Two-dimensional photonic crystal laser

Abstract

A two-dimensional photonic crystal laser (10) comprises: an active layer (11); a two-dimensional photonic crystal layer (12) in which different-refractive-index parts (122) having a different refractive index from a plate-shaped base member (121) are disposed in the base member (121) so as to respectively be offset from lattice points (124) in a two-dimensional lattice (123) that are arranged in two prescribed directions on a cycle corresponding to the prescribed wavelength and / or said different-refractive-index parts (122) are respectively disposed on the lattice points in prescribed areas, and in which the offset amount and / or the area are modulated in a prescribed modulation cycle; and one pair of electrodes (17, 18). The active layer (11) includes: a saturable absorption region (112) that is a portion of a prescribed range and is a linear region having no portion parallel to either of the prescribed two directions; a gain region (111) that occupies the portions of the prescribed range other than the saturable absorption region; and a non-gain region (113) that is formed surrounding the prescribed range. The saturable absorption region (112) has lower conductivity and a shorter carrier lifetime than the gain region (111).

Description

Two-dimensional photonic crystal laser

[0001] The present invention relates to a two-dimensional photonic crystal laser.

[0002] In the fields of sensors using laser light and processing of workpieces, there has been a demand for laser light sources that emit pulsed laser light with a small pulse width (for example, less than 1 nanosecond). Such pulsed laser light sources have the advantage of being able to achieve high-precision detection of objects and powerful processing of workpieces by instantaneously irradiating high-intensity laser light, while also being able to reduce damage to the objects by shortening the time of laser light irradiation.

[0003] Non-Patent Document 1 describes a two-dimensional photonic crystal laser that can emit pulsed laser light having a pulse width of less than 1 nanosecond.

[0004] Generally, a two-dimensional photonic crystal laser has an active layer, a two-dimensional photonic crystal layer in which regions (hereinafter referred to as "modified refractive index portions", typically consisting of holes) with a refractive index different from that of the base material are arranged in a two-dimensional pattern at a predetermined periodic length within a plate-shaped base material arranged parallel to the active layer, and a pair of electrodes arranged to sandwich the two layers in a direction perpendicular to the two layers. In such a general two-dimensional photonic crystal laser, light is generated in the active layer when a current is passed between the pair of electrodes, and light of the light having a wavelength corresponding to the periodic length of the arrangement of the modified refractive index portions is amplified in the two-dimensional photonic crystal layer and emitted as laser light in a direction perpendicular to the two-dimensional photonic crystal layer.

[0005] In contrast, in the two-dimensional photonic crystal laser described in Non-Patent Document 1, a saturable absorption region is formed in a portion of the active layer within a region (called the "gain region") where light is generated by current injection, using a technique described below. The saturable absorption region has the characteristic that when light flows in, it initially absorbs the light and converts it into carriers. Then, as the carriers accumulate and reach a saturated state, light absorption ceases. In a two-dimensional photonic crystal laser with such a saturable absorption region formed in the active layer, immediately after current injection begins, light generated in the gain region flows into and is absorbed in the saturable absorption region, preventing laser oscillation. Subsequently, when the saturable absorption region becomes saturated, light is amplified not only in the gain region but also in the saturable absorption region without being absorbed, resulting in laser oscillation. Furthermore, when laser oscillation occurs in the saturable absorption region, the carriers in the saturable absorption region decrease, and light is again absorbed in the saturable absorption region, causing laser oscillation to cease. By repeating these operations, the two-dimensional photonic crystal laser described in Non-Patent Document 1 repeatedly emits pulsed laser light having a short pulse width (less than 1 nanosecond).

[0006] The two-dimensional photonic crystal laser described in Non-Patent Document 1 uses an electrode in which concentric conductive and non-conductive regions are alternately formed. This non-conductive region is formed by irradiating a conductor with a proton ion beam. Some of the irradiated proton ions pass through the conductor and each layer up to the active layer and reach the active layer. As a result, a saturable absorption region of the same shape as the non-conductive region of the electrode is formed in the active layer. Furthermore, the portion of the electrode corresponding to the conductive region becomes a gain region in the active layer. By forming such concentric gain and saturable absorption regions, laser oscillation in a higher-order mode having two or more peaks parallel to the active layer is suppressed. Therefore, the pulsed laser beam emitted from the two-dimensional photonic crystal laser exhibits an intensity distribution called a unimodal distribution, in which the intensity is high near the center of the cross section and decreases with increasing distance from the center.

[0007] The area outside the outermost conductive region of the electrode may be a non-conductive region irradiated with a proton ion beam, or may be a region without the conductor. In the former case, the area in the active layer corresponding to the non-conductive region becomes a saturable absorption region, while in the latter case, the area is neither a gain region nor a saturable absorption region, but simply a region where light is diffused (propagated) and / or absorbed.

[0008] International Publication No. WO2014 / 136607 U.S. Patent Publication No. US2016 / 0248224 International Publication No. WO2022 / 181723 International Publication No. WO2021 / 039316

[0009] Ryohei Morita and four others, "Photonic-crystal lasers with two-dimensionally arranged gain and loss sections for high-peak-power short-pulse operation," Nature Photonics, Nature Research (UK), March 4, 2021, Vol. 15, pp. 311-318. Ryohei Morita and nine others, "200-W short-pulse operation of photonic-crystal lasers based on simultaneous absorptive and radiative Q-switching," Optics Express, Optical Society of America, September 6, 2023.

[0010] In the fields of sensors and workpiece processing, there is a demand for a laser device that emits multiple pulsed laser beams in different directions. Such a laser device forms multiple pulsed laser beam spots distributed over a wide range at the position of the sensor or workpiece, which is located away from the laser device, thereby expanding the sensor's detection range and processing range. However, the two-dimensional photonic crystal laser described in Non-Patent Document 1 can only emit pulsed laser beams in a direction perpendicular to the two-dimensional photonic crystal layer.

[0011] The problem to be solved by the present invention is to provide a two-dimensional photonic crystal laser that can be used in a laser device that emits multiple pulsed laser beams with short pulse widths (less than 1 nanosecond) in different directions.

[0012] The two-dimensional photonic crystal laser according to the present invention, which has been made to solve the above-mentioned problems, comprises: an active layer which generates light of a predetermined wavelength, which is the oscillation wavelength of the laser, when a current is injected therein; a two-dimensional photonic crystal layer in which modified refractive index sections, which have a refractive index different from that of the base material and are disposed within a plate-like base material arranged parallel to the active layer, are shifted by a predetermined amount from each lattice point of a two-dimensional lattice which is arranged in two predetermined directions at a period corresponding to the predetermined wavelength, and / or which are disposed at each lattice point with a predetermined area, and the shift amount and / or area are modulated at a predetermined modulation period; and a pair of electrodes provided in a direction perpendicular to the active layer and the two-dimensional photonic crystal layer so as to sandwich the active layer and the two-dimensional photonic crystal layer therebetween, wherein the active layer has a saturable absorption region which is a linear region that is part of a predetermined range and has no portion parallel to either of the two predetermined directions, a gain region which occupies the part of the predetermined range other than the saturable absorption region, and a non-gain region formed around the predetermined range, and the saturable absorption region is a region which has lower conductivity and a shorter carrier lifetime than the gain region.

[0013] Here, the "non-gain region" may be any region other than the gain region, and may be a region having characteristics similar to the saturable absorption region, or may be a region in which light is simply diffused (propagated) and / or absorbed, unlike both the gain region and the saturable absorption region.

[0014] Among two-dimensional photonic crystal lasers that emit continuous (non-pulse laser beams) laser beams, modulated two-dimensional photonic crystal lasers have been known, which include a two-dimensional photonic crystal layer in which the positions of modified refractive index sections are shifted from reference positions corresponding to the positions of lattice points of a two-dimensional lattice and the amount of shift (a vector quantity combining the magnitude and direction of the shift) is modulated (changed) at a predetermined modulation period, or the area of ​​the modified refractive index sections is modulated at a predetermined modulation period (see, for example, Patent Documents 1 and 2). Such modulated two-dimensional photonic crystal lasers emit continuous laser beams tilted from a direction perpendicular to the two-dimensional photonic crystal layer in a direction determined by the modulation period. A laser device combining multiple modulated two-dimensional photonic crystal lasers with different modulation periods can emit multiple continuous laser beams in different directions according to the modulation periods. Furthermore, a single modulated two-dimensional photonic crystal laser can emit multiple continuous laser beams in different directions by modulating the amount of shift and / or the area using a composite modulation period in which multiple different modulation periods are superimposed (see Patent Document 3).

[0015] The modulated two-dimensional photonic crystal lasers described in Patent Documents 1 to 3 cannot emit a pulsed laser beam as they are. Furthermore, the present inventors performed a simulation on a modulated two-dimensional photonic crystal laser described in Patent Documents 1 to 3 in which the concentric gain region and the saturable absorption region described in Non-Patent Document 1 were alternately formed in the active layer, and it was found that a continuous laser beam was emitted instead of a pulsed laser beam.

