FIG. 8 is a diagram showing the electric field distribution
in the far field pattern in the modes A and D of the
two-dimensional photonic crystal of the first embodiment of the
invention.
[0044] FIG. 9 is a diagram showing the electric field distribution
in the far field pattern in the modes B and C of the
two-dimensional photonic crystal of the first embodiment of the
invention.
[0045] FIG. 10 is a diagram showing the diffracting effect obtained
by appropriately positioning defects in the two-dimensional
photonic crystal of the first embodiment of the invention.
[0046] FIG. 11 is a diagram showing the effect of uniformizing the
directions of electric fields obtained by appropriately positioning
defects in the two-dimensional photonic crystal of the first
embodiment of the invention.
[0047] FIG. 12 is a plan view of the two-dimensional photonic
crystal of a second embodiment of the invention.
[0048] FIG. 13 is a plan view of the two-dimensional photonic
crystal of a third embodiment of the invention.
[0049] FIG. 14 is a plan view illustrating the symmetry obtained by
appropriately positioning defects in a two-dimensional photonic
crystal.
[0050] FIG. 15 is a plan view illustrating the symmetry obtained by
appropriately arraying defects in a two-dimensional photonic
crystal.
[0051] FIG. 16 is a plan view illustrating the symmetry obtained by
appropriately arraying defects in a two-dimensional photonic
crystal.
[0052] FIG. 17 is a plan view illustrating the symmetry obtained by
appropriately arraying defects in a two-dimensional photonic
crystal.
[0053] FIG. 18 is a plan view of the two-dimensional photonic
crystal of a fourth embodiment of the invention.
[0054] FIG. 19 is a band diagram of the second order .GAMMA. point
of the two-dimensional photonic crystal of the fourth embodiment of
the invention.
[0055] FIG. 20 is a diagram showing the electric field distribution
in the near field pattern in the mode A of the two-dimensional
photonic crystal of the fourth embodiment of the invention.
[0056] FIG. 21 is a diagram showing the electric field distribution
in the near field pattern in the mode B of the two-dimensional
photonic crystal of the fourth embodiment of the invention.
[0057] FIG. 22 is a diagram showing the electric field distribution
in the near field pattern in the mode C of the two-dimensional
photonic crystal of the fourth embodiment of the invention.
[0058] FIG. 23 is a diagram showing the electric field distribution
in the near field pattern in the mode D of the two-dimensional
photonic crystal of the fourth embodiment of the invention.
[0059] FIG. 24 is a plan view of the two-dimensional photonic
crystal of a fifth embodiment of the invention.
[0060] FIG. 25 is a plan view of the two-dimensional photonic
crystal of a sixth embodiment of the invention.
[0061] FIG. 26 is an exploded perspective view of the
two-dimensional photonic crystal surface-emitting laser of a
seventh embodiment of the invention.
[0062] FIG. 27 is a plan view of the two-dimensional photonic
crystal of the seventh embodiment of the invention.
[0063] FIG. 28 is a plan view of the two-dimensional photonic
crystal of the seventh embodiment of the invention.
[0064] FIG. 29 is a band diagram of the second order .GAMMA. point
of the two-dimensional photonic crystal of the seventh embodiment
of the invention.
[0065] FIG. 30 is a diagram showing the electric field distribution
in the near field pattern in the mode A of the two-dimensional
photonic crystal of the seventh embodiment of the invention.
[0066] FIG. 31 is a diagram showing the electric field distribution
in the near field pattern in the mode B of the two-dimensional
photonic crystal of the seventh embodiment of the invention.
[0067] FIG. 32 is a diagram showing the electric field distribution
in the near field pattern in the mode C of the two-dimensional
photonic crystal of the seventh embodiment of the invention.
[0068] FIG. 33 is a diagram showing the electric field distribution
in the near field pattern in the mode D of the two-dimensional
photonic crystal of the seventh embodiment of the invention.
[0069] FIG. 34 is a diagram showing the relationship between the
gain and the frequency of the light emitted from the active
layer.
[0070] FIG. 35 is a plan view of a conventional two-dimensional
photonic crystal.
[0071] FIG. 36 is a band diagram of a conventional two-dimensional
photonic crystal.
