The present invention relates to a two-dimensional photonic crystal surface-emitting laser, and more particularly to a two-dimensional photonic crystal surface-emitting laser having a photonic crystal periodic structure with a two-dimensionally periodic refractive index distribution in or near an active layer which emits light when carriers are injected thereto.
Conventionally, lasers of a surface-emitting type which emits laser beams from a surface of the substrate in a direction perpendicular to the surface have been studied and developed into various kinds. Such a surface-emitting laser contains a large number of elements arrayed in one substrate and is capable of emitting parallel coherent light from the respective elements. Therefore, such surface-emitting lasers are expected to be used in the fields of parallel light pick-up, parallel light transmission and optical parallel information processing.
For example, Japanese Patent Laid-Open Publication No. 2000-332351 discloses a two-dimensional photonic crystal surface-emitting laser using a photonic crystal. The photonic crystal is a crystal with a refractive index period which is substantially equal to or smaller than the wavelength of light. By the law that in a multidimensional periodic structure of a dielectric, a band gap occurs in ions in a conductor crystal, in a photonic crystal, a band which inhibits radiation of light (photonic band gap) occurs, and thereby, it is possible to confine light two-dimensionally or three-dimensionally.
The two-dimensional photonic crystal surface-emitting laser disclosed by Japanese Patent Laid-Open Publication No. 2000-332351 has a photonic crystal periodic structure with a two-dimensionally periodic refractive index distribution near an active layer which emits light when carriers are injected thereto. Light resonates in the photonic crystal, and thereby, the laser emits light from a surface.
More specifically, as
The substrate 11 is, for example, made of a semiconductor material such as n-type InP. The lower clad layer 12 and the upper clad layer 14 are semiconductor layers made of, for example, n-type InP and p-type InP, respectively, and the refractive indices of the clad layers 12 and 14 are lower than that of the active layer 13. The two-dimensional photonic crystal 20 has hollow holes made in the lower clad layer 12. Thereby, a photonic crystal periodic structure 21 composed of the hollow holes is formed. The hollow holes are arrayed into a square lattice or a triangular lattice so that a medium with a refractive index different from that of the lower clad layer 12 is scattered in the lower clad layer 12 with two-dimensional periodicity. In the hollow holes, a material such as SiN or the like may be filled. The active layer 13 is, for example, of a multiple quantum well structure of a semiconductor material such as InGaAs/InGaAsP, and when carriers are injected into the active layer 13, the active layer 13 emits light.
The active layer 13 is located between the lower clad layer 12 and the upper clad layer 14, so that a double heterojunction is formed, and in this structure, the carriers, which contribute to light emission, gather in the active layer 13.
A lower electrode 16 and an upper electrode 17 are formed of gold or the like on the lower surface of the substrate 11 and on the upper surface of the upper clad layer 14, respectively. When voltages are applied to the electrodes 16 and 17, the active layer 13 emits light, and a component leaking from the active layer 13 enters the two-dimensional photonic crystal 20. Light with a wavelength coincident with the intervals among the lattice points (hollow holes) is resonated and amplified by the two-dimensional photonic crystal 20. Thereby, coherent light is surface-emitted from the upper surface (an emitting area 18 around the electrode 17) of the upper clad layer 14.
Now referring to
The two-dimensional photonic crystal 20 is a square lattice wherein lattice points of a second medium 21, such as hollow holes, are placed in the first medium 12 at uniform intervals in two orthogonal directions, the intervals in one direction and the intervals in the other direction being equal to each other. The square lattice has representative directions, namely, Γ-X direction and Γ-M direction. If the intervals among the lattice points of the second medium 21 in the Γ-X direction are a, a fundamental lattice E composed of the lattice points of the second medium 21 is a square with four sides having a length of a.
When light L with a wavelength λ coincident with the lattice interval a travels in the Γ-X direction, the light L is diffracted on the lattice points. Of the light L, only the components diffracted in directions at 0 degree, ±90 degrees and 180 degrees to the traveling direction fulfill the Bragg condition. Then, since lattice points exist also in the traveling directions of the components diffracted at 0 degree, ±90 degrees and 180 degrees, the components are diffracted again on the lattice points in directions at 0 degree, ±90 degrees and 180 degrees to the respective traveling directions.