[0016] Therefore, we investigated the laser oscillation state of the aforementioned modulated two-dimensional photonic crystal laser in which concentric gain regions and saturable absorption regions are alternately formed in the active layer, and the two-dimensional photonic crystal laser emitting pulsed laser light (without modulation in the two-dimensional photonic crystal layer) described in Non-Patent Document 1. As a result, it was found that in the latter, light propagating in two predetermined directions, in which lattice points are arranged in the two-dimensional photonic crystal layer at a period corresponding to a predetermined wavelength, repeatedly changes its propagation direction to the other direction, resulting in two-dimensional (planar) amplification, whereas in the former, light propagating in one direction without changing its propagation direction occurs. In such a modulated two-dimensional photonic crystal laser, if the saturable absorption region has a portion parallel to either of the two predetermined directions, one-dimensional oscillation occurs in which only light propagating one-dimensionally in each of the two directions is amplified, resulting in continuous emission of laser light without interruption.

[0017] Therefore, in the two-dimensional photonic crystal laser according to the present invention, an active layer having a linear saturable absorption region that has no portion parallel to either of the two predetermined directions is used to prevent the intensive amplification of only the light propagating one-dimensionally in either of the two directions, thereby preventing continuous laser oscillation. As a result, a pulsed laser beam tilted in the direction determined by the modulation period is obtained. The pulse width of this pulsed laser beam is short, less than 1 nanosecond, similar to the two-dimensional photonic crystal laser described in Non-Patent Document 1.

[0018] The two predetermined directions are, for example, a square lattice in which lattice points are arranged with a period length a in the x direction and the y direction perpendicular thereto, and the predetermined wavelength in the two-dimensional photonic crystal layer is 2 1 / 2 a (wavelength in vacuum is multiplied by the effective refractive index n of the two-dimensional photonic crystal layer) eff multiplied by 2 1 / 2 a×n eff), the directions are inclined at 45° from the x and y directions. In this case, the linear saturable absorption region has a shape that does not have any portions parallel to the directions inclined at 45° from the x and y directions. For example, one linear saturable absorption region parallel to the x or y direction or multiple linear saturable absorption regions parallel to each other may be provided, or a cross or mesh shape may be provided with one or multiple linear saturable absorption regions parallel to the x and y directions, respectively. In this case, the saturable absorption regions extending in the x direction and the y direction may be provided with different numbers and intervals, or may be provided at unequal intervals rather than equal intervals. For example, by providing saturable absorption regions extending in a certain direction at approximately equal intervals and appropriately adjusting the number and intervals of saturable absorption regions extending in a direction different by 90° from that, it is possible to suppress oscillation in higher-order modes or, conversely, intentionally induce oscillation in higher-order modes. Furthermore, the direction in which the linear saturable absorption region extends is not limited to the x or y direction, but may be any direction other than directions inclined at 45° from the x and y directions.

[0019] By combining multiple two-dimensional photonic crystal lasers according to the present invention, each having a different modulation period, it is possible to obtain a laser device that emits multiple pulsed laser beams with short pulse widths (less than 1 nanosecond) in different directions.

[0020] The saturable absorption region of the active layer of the two-dimensional photonic crystal laser according to the present invention can be formed by a method similar to that described in Non-Patent Document 1. That is, by irradiating the region where the saturable absorption region is to be formed with proton ions from one side of a pair of electrodes, the proton ions pass through each layer from the electrode to the active layer and reach the active layer, thereby forming the saturable absorption region. Note that the ions to be irradiated are not limited to proton ions; oxygen ions, etc., can also be used. Furthermore, the method for forming the saturable absorption region is not limited to such ion irradiation method, and any method can be used as long as the region where the saturable absorption region is to be formed in the active layer is treated so that it has lower conductivity and a shorter carrier lifetime than the surrounding region (the region that will become the gain region).

[0021] In the two-dimensional photonic crystal laser according to the present invention, the shift amount and / or the area may be modulated by a composite modulation period in which a plurality of different modulation periods are superimposed, thereby enabling a single modulated two-dimensional photonic crystal laser to emit a plurality of pulsed laser beams that are emitted in different emission directions according to the respective modulation periods.

[0022] The two-dimensional photonic crystal laser according to the present invention can further be configured such that second lattice points are arranged at the same period as the lattice points of the two-dimensional lattice and are provided at positions shifted from the two-dimensional lattice, and second modified refractive index portions having a refractive index different from that of the base material are arranged at the second lattice points of a second two-dimensional lattice, or at different shift amounts or / and different areas from the second lattice points.

[0023] By adding such a second modified refractive index section, light amplified by the periodic structure formed by the (first) modified refractive index section and light amplified by the periodic structure formed by the second modified refractive index section interfere with each other. This interference occurs so that the intensities of the light are destructive or constructive, depending on the magnitude and direction of the misalignment between the (first) two-dimensional lattice and the second two-dimensional lattice. When constructive interference occurs that destructively interferes with the intensities of the light, the intensity of the emitted pulsed laser light also increases. On the other hand, when destructive interference occurs, stable laser oscillation occurs over a wider area in the two-dimensional photonic crystal layer than in the absence of the second modified refractive index section, and as a result, the intensity of the emitted pulsed laser light can be increased.

[0024] The two-dimensional photonic crystal laser according to the present invention may have a configuration in which an intermediate layer is provided between one of the pair of electrodes and the active layer, a low-conductivity region having lower conductivity than other parts of the intermediate layer is formed in a part of the intermediate layer, and the part in which the low-conductivity region is formed is connected to the saturable absorption region of the active layer.

[0025] By forming such a low-conductivity region in the intermediate layer, current flows more easily from one electrode to the gain region than to the saturable absorption region, thereby enabling efficient light emission in the gain region. The intermediate layer is not limited to a single layer, but may be formed from multiple layers made of different materials. In this case, the multiple layers are collectively considered to be one "intermediate layer." The intermediate layer may include the two-dimensional photonic crystal layer.

[0026] According to the present invention, a two-dimensional photonic crystal laser can be obtained that can be used in a laser device that emits a plurality of pulsed laser beams with short pulse widths (less than 1 nanosecond) in different directions.