[0072] FIG. 37 is a band diagram of the second order .GAMMA. point
of a conventional two-dimensional photonic crystal.
[0073] FIG. 38 is a diagram showing the electric field distribution
in the near field pattern in the mode A of a conventional
two-dimensional photonic crystal.
[0074] FIG. 39 is a diagram showing the electric field distribution
in the near field pattern in the mode B of a conventional
two-dimensional photonic crystal.
[0075] FIG. 40 is a diagram showing the electric field distribution
in the far field pattern in the mode A of a conventional
two-dimensional photonic crystal.
[0076] FIG. 41 is a diagram showing the electric field distribution
in the far field pattern in the mode B of a conventional
two-dimensional photonic crystal.
BEST MODE FOR CARRYING OUT THE INVENTION
[0077] Hereinafter, embodiments of the present invention will be
described with reference to the drawings. For convenience's sake,
in the following descriptions, such elements as are found also in
the conventional example shown in FIG. 35 are identified with the
same reference numerals. FIG. 1 is a perspective view of the
two-dimensional photonic crystal surface-emitting laser of a first
embodiment of the invention. The two-dimensional photonic crystal
surface-emitting laser 1 has a lower clad layer 3, an active layer
4, and an upper clad layer 5 laid on top of a substrate 2. The
lower clad layer 3 incorporates a two-dimensional photonic crystal
10 near the active layer 4.
[0078] The two-dimensional photonic crystal 10 is produced by
forming hollow holes in a semiconductor layer of, for example,
n-type InP, and is formed as a square lattice having media having
different refractive indices arrayed with a predetermined
two-dimensional period. The hollow holes may be filled with SiN or
the like. The active layer 4 is formed as a multiple quantum well
structure using, for example, an InGaAs/InGaAsP-based semiconductor
material, and emits light when carriers are injected into it.
[0079] The lower clad layer 3 is formed of, for example, an n-type
InP semiconductor as described above, and the upper clad layer 5 is
formed of, for example, a p-type InP semiconductor. The active
layer 4 is sandwiched between the lower and upper clad layers 3 and
5 to form a double hetero junction and thereby confine carriers so
that the carriers that contribute to light emission concentrate in
the active layer. On the top surface of the upper clad layer 5 and
on the bottom surface of the substrate 2, there are formed
electrodes 6 and 7 of gold or the like. When a voltage is applied
between the electrodes 6 and 7, the active layer 4 emits light.
[0080] FIG. 2 is a plan view of the two-dimensional photonic
crystal 10. The two-dimensional photonic crystal 10 is formed as a
square lattice having a second medium 12 in the form of hollow
holes or the like formed with equal periods (lattice constant "a")
in two mutually perpendicular directions within a first medium 11.
The basic lattice constant "a" may be equal to any period that
fulfils the conditions for the wave-number vector at the point
.GAMMA.. In this embodiment, a description is given of a case where
the oscillation wavelength is equal to the lattice constant "a"
that corresponds to the second order .GAMMA. point (see FIG. 36,
the part S), as most commonly practiced.
[0081] In the two-dimensional photonic crystal 10, there are formed
fundamental lattices E1 of which each has the shape of a square
that has its vertices at patches of the second medium 12 and of
which the length of each side equals the minimum period (lattice
constant "a") of equally sized patches of the second medium 12.
Moreover, there are formed patches of a third medium 13 at
predetermined positions, one every two periods in each of two
mutually perpendicular directions with respect to the second medium
12.
[0082] As described earlier, an evanescent component that leaks out
of the active layer 4 enters the two-dimensional photonic crystal
10. When the lattice constant "a" of the two-dimensional photonic
crystal 10 equals a predetermined length, the light resonates with
the two-dimensional photonic crystal 10 and is thereby amplified.
As a result, the two-dimensional photonic crystal surface-emitting
laser 1 achieves surface light emission, emitting coherent light.
Here, within each fundamental lattice E1 that includes a patch of
the third medium 13, the patch of the third medium 13 is located at
a position asymmetric with respect to both of the diagonal lines C1
and C2 of the fundamental lattice E1.
