Two-dimensional photonic crystal laser

Abstract

A two-dimensional photonic crystal laser (10) is provided with: a light-emitting layer (11) having a plurality of light-emitting regions (111) that are regions made of a material that, upon injection of electric current, emits light including a predetermined frequency, and a transparent region (112) that is provided so as to include the plurality of light-emitting regions (111) and is made of a material transparent to light of the predetermined frequency; a two-dimensional photonic crystal layer (12) having a plate-shaped base material (121) provided in parallel with the light-emitting layer (11) and different refractive index portions (122) having a refractive index different from that of the base material (121) and arranged periodically within the base material with a period at which the light of the predetermined frequency is amplified; and electrodes (171, 172) provided so as to sandwich the light-emitting layer (11) and the two-dimensional photonic crystal layer (12) to inject electric current into the plurality of light-emitting regions.

Description

Two-dimensional photonic crystal laser

[0001] The present invention relates to a two-dimensional photonic crystal laser (also called a "two-dimensional photonic crystal surface-emitting laser") that amplifies light using a two-dimensional photonic crystal.

[0002] Semiconductor lasers generally emit laser beams with a divergence angle of 10° or more, so they are used in combination with an optical system to reduce the divergence angle. Lasers used in sensing technology called LiDAR (Light Detection And Ranging), which detects and measures distance from obstacles in self-driving cars, etc., require a very small divergence angle of the laser beam (for example, less than 0.05°), which requires a high-precision optical system. This requires advanced technology and is costly. A similar problem occurs with lasers used in inter-satellite optical communications.

[0003] In recent years, two-dimensional photonic crystal lasers (see Patent Document 1) have been put into practical use as lasers capable of achieving a smaller divergence angle than conventional semiconductor lasers. Two-dimensional photonic crystal lasers include an active layer, which is a light-emitting layer; a two-dimensional photonic crystal layer, which is a plate-shaped base material with periodic arrangement of modified refractive index portions having a different refractive index from that of the active layer; and electrodes sandwiching the two layers in the thickness direction for injecting current into the active layer. The modified refractive index portions are typically formed of holes formed in the base material. In two-dimensional photonic crystal surface-emitting lasers, light of a predetermined frequency band is generated by injecting current into the active layer. Of this light, only light of a specific frequency corresponding to the length of the period (the wavelength of light having this specific frequency within the two-dimensional photonic crystal layer) is amplified by the periodic structure of the two-dimensional photonic crystal layer and oscillates as a laser. The laser beam generated in this way is emitted in a direction perpendicular to the two-dimensional photonic crystal layer.

[0004] In surface-emitting lasers, Fresnel diffraction generally causes the laser beam divergence angle to decrease as the region where light is amplified (light amplification region) becomes larger. In conventional two-dimensional photonic crystal lasers, in order to widen the light amplification region, modified refractive index portions are arranged in the two-dimensional photonic crystal layer over a range wider than the region in the active layer where current is injected from the electrode (current injection region). However, since the light amplified in the two-dimensional photonic crystal layer also spreads in the thickness direction from the layer, it is absorbed in the region of the active layer outside the current injection region (non-current injection region). (Note: In the current injection region, electron energy is excited by current injection, so light absorption (and electron energy excitation) does not occur.) Therefore, even if modified refractive index portions are arranged over a range wider than the current injection region, the current injection region of the active layer and the light amplification region of the two-dimensional photonic crystal layer approximately coincide, and the light amplification region cannot be wider than the current injection region. On the other hand, widening the current injection region can widen the light amplification region, but requires an increased injected current.

[0005] In the two-dimensional photonic crystal laser described in Non-Patent Document 1, impurities are diffused into the non-current injection region of the active layer. This prevents light amplified in the two-dimensional photonic crystal layer from being absorbed in the non-current injection region, making it possible to widen the optical amplification region compared to conventional two-dimensional photonic crystal lasers in which impurities are not diffused into the non-current injection region.

[0006] International Publication No. WO2022 / 181722

[0007] Chia-Jui Chang et al., "Design of low-threshold photonic-crystal surface-emitting lasers with confined gain regions by using selective area intermixing," Discover Nano, Springer Nature, (UK, Germany), Vol. 18, Article No. 134, published October 30, 2023; Takuya Inoue et al., "General recipe to realize photonic-crystal surface-emitting lasers with 100-W-to-1-kW single-mode operation," Nature Communications, Springer Nature, (UK, Germany), Vol. 13, Article No. 3262, published July 4, 2022

[0008] In the two-dimensional photonic crystal layer, light having the predetermined frequency introduced into a certain position in the two-dimensional photonic crystal layer is reflected by the modified refractive index section and its propagation direction repeatedly changes by 90° or 180°, thereby remaining within a certain range from that position in the in-plane direction and being amplified. Therefore, in the two-dimensional photonic crystal laser described in Non-Patent Document 1, the optical amplification region of the two-dimensional photonic crystal layer does not extend to the entire region corresponding to the current injection region and non-current injection region of the active layer, but remains within a region corresponding to a certain range extended from the current injection region. Therefore, the optical amplification region cannot be made sufficiently wide, and the divergence angle cannot be made sufficiently small.