[0027] 1 is a perspective view showing a schematic configuration of a first embodiment of a two-dimensional photonic crystal laser according to the present invention. FIG. 2 is a plan view showing a gain region, a saturable absorption region, and a non-gain region formed in an active layer in the two-dimensional photonic crystal laser of the first embodiment. FIG. 3 is a partial plan view showing the configuration of a two-dimensional photonic crystal layer in the two-dimensional photonic crystal laser of the first embodiment. FIG. 4 is a partial enlarged view of the two-dimensional photonic crystal layer in the two-dimensional photonic crystal laser of the first embodiment. FIG. 5 is a plan view showing the configuration of a first electrode in the two-dimensional photonic crystal laser of the first embodiment. FIG. 6 is a longitudinal sectional view showing a conductive region, a non-conductive region, and an outer edge non-conductive region formed in the first electrode, and a gain region, a saturable absorption region, and a non-gain region formed in the active layer in the two-dimensional photonic crystal laser of the first embodiment. FIG. 7 is a graph showing the results of calculations of the change in intensity of pulsed laser light over time for the two-dimensional photonic crystal laser of the first embodiment. 1 is a diagram showing the results of calculation of the near-field pattern (left) and far-field pattern (right) of a pulsed laser beam for the two-dimensional photonic crystal laser of the first embodiment. FIG. 2 is a diagram showing the results of calculation of the near-field pattern (left) and far-field pattern (right) of a laser beam for the two-dimensional photonic crystal laser of the comparative example. FIG. 3 is a graph showing the results of calculation of the time change in intensity of laser light for two modified examples of the two-dimensional photonic crystal laser of the first embodiment, which have different periods for the arrangement of non-conductive regions. FIG. 4 is a graph showing the results of calculation of the time change in intensity of laser light for two modified examples of the two-dimensional photonic crystal laser of the first embodiment, which have different widths for the non-conductive regions. FIG. 5 is a diagram showing the results of calculation of the near-field pattern and far-field pattern of a laser beam for two modified examples of the two-dimensional photonic crystal laser of the first embodiment, which have different extension directions of the non-conductive regions. FIG. 6 is a diagram showing modified examples of the first electrode in the two-dimensional photonic crystal laser of the first embodiment. FIG. 7 is a diagram showing other modified examples of the first electrode in the two-dimensional photonic crystal laser of the first embodiment. 1A and 1B are a perspective view showing a schematic configuration of a second embodiment of a two-dimensional photonic crystal laser according to the present invention, and a partial plan view showing the configuration of a two-dimensional photonic crystal layer in the two-dimensional photonic crystal laser of the second embodiment.1 is a diagram for explaining the light amplification effect in a two-dimensional photonic crystal layer of the two-dimensional photonic crystal laser of the second embodiment. FIG. 2 is a graph showing the calculated results of the change in intensity of pulsed laser light over time for the two-dimensional photonic crystal laser of the second embodiment. FIG. 3 is a diagram showing the calculated results of the emission pattern of a pulsed laser beam for the two-dimensional photonic crystal laser of the second embodiment, and a partially enlarged view thereof. FIG. 4 is a graph showing the calculated results of the change in intensity of laser light over time for five modified examples of the two-dimensional photonic crystal laser of the second embodiment, which have different periods for the arrangement of non-conductive regions. FIG. 5 is a graph showing the calculated results of the change in intensity of laser light for two modified examples of the two-dimensional photonic crystal laser of the second embodiment, which have different widths for the non-conductive regions. FIG. 6 is a micrograph of a first electrode in the fabricated two-dimensional photonic crystal laser of the second embodiment. FIG. 7 is a photograph of a near-field image of a pulsed laser beam for the fabricated two-dimensional photonic crystal laser of the second embodiment. FIG. 8 is a photograph of a far-field image of a pulsed laser beam for the fabricated two-dimensional photonic crystal laser of the second embodiment, and a partially enlarged photograph thereof. 27 is a graph showing experimental results of the change in intensity of pulsed laser light over time for a fabricated two-dimensional photonic crystal laser of the second embodiment. A graph showing experimental results of the relationship between the magnitude of current supplied to a fabricated two-dimensional photonic crystal laser of the second embodiment and the peak value (peak power) of the intensity of a pulsed laser beam. A graph showing experimental results of the relationship between the magnitude of current supplied to a fabricated two-dimensional photonic crystal laser of the second embodiment and the pulse repetition frequency. A graph showing calculated results of the relationship between the magnitude of current injected into the active layer and the carrier lifetime and peak power in the saturable absorption region. A graph showing experimental results of the relationship between the magnitude of current supplied to a saturable absorption region and the peak power for an example in which the thickness of the saturable absorption region is made thicker (the depth of ions implanted into the active layer is deeper) than in the example of FIG. 27. An optical microscope photograph of a first electrode in a fabricated modified example of the second embodiment. A plan view showing the saturable absorption region and the like formed in the active layer in a modified example of the second embodiment. An electron microscope photograph of a two-dimensional photonic crystal layer in a fabricated modified example of the second embodiment.36 is a graph showing the results of a calculation of the change in intensity of pulsed laser light over time when the current shown in FIG. 36 is injected into the two-dimensional photonic crystal laser of the second embodiment. 37 is a photograph of the far-field pattern of a pulsed laser beam from a fabricated two-dimensional photonic crystal laser according to a modified example of the second embodiment. 38 is a graph showing the results of an experiment to determine the change in intensity of laser light over time from a fabricated two-dimensional photonic crystal laser according to a modified example of the second embodiment. 39 is a graph showing an example of the change in intensity of current over time injected into the two-dimensional photonic crystal laser of this embodiment to emit only one pulse of pulsed laser light. 39 is a graph showing the results of a calculation to determine the change in intensity of pulsed laser light over time when the current shown in FIG. 36 is injected into the two-dimensional photonic crystal laser of the second embodiment. 39 is a graph showing the results of measuring, with an oscilloscope, the intensity of a pulsed current injected to emit only one pulse of pulsed laser light from the fabricated two-dimensional photonic crystal laser of the second embodiment, and measuring, with a photodiode, the intensity of a pulsed laser beam obtained by the injection of the current. 39 is a graph showing the results of measuring the intensity of the pulsed laser beam shown in FIG. 38 with a streak camera. a two-dimensional photonic crystal laser having a plurality of two-dimensional photonic crystal lasers according to the first or second embodiment, each having a different amount of displacement from a lattice point of the two-dimensional lattice and / or a different area of ​​the modified refractive index portion; ...

[0028] 1 to 41, an embodiment of a two-dimensional photonic crystal laser according to the present invention will be described.

[0029] (1) Two-dimensional photonic crystal laser of the first embodiment (1-1) Configuration of the two-dimensional photonic crystal laser of the first embodiment As shown in FIG. 1 , the two-dimensional photonic crystal laser 10 of the first embodiment has a configuration in which a first electrode 17, a first cladding layer 141, a two-dimensional photonic crystal layer 12, a spacer layer 13, an active layer 11, a second cladding layer 142, a substrate 16, and a second electrode 18 are stacked in this order. However, the order of the active layer 11 and the two-dimensional photonic crystal layer 12 may be reversed. For convenience, FIG. 1 illustrates the first electrode 17 on the lower side and the second electrode 18 on the upper side. However, the orientation of the two-dimensional photonic crystal laser 10 during use is not limited to that shown in this figure. Also, for convenience, FIG. 1 illustrates the first electrode 17 spaced downward from the first cladding layer 141; however, in reality, the first electrode 17 is in contact with the lower surface of the first cladding layer 141. The configuration of each layer and electrode will be described below.

[0030] The active layer 11 emits light having a predetermined wavelength band when electric charges are injected from the first electrode 17 and the second electrode 18. The material of the active layer 11 can be, for example, an InGaAs / AlGaAs multiple quantum well (emission wavelength band: 935 to 945 nm).

[0031] The active layer 11 has three regions formed therein: a gain region 111, a saturable absorption region 112, and a non-gain region 113 (see FIGS. 2 and 6). Details of these regions will be described later together with the description of the first electrode 17.

[0032] As shown in Fig. 3, the two-dimensional photonic crystal layer 12 is formed by arranging modified refractive index portions 122, each having a refractive index different from that of the base material 121, at each lattice point 124 of a two-dimensional lattice 123 in a plate-shaped base material 121. Note that Fig. 3 shows an enlarged view of only a portion of the two-dimensional photonic crystal layer 12. The base material 121 may be made of, for example, p-type GaAs (a p-type semiconductor). The modified refractive index portions 122 are typically made of voids (air) provided in the base material 121, but may also be formed by embedding a member having a refractive index different from that of the base material 121 in the base material 121.

[0033] In this embodiment, the planar shape of the modified refractive index portion 122 is an ellipse, but it can be various shapes such as a circle, an equilateral triangle or other triangles, or a polygon other than a triangle. In addition, in this embodiment, the two-dimensional grating 123 is a square lattice, but other two-dimensional gratings such as a rectangular lattice or a triangular lattice may be used. The length (periodic length) a of the lattice point period of the two-dimensional grating 123 is determined depending on the material of the base material 121 and the emission wavelength in the active layer 11. In this embodiment, 2 of the periodic length a 1 / 2 times (2 1 / 2 a) is determined so that it coincides with the wavelength λ of the light emitted from the active layer 11 in the two-dimensional photonic crystal layer 12. The effective refractive index of the two-dimensional photonic crystal layer 12 is n eff Then, the wavelength λ0 of light in a vacuum, which has a wavelength λ in the two-dimensional photonic crystal layer 12, is λ×n eff , i.e. 2 1 / 2 a×n eff If the two directions in which the lattice points 124 are arranged with a periodic length a in the two-dimensional lattice 123 are the x direction and the y direction, then in such a two-dimensional photonic crystal layer 12, light of wavelength λ traveling in two directions (defined as the X' direction and the Y' direction) tilted at 45 degrees from the x direction and the y direction, respectively, is amplified. Note that the modified refractive index portion 122, which is elliptical as described above, is arranged so that its major axis faces the x direction and its minor axis faces the y direction.

[0034] In this embodiment, as shown in the enlarged view of FIG. 4 , the modified refractive index portion 122 is disposed such that the center (center of gravity) 125 of its planar shape is offset from the lattice point 124 of the two-dimensional lattice 123. The magnitude and direction of this offset are collectively referred to as the "amount of offset," and in FIG. 4 , the amount of offset is represented by a vector 126. The amount of offset differs for each lattice point 124. Furthermore, in this embodiment, the area of ​​the planar shape of the modified refractive index portion 122 differs for each modified refractive index portion 122 (lattice point 124). These amounts of offset and area change at a predetermined period along each of the x and y directions. The period of this change in the amount of offset or / and area is referred to as the "modulation period," and the amount of offset or / and area of ​​each modified refractive index portion modulated by this modulation period is referred to as the "modulation phase." Furthermore, a combination of multiple different modulation periods is referred to as the "composite modulation period," and the amount of offset or / and area of ​​each modified refractive index portion modulated by this composite modulation period is referred to as the "composite modulation phase."

[0035] As described in Patent Documents 1 to 3, the modulation phase or composite modulation phase is a parameter that determines the emission direction of a laser beam emitted from a general modulated two-dimensional photonic crystal laser. This is also true in this embodiment, and the emission direction of a pulsed laser beam emitted from two-dimensional photonic crystal laser 10 is determined by the modulation phase or composite modulation phase. Below, the composite modulation phase will be explained, and then the modulation phase will be explained as a special example thereof.

[0036] As described in Patent Document 3, the composite modulation phase ψ(r↑) (r↑ is a vector indicating the position of the lattice point 124) is expressed by the vectors k↑ and k↑, which will be described later. n Using ↑, It is expressed as:

[0037] In equation (1), "arg" is the argument of a complex number. n teeth, In equation (2), A n is the amplitude, α n is the phase.