[0083] Incidentally, the fundamental lattice denotes the square
lattice section having the minimum period that has its vertices at
patches of the same medium, and is therefore different from the
so-called unit lattice, which is the minimum unit of a periodical
structure. Accordingly, lattice sections E2 having its vertexes at
patches of the third medium 13 may be regarded as the fundamental
lattice. Even then, patches of the second medium 12 are arrayed at
positions asymmetric with respect to both of the two diagonal lines
of the fundamental lattice.
[0084] FIG. 3 is a band diagram near the point .GAMMA. of the
two-dimensional photonic crystal 10 structured as described above.
The figure shows the second order .GAMMA. point (see FIG. 36, the
part S) in which the wavelength ".lambda." is coincident with the
lattice constant "a." Along the vertical axis is taken the
normalized frequency, i.e., the frequency of light normalized by
being multiplied by "a/c," where "c" represents the speed of light
(in m/sec) and "a" represents the lattice constant (in m). Along
the horizontal axis is taken the wave-number vector of light.
[0085] This figure shows that, provided that the refractive indices
and sizes of the media and other parameters are appropriately set,
the two-dimensional photonic crystal 10 has, at the second order
.GAMMA. point, four different resonant frequencies and thus four
different resonant modes corresponding to those resonant
frequencies. That is, the modes C and D have different resonant
frequencies, and thus mode degeneration as observed in the
conventional two-dimensional photonic crystal shown in FIG. 37 is
resolved.
[0086] Incidentally, mode degeneration in a two-dimensional
photonic crystal can be resolved by distributing refractive indices
asymmetrically with respect to at least one of the diagonal lines
of the fundamental lattice E1. For example, within the fundamental
lattice E1 shown in FIG. 2, instead of forming a patch of the third
medium as indicated by 13a, one may be placed as indicated by 13b,
i.e., on one diagonal line C2.
[0087] FIGS. 4 to 7 show the electric field distributions in the
two-dimensional photonic crystal 10 when it is in the mode-A,
mode-B, mode-C, and mode-D resonant states, respectively; that is,
these figures show the near field pattern images at the time of
laser oscillation. Arrows indicate the directions and magnitudes of
electric fields. As shown in these figures, in all the modes, the
directions of electric fields are uniform. That is, the
polarization direction is uniform.
[0088] Accordingly, as shown in FIG. 8, which shows the electric
field distribution in the far field pattern in the modes A and D,
and in FIG. 9, which shows the electric field distribution in the
far field pattern in the modes B and C, it is possible to obtain a
two-dimensional photonic crystal surface-emitting laser 1 that
emits light of which the polarization direction (the directions of
electric fields) is uniform.
[0089] Incidentally, in the modes A and B, in a central portion of
the light-emitting region, electric fields cancel out one another.
As a result, the central portion of the light-emitting region
covered with the electrodes 7 is a non-light-emitting region. Thus,
light emission takes place in a ring-shaped region with a bright
peripheral portion and a dim central portion. By contrast, in the
modes C and D, electric fields do not cancel out one another, and
thus light emission takes place all over the light-emitting region.
Accordingly, two-dimensional photonic crystal surface-emitting
lasers are better used in the modes C and D than in the modes A and
B.
[0090] The third medium 13 has a different refractive index from
the first medium 11, but may have the same refractive index as the
second medium 12. To resolve mode degeneration, the third medium 13
may be arrayed at any positions, provided that it is arrayed
asymmetrically with respect to at least one of the diagonal lines
C1 and C2. The third medium 13 may be arrayed at random positions,
but it is preferable to array it at positions as described
below.
[0091] FIG. 10 is a diagram showing, in the form of contour lines,
the degree of how light is affected by the presence of the photonic
crystal when the third medium 13 is arrayed within the fundamental
lattice E1. Hatching indicates where the effect is great. This
figure shows that arraying the third medium 13 on a side of the
fundamental lattice E1 or on the bisecting normal to a side thereof
is most effective, resulting in the presence of the photonic
crystal exerting the greatest effect. Arraying the third medium 13
within a width of "0.1a" (=a/10) of a side of the fundamental
lattice E1 or the bisecting normal to a side thereof produces a
satisfactory effect.