[0009] The problem to be solved by the present invention is to provide a two-dimensional photonic crystal laser that can reduce the divergence angle of the emitted laser beam without increasing the injected current.

[0010] The two-dimensional photonic crystal laser according to the present invention, which has been made to solve the above-mentioned problems, is characterized by comprising: a) a light-emitting layer having a plurality of light-emitting regions, which are regions made of a material that emits light including a predetermined frequency when a current is injected therein, and a transparent region that is arranged to encompass the plurality of light-emitting regions and is made of a material that is transparent to light of the predetermined frequency; b) a two-dimensional photonic crystal layer having a plate-shaped base material arranged parallel to the light-emitting layer and modified refractive index portions that have a refractive index different from that of the base material and are periodically arranged within the base material at a periodic length that amplifies the light of the predetermined frequency; and c) electrodes that are arranged to sandwich the light-emitting layer and the two-dimensional photonic crystal layer and that inject a current into the plurality of light-emitting regions.

[0011] In the two-dimensional photonic crystal laser according to the present invention, the light-emitting layer has multiple light-emitting regions and a transparent region that encompasses the multiple light-emitting regions. That is, the light-emitting layer has multiple light-emitting regions within a predetermined region (referred to as the "inclusion region"), and the portion of the inclusion region excluding the light-emitting regions is the transparent region. When a current is injected from an electrode into each light-emitting region, light having a predetermined frequency is generated. The light of the predetermined frequency is distributed in a region (referred to as the "extended region") that extends a predetermined distance from the outer edge of the region corresponding to each light-emitting region (the "emission-corresponding region") within the two-dimensional photonic crystal layer, and also extends in the thickness direction. This predetermined distance is determined by the intensity of the light in the light-emitting region and the structure of the two-dimensional photonic crystal layer (the shape of the modified refractive index portion, the base material, the refractive index of the modified refractive index portion, etc.), and the intensity of the light is determined by the current density injected into the light-emitting region. The light that extends in the thickness direction is not absorbed in the light-emitting region because electrons are excited by the current injection in the light-emitting region, and is not absorbed in the transparent region because the transparent region is transparent to light of the predetermined frequency. As a result, the light amplified in the two-dimensional photonic crystal layer spreads throughout the entire containment region without being absorbed by the light-emitting layer. The extended region extending from each light-emitting corresponding region remains within a certain range by repeatedly changing the propagation direction by 90° or 180° due to reflections at the modified refractive index sections.

[0012] According to the two-dimensional photonic crystal laser of the present invention, by providing a plurality of light-emitting regions in the light-emitting layer, the combined area of ​​the plurality of regions in the two-dimensional photonic crystal layer corresponding to the plurality of light-emitting regions and the extended regions extending in the in-plane direction from each of the regions forms the optical amplification region. Therefore, the area of ​​the extended region in the two-dimensional photonic crystal layer can be made larger than when only one light-emitting region having the same area as the sum of the areas of the plurality of light-emitting regions is provided in the light-emitting layer and a current of the same current density is injected (to emit light with the same intensity), thereby making it possible to widen the optical amplification region. Therefore, the divergence angle of the emitted laser beam can be made smaller without increasing the area of ​​the light-emitting region, i.e., without increasing the injected current.

[0013] On the other hand, if the area of ​​the optical amplification region is made approximately the same as that of a conventional two-dimensional photonic crystal laser, the area of ​​the light-emitting region can be made smaller than before, thereby making it possible to reduce the amount of current injected into the light-emitting region more than before.

[0014] The light-emitting layer and the base material may be in contact with each other, or an intervening layer may be present between them. The inclusion region may be provided only in a portion of the light-emitting layer, or the entire light-emitting layer may be the inclusion region.

[0015] The material of the light emitting region can be the same as the material of the active layer in a conventional two-dimensional photonic crystal laser, such as a material having a quantum well structure.

[0016] Any material that is transparent to light of the predetermined frequency can be used for the material of the transparent region, but a material different from the active layer may be used, or a material that has been made to lose its light-emitting and light-absorbing functions by implanting ions into the material of the active layer (particularly a material having a quantum well structure) as in the method described in Non-Patent Document 1. When a material different from the active layer is used, for example, the active layer can be fabricated over the entire region where the light-emitting layer is to be provided, and then a mask can be formed on the light-emitting region (when the inclusion region is provided only in a portion of the light-emitting layer, on the light-emitting region and outside the inclusion region), and the active layer can be etched, and the material of the transparent region can be deposited in this etched region, thereby fabricating the light-emitting layer.