[0038] Returning to equation (1), k n ↑ is the tilt angle θ of the pulse laser beam to be emitted n(the inclination angle of the pulse laser beam from the normal to the two-dimensional photonic crystal layer 12) and the azimuth angle φ n (the angle representing the direction in a plane of the projection of the pulsed laser beam onto the plane parallel to the two-dimensional photonic crystal layer 12), and the effective refractive index n eff is a vector expressed using δ(k↑-k n ↑) is the delta function, and k↑=k n When ↑, δ(k↑-k n ↑)=1, otherwise δ(k↑-k n ↑)=0.

[0039] When the two-dimensional lattice is a square lattice with a periodic length a as in this embodiment, k n ↑ is It is expressed as:

[0040] From these formulas (1) to (3), the two-dimensional photonic crystal laser 10, whose two-dimensional photonic crystal layer 12 has the composite modulation phase ψ(r↑) shown in formula (1), emits n pulsed laser beams with different tilt angles θ and / or azimuth angles φ (in practice, in addition to these n pulsed laser beams, a pulsed laser beam with the same tilt angle as each pulsed laser beam but with an azimuth angle 180° different from each other is also emitted). n and azimuth angle φ n The above modulation phase corresponds to the case where there is only one combination (only n=1). The amount of deviation and / or area at each lattice point can be determined by these modulation phases or composite modulation phases.

[0041] In this embodiment, both the amount of shift and the area are made different for each modified refractive index section 122 (lattice point 124), but it is also possible to make only one of the amount of shift and the area different for each modified refractive index section 122 (lattice point 124).

[0042] The first cladding layer 141 and the second cladding layer 142 have the role of injecting charges from the first electrode 17 and the second electrode 18, and also the role of preventing in-plane guided light, which is guided parallel to the layers in the two-dimensional photonic crystal layer 12, from leaking from the layers. To fulfill the former role, the first cladding layer 141 is made of a p-type semiconductor (e.g., p-type Al 0.37 Ga 0.63 As) in the second cladding layer 142, and an n-type semiconductor (e.g., n-type Al 0.37 Ga 0.63 As) are used (the reason why the material of the base material 121 of the two-dimensional photonic crystal layer 12 is a p-type semiconductor is the same as this).

[0043] The spacer layer 13 is provided to allow holes injected from the first electrode 17 to pass through and be introduced into the active layer 11, while preventing electrons injected from the second electrode 18 from passing through the active layer 11 (and thereby combining with holes on the first electrode 17 side of the active layer 11). The spacer layer 13 is made of a material such as p-type Al 0.45 Ga 0.55 As can be used.

[0044] 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.

[0045] As shown in FIG. 5 , the first electrode 17 includes a linear non-conductive region 172 formed within a circular area, a conductive region 171 occupying the remaining portion of the circular area except for the non-conductive region 172, and a square outer non-conductive region 173 formed around the periphery of the circular area. The non-conductive regions 172 extend in the x direction (a direction tilted 45° relative to the X' and Y' directions) and are spaced apart from one another in the y direction. The non-conductive regions 172 do not have any portions parallel to the X' and Y' directions. The first electrode 17 can be formed by implanting ions such as hydrogen ions into the portions of a conductive plate that form the non-conductive region 172 and the outer non-conductive region 173 to render them non-conductive. Alternatively, ions may be implanted into the non-conductive region 172 of a plate having the same shape as the circular area. In this case, the outer non-conductive region 173 is not provided. The shape of the non-conductive region 172 given here is merely an example, and various shapes of the non-conductive region 172 can be used as long as they do not have portions parallel to the X' direction and the Y' direction (details will be described later). The combined shape of the conductive region 171 and the non-conductive region 172 is circular in this embodiment, but other shapes are also possible. Furthermore, the shape of the outer edge non-conductive region 173 is not limited to the square shape of this embodiment, and various shapes, such as a circle, can be used.

[0046] When ions such as hydrogen ions are implanted into the portion forming the non-conductive region 172, some of the ions pass through the first electrode 17, the first cladding layer 141, the two-dimensional photonic crystal layer 12, and the spacer layer 13 to reach the active layer 11. As a result, the ions are implanted into the region of the active layer 11 corresponding to the non-conductive region 172 of the first electrode 17, reducing the conductivity of the region compared to the other regions of the active layer 11 and shortening the lifetime of carriers generated when light is absorbed in the region. This results in the region becoming the saturable absorption region 112 (see FIGS. 2 and 6). On the other hand, the ions do not reach the region of the active layer 11 corresponding to the conductive region 171 of the first electrode 17, and the region becomes the gain region 111 (see the same figures). Furthermore, the region outside the circular range formed by combining the gain region 111 and the saturable absorption region 112 has the same characteristics as the saturable absorption region 112 when the outer non-conductive region 173 is formed in the first electrode 17, but does not have such characteristics when the outer non-conductive region 173 is not formed. However, in either case, no current is injected into the region, and therefore the region outside the circular range is called the "non-gain region 113" (see the same figure).

[0047] When the saturable absorption region 112 is formed in the active layer 11 by ion implantation as described above, some ions are trapped in the region through which the ions attempt to pass (the region marked with reference numeral 192 in FIG. 6 ) in the first cladding layer 141, the two-dimensional photonic crystal layer 12, and the spacer layer 13 (collectively referred to as the "intermediate layer 19") between the first electrode 17 and the active layer 11. As a result, the region 192 through which the ions pass in the intermediate layer 19 also has lower conductivity than other regions in the same layer. This region 192 is referred to as the "low conductivity region 192." The low conductivity region 192 is formed to connect the non-conductive region 172 of the first electrode 17 and the saturable absorption region 112 of the active layer 11. On the other hand, the region between the low conductivity regions 192 (the high conductivity region 191) has higher conductivity than the low conductivity region 192 and is formed to connect the conductive region 171 of the first electrode 17 and the gain region 111 of the active layer 11. These high conductivity region 191 and low conductivity region 192 serve to selectively introduce the current flowing between the first electrode 17 and the second electrode into the gain region 111 rather than into the saturable absorption region 112 of the active layer 11 .

[0048] As shown as a comparative example in FIG. 7 , an electrode 97 in which linear non-conductive regions 972 are formed concentrically within a plate material made of a conductive material (concentric conductive regions 971 and non-conductive regions 972 are formed alternately) has portions parallel to the X′ direction and portions parallel to the Y′ direction within the circumference of the non-conductive region 972 (portions surrounded by dashed-line circles in FIG. 7 ), and therefore cannot be used as the first electrode of this embodiment.

[0049] The second electrode 18 has a configuration in which a circular plate made of a conductive material has a circular hole cut out in the center. The cut-out portion of the plate is called a window portion 182, and the remaining portion of the plate is called a frame portion 181. The window portion 182 is provided to allow the pulsed laser beam emitted from the two-dimensional photonic crystal layer 12 to pass through. The overall shape of the second electrode 18 may be a shape other than a circle, but it is desirable that the shape be similar to the overall shape of the first electrode 17.

[0050] The thickness of the first cladding layer 141 is sufficiently thinner than that of the second cladding layer 142. Therefore, the distance between the active layer 11 and the first electrode 17 is sufficiently shorter than the distance between the active layer 11 and the second electrode 18. As a result, when a current is passed between the first electrode 17 and the second electrode 18, the current distribution in the active layer 11 becomes similar to the shape of the conductive region 171 of the first electrode 17.

[0051] (1-2) Operation of the Two-Dimensional Photonic Crystal Laser of the First Embodiment The two-dimensional photonic crystal laser 10 of the first embodiment operates as follows when a current is passed between the first electrode 17 and the second electrode 18. When current injection begins, light having a predetermined wavelength is generated in the gain region 111 of the active layer 11. At that time, the current (holes) injected from the first electrode 17 side mainly passes through the highly conductive region 191 and flows into the gain region 111 connected thereto, so that holes can be efficiently injected into the gain region 111.

[0052] The light generated in the gain region 111 is initially absorbed in the saturable absorption region 112 and converted into electron-hole pairs (carriers). If the current injection continues thereafter, carriers accumulate in the saturable absorption region 112, making it unable to absorb light. As a result, light is introduced into the two-dimensional photonic crystal layer 12 not only from the gain region 111 but also from the saturable absorption region 112, and the light is amplified by the periodic structure of the two-dimensional photonic crystal layer 12. As a result, light is amplified over a wide range that includes the gain region 111 and the saturable absorption region 112, and laser light is generated. The generated laser light is irradiated at an inclination angle θ determined by the modulation phase or composite modulation phase of the two-dimensional photonic crystal layer 12. n and azimuth angle φ n and (φ n +180°).

[0053] When light is amplified in saturable absorption region 112, the carriers accumulated in saturable absorption region 112 are rapidly reduced, and the amplification of light stops in a short time. As a result, the laser beam also stops in a short time. Then, carriers are accumulated again in saturable absorption region 112, and 2n laser beams are emitted through the same process as above, after which the laser beam stops in a short time. By repeating the above operation, the two-dimensional photonic crystal laser 10 of the first embodiment repeatedly emits 2n pulsed laser beams tilted with respect to the normal to the two-dimensional photonic crystal layer 12.