[0092] FIG. 11 is a diagram showing, in the form of contour lines,
how great is the effect of the third medium 13 making the
differences between the frequencies of the different modes greater
when the third medium 13 is arrayed within the fundamental lattice
E1. Hatching indicates where the effect is great. This figure shows
that, assuming that two adjacent sides of the fundamental lattice
E1 are the X- and Y-axes with the origin set at the position of a
patch of the second medium 12, arraying the third medium 13 at
points of which the X- and Y-coordinates are
(na/4, ma/4), where n=0, 2, and 4, and m=1 and 3
or
(na/4, ma/4), where n=1 and 3, and m=0, 2, and 4.
[0093] is most effective in making the differences between the
frequencies of the different modes greater. Arraying the third
medium 13 within a radius of "0.1a" (=a/10) of such a point
produces a satisfactory effect.
[0094] For example, when the third medium 13 is arrayed at
positions of which the X- and Y-coordinates are (0, a/4), (0,
3a/4), and (a/4, a/2), light is satisfactorily affected by the
photonic crystal, resulting in a great effect of the photonic
crystal and large differences between the frequencies of the
different modes.
[0095] As in a second embodiment of the invention shown in FIG. 12,
patches of the third medium 13 may be arrayed every period in two
mutually perpendicular directions. In that case, even if a lattice
section E3 is regarded as the fundamental lattice, patches of the
third medium 13 is arrayed asymmetrically with respect to the two
diagonal lines. As in a third embodiment of the invention shown in
FIG. 13, within a fundamental lattice E1' having patches of the
second medium 12 periodically arrayed, the two patches located at
non-diagonal positions may be replaced with patches of the third
medium 13. The resulting fundamental lattice E1 has a shorter side
length than the original fundamental lattice E1', and is asymmetric
with respect to the diagonal lines. The same holds even if a
lattice section E5 having its vertices at patches of the third
medium 13 is regarded as the fundamental lattice.
[0096] In the first to third embodiments, as compared with arraying
the third medium 13 with a long period of the second medium 12,
arraying the third medium 13 with a short period of the second
medium 12 produces a greater effect of resolving mode degeneration
and making the differences between the different resonant
frequencies greater. This permits easier selection of the resonance
mode used. Moreover, as compared with making patches of the third
medium 13 small, making them large results in a greater effect of
resolving mode degeneration.
[0097] In FIGS. 14 to 17, the refractive index distribution is
symmetrical with respect to both the diagonal lines of the
fundamental lattice. As shown in FIG. 14, in a case where patches
of the third medium 13 are arrayed at midpoints of the diagonal
lines of the fundamental lattice E1, the refractive index
distribution is symmetric with respect to both the diagonal lines
of the fundamental lattice E1. This makes it impossible to make the
polarization direction of the emitted light uniform. The same holds
even if a lattice section E6 having its vertices at patches of the
third medium 13 is regarded as the fundamental lattice.
[0098] Likewise, as shown in FIG. 15, in a case where patches of
the third medium 13 are arrayed at midpoints of the diagonal lines
of the fundamental lattice E1 every two periods thereof, the
refractive index distribution is symmetric with respect to both the
diagonal lines of the fundamental lattice E1. The same holds even
if a lattice section E7 or E8 is regarded as the fundamental
lattice. Incidentally, here, a lattice section E20 has its vertices
at patches of the second medium 12, but its side length is not
equal to the minimum period of the second medium 12, and therefore
the lattice section E20 cannot be regarded as a fundamental
lattice.
[0099] Likewise, as shown in FIG. 16, in a case where, within a
fundamental lattice E1' having patches of the second medium 12
periodically arrayed, one patch is replaced with a patch of the
third medium 13 every two period, the refractive index distribution
is symmetric with respect to both the diagonal lines of the new
fundamental lattice E1. The same holds even if a lattice section
E10 is regarded as the fundamental lattice.
[0100] Likewise, as shown in FIG. 17, in a case where, within a
fundamental lattice E1' having patches of the second medium 12
periodically arrayed, one patch is replaced with a patch of the
third medium 13, formed of the same material but in a different
size, every two period, the refractive index distribution is
symmetric with respect to both the diagonal lines of the new
fundamental lattice E1. The same holds even if a lattice section E9
is regarded as the fundamental lattice. Incidentally, here, a
lattice section E21 has its vertices at patches of the second and
third media 12 and 13, which are of the same material, but those
patches have different sizes, and therefore the lattice section E21
cannot be regarded as a fundamental lattice.