[0017] It is desirable that the material of the transparent region has a higher electrical resistivity than the material of the light-emitting region, so that the current flows through the light-emitting region without leaking into the transparent region, reducing current waste and reducing the current supplied from the electrodes.

[0018] In the two-dimensional photonic crystal laser according to the present invention, it is desirable that the distance between each of the plurality of light-emitting regions and the nearest light-emitting region is shorter than the propagation length of light of the predetermined frequency in the two-dimensional photonic crystal layer. Here, the propagation length refers to the average length of propagation of light introduced into the two-dimensional photonic crystal layer within the two-dimensional photonic crystal layer, and is a parameter that determines the predetermined distance (although not the specific distance itself). As described above, light propagating within the two-dimensional photonic crystal layer is reflected by modified refractive index sections, causing its propagation direction to repeatedly change by 90° or 180°, eventually resulting in laser oscillation and emission to the outside, and therefore the propagation length has a finite value. By making the distance between the light-emitting regions shorter than the propagation length, the light intensity within the optical amplification region can be made nearly uniform, resulting in stable laser oscillation.

[0019] In the two-dimensional photonic crystal laser according to the present invention, it is desirable that the average refractive index in the combination of the light-emitting region of the light-emitting layer and the light-emission corresponding region of the two-dimensional photonic crystal layer and the resonant frequency determined by the periodic length within the light-emission corresponding region match the average refractive index in the combination of the transparent region of the light-emitting layer and the region outside the light-emission corresponding region of the two-dimensional photonic crystal layer and the resonant frequency determined by the periodic length in the region outside the light-emission corresponding region.

[0020] Since the refractive indexes of the light-emitting region and the transparent region of the light-emitting layer are usually different, even if modified refractive index sections having the same material and size and the same periodic length are arranged in the light-emitting region and the region outside of the two-dimensional photonic crystal layer, the resonant frequencies of these regions do not match. In contrast, by using different materials, sizes, and / or periodic lengths for the modified refractive index sections between these two regions, or by using different materials for the base material of the two-dimensional photonic crystal layer, the resonant frequencies of these two regions can be made to match even if the refractive indexes of the light-emitting region and the transparent region of the light-emitting layer are different. This prevents unnecessary reflections from occurring at the boundary between the light-emitting region and the transparent region of the light-emitting layer and at the boundary between the light-emitting region and the region outside of it in the two-dimensional photonic crystal layer, thereby enabling the stable formation of an optical amplification region.

[0021] The two-dimensional photonic crystal laser according to the present invention can reduce the divergence angle of the emitted laser beam without increasing the injected current.

[0022] 6 is a schematic perspective view showing the overall configuration of one embodiment of a two-dimensional photonic crystal laser according to the present invention. FIG. 7 is a plan view showing the schematic configuration of an emission layer in the two-dimensional photonic crystal laser of this embodiment. FIG. 8 is a plan view showing the schematic configuration of a portion of the two-dimensional photonic crystal layer in the two-dimensional photonic crystal laser of this embodiment. FIG. 9 is a diagram showing the relationship between the emission region and transparent region of the emission layer and the optical amplification region of the two-dimensional photonic crystal layer in the two-dimensional photonic crystal laser of this embodiment. FIG. 10 is a diagram showing the relationship between the emission region and transparent region of the emission layer and the optical amplification region of the two-dimensional photonic crystal layer in a comparative two-dimensional photonic crystal laser having only one emission region in the emission layer. FIG. 11 is a schematic perspective view showing the overall configuration of a modified two-dimensional photonic crystal laser according to the present invention. FIG. 12 is a plan view showing the schematic configuration of the emission layer in the two-dimensional photonic crystal laser of the modified two-dimensional photonic crystal laser of FIG. 6. FIG. 13 is a schematic perspective view showing the overall configuration of another modified two-dimensional photonic crystal laser according to the present invention. FIG. 14 is a plan view showing the schematic configuration of the emission layer in the two-dimensional photonic crystal laser of the modified two-dimensional photonic crystal laser of FIG. Fig. 6 is a diagram showing the results of a simulation of a near-field pattern of the cross section of a laser beam emitted from the two-dimensional photonic crystal laser of this embodiment. Fig. 7 is a diagram showing the results of a simulation of a far-field pattern of the cross section of a laser beam emitted from the two-dimensional photonic crystal laser of this embodiment. Fig. 8 is a diagram showing the results of a simulation of a near-field pattern of the cross section of a laser beam emitted from the two-dimensional photonic crystal laser of the modified example of Fig. 6. Fig. 9 is a diagram showing the results of a simulation of a far-field pattern of the cross section of a laser beam emitted from the two-dimensional photonic crystal laser of the modified example of Fig. 6.

[0023] An embodiment of a two-dimensional photonic crystal laser according to the present invention will be described with reference to FIGS.