When the diffraction of the light L is performed once or repeated a plurality of times, the diffracted light returns to the initial lattice point, which causes resonance. Also, the component diffracted in the direction perpendicular to the surface of the paper of
In order to study the two-dimensional resonance phenomenon by use of a photonic crystal more quantitatively, the dispersion relation of light in the two-dimensional square lattice photonic crystal is shown by
Since the group velocity vg, which is the propagation velocity of light energy, is expressed by ∂ω/∂k, at a band edge where the gradient in the graph of
Because the two band ends III and IV of these four modes are degenerate, the electric field distribution depends on the characteristics of the degeneration and therefore is not definite and is unstable. In the other non-degenerate two modes I and II, the polarization is peculiar as shown by
As is apparent from
In the double-degenerate modes III and IV, the electric field distribution is not definite as mentioned above, and therefore, the polarization in the modes III and IV is unstable. In view of this problem existing in the prior art, the present inventors had tried to achieve polarization in a uniform direction, and as disclosed by Japanese Patent Laid-Open Publication No. 2003-23193, they found that polarization in a uniform direction can be achieved by designing the shapes of the lattice points of the two-dimensional photonic crystal appropriately.
As an example,
Referring to
As is apparent from
In the modes III′ and IV′, in the entire emitting surface, the polarization is uniform in phase as well as in direction. In the modes I′ and II′, however, although the direction of polarization is uniform, the phase is opposite (rotates at 180 degrees) between the upper area and the lower area (in the mode I′) or between the right area and the left area (in the mode II′). Therefore, in the modes I′ and II′, electric fields offset each other in the center of the emitting surface, and two-lobed emission occurs, resulting in a dark center area.
Further, because the photonic crystal has a characteristic as a resonator, the modes I′ and II′ have higher Q-values than the modes III′ and IV′. Therefore, in a case of selecting the modes III′ and IV′ as oscillation modes, the threshold is higher compared with a case of selecting the modes I′ and II′ as an oscillation mode. Thus, it is difficult to achieve the both merits, namely, single-lobed linear polarization and a low threshold (a high Q-value).
It is an object of the present invention to provide a two-dimensional photonic crystal surface-emitting laser which emits single-lobed linearly polarized light and which has a high Q-value.
In order to attain the object, a first aspect of the present invention provides a two-dimensional photonic crystal surface-emitting laser comprising a photonic crystal which has a photonic crystal periodic structure located in or near an active layer which emits light when carriers are injected thereto, the photonic crystal periodic structure having media with different refractive indices in two-dimensional periodic array, and the photonic crystal periodic structure is of a square lattice structure or a rectangular lattice structure which has translation symmetry but does not have rotation symmetry.
A second aspect of the present invention provides a two-dimensional photonic crystal surface-emitting laser comprising a photonic crystal which has a photonic crystal periodic structure located in or near an active layer which emits light when carriers are injected thereto, the photonic crystal periodic structure having media with different refractive indices in two-dimensional periodic array, and the photonic crystal periodic structure is of a square lattice structure or a rectangular lattice structure which is classified into p1, pm, pg or cm by a classification method under IUC (International Union of Crystallography in 1952).
In the two-dimensional photonic crystal surface-emitting laser according to the first aspect or the second aspect of the present invention, the photonic crystal has a photonic crystal periodic structure which is of a lattice structure having translation symmetry and not having rotation symmetry, that is, of a structure classified into pl, pm, pg or cm according to the two-dimensional pattern classification method. Thereby, the light emitted from a surface of the laser is single-lobed linearly polarized light which has a high Q value (a low threshold).