[0101] FIG. 18 is a plan view of the two-dimensional photonic
crystal incorporated in the two-dimensional photonic crystal
surface-emitting laser of a fourth embodiment of the invention. The
two-dimensional photonic crystal 20 of this embodiment has a second
medium 12 in the form of patches having an elliptic cross-sectional
shape formed with equal periods in two mutually perpendicular
directions in a first medium 11. In other respects, the structure
here is the same as that of the first embodiment.
[0102] In this embodiment, the first and second media 11 and 12
have different refractive indices, and therefore the refractive
index distribution is asymmetric with respect to both the diagonal
lines C1 and C2 of the fundamental lattice E1, which has the shape
of a square having its vertices at patches of the second medium
12.
[0103] FIG. 19 is a band diagram of the second order .GAMMA. point
(see FIG. 36, the part S) of the two-dimensional photonic crystal
20 structured as described above. Along the vertical axis is taken
the normalized frequency, i.e., the frequency of light normalized
by being multiplied by "a/c," and along the horizontal axis is
taken the wave-number vector of light. Provided that the refractive
indices and sizes of the media and other parameters are
appropriately set, the two-dimensional photonic crystal 20 has, at
the second order .GAMMA. point, four different resonant frequencies
and thus four different resonant modes corresponding to those
resonant frequencies. That is, the modes C and D have different
resonant frequencies, and thus, in the two-dimensional photonic
crystal 20, mode degeneration is resolved.
[0104] FIGS. 20 to 23 show the electric field distributions in the
two-dimensional photonic crystal 20 when it is in the mode-A,
mode-B, mode-C, and mode-D resonant states, respectively; that is,
these figures show the near field pattern images at the time of
laser oscillation. Arrows indicate the directions and magnitudes of
electric fields. As shown in these figures, in all the modes, the
directions of electric fields are uniform. That is, the
polarization direction is uniform. This makes it possible to obtain
a two-dimensional photonic crystal surface-emitting laser 1 that
emits light of which the polarization direction (the directions of
electric fields) is uniform.
[0105] As in a fifth embodiment of the invention shown in FIG. 24,
the patches of the second medium 12, each having an elliptic
cross-sectional shape, may be arrayed with an inclination relative
to the directions of the period. As in a sixth embodiment of the
invention shown in FIG. 25, giving the patches of the second medium
12 a rectangular cross-sectional shape results in obtaining the
same effect.
[0106] FIG. 26 is an exploded perspective view of the
two-dimensional photonic crystal surface-emitting laser of a
seventh embodiment of the invention. For convenience' sake, in the
following descriptions, such elements as find their counterparts in
the first embodiment shown in FIG. 1 and described earlier are
identified with the same reference numerals. The two-dimensional
photonic crystal surface-emitting laser 9 has lower clad layers 3
and 8, an active layer 4, and an upper clad layer 5 laid on top of
a substrate 2. These are formed of the same materials and in the
same manners as in the first embodiment. The lower clad layers 3
and 8 may be formed of the same material or different materials. On
the bottom surface of the substrate 2 and on the top surface of the
upper clad layer 5, there are formed electrodes 6 and 7 of gold or
the like.
[0107] On the top surface of the lower clad layer 8 and on the
bottom surface of the upper clad layer 3, there are formed
one-dimensional diffraction gratings 8a and 3a, respectively, that
have grooves formed therein with the same one-dimensional period.
The one-dimensional diffraction gratings 8a and 3a are cemented
together in such a way that the directions of their periods are
perpendicular to each other. In this way, a two-dimensional
photonic crystal 30 having a two-dimensional periodic structure is
formed.