[0024] (1) Configuration of the Two-Dimensional Photonic Crystal Laser of the Present Embodiment The two-dimensional photonic crystal laser of the present embodiment has a configuration in which a first electrode 171, a first cladding layer 141, a two-dimensional photonic crystal layer 12, a spacer layer 13, an emission layer 11, a second cladding layer 142, a substrate 16, and a second electrode 172 are laminated in this order, as shown in Fig. 1. However, the order of the emission layer 11 and the two-dimensional photonic crystal layer 12 may be reversed. For convenience, Fig. 1 shows the first electrode 171 on the lower side and the second electrode 172 on the upper side, but the orientation of the two-dimensional photonic crystal laser 10 during use is not limited to that shown in this figure.

[0025] 2, the light-emitting layer 11 has a plurality of light-emitting regions 111 and transparent regions 112 that are provided to encompass the plurality of light-emitting regions 111. In this embodiment, nine circular light-emitting regions 111 having a radius of R are arranged in a two-dimensional array, three vertically and three horizontally, at predetermined intervals, but the shape, number, and arrangement of the light-emitting regions 111 are not limited to this example. The size of each light-emitting region 111 and the spacing between the light-emitting regions 111 will be described after explaining the configuration of the two-dimensional photonic crystal layer 12.

[0026] The material of each light-emitting region 111 is one that emits light containing a predetermined frequency, as described below, when a current is injected. In this embodiment, a semiconductor material having a multiple quantum well structure is used. One example of such a material is an InGaAs / AlGaAs multiple quantum well, which is formed by alternately stacking multiple layers of InGaAs and AlGaAs, and is used as the active layer material in conventional two-dimensional photonic crystal lasers and other semiconductor lasers.

[0027] The transparent region 112 is made of a material that is transparent to light of a predetermined frequency. In this embodiment, the light-emitting region 111 is made of a semiconductor material having a multiple quantum well structure, which is implanted with ions to eliminate its light-emitting and light-absorbing functions. For example, an InGaAs / AlGaAs multiple quantum well is irradiated with an ion beam consisting of hydrogen ions, oxygen ions, or the like.

[0028] 3 , the two-dimensional photonic crystal layer 12 has a plate-shaped base material 121 provided parallel to the light-emitting layer 11, and modified refractive index portions 122 made of air holes periodically arranged in the base material 121. In this embodiment, the modified refractive index portions 122 are made of air holes, and therefore their refractive index (the refractive index of air) differs from that of the base material 121. Instead of air holes, the modified refractive index portions 122 may be formed by embedding a member having a refractive index different from that of the base material 121 in the base material 121. Furthermore, in this embodiment, a combination of elliptical and circular modified refractive index portions 122 with smaller planar shapes is used, but the shape of the modified refractive index portions 122 is not limited to this.

[0029] In this embodiment, the modified refractive index portions 122 are arranged in a square lattice pattern. When the modified refractive index portions 122 are arranged in this square lattice pattern, the periodic length a of the arrangement is adjusted to the wavelength λ of light having a predetermined frequency f in the two-dimensional photonic crystal layer 12, among the light generated in the light-emitting region 111 of the light-emitting layer 11. The wavelength λ of light in the two-dimensional photonic crystal layer 12 is λ = c / (nf), which is the value obtained by dividing the speed of light c in a vacuum by the refractive index n and the frequency f. Here, the refractive index n is the average refractive index within the range in which the light exists. However, because the light spreads in the thickness direction not only within the two-dimensional photonic crystal layer 12 but also into the light-emitting layer 11, the refractive index n differs depending on whether the position in the in-plane direction corresponds to the light-emitting region 111 or the transparent region 112 of the light-emitting layer 11. In the light-emitting layer 11 of this embodiment, the refractive index of the light-emitting region 111 is lower than that of the transparent region 112 when a current is injected. Therefore, in the two-dimensional photonic crystal layer 12, the periodic length a in the emission corresponding region 123 corresponding to the emission region 111 is made longer by Δa (>0) than the periodic length a' in other regions (a=a'+Δa). Specifically, the periodic length a' in regions other than the emission corresponding region 123 is set to 0.278 μm, and the periodic length a in the emission corresponding region 123 is made longer by 0.02% (Δa / a'=2×10 -4) is performed. This causes the resonance frequency in the emission corresponding region 123 to match the resonance frequency in regions other than the emission corresponding region 123. Here, the difference between the periodic length a in the emission corresponding region 123 and the periodic length a' in other regions is slight (0.02%), but the modified refractive index portions 122 are arranged over several hundred periods in each of the emission corresponding regions 123 and other regions, and the difference in lattice constant for these several hundred portions amounts to approximately several tens of nm when added up. Therefore, the two-dimensional photonic crystal layer 12 can be fabricated so that a significant difference is imparted to the lattice constant when the entire regions are compared.

