Two-dimensional photonic crystal laser

Abstract

A two-dimensional photonic crystal laser (10) is provided with: an active layer (11); a two-dimensional photonic crystal layer (12) which is provided parallel to the active layer (11) and in which different refractive index regions (122) having a refractive index different from that of a plate-shaped base material (121) are periodically and two-dimensionally arranged within the base material (121); and a first electrode (171) and a second electrode (172) which are provided so as to sandwich the active layer (11) and the two-dimensional photonic crystal layer (12) in the thickness direction. The first electrode (171) includes a window frame portion (1711) made of an electrically conductive plate material, a window portion (1713) provided inside the window frame portion (1711), and a plurality of linear electrode portions (1712) made of electrically conductive linear members and extending radially from the window frame portion (1711) toward a prescribed focusing point (1714) within the window portion (1713), to a terminal portion (1715) which is positioned between the window frame portion (1711) and the focusing point (1714) or positioned at the focusing point (1714). The width of the linear electrode portions (1712) varies from the window frame portion (1711) toward the terminal portion (1715).

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] A two-dimensional photonic crystal laser generally has a stack in which an active layer and a two-dimensional photonic crystal layer are sandwiched between a pair of cladding layers, and further has a pair of electrodes (electrode pair) sandwiching the stack. The active layer generates light in a specific emission wavelength band when carriers (holes, electrons) are injected from the electrode pair. The two-dimensional photonic crystal layer has a structure in which modified refractive index areas with a different refractive index from that of a plate-shaped base material are periodically arranged in a two-dimensional pattern within the base material. The modified refractive index areas consist of voids (air) formed in the base material, or a material with a different refractive index from that of the base material that is embedded in the base material.

[0003] In such a two-dimensional photonic crystal laser, only light of a predetermined wavelength corresponding to the periodic length of the arrangement of the modified refractive index areas among the light generated in the active layer is amplified in the two-dimensional photonic crystal layer to cause laser oscillation, and a laser beam is emitted in a direction perpendicular to the two-dimensional photonic crystal layer. However, since electrodes are present in the direction perpendicular to the two-dimensional photonic crystal layer, there is a risk that the laser beam will be blocked by the electrodes depending on their positions and shapes.

[0004] Therefore, the two-dimensional photonic crystal laser described in Patent Document 1 uses a window-shaped electrode in one of a pair of electrodes, in which a window is provided by hollowing out a part of a conductive plate material. In this two-dimensional photonic crystal laser, the laser beam emitted in a direction perpendicular to the two-dimensional photonic crystal layer passes through the window and is not blocked by the electrode.

[0005] However, when a window-shaped electrode is used, the density of carriers injected into the active layer exhibits a doughnut-shaped distribution, with the center being lower than the periphery, which results in a non-uniform laser beam intensity in the center of the cross section perpendicular to the beam.

[0006] Therefore, in the two-dimensional photonic crystal laser described in Non-Patent Document 1, one of a pair of electrodes is an electrode having a plurality of conductive wires extending radially from a ring-shaped conductive plate (hereinafter referred to as the "window frame") corresponding to the window frame of the window-shaped electrode toward the center of the window. By using such wires, it is possible to minimize the obstruction of the emitted laser beam by the electrode while making the carrier density injected into the active layer nearly uniform, thereby obtaining a laser beam with nearly uniform light intensity in a cross section perpendicular to the beam.

[0007] However, if all the wires are the same length, the spatial density occupied by the wires increases toward the center of the window, resulting in non-uniformity in the carrier density. Therefore, in the two-dimensional photonic crystal laser described in Non-Patent Document 1, multiple types of wires with different lengths are combined, and the shorter wires are provided only near the window frame, thereby adjusting the distribution of the spatial density of the wires.

[0008] International Publication No. WO2007 / 029538

[0009] Tatsuya Abe et al., "Investigation of the introduction of split electrodes on the output side of a large-area photonic crystal laser," Proceedings of the 83rd Autumn Meeting of the Japan Society of Applied Physics, August 26, 2022, Lecture No. 20p-A101-12

[0010] When the inventors of the present application measured the current density in the active layer of the two-dimensional photonic crystal laser described in Non-Patent Document 1, from the window frame toward the position corresponding to the center of the window, they found that the current density in the active layer suddenly decreased at the positions corresponding to the ends of the wires of different lengths. Such a sudden change in current density depending on the position causes non-uniformity in the light intensity of the emitted laser beam in a cross section perpendicular to the beam.

[0011] The problem to be solved by the present invention is to provide a two-dimensional photonic crystal laser in which the current density distribution can be set without the current density changing suddenly depending on the position within the active layer.

[0012] The two-dimensional photonic crystal laser of the present invention, which has been made to solve the above-mentioned problems, comprises: a) an active layer; b) a two-dimensional photonic crystal layer, which is provided parallel to the active layer and has modified refractive index areas, which have a refractive index different from that of the base material, periodically arranged in a two-dimensional pattern within a plate-shaped base material; and c) a first electrode and a second electrode, which are provided so as to sandwich the active layer and the two-dimensional photonic crystal layer in the thickness direction, wherein the first electrode has a window frame portion made of a conductive plate material, a window portion provided inside the window frame portion, and a plurality of linear electrode portions made of linear conductive members, which extend radially from the window frame portion toward a predetermined focal point within the window portion, between the window frame portion and the focal point or to an end portion located at the focal point, and the width of the linear electrode portions changes from the window frame portion toward the end portion.