[0108] FIG. 27 is a plan view of the two-dimensional photonic
crystal 30. The one-dimensional diffraction gratings 8a and 3a have
groove portions 8b and 3b and ridge portions 8c and 3c,
respectively. The portions F1 where the groove portions 8b overlap
the groove portions 3b have the same refractive index as air. The
portions F2 where the ridge portions 8c overlap the groove portions
3b have the average refractive index between the refractive index
of the lower clad layer 8 and that of air. The portions F3 where
the groove portions 8b overlap the ridge portions 3c have the
average refractive index between the refractive index of the lower
clad layer 3 and that of air. The portions F4 where the ridge
portions 8c overlap the ridge portions 3c have the average
refractive index between the refractive index of the lower clad
layer 8 and that of the lower clad layer 3.
[0109] The distance from the active layer 4 to the one-dimensional
diffraction grating 3a is shorter than the distance from the active
layer 4 to the one-dimensional diffraction grating 8a. Thus, the
intensity of the light reaching the one-dimensional diffraction
grating 3a is higher than that of the light reaching the
one-dimensional diffraction grating 8a. Accordingly, the average
refractive index of the portions F3 is affected more by the
refractive index of the lower clad layer 3 than by the refractive
index of air, and the average refractive index of the portions F2
is affected more by the refractive index of air than by the
refractive index of the lower clad layer 8. Thus, even though the
lower clad layers 8 and 3 have the same refractive index, the
portions F2 and the portions F3 have different refractive
indices.
[0110] As a result, as shown in FIG. 28, the two-dimensional
photonic crystal 30 has a first, a second, a third, and a fourth
medium 31, 32, 33, and 34 in the form of blocks having different
refractive indices and having a rectangular cross-sectional shape
arrayed in close contact with one another in a checkered pattern.
Thus, for example, blocks of the first medium 31 are arrayed
periodically in two mutually perpendicular directions, and
therefore the two-dimensional photonic crystal 30 is formed as a
square lattice. Moreover, the fundamental lattice E1, which has the
shape of a square having its vertices at blocks of the first medium
31, exhibits a refractive index distribution that is asymmetric
with respect to both of the two diagonal lines of the fundamental
lattice E1.
[0111] FIG. 29 is a band diagram of the second order .GAMMA. point
(see FIG. 36, the part S) of the two-dimensional photonic crystal
30. Along the vertical axis is taken the normalized frequency,
i.e., the frequency of light normalized by being multiplied by
"a/c," and along the horizontal axis is taken the wave-number
vector of light. Provided that the refractive indices and sizes of
the media and other parameters are appropriately set, the
two-dimensional photonic crystal 30 has, at the second order
.GAMMA. point, four different resonant frequencies and thus four
different resonant modes corresponding to those resonant
frequencies. That is, the modes C and D have different resonant
frequencies, and thus, in the two-dimensional photonic crystal 30,
mode degeneration is resolved.
[0112] FIGS. 30 to 33 show the electric field distributions in the
two-dimensional photonic crystal 30 when it is in the mode-A,
mode-B, mode-C, and mode-D resonant states, respectively; that is,
these figures show the near field pattern images at the time of
laser oscillation. Arrows indicate the directions and magnitudes of
electric fields. As shown in these figures, in all the modes, the
directions of electric fields are uniform. That is, the
polarization direction is uniform. This makes it possible to obtain
a two-dimensional photonic crystal surface-emitting laser 9 that
emits light of which the polarization direction (the directions of
electric fields) is uniform.
[0113] Incidentally, the one-dimensional diffraction gratings 8a
and 3a have simply to have equal periods, and do not necessarily
have equal duty ratios. Instead of cementing together
one-dimensional diffraction gratings as in this embodiment, media
having a rectangular cross-sectional shape as seen in a plan view
and having different refractive indices may be arrayed in close
contact with one another as shown in FIG. 28 described above so
that, with respect to blocks of one medium (for example, the first
medium), blocks of two other media (for example, the second and
third media 32 and 33) adjacent thereto in two mutually
perpendicular directions have different refractive indices. Also in
this way, it is possible to obtain a two-dimensional photonic
crystal free from mode degeneration.
[0114] Here, blocks of the one medium are arrayed at intervals
equal to the wavelength or equal to a period that fulfills the
conditions for the wave-number vector at the point .GAMMA. defined
earlier. While the fundamental lattice is square, blocks of the
individual media do not have to be square as seen in a plan view.
The fourth medium 34 of which blocks are arrayed in oblique
directions with respect to the one medium (the first medium 31) may
have the same refractive index as one of the first to third media
31 to 33.