[0030] 3 shows a partially enlarged view of the vicinity of one of the nine emission-corresponding regions 123 in the two-dimensional photonic crystal layer 12. Furthermore, in order to clearly show the shape of the modified refractive index portions 122, the ratio of the area of ​​the modified refractive index portions 122 to the area of ​​the emission-corresponding region 123 is depicted larger than it actually is, and accordingly the interval at which the modified refractive index portions 122 are arranged is depicted larger than the actual periodic length a.a'. Therefore, in the actual two-dimensional photonic crystal layer 12, a larger number of modified refractive index portions 122 are arranged in the emission-corresponding region 123 than shown in FIG.

[0031] The distance X (see FIG. 2 ) between the light-emitting regions 111 in the light-emitting layer 11 (the shortest distance between the outer edges of the light-emitting regions 111) is set so as to be shorter than the propagation length (the average length of propagation in the two-dimensional photonic crystal layer 12) of light having the resonance frequency (predetermined frequency) propagating through the two-dimensional photonic crystal layer 12. When the resonance frequency in the light-emitting corresponding region 123 and the resonance frequency outside the light-emitting corresponding region 123 are approximately the same, the propagation length λ can be calculated as follows: λ=|Re((iμ(R-(R 2 +I 2 ) 1 / 2 )) -1 / 2)|. Both the Hermite coupling coefficient and the non-Hermitian coupling coefficient are coefficients determined by the shape, size, and refractive index of the modified refractive index section 122. The Hermite coupling coefficient corresponds to the diffraction intensity at which light propagating parallel to the two-dimensional photonic crystal layer changes its propagation direction without loss at the modified refractive index section, and the non-Hermitian coupling coefficient corresponds to the diffraction intensity at which propagating light similarly changes its propagation direction but with loss as a result of part of the light being radiated in a direction perpendicular to the layer at the modified refractive index section. Details of the Hermite coupling coefficient and the non-Hermitian coupling coefficient are described in Non-Patent Document 2.

[0032] Here, the propagation length λ is approximately λ = |I| -1 |R / μ| 1 / 2 According to this approximation, the closer the magnitude μ of the non-Hermitian coupling coefficient or the imaginary part I of the Hermitian coupling coefficient is to 0, the longer the propagation length λ. For example, by reducing the thickness of the two-dimensional photonic crystal layer 12 or by reducing the volume ratio of the modified refractive index section 122 to the entire two-dimensional photonic crystal layer 12, the magnitude μ of the non-Hermitian coupling coefficient can be made closer to 0, thereby lengthening the propagation length λ. Furthermore, when the modified refractive index section 122 is composed of a pair of modified refractive index areas formed by a first modified refractive index section 1221 and a second modified refractive index section 1222 (see FIG. 3 ) that are spaced apart from each other, the value of the imaginary part I of the Hermitian coupling coefficient can be made closer to 0 by adjusting the difference in the area of ​​the planar shape of the first modified refractive index section 1221 and the second modified refractive index section 1222, thereby lengthening the propagation length λ.

[0033] The first cladding layer 141 and the second cladding layer 142 have the role of injecting holes from the first electrode 171 and electrons from the second electrode 172 as electric charges, respectively, and also have the role of preventing light propagating parallel to the two-dimensional photonic crystal layer 12 from leaking from the two-dimensional photonic crystal layer 12 and the light-emitting layer 11. To perform the former role of injecting electric charges, a p-type semiconductor is used for the first cladding layer 141, and an n-type semiconductor is used for the second cladding layer 142. For the same reason, a p-type semiconductor is used as the material for the base material 121 of the two-dimensional photonic crystal layer 12.

[0034] The spacer layer 13 is provided to allow holes injected from the first electrode 171 to pass through and be introduced into the light-emitting layer 11, while suppressing electrons injected from the second electrode 172 from passing through the light-emitting layer 11 (thereby suppressing combination with holes on the first electrode 171 side of the light-emitting layer 11). In order to fulfill this role, a p-type semiconductor is used for the spacer layer 13.

[0035] The substrate 16 is made sufficiently thicker than the other layers in order to maintain the mechanical strength of the entire two-dimensional photonic crystal laser 10. For the same reason as for the second cladding layer 142, an n-type semiconductor is used as the material for the substrate 16.

[0036] The first electrode 171 is composed of first partial electrodes 1711 having the same shape and number as the light-emitting regions 111 of the light-emitting layer 11, and all of them are provided on the lower surface of the first clad layer. In FIG. 1, the first electrodes 171 are depicted separated from the lower surface of the first clad layer to show the shape of the first electrodes 171, but in reality, they are in contact with the lower surface of the first clad layer. One first partial electrode 1711 is provided for each light-emitting region 111, and is located directly below the corresponding light-emitting region 111.

[0037] The second electrode 172 is a circular electrode made of a conductive material and is provided on the upper surface of the substrate 16. The area inside the ring where no conductive material exists serves as a window 1722 through which the laser beam passes. Although the second electrode 172 has a different shape from the light-emitting region 111, the substrate 16 is sufficiently thicker than the other layers, so that the current flowing between the first electrode 171 and the second electrode 172 reaches the light-emitting layer 11 with substantially the same shape as each first partial electrode 1711 and with a substantially uniform current density distribution. Therefore, the light-emitting region 111 of the above shape is formed in the light-emitting layer 11 with a substantially uniform current density distribution.