[0054] Next, we will explain the results of calculations of the temporal change in intensity of emitted laser light and the intensity distribution in the cross section of the laser beam for the two-dimensional photonic crystal laser 10 of the first embodiment and a comparative two-dimensional photonic crystal laser having the electrode 97 shown in Figure 7 instead of the first electrode 17. In these calculations, in the two-dimensional photonic crystal laser 10, the overall diameter of the first electrode 17 was 150 µm, the width of the non-conductive regions 172 was 2.75 µm, and seven non-conductive regions 172 were arranged at a period of 22 µm. In the comparative two-dimensional photonic crystal laser, the overall diameter of the electrode 97 was 150 µm (same as in the first embodiment), the width of the non-conductive regions 972 was 2.75 µm (same as in the first embodiment), and four non-conductive regions 972 were arranged concentrically at a period of 35 µm. In both the first embodiment and the comparative example, the two-dimensional photonic crystal layer 12 was set to have a modulation phase so that a pulse laser beam was emitted with a tilt angle θ of 54° and an azimuth angle φ of 0° and 180°.

[0055] The calculation results of the change in laser light intensity over time are shown in graphs in Fig. 8 for the first embodiment and in Fig. 9 for the comparative example. The horizontal axis of the graph represents the elapsed time from the start of current injection, and the vertical axis represents the intensity of the laser light. The two-dimensional photonic crystal laser 10 of the first embodiment repeatedly oscillates pulsed laser light with a pulse repetition period of approximately 0.3 nanoseconds and a pulse width of approximately 20 to 30 picoseconds. In contrast, the two-dimensional photonic crystal laser of the comparative example continuously oscillates laser light with an intensity one order of magnitude weaker than that of the first embodiment, and no pulsed laser light is obtained.

[0056] The calculation results of the intensity distribution in the cross section of the laser beam are shown in Fig. 10 for the first embodiment and in Fig. 11 for the comparative example. In both Fig. 10 and Fig. 11, the intensity distribution (near-field pattern) near the second electrode 18 is shown in the left diagram, and the intensity distribution (far-field pattern) at a position sufficiently distant from the second electrode 18 is shown in the right diagram. In both cases, two pulsed laser beams with the same inclination angle but azimuth angles differing by 180° are actually emitted, but Fig. 10 and Fig. 11 show only one of the two. In addition, in each figure, the X' direction is the horizontal direction and the Y' direction is the vertical direction (orientations rotated -45° from the orientations in Figs. 3 to 7).

[0057] It can be seen that the two-dimensional photonic crystal laser 10 of the first embodiment has a single-peak intensity distribution, with the intensity highest at the center of the beam cross section, particularly in the far-field pattern. In contrast, the two-dimensional photonic crystal laser of the comparative example has a beam cross section that extends one-dimensionally in both the X' and Y' directions, particularly in the far-field pattern. This intensity distribution of the comparative example indicates the occurrence of one-dimensional oscillation, in which only light propagating one-dimensionally in each of the two directions is amplified. It is believed that the occurrence of such one-dimensional oscillation is the reason why laser light is continuously emitted without interruption in the two-dimensional photonic crystal laser of the comparative example.

[0058] The shape of first electrode 17 is not limited to the above example. For example, when the period at which non-conductive regions 172 in the two-dimensional photonic crystal laser 10 are arranged is changed from 22 μm to 17 μm and 28 μm (the width is unchanged from 2.75 μm), the change in the intensity of the laser light over time is calculated, and it is confirmed that pulsed laser light is oscillated in all cases, as shown in Fig. 12. The longer the period at which non-conductive regions 172 are arranged, the shorter the pulse repetition period becomes, and the smaller the peak intensity value per pulse becomes.

[0059] Next, we calculated the temporal change in laser light intensity for the two-dimensional photonic crystal laser 10 in which the width of non-conductive region 172 was changed from 2.75 μm to 4.1 μm and 5.5 μm (while the period at which non-conductive region 172 was arranged remained unchanged from 22 μm). As shown in Figure 13, we confirmed that pulsed laser light was generated in all cases. The larger the width of non-conductive region 172, the longer the pulse repetition period. The peak intensity per pulse was greater when the width of non-conductive region 172 was 4.1 μm and 5.5 μm than when it was 2.75 μm (there was no significant difference between the 4.1 μm and 5.5 μm cases).

[0060] In the examples of FIGS. 12 and 13, one of the period at which the non-conductive regions 172 (and the saturable absorption regions 112) are arranged and the width of the non-conductive regions 172 (same as above) is changed, but both of them may also be changed.

[0061] In the two-dimensional photonic crystal laser 10 described above, the non-conductive region 172 extends in a direction tilted at 45° with respect to the X' and Y' directions. However, it may extend in any other direction as long as it is not parallel to the X' or Y' directions. Figure 14 shows the near-field and far-field patterns of the laser beam calculated for examples in which the non-conductive region 172 is tilted at an angle α of 40° with respect to the X' direction (50° with respect to the Y' direction) and 30° with respect to the X' direction (60° with respect to the Y' direction). Pulsed laser oscillation was obtained in all of these examples. Furthermore, the near-field and far-field patterns are similar to those in the example in which the non-conductive region 172 extends in a direction tilted at 45° with respect to the X' and Y' directions. It can be seen from the far-field pattern that an intensity distribution closer to a single peak is obtained.

[0062] So far, we have described the case where the first electrode 17 (and the active layer 11) has a linear non-conductive region 172 (and the saturable absorption region 112). However, the shape of the non-conductive region (and the saturable absorption region 112) may be any shape as long as it does not have any portions parallel to the X' direction or the Y' direction. For example, as shown in FIG. 15 , linear non-conductive regions 172 extending in the x direction and the y direction (both directions inclined at 45° with respect to the X' direction and the Y' direction) may be arranged in a grid pattern. Alternatively, as shown in FIG. 16 , multiple non-conductive regions 172 may be arranged concentrically in the shape of a quadrilateral (square) frame whose four sides extend in the x direction or the y direction and whose sides have different lengths. In both the examples shown in FIGS. 15 and 16 , the non-conductive regions 172 may be formed so as to be inclined with respect to the x direction and / or the y direction so as not to be parallel to the X' direction and the Y' direction.

[0063] (2) Two-dimensional photonic crystal laser of second embodiment The two-dimensional photonic crystal laser 20 of the second embodiment has a configuration similar to that of the two-dimensional photonic crystal laser 10 of the first embodiment, except that the configuration of the two-dimensional photonic crystal layer 22 differs from that of the two-dimensional photonic crystal layer 12 of the first embodiment. Therefore, the components other than the two-dimensional photonic crystal layer 22 are denoted by the same reference numerals as the components of the two-dimensional photonic crystal laser 10 of the first embodiment and are shown in the perspective view of Fig. 17, and detailed description thereof will be omitted. The configuration of the two-dimensional photonic crystal layer 22 will be described below using the plan view of Fig. 17.

[0064] The two-dimensional photonic crystal layer 22 is configured by arranging, in a plate-shaped base material 221, first modified refractive index sections 2221 having a refractive index different from that of the base material 221 at each first lattice point 2241 of a first two-dimensional lattice 2231, and second modified refractive index sections 2222 having a refractive index different from that of the base material 221 at each second lattice point 2242 of a second two-dimensional lattice 2232. Both the first two-dimensional lattice 2231 and the second two-dimensional lattice 2232 are square lattices in which the first lattice points 2241 and the second lattice points 2242 are arranged with a periodic length a in the x and y directions. The second two-dimensional lattice 2232 is arranged with a shift of a / 2 in the y direction relative to the first two-dimensional lattice 2231. There is no shift in the x direction between the first two-dimensional lattice 2231 and the second two-dimensional lattice 2232. Each first modified refractive index portion 2221 is arranged such that the amount of deviation or / and area from the first lattice point 2241 is modulated at a predetermined modulation period or composite modulation period. Similarly, each second modified refractive index portion 2222 is arranged such that the amount of deviation or / and area from the second lattice point 2242 is modulated at a predetermined modulation period or composite modulation period. The modulation periods or composite modulation periods of the first modified refractive index portion 2221 and the second modified refractive index portion 2222 may be the same or different. Furthermore, the second modified refractive index portion 2222 may be arranged without being modulated.