In the two-dimensional photonic crystal surface-emitting laser according to the first aspect or the second aspect of the present invention, it is preferred that the lattice structure of the photonic crystal has substantially triangular lattice points. Alternatively, each of the lattice points may be a combination of a relatively large circle and a relatively small circle. Also, each of the lattice points may be made of two or more media which are different in refractive index or may be made of a medium with a refractive index distribution.
a,
6
b and 6c are charts which show electric field distribution in mode I when a two-dimensional photonic crystal surface-emitting laser has circular lattice points.
a,
7
b and 7c are charts which show electric field distribution in mode I′ when a two-dimensional photonic crystal surface-emitting laser has elliptic lattice points.
a,
8
b and 8c are charts which show electric field distribution in mode I″ when a two-dimensional photonic crystal surface-emitting laser has triangular lattice points.
a and 9b are charts which show electric field distribution in mode I when a two-dimensional photonic crystal surface-emitting laser has circular lattice points,
a and 10b are charts which show electric field distribution in mode I″ when a two-dimensional photonic crystal surface-emitting laser has triangular lattice points,
a,
11
b and 11c are charts which show electric field distribution in mode II when a two-dimensional photonic crystal surface-emitting laser has circular lattice points.
a,
12
b and 12c are charts which show electric field distribution in mode II′ when a two-dimensional photonic crystal surface-emitting laser has elliptic lattice points.
a,
13
b and 13c are charts which show electric field distribution in mode II″ when a two-dimensional photonic crystal surface-emitting laser has triangular lattice points.
a and 14b are illustrations showing reflection and shear reflection respectively.
a and 32b are charts showing electric field distribution in mode I′ when a two-dimensional photonic crystal surface-emitting laser has elliptic lattice points,
a and 35b are charts showing electric field distribution in mode IV′ when a two-dimensional photonic crystal surface-emitting laser has elliptic lattice points,
Preferred embodiments of a two-dimensional photonic crystal surface-emitting laser according to the present invention are hereinafter described with reference to the accompanying drawings.
Triangular Lattice Points
A two-dimensional photonic crystal surface-emitting laser according to the present invention, of which surface is shown by
The two-dimensional photonic crystal 20 shown by
The modes I″, II″, III″ and IV′ are similar to the modes I′, II′, III′ and IV′ wherein each of the lattice points is elliptic (see
The above-described phenomenon is understood as follows. A two-dimensional photonic crystal is a laser which emits light in a direction perpendicular to an emission surface. The photonic crystal has a periodically changing refractive index, and the polarization of light depends on the direction of electric field in areas with a lower refractive index. When the lattice points are elliptic, for example, in the mode I′, as shown in
On the other hand, for example, in the mode IV′, as shown in
Thus, in order to achieve an oscillation mode with an electric field distribution with linear polarization and single-lobe, an electric field distribution with electric fields in one direction extending over the areas of the second medium with a lower refractive index is formed.
a to 6c, 7a to 7c and 8a to 8c schematically show electric field distributions in the modes I, I′′and I″ when the lattice points are true circular, elliptic and triangular, respectively. Each of
When the lattice points are triangular, the electric field distribution of components which are taken out in the direction perpendicular to the emitting surface, as shown in
In the same way as
As is apparent from
The lattice structure shall be of a square lattice or a rectangular lattice, which does not have rotational symmetry. It is generally known that two-dimensional periodical patterns are classified into 17 kinds under IUC (International Union of Crystallography in 1952). These 17 kinds are p1, pm, pg, cm, p2, pmm, pgg, cmm, pmg, p4, p4m, p4g, p3, p31m, p3m1, p6 and p6m. As shown in Table 1 below, of these 17 kinds, those which do not have rotational symmetry are p1, pm, pg and cm. The lattice structure with triangular lattice points corresponds to pm.
Reflection is a pattern which is line symmetrical on an axis of reflection as shown in
Possible shapes of lattice points and possible patterns (p1, pm, pg, cm) are shown in
As
Also, as
Although the present invention has been described in connection with the preferred embodiments above, it is to be noted that various changes and modifications are possible to those who are skilled in the art. Such changes and modifications are to be understood as being within the scope of the present invention.
The materials of the semiconductor layer, the photonic crystal and the electrodes may be selected arbitrarily, and the structure for achieving uniform polarization direction may be designed arbitrarily. Also, the photonic crystal periodic structure is not necessarily formed in the lower clad layer and may be provided in or near the active layer of the upper clad layer.
In the above-described embodiments, the refractive index of the second medium is lower than that of the first medium. However, the refractive index of the second medium may be higher than that of the first medium.
Number | Date | Country | Kind |
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2003-083707 | Mar 2003 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP04/03987 | 3/23/2004 | WO | 9/25/2006 |