[0038] (2) Operation of the Two-Dimensional Photonic Crystal Laser of the Present Embodiment The operation of the two-dimensional photonic crystal laser 10 of the present embodiment will be described with reference to FIGS.

[0039] When a voltage is applied between the first electrode 171 and the second electrode 172 by a power source (not shown), a current flows in the light-emitting region 111, which has substantially the same shape as each of the first partial electrodes 1711 and a substantially uniform current density distribution, as described above. This causes light in a frequency band including a predetermined frequency to be generated from the light-emitting region 111. The light having the predetermined frequency is amplified in the light-emission-corresponding region 123 in the two-dimensional photonic crystal layer 12 by the periodic arrangement of the modified refractive index sections 122 with a periodic length a corresponding to the predetermined frequency (resonance frequency). Within the two-dimensional photonic crystal layer 12, the light further expands from the light-emission-corresponding region 123 to a region corresponding to the transparent region 112 of the light-emitting layer 11, but is not absorbed by the transparent region 112 in this region. Therefore, an extension region 124 is formed, in which the light expands a predetermined distance L from the outer edge of the light-emission-corresponding region 123 (see FIG. 4 ). The combined region of the light-emission-corresponding region 123 and the extension region 124 constitutes the light amplification region 125. Furthermore, by periodically arranging the modified refractive index portions 122 in the base material 121 of the two-dimensional photonic crystal layer 12, light of a predetermined frequency is amplified in the optical amplification region 125, resulting in laser oscillation. The generated laser beam passes from the optical amplification region 125 through the window 1722 of the second electrode 172 and is emitted in a direction perpendicular to the two-dimensional photonic crystal layer 12.

[0040] For comparison with this embodiment, the total area of ​​nine circular light emitting regions 111 with a radius R in this embodiment is 9πR 2 Consider a two-dimensional photonic crystal laser having only one circular light-emitting region of radius 3R, which has the same area as the light-emitting region of radius 3R. By making the light-emitting region of this embodiment and the comparative example the same area, when a current is injected into the light-emitting region at the same current density, the magnitude of the current injected into the entire light-emitting region is the same. In the two-dimensional photonic crystal laser of this comparative example, as shown in FIG. 5, a circular light-emitting region of radius 3R and area π(3R) corresponding to the light-emitting region is formed in two-dimensional photonic crystal layer 92 (the structure of which is the same as that of two-dimensional photonic crystal layer 12 in this embodiment). 2 =9πR 24 and 5 , although the total area of ​​the light-emission corresponding regions 123, 923 (and the corresponding light-emitting regions of the light-emitting layer) is the same between this embodiment and the comparative example, the area of ​​the light-amplification regions 125, 925 is larger in this embodiment than in the comparative example. Therefore, the spread angle of the laser beam emitted from the light-amplification regions 125, 925 can be made smaller in this embodiment than in the comparative example.

[0041] Furthermore, as described above, in the two-dimensional photonic crystal laser 10 of this embodiment, the periodic length for arranging the modified refractive index portions 122 is set to a' outside the emission corresponding region 123, and to a (=a'+Δa) which is longer than a' by Δa in the emission corresponding region 123, thereby compensating for the difference in refractive index between the emission region 111 and the transparent region 112 of the emission layer 11 and matching the resonance frequency in the emission corresponding region 123 with the resonance frequency outside the emission corresponding region 123. This prevents unnecessary reflection from occurring at the boundary between the emission corresponding region 123 and the region outside it, and enables the optical amplification region 125 to be formed stably.

[0042] Furthermore, in the two-dimensional photonic crystal laser 10 of this embodiment, by arranging the first partial electrode 1711 so that the distance between the light-emitting regions 111 is shorter than the propagation length of light having a predetermined frequency propagating within the two-dimensional photonic crystal layer 12, or by adjusting the structure of the two-dimensional photonic crystal layer 12 as described above so that the propagation length is shorter than the distance between the light-emitting regions 111, the intensity of light within the optical amplification region 125 can be made closer to uniform, and stable laser oscillation can be obtained.

[0043] So far, we have explained that this embodiment allows the optical amplification region to be wider than conventionally, thereby reducing the divergence angle of the laser beam. On the other hand, in this embodiment, if the divergence angle of the laser beam is to be the same as conventionally, the area of ​​the emission-corresponding region required to form an optical amplification region of the same width as conventionally can be reduced. This allows the current injected into the emission-corresponding region to be reduced. For example, in some laser applications, such as when irradiating a laser beam on an object that would be damaged if high energy were applied, it is necessary to narrow the divergence angle of the laser beam while suppressing the output. In such applications, it is useful to be able to reduce the current injected into the emission-corresponding region.