[0065] The amplification of light in the two-dimensional photonic crystal layer 22 having such a configuration will be described. 1 / 2 Among the light beams a, the first light beam is reflected by a first modified refractive index portion 2221 arranged at a certain first lattice point 2241 and its traveling direction is changed by 180° (thick solid arrow in FIG. 18), and the second light beam is reflected by a second modified refractive index portion 2222 arranged at a certain second lattice point 2242 and its traveling direction is changed by 180° (thick dashed arrow in FIG. 18). The difference in optical path length between the first light beam and the second light beam is 2. -1 / 2 a=(2 1 / 2a) / 2=λ / 2. As a result, the first light and the second light are weakened (or vanished in this example) by interference. Therefore, the intensity of the light whose propagation direction changes to a direction other than 180° in the first modified refractive index section 2221 or the second modified refractive index section 2222 becomes relatively large, and stable laser oscillation can be obtained over a wide area in the two-dimensional photonic crystal layer 22. As a result, the intensity of the pulsed laser light emitted from the two-dimensional photonic crystal laser 20 can be increased.

[0066] Although the case where the first light and the second light are weakened by interference has been described here, the first light and the second light may be strengthened by interference depending on the magnitude and direction of the misalignment between first two-dimensional lattice 2231 and second two-dimensional lattice 2232. In that case, the intensity of the pulsed laser light emitted from two-dimensional photonic crystal laser 20 increases due to the effect of interference itself.

[0067] Next, calculation results of the temporal change in intensity of the emitted pulsed laser light and the emission pattern of the pulsed laser beam for the two-dimensional photonic crystal laser 20 of the second embodiment will be described. In these calculations, the overall diameter of the first electrode 17 was 250 μm, the width of the non-conductive regions 172 was 6.6 μm, and seven non-conductive regions 172 were arranged at a 44 μm interval. The overall diameter of the first electrode 17 listed here was larger than the value (150 μm) used in the calculations for the first embodiment. This is because, as described above, the second embodiment can obtain stable laser oscillation over a wide area within the two-dimensional photonic crystal layer 22. The areas of the first modified refractive index portion 2221 and the second modified refractive index portion 2222 were modulated (without deviation from the first lattice point 2241 and the second lattice point 2242). The area of ​​the first modified refractive index portion 2221 was modulated within a range of 8±3% (5 to 11%) of the total area of ​​the two-dimensional photonic crystal layer 22, and the area of ​​the second modified refractive index portion 2222 was modulated within a range of 6.5±2.5% (4 to 9%) of the total area of ​​the two-dimensional photonic crystal layer 22. The modulation phase was set so that a pulsed laser beam was emitted with a tilt angle θ of 54° and azimuth angles φ of 0° and 180°.

[0068] Figure 19 shows the calculation results for the temporal change in the intensity of the pulsed laser beam. A pulsed laser beam was obtained with a pulse repetition period of approximately 1 nanosecond. The peak intensity of the pulsed laser beam was approximately 6 W, which is higher than the calculated value of approximately 2 W for the two-dimensional photonic crystal laser 10 of the first embodiment (Figures 8, 12, and 13). The pulse width of one pulse was 25 picoseconds.

[0069] Figure 20 shows the calculated emission pattern of a pulsed laser beam. In the left image of Figure 20, two beam spots can be seen, one in each of the two rectangular areas. One of these beam spots is shown enlarged in the right image of Figure 20. From the left image, it can be seen that two pulsed laser beams are emitted with an inclination angle θ = 54° and azimuth angles φ = 0° and 180°. Furthermore, from the right image, it can be seen that the beam divergence angle is sufficiently smaller than 1°, allowing a high-intensity pulsed laser beam to reach long distances.

[0070] In the two-dimensional photonic crystal laser 20 of the second embodiment, as in the first embodiment, the non-conductive regions 172 of the first electrode 17 (and the saturable absorption regions 112 of the active layer 11) can have various shapes. For example, when the period at which the non-conductive regions 172 in the two-dimensional photonic crystal laser 20 are arranged is changed from 44 μm to 27 μm, 35 μm, 53 μm, 62 μm, and 71 μm (with the width remaining unchanged from 6.6 μm), the change in the intensity of the laser light over time was calculated, and it was confirmed that pulsed laser light was oscillated in all cases, as shown in FIG. 21 . The longer the period at which the non-conductive regions 172 are arranged, the shorter the pulse repetition period.

[0071] It was also confirmed that pulsed laser light was emitted in all cases where the width of non-conductive region 172 in the above-mentioned two-dimensional photonic crystal laser 20 was changed from 6.6 μm to 4.4 μm and 8.8 μm (with the arrangement period remaining unchanged from 44 μm), as shown in Fig. 22. The larger the width of non-conductive region 172, the longer the pulse repetition period.

[0072] Furthermore, the two-dimensional photonic crystal laser 20 of the second embodiment may also be adapted to change both the period at which the non-conductive regions 172 (and the saturable absorption regions 112; the same applies below) are arranged and the width of the non-conductive regions 172, change the direction in which the non-conductive regions 172 extend, or use non-conductive regions 172 having a shape other than linear, as described in the first embodiment.

[0073] Next, an example of an actually fabricated two-dimensional photonic crystal laser 20 according to the second embodiment is shown. In this example, a first electrode 17 shown in the micrograph of FIG. 23 was fabricated. This first electrode 17 is the same as the example shown in FIG. 5 , except that the shape of the outer non-conductive region 173 is changed to a circular shape. The first electrode 17 was formed by implanting hydrogen ions into the portion of a circular gold plate larger than the outer non-conductive region 173, where the non-conductive region 172 and the outer non-conductive region 173 are to be formed. The hydrogen ions also reach the active layer 11, forming a saturable absorption region 112 in the active layer 11. Note that although a portion 174 outside the outer non-conductive region 173 is conductive, when the two-dimensional photonic crystal laser 20 is in use, no current is injected into this portion 174. Instead, current is injected only into the conductive region 171. The width of the non-conductive region 172 was 6 μm, and the period of the non-conductive region 172 was 45 μm. The two-dimensional photonic crystal layer 22 was designed to emit a total of 28 laser beams, including 14 beams with different tilt angles and / or azimuth angles, and 14 beams with the same tilt angle but azimuth angles differing by 180°, by modulating the areas of the first modified refractive index portion 2221 and the second modified refractive index portion 2222 using the technique described in Patent Document 3. The range of area modulation is expressed using the period a of the square lattice, and is (0.08±0.03)a for the first modified refractive index portion 2221. 2 , (0.07±0.02)a in the second modified refractive index portion 2222 2 It was decided.

[0074] The near-field image of the pulsed laser beam obtained from the fabricated two-dimensional photonic crystal laser 20 is shown in Figure 24, and the far-field image is shown in Figure 25. The far-field image reveals that more than two beam spots are formed at different positions, and multiple pulsed laser beams with different tilt angles θ and / or azimuth angles φ are emitted. An enlarged view of one of the spots is shown (right image of Figure 25). The beam divergence angle is approximately 0.46°, which is almost the diffraction limit, allowing the pulsed laser beam to reach long distances.

[0075] Next, Fig. 26 shows the temporal change in the intensity of pulsed laser light obtained from the fabricated two-dimensional photonic crystal laser 20. Fig. 26 shows two examples in which currents of 0.61 A and 0.87 A, respectively, were passed between the first electrode 17 and the second electrode 18. The intensity shown here is the sum of the individual intensities of the multiple pulsed laser beams obtained. In both cases, pulsed laser light was obtained with a pulse repetition period of approximately 2 nanoseconds (slightly longer when the current was 0.61 A).

[0076] Similar measurements were also performed at several other current values, and the peak intensity (peak power) and pulse repetition period of the pulsed laser beam were determined for each measurement. The experimental results are shown in Figure 27 for the peak power and in Figure 28 for the pulse repetition period. The peak power increases until the current injected into the active layer 11 is increased to approximately 0.8 A, but then decreases as the current is further increased. The pulse repetition period decreases monotonically with increasing current. These results indicate that as the current is increased, the laser oscillation changes from pulsed to a state closer to continuous oscillation, thereby decreasing the peak power and pulse repetition period.

[0077] Therefore, the relationship between the magnitude of the current injected into the active layer 11, the carrier lifetime in the saturable absorption region 112, and the peak power was calculated. The calculation results are shown in Figure 29. In this figure, pulsed laser light is obtained outside the ranges marked "non-oscillation" or "continuous wave oscillation via relaxation oscillation." Within the range in which pulsed laser light is obtained, the brighter the color, the greater the peak power. The calculation results show that in order to increase the peak power, it is effective to increase the current (toward the right in Figure 29) and shorten the carrier lifetime (toward the top of the figure).

[0078] Increasing the thickness of saturable absorption region 12 in active layer 11 is an effective way to extend the carrier lifetime. Therefore, in order to increase the thickness of saturable absorption region 12, a two-dimensional photonic crystal laser 20 was fabricated in which the hydrogen ions were implanted deeper into active layer 11 by increasing the hydrogen ion implantation energy compared to the examples described so far. The results of measuring the peak power of this two-dimensional photonic crystal laser 20 while varying the magnitude of the current injected into active layer 11 are shown in FIG. 30 . The results show that the peak power increased even in the region exceeding 0.8 A, where the peak power began to decrease with increasing current in the example shown in FIG. 27 , and operation at a high peak power of 15 W to 20 W was achieved.