[0115] FIG. 34 shows the frequency response of the gain of the
active layer 4 in the first to seventh embodiments. Along the
vertical axis is taken the gain of the active layer, and along the
horizontal axis is taken the frequency. As shown in this figure,
the active layer 4 exhibits different gains at different
frequencies. Accordingly, in the first to seventh embodiments, by
appropriately setting the parameters of the two-dimensional
photonic crystal, such as the refractive indices and sizes of the
media and the lattice constant, so that resonance occurs at the
frequency at which the gain of the active layer 4 is at the
maximum, it is possible to let the two-dimensional photonic crystal
emit light efficiently and thereby save power consumption.
Moreover, making the differences between the frequencies of the
different modes greater makes it easy to select the resonance mode
used.
INDUSTRIAL APPLICABILITY
[0116] As described above, according to the present invention, in a
two-dimensional photonic crystal, the fundamental lattice has a
refractive index distribution that is asymmetric with respect to
the diagonal lines thereof. This helps resolve mode degeneration,
and thus makes it possible to obtain a two-dimensional photonic
crystal surface-emitting laser that emits light of which the
polarization direction is uniform.
[0117] According to the present invention, by resolving mode
degeneration in the two-dimensional photonic crystal, it is
possible to obtain a two-dimensional photonic crystal
surface-emitting laser that emits light of which the polarization
direction is uniform.
[0118] According to the present invention, the frequency at which
the active layer exhibits the maximum gain is made coincident with
the frequency at which the two-dimensional photonic crystal
resonates. This makes it possible to let the two-dimensional
photonic crystal emit light efficiently and thereby save power
consumption.
[0119] According to the present invention, a third medium is
arrayed asymmetrically with respect to the two diagonal lines of
the fundamental lattice that has its vertices at patches of a
second medium arrayed in a first medium. This makes it easy to
obtain a two-dimensional photonic crystal having an asymmetric
refractive index distribution.
[0120] According to the present invention, assuming that the length
of each side of the fundamental lattice is "a," patches of the
third medium are arrayed within a width of "0.1a" of one side of
the fundamental lattice or within a width of "0.1a" of the
bisecting normal to one side of the fundamental lattice. This makes
the effect of the photonic crystal on light greater, and makes it
easy to resolve mode degeneration.
[0121] According to the present invention, assuming that the length
of each side of the fundamental lattice is "a" and that the first
and second directions are, with an origin set at a patch of the
second medium, the X- and Y-axes, patches of the third medium are
arrayed within a radius of "0.1a" of points of which the X- and
Y-coordinates are
(na/4, ma/4), where n=0, 2, and 4, and m=1 and 3
or
(na/4, ma/4), where n=1 and 3, and m=0, 2, and 4.
[0122] This helps introduce defects and thereby achieve a greater
effect of resolving mode degeneration.
[0123] According to the present invention, the patches of the
second medium and the patches of the third medium are differently
sized. This makes it easy to form an asymmetric refractive index
distribution even when the second and third media have the same
refractive index.
[0124] According to the present invention, the second medium and
the third medium are formed of the same material. This makes it
possible to form patches of the second and third media
simultaneously, and makes it easier to form an asymmetric
refractive index distribution.
[0125] According to the present invention, the patches of the
second medium, as seen in a plan view, are given an asymmetrical
shape with respect to the two diagonal lines of the fundamental
lattice that has its vertices at patches of the second medium
arrayed in the first medium. This makes it easy to obtain a
two-dimensional photonic crystal having an asymmetric refractive
index distribution.
[0126] According to the present invention, patches of different
media, each rectangular in shape as seen in a plan view, are
arrayed in close contact with one another in a checkered pattern,
and, with respect to the patches of one medium, the patches of two
other media adjacent thereto in two mutually perpendicular
directions have different refractive indices. This makes it easy to
obtain a two-dimensional photonic crystal having an asymmetric
refractive index distribution.
[0127] According to the present invention, two one-dimensional
diffraction gratings are cemented together in such a way that the
directions of the periods thereof are perpendicular to each other.
This makes it easy to obtain a two-dimensional photonic crystal
having different media arrayed in a checkered pattern.
* * * * *