[0044] (3) Modified Two-Dimensional Photonic Crystal Laser Figures 6 and 7 show a first modified two-dimensional photonic crystal laser according to the present invention. The two-dimensional photonic crystal laser 10A of the first modified example includes a light-emitting layer 11A, instead of the light-emitting layer 11 used in the two-dimensional photonic crystal laser 10 of the above embodiment. The light-emitting layer 11A includes three rectangular light-emitting regions 111A spaced apart from one another in a one-dimensional array along the direction of the short sides of the rectangles, with transparent regions 112A surrounding the light-emitting regions 111A. The two-dimensional photonic crystal laser 10A of the first modified example also includes a first electrode 171A, instead of the first electrode 171 used in the above embodiment. The first electrode 171A is composed of first partial electrodes 1711A having the same shape and number as the light-emitting regions 111A. The remaining configuration of the two-dimensional photonic crystal laser 10A is the same as that of the above embodiment. Simulations were performed on this two-dimensional photonic crystal laser 10A of the first modified example, along with the two-dimensional photonic crystal laser 10 of the above embodiment, and the results will be described later.

[0045] 8 and 9 show a second modified example of the two-dimensional photonic crystal laser according to the present invention. The two-dimensional photonic crystal laser 10B of the second modified example includes a light-emitting layer 11B and a first electrode 171B described below, instead of the light-emitting layer 11 and the first electrode 171 used in the two-dimensional photonic crystal laser 10 of the above embodiment. The remaining configuration of the two-dimensional photonic crystal laser 10B is the same as that of the above embodiment.

[0046] The light-emitting layer 11B includes a plurality of light-emitting regions 111B made of a material having a quantum well structure and a transparent region 112B formed to encompass the plurality of light-emitting regions 111B and made of a material having a higher electrical resistivity than the light-emitting region 111B (hereinafter referred to as a "high-resistance material"). This light-emitting layer 11B can be fabricated by first forming a layer made of a material having a quantum well structure, masking the portions where the light-emitting regions 111B will be formed, etching the portions other than the mask, depositing the material of the transparent region 112B in the etched areas, and finally removing the mask. The high-resistance material constituting the transparent region 112B can be, for example, an intrinsic semiconductor without impurity doping or an insulator. In this modification, the shape and number of the light-emitting regions 111B are the same as in the above embodiment, but other shapes and numbers (e.g., the same as the light-emitting region 111A in the first modification) may also be used.

[0047] The first electrode 171B is a single circular electrode as a whole. The shape of the first electrode 171B is different from the shape of each light-emitting region 111B, and a projection 116B of the first electrode 171B onto the light-emitting layer 11B has a shape that encompasses all of the light-emitting regions 111B.

[0048] In two-dimensional photonic crystal laser 10B of the second modification, when a current is passed between first electrode 171B and second electrode 172, the current in light-emitting layer 11 flows only to light-emitting region 111B, but not to transparent region 112B made of a highly resistive material. This allows current to be efficiently supplied to light-emitting region 111B. The laser oscillation operation in two-dimensional photonic crystal laser 10B of the second modification is similar to that of two-dimensional photonic crystal laser 10 of the above embodiment.

[0049] (4) Simulation of the two-dimensional photonic crystal laser of this embodiment and the first modified example The near-field pattern and far-field pattern of the emitted laser beam for the two-dimensional photonic crystal lasers 10 and 10A of this embodiment and the first modified example were determined by simulation under the following conditions.

[0050] In this embodiment, the diameter 2R of the light emitting region 111 is 0.37 mm, and the distance (center-to-center distance) X between the light emitting regions 111 is 0.93 mm. The current injected into the light emitting region 111 is 10 A. The periodic length of the arrangement of the modified refractive index portions 122 of the two-dimensional photonic crystal layer 12 is a' = 0.278 μm in the region other than the light emission corresponding region 123, and a = a' + Δa, Δa / a' = 2 × 10 -4 The real part R of the Hermite coupling coefficient calculated from the shape and refractive index of the modified refractive index portion 122 and the refractive index of the base material 121 is 80 cm -1 , the imaginary part of the Hermite coupling coefficient I is 27cm -1 , the magnitude of the non-Hermitian coupling coefficient μ is 2cm -1 In this case, the propagation length λ in the transparent region 112 is calculated to be 2.9 mm. The distance (0.93 mm) between the light emitting regions 111 is shorter than the calculated propagation length λ.

[0051] In the first modification, the light emitting region 111A has a long side length of 1.45 mm, a short side length of 0.21 mm, and a distance between the light emitting regions 111A (center-to-center distance in the short side direction) of 0.94 mm. The current injected into the light emitting region 111A is 10 A. The periodic length of the modified refractive index portions 122 of the two-dimensional photonic crystal layer 12 is a' = 0.278 μm, a = a' + Δa, and Δa / a' = 2 × 10 in the region other than the light emitting corresponding region 123. -4 The real part of the Hermite coupling coefficient R is 73 cm -1 , the imaginary part of the Hermite coupling coefficient I is 45cm -1 , the magnitude of the non-Hermitian coupling coefficient μ is 1.2 cm -1 In this case, the propagation length λ in the transparent region 112A is calculated to be 1.8 mm. The distance (0.94 mm) between the light emitting regions 111 is shorter than the calculated propagation length λ.