[0079] Next, an experiment was conducted to increase the peak power of pulsed laser light by widening the first electrode 17 (the region inside the outer edge non-conductive region 173) in the two-dimensional photonic crystal laser 20 of the second embodiment, thereby increasing the current injected into the active layer 11. In this experiment, the diameter of the first electrode 17 was increased to 1 mm (from 250 μm in the previously described experimental examples). However, while increasing the current can increase the peak power, it also creates the problem of increasing the likelihood of unwanted higher-order mode laser oscillation. To suppress such higher-order mode laser oscillation, the ratio b / a of the major axis a to the minor axis b of the ellipse was adjusted for the first modified refractive index portion 2221 and the second modified refractive index portion 2222 provided in the two-dimensional photonic crystal layer 22. Furthermore, linear second non-conductive regions 1722 extending in the y direction were provided in the first electrode 17 in addition to the linear non-conductive regions 172 extending in the x direction (see the optical microscope photograph in FIG. 31 ), and the number and positions of the second non-conductive regions 1722 were adjusted. As a result, in the active layer 11, a first saturable absorption region 1121 and a second saturable absorption region 1122 are formed, which extend in the x direction and the y direction, respectively, which are directions inclined at 45° from the X' direction and the Y' direction, and in which the first modified refractive index section 2221 and the second modified refractive index section 2222 are arranged at a period corresponding to a predetermined wavelength λ (Figure 32).

[0080] An electron microscope photograph of the fabricated two-dimensional photonic crystal layer 22 is shown in Figure 33. In this example, the first modified refractive index portion 2221 and the second modified refractive index portion 2222 of the two-dimensional photonic crystal layer 22 have an ellipse with a ratio b / a of the major axis a to the minor axis b of the ellipse set to 0.42. As shown in Figure 31, two second non-conductive regions 1722 were provided near each end in the x direction. The non-conductive regions 172 were arranged at equal intervals throughout the entire y direction. By injecting a current from the first electrode 17 provided with the non-conductive regions 172 and second non-conductive regions 1722 in this manner, a saturable absorption region having the same shape as the non-conductive regions 172 and second non-conductive regions 1722 is formed in the active layer 11.

[0081] The spacing between the second non-conductive regions 1722 was 130 μm between two adjacent regions near both ends in the x direction (the first and second regions from the left in FIG. 31 , and the third and fourth regions from the left in FIG. 31 ), and 620 μm between two adjacent regions sandwiching the center of the first electrode 17 (the second and third regions from the left in FIG. 31 ). The width of each second non-conductive region 1722 was 30 μm. The non-conductive regions 172 were arranged at intervals of 120 μm, and the width of each non-conductive region 172 was 30 μm.

[0082] Figure 34 shows a photograph of the far-field pattern of the laser beam obtained by the two-dimensional photonic crystal laser 20 having a first electrode 17 with a diameter of 1 mm, fabricated as described above, and Figure 35 shows a graph measuring the change in the intensity of the laser beam over time. Ten laser beams are clearly visible in Figure 34. Figure 35 shows that a pulsed laser beam with a high peak power of up to approximately 100 W and a narrow pulse width of 50 picoseconds (psec) or less can be obtained.

[0083] In the examples shown so far, pulsed laser light is repeatedly emitted by continuously injecting current, but it is also possible to emit a single pulse of pulsed laser light by injecting current in a short pulsed manner. For example, according to calculations by the inventors of the present application, if one pulse of current shown in Fig. 36 is injected into two-dimensional photonic crystal laser 20, which emits pulsed laser light having the intensity varying over time as shown in Fig. 19 by continuously injecting current, a single pulse of pulsed laser light as shown in Fig. 37 can be obtained.

[0084] Next, a pulsed power supply was connected to the actually fabricated two-dimensional photonic crystal laser 20 of the second embodiment, a pulsed current was passed between the first electrode 17 and the second electrode 18, and the temporal change in the intensity of the output pulsed laser beam was measured using a photodiode and a streak camera. The peak intensity of the pulsed current was approximately 1.04 A. The applied current and the intensity of the pulsed laser beam measured using the photodiode are shown in FIG. 38, and the intensity of the pulsed laser beam measured using the streak camera is shown in FIG. 39. These results confirmed that the actual two-dimensional photonic crystal laser 20 can produce a pulsed laser beam with only one pulse.

[0085] (3) Other Embodiments Up to this point, the configurations of the individual two-dimensional photonic crystal lasers 10 and 20 in each embodiment have been described. However, a plurality of these two-dimensional photonic crystal lasers 10 and 20 may be combined for use. For example, a single laser device 30 ( FIGS. 40 and 41 ) can be configured by combining a plurality of two-dimensional photonic crystal lasers 10 having different amounts of deviation of the positions of the modified refractive index portions 122 from the lattice points 124 and / or different modulation periods for the areas of the modified refractive index portions 122, or a plurality of two-dimensional photonic crystal lasers 20 having similarly different modulation periods. This allows each two-dimensional photonic crystal laser 10 and 20 to emit a plurality of pulsed laser beams in different directions according to the modulation period. In the example shown in FIG. 40 , a plurality of two-dimensional photonic crystal lasers 10 and 20 are arranged one-dimensionally. However, a plurality of two-dimensional photonic crystal lasers 10 and 20 may also be arranged two-dimensionally, as shown in FIG. 41 .

[0086] Although a number of embodiments of the two-dimensional photonic crystal laser according to the present invention have been described above, it goes without saying that the present invention is not limited to these embodiments and various modifications are possible.

[0087] (4) Reference Example Up to this point, we have described a two-dimensional photonic crystal laser that emits pulsed laser light in a direction tilted from the normal to the two-dimensional photonic crystal layer by using a two-dimensional photonic crystal layer in which a modulated two-dimensional photonic crystal is formed and an electrode having a linear non-conductive region extending in a predetermined direction. Below, as a reference example, we will describe a two-dimensional photonic crystal laser that emits pulsed laser light in the same direction as this embodiment by devising the configuration of the two-dimensional photonic crystal layer instead of using such electrodes.

[0088] Patent Document 4 describes a two-dimensional photonic crystal laser in which a two-dimensional photonic crystal layer without modulation of the positional shift or area of ​​a modified refractive index portion is formed so as to have a frequency gradient in which the band edge frequency (a frequency corresponding to the energy called the band edge, which is the boundary between the photonic band and the photonic band gap of the two-dimensional photonic crystal) monotonically decreases in one direction parallel to the base material. Non-Patent Document 2 (which was published after the priority date of the present application (before the international filing date)) describes a two-dimensional photonic crystal laser in which a two-dimensional photonic crystal layer without modulation of the positional shift or area of ​​a modified refractive index portion is formed so as to have a frequency gradient in which the band edge frequency monotonically decreases (e.g., like a quadratic function with the reference position as the origin) with increasing distance from a predetermined reference position. The example in Patent Document 4 is also interpreted as one end in the one direction being a reference position, and the frequency monotonically decreasing with increasing distance from that position. These frequency gradients can be formed by increasing the periodic length of the modified refractive index portion or decreasing the area of ​​the modified refractive index portion with increasing distance from the reference position. According to the two-dimensional photonic crystal lasers described in these documents, pulsed laser light is emitted in the normal direction of the two-dimensional photonic crystal layer.

[0089] By introducing such a frequency gradient into the modulated two-dimensional photonic crystal, it is possible to emit a pulsed laser beam in a direction tilted from the normal to the two-dimensional photonic crystal layer. Specifically, the following two modes can be adopted.

[0090] A first aspect of the two-dimensional photonic crystal laser of the reference example is a two-dimensional photonic crystal laser comprising: an active layer that generates light of a predetermined wavelength when a current is injected therein; a two-dimensional photonic crystal layer in which modified refractive index sections, which have a refractive index different from that of a plate-shaped base material arranged parallel to the active layer, are positioned at predetermined offset amounts from each lattice point of the two-dimensional lattice, the modified refractive index sections being arranged at a period that monotonically increases with distance from a predetermined reference position, and / or the two-dimensional photonic crystal layer is positioned at each lattice point with a predetermined area, the offset amount and / or area being modulated at a predetermined modulation period; and a pair of electrodes arranged to sandwich the active layer and the two-dimensional photonic crystal layer in a direction perpendicular to the active layer and the two-dimensional photonic crystal layer.

[0091] A second aspect of the two-dimensional photonic crystal laser of the reference example is a two-dimensional photonic crystal laser comprising: an active layer that generates light of a predetermined wavelength when a current is injected therein; a two-dimensional photonic crystal layer in which modified refractive index portions having a refractive index different from that of the base material are arranged at each lattice point of the two-dimensional lattice, and the modified refractive index portions are arranged in a plate-like base material that is arranged parallel to the active layer, and the modified refractive index portions are modulated at a predetermined modulation period from a reference area at each lattice point, the reference area being an area that monotonically decreases with increasing distance from a predetermined reference position, or the modified refractive index portions are arranged shifted from each lattice point by a shift amount that modulates at the predetermined modulation period, and have an area modulated at the predetermined modulation period from the reference area at each lattice point; and a pair of electrodes arranged to sandwich the active layer and the two-dimensional photonic crystal layer in a direction perpendicular to the active layer and the two-dimensional photonic crystal layer.