[0052] The simulation results are shown in Fig. 10 for the near-field image of this embodiment, Fig. 11 for the far-field image of this embodiment, Fig. 12 for the near-field image of the first modified example, and Fig. 13 for the far-field image of this embodiment. The near-field images show that laser light is emitted from the surfaces of the two-dimensional photonic crystal layers 12, 12A over a range of 2 mm or more in diameter in both this embodiment and the first modified example. These results show that a wide optical amplification region with a diameter of 2 mm or more is formed in both cases. Furthermore, the far-field images show that a narrow divergence angle of 0.05° or less, which is required for sensing technology, is achieved, with a divergence angle of 0.0195° in this embodiment and 0.0258° in the first modified example.

[0053] In conventional two-dimensional photonic crystal lasers, to achieve such a narrow divergence angle of 0.0195° or 0.0258°, an emission region with a diameter of about 3 mm is required, which requires a current of 30 A or more (for a divergence angle of 0.0195°) or 20 A or more (for a divergence angle of 0.0258°) to be injected into the emission region. In contrast, in this embodiment and the first modification, the above narrow divergence angle can be achieved with a current of 10 A, which is smaller than conventional currents.

[0054] Although the embodiments and modifications of the two-dimensional photonic crystal laser according to the present invention have been described above, the present invention is not limited to these embodiments and modifications, and further modifications are possible.

[0055] 10, 10A, 10B... Two-dimensional photonic crystal laser 11, 11A, 11B... Light-emitting layer 111, 111A, 111B... Light-emitting region 112, 112A, 112B... Transparent region 116B... Projection of first electrode onto light-emitting layer 12, 92... Two-dimensional photonic crystal layer 121... Base material 122... Modified refractive index section 1221... First modified refractive index section 1222... Second modified refractive index section 123, 923... Light-emitting corresponding region 124, 924... Extension region 125, 925... Optical amplification region 13... Spacer layer 141... First cladding layer 142... Second cladding layer 16... Substrate 171, 171A, 171B... First electrode 1711, 1711A... First partial electrode 172... Second electrode 1722... Window

Claims

1. A two-dimensional photonic crystal laser comprising: a) a light-emitting layer having a plurality of light-emitting regions, which are regions made of a material that emits light including a predetermined frequency when an electric current is injected therein, and a transparent region that is arranged to encompass the plurality of light-emitting regions and is made of a material that is transparent to light of the predetermined frequency; b) a two-dimensional photonic crystal layer having a plate-shaped base material arranged parallel to the light-emitting layer and modified refractive index portions that have a refractive index different from that of the base material and are periodically arranged within the base material at a periodic length that amplifies the light of the predetermined frequency; and c) electrodes that are arranged to sandwich the light-emitting layer and the two-dimensional photonic crystal layer and that inject electric current into the plurality of light-emitting regions.

2. The two-dimensional photonic crystal laser according to claim 1, wherein the material of said transparent region is made of a material having a higher electrical resistivity than the material of said light-emitting region.

3. A two-dimensional photonic crystal laser as described in claim 1 or 2, characterized in that the distance between each of the plurality of light-emitting regions and the nearest light-emitting region is shorter than the propagation length of light of the specified frequency in the two-dimensional photonic crystal layer.

4. The distance between each of the plurality of light-emitting regions and the nearest light-emitting region is calculated using the real part R and imaginary part I of the Hermitian coupling coefficient and the magnitude μ of the non-Hermitian coupling coefficient in the two-dimensional photonic crystal layer, |I| -1 |R / μ| 1 / 2 4. The two-dimensional photonic crystal laser according to claim 3, wherein the wavelength is shorter than 5. A two-dimensional photonic crystal laser as described in claim 1 or 2, characterized in that the resonance frequency determined by the average refractive index and periodic length within the light-emitting region of the light-emitting layer combined with the light-emitting corresponding region of the two-dimensional photonic crystal layer corresponding to the light-emitting region is consistent with the resonance frequency determined by the average refractive index and periodic length within the light-emitting corresponding region of the light-emitting layer combined with the transparent region of the light-emitting layer combined with the region outside the light-emitting corresponding region of the two-dimensional photonic crystal layer.

6. The two-dimensional photonic crystal laser according to claim 1 or 2, wherein the plurality of light-emitting regions are arranged in a two-dimensional array.

7. A two-dimensional photonic crystal laser according to claim 1 or 2, wherein the plurality of light-emitting regions are arranged in a one-dimensional array.

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