[0092] In the first and second aspects of the reference example, the modulation period may be a composite modulation period in which a plurality of different modulation periods are superimposed.

[0093] As a specific example of the reference example, a calculation was performed for the case where the area of ​​the modified refractive index portion in the first embodiment is modulated. In this calculation, a configuration was used in which modified refractive index portions with modulated areas are arranged at the first lattice points of a first lattice in which the periodic length of a square lattice is monotonically increased in the y direction, and at the second lattice points of a second lattice having the same configuration as the first lattice but shifted from the first lattice by half the periodic length in the y direction. The difference Δa between the periodic lengths of the first lattice and the second lattice in the y direction and the periodic lengths between adjacent lattice points in the y direction is expressed as follows: Δa=a1×(y+L / 2) / L+a2×x 2 / L 2 (a1, a2, and L are all constants having the dimension of length; in this calculation example, a1 = 0.08 nm, a2 = 0.02 nm, and L = 250 μm.) The area of ​​the first modified refractive index portion was modulated within a range of 8 ± 2% (6 to 10%) of the area of ​​the entire two-dimensional photonic crystal layer, and the area of ​​the second modified refractive index portion 2222 was modulated within a range of 6.5 ± 1.5% (5 to 8%) of the area of ​​the entire two-dimensional photonic crystal layer 22. The modulation phase was set so that a pulsed laser beam was emitted with a tilt angle θ = 54° and azimuth angles φ = 0° and 180°. Figure 42 shows the difference in Δa at each position within the two-dimensional photonic crystal layer 22 designed as described above, represented by color shading. In the example shown in the figure, Δa (and a) increase in the x direction as one moves away from the reference position where x = 0, and increase in the y direction as one moves away from the reference position where y is at its minimum value (negative and has its absolute maximum value).

[0094] The time variation of the intensity of the pulsed laser beam obtained by this calculation is shown in the graph in Figure 43. Two calculations were performed, with an injected current of 2 A and 3 A, and it was found that in both cases, pulsed laser beams with pulse widths significantly narrower than 1 nanosecond were repeatedly emitted. The beam was emitted in two directions, with an inclination angle of θ = 54° and azimuth angles of φ = 0° and 180°.

[0095] Figure 42 shows an example in which Δa is changed for each position in the two-dimensional photonic crystal layer 22, but similarly, the area (change δS from the reference area) of the modified refractive index sections (first modified refractive index section 2221 and second modified refractive index section 2222) may be changed for each position in the two-dimensional photonic crystal layer 22 (Figure 44).

[0096] Aspects It will be apparent to those skilled in the art that the exemplary embodiments described above are examples of the following aspects.

[0097] (Item 1) A two-dimensional photonic crystal laser according to one aspect of the present invention comprises: an active layer that generates light of a predetermined wavelength, which is the oscillation wavelength of the laser, when a current is injected into it; a two-dimensional photonic crystal layer in which modified refractive index sections, which have a refractive index different from that of the base material and are disposed within a plate-like base material arranged parallel to the active layer, are shifted by a predetermined amount from each lattice point of a two-dimensional lattice that is arranged in two predetermined directions at a period corresponding to the predetermined wavelength, and / or are disposed at each of the lattice points with a predetermined area, and the shift amount and / or area are modulated at a predetermined modulation period; and a pair of electrodes provided in a direction perpendicular to the active layer and the two-dimensional photonic crystal layer so as to sandwich the active layer and the two-dimensional photonic crystal layer therebetween, wherein the active layer has a saturable absorption region that is a linear region that is part of a predetermined range and has no portion parallel to either of the two predetermined directions, a gain region that occupies the part of the predetermined range other than the saturable absorption region, and a non-gain region formed around the predetermined range, and the saturable absorption region has lower conductivity and a shorter carrier lifetime than the gain region.

[0098] (Item 2) The two-dimensional photonic crystal laser according to item 2 is the two-dimensional photonic crystal laser according to item 1, further comprising second modified refractive index sections, the refractive index of which differs from that of the base material, arranged at the second lattice points of a second two-dimensional lattice, the second lattice points being arranged at the same period as the lattice points of the two-dimensional lattice and being positioned offset from the two-dimensional lattice, or at different offset amounts and / or with different areas from the second lattice points.

[0099] (Item 3) The two-dimensional photonic crystal laser according to item 3 is the two-dimensional photonic crystal laser according to item 1 or 2, further comprising an intermediate layer between one of the pair of electrodes and the active layer, a low-conductivity region having lower conductivity than other parts of the intermediate layer is formed in a part of the intermediate layer, and the part where the low-conductivity region is formed is connected to the saturable absorption region of the active layer.

[0100] (Item 4) The two-dimensional photonic crystal laser according to item 4 is a two-dimensional photonic crystal laser according to any one of items 1 to 3, in which the shift amount and / or the area are modulated with a composite modulation period in which a plurality of modulation periods different from each other are superimposed.

[0101] (Item 5) The two-dimensional photonic crystal laser according to item 5 is the two-dimensional photonic crystal laser according to any one of items 1 to 4, wherein the saturable absorption region has a first saturable absorption region that extends linearly in a first direction without having a portion parallel to either of the two specified directions, and a second saturable absorption region that extends linearly in a second direction perpendicular to the first direction without having a portion parallel to either of the two specified directions.

[0102] (Item 6) A laser device according to item 6 includes a plurality of two-dimensional photonic crystal lasers according to any one of items 1 to 5, each having a different amount of deviation and / or a different area.

[0103] 10, 20... Two-dimensional photonic crystal laser 11... Active layer 111... Gain region 112... Saturable absorption region 1121... First saturable absorption region 1122... Second saturable absorption region 113... Non-gain region 12, 22... Two-dimensional photonic crystal layer 121, 221... Base material 122... Modified refractive index section 123... Two-dimensional lattice 124... Lattice point 125... Center (centre of gravity) of planar shape of modified refractive index section 126... Vector indicating displacement amount 13... Spacer layer 141... First cladding layer 142... Second cladding layer 16... Substrate 17... First electrode 171, 971... Conductive region 172, 972... Non-conductive region 1722... Second non-conductive region 173... Outer edge non-conductive region 174... Portion outside the circumference of the outer edge non-conductive region 18... Second electrode 181...frame portion 182...window portion 19...intermediate layer 191...highly conductive region 192...lowly conductive region 2221...first modified refractive index portion 2222...second modified refractive index portion 2231...first two-dimensional grating 2232...second two-dimensional grating 2241...first lattice point 2242...second lattice point 30...laser device 97...electrode

Claims

a two-dimensional photonic crystal layer in which modified refractive index sections having a refractive index different from that of a plate-shaped base material arranged parallel to the active layer are disposed at predetermined offset amounts from each lattice point of a two-dimensional lattice arranged at a period corresponding to the predetermined wavelength in two predetermined directions and / or are disposed at each lattice point with a predetermined area, the offset amount and / or area being modulated at a predetermined modulation period; and a pair of electrodes arranged in a direction perpendicular to the active layer and the two-dimensional photonic crystal layer so as to sandwich the active layer and the two-dimensional photonic crystal layer therebetween, wherein the active layer has a saturable absorption region which is a linear region that is part of a predetermined range and has no portion parallel to either of the two predetermined directions, a gain region that occupies a portion of the predetermined range other than the saturable absorption region, and a non-gain region formed around the predetermined range, wherein the saturable absorption region has lower conductivity and a shorter carrier lifetime than the gain region.

2. The two-dimensional photonic crystal laser according to claim 1, further characterized in that second lattice points are arranged at the same period as the lattice points of the two-dimensional lattice and are provided at positions shifted from the two-dimensional lattice, and second modified refractive index sections having a refractive index different from that of the base material are arranged at the second lattice points of a second two-dimensional lattice, or at different offsets and / or with different areas from the second lattice points.

3. A laser device comprising the two-dimensional photonic crystal laser described in claim 1, further comprising an intermediate layer between one of the pair of electrodes and the active layer, a low conductivity region having a lower conductivity than other parts of the intermediate layer being formed in a part of the intermediate layer, and the part in which the low conductivity region is formed being connected to the saturable absorption region of the active layer.

4. A two-dimensional photonic crystal laser as claimed in claim 1 or 2, characterized in that the offset and / or the area are modulated with a composite modulation period in which a plurality of different modulation periods are superimposed.

5. A two-dimensional photonic crystal laser as described in claim 1 or 2, characterized in that the saturable absorption region has a first saturable absorption region extending linearly in a first direction having no portion parallel to either of the two specified directions, and a second saturable absorption region extending linearly in a second direction perpendicular to the first direction having no portion parallel to either of the two specified directions.

6. A laser device comprising a plurality of two-dimensional photonic crystal lasers according to claim 1 or 2, the offset amounts and / or areas of which are different from one another.

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