[0013] In the present invention, "linear" means that the ratio of the maximum width to the length is sufficiently small, specifically, 1 / 10 or less. By making the maximum width sufficiently small in this way, it is possible to prevent the laser beam emitted from the window from being blocked by the linear electrode portion. Furthermore, "linear" and "radial" do not limit the linear electrode portion to being straight, and the linear electrode portion may also be curved.

[0014] The window portion is typically circular, but is not limited thereto and can be any shape, such as a square or other quadrangle, or a polygon other than a quadrangle. The window frame is typically provided around the entire outer periphery of the window portion, but may not be provided on part of the outer periphery of the window portion (for example, a shape in which a portion of a ring is cut off). The focal point is typically the center of the circle when the window portion is circular, but is not limited thereto and can be a point at any position within the window portion. Each linear electrode portion may extend from the window frame portion to a position just before the focal point (with its terminal end located between the window frame portion and the focal point) or from the window frame portion to the focal point (with its terminal end located at the focal point).

[0015] In the two-dimensional photonic crystal laser described in Non-Patent Document 1, the individual wires (linear electrode portions) are formed with the same width from the window frame portion to the terminal end, and their spatial density is adjusted by the length of the wire. In contrast, in the two-dimensional photonic crystal laser according to the present invention, the width of the linear electrode portions is changed from the window frame portion to the terminal end, thereby adjusting the spatial density of the linear electrode portions in the first electrode. This makes it possible to prevent the density of the current injected from the first electrode and the second electrode from changing suddenly depending on the position.

[0016] The width of the linear electrode portions is typically decreased from the window frame portion toward the terminal end portion, which makes the spatial density of the linear electrode portions in the first electrode nearly uniform, and also makes the density of the current injected from the first electrode and the second electrode into the active layer nearly uniform.

[0017] On the other hand, due to the electrical resistance of the linear electrode portion, a voltage drop occurs in which the voltage between each point on the linear electrode portion and the second electrode decreases from the window frame side to the terminal end. If the effect of this voltage drop is significant, the width of the linear electrode portion may be increased from the window frame side to the terminal end, or may be changed by a combination of increasing and decreasing. In addition to the effect of voltage drop due to electrical resistance, the width of the linear electrode portion may be increased from the window frame side to the terminal end, or may be changed by a combination of increasing and decreasing, when intentionally forming a spatial distribution of the current density in the active layer or the intensity of the laser light in the cross section of the laser beam.

[0018] It is not essential that the width of each linear electrode portion changes toward the end (the rate of change is not 0) at all positions, and it is also acceptable that the width does not change (the rate of change is 0) at some positions of each linear electrode portion.

[0019] In the two-dimensional photonic crystal laser according to the present invention, it is preferable that the electrode occupancy rate varies so as to compensate for the voltage change depending on the position within the linear electrode portion as described above. Here, the electrode occupancy rate is expressed as a function of the distance from the focal point and is defined as the proportion of the linear electrode portion occupying the circumference of a circle centered at the focal point and having a radius equal to the distance. Specifically, it is preferable that the electrode occupancy rate varies from the window frame side toward the terminal end side so as to have a sign opposite to the positive or negative sign of the rate of change of voltage between the second electrode and the linear electrode portion.

[0020] In addition, if the voltage drop is made small enough to be negligible by using a conductor with sufficiently low electrical resistivity as the material for the linear electrode portion or by making the linear electrode portion sufficiently thick, the electrode occupancy rate may be uniform between the window frame portion and the terminal portion.

[0021] In the two-dimensional photonic crystal laser according to the present invention, two (or three or more) adjacent linear electrodes of the plurality of linear electrodes may be configured to join at a position between the window frame and the terminal end (i.e., the two linear electrodes are integrated without any gaps). In this case, the width of each linear electrode between the position where the two linear electrodes join (integrate) and the terminal end is considered to be half the width of the joined (integrated) line (or, in the case of three or more linear electrodes, one for each joined line). By joining the two linear electrodes at a position near the terminal end in this manner, it is not necessary to form independent linear electrodes with extremely small widths, and the linear electrodes can be easily fabricated and the mechanical strength of the linear electrodes can be increased.

[0022] In the two-dimensional photonic crystal laser according to the present invention, the linear electrode portion is preferably thick to suppress the effect of voltage drop. On the other hand, if the entire first electrode is too thick, the difference in thermal expansion coefficient between the first electrode and the member in contact with it (usually a semiconductor substrate) may cause the first electrode to peel off from the member as the temperature drops when the first electrode is fabricated using a plating method or the like. Therefore, the thickness of the linear electrode portion is preferably thicker than the thickness of the window frame portion. This suppresses the effect of thermal expansion (contraction) of the entire first electrode, thereby preventing peeling of the first electrode and suppressing the effect of voltage drop in the linear electrode portion. Furthermore, because the thickness of the window frame portion is thinner than the linear electrode portion, the amount of material (e.g., gold) used for the first electrode can be reduced compared to when the entire first electrode is thick, thereby reducing material costs.

[0023] A first electrode having a linear electrode portion thicker than the window frame portion can be easily fabricated by, for example, forming the entire shape of the first electrode by the thickness of the window frame portion using a vapor deposition method, and then forming only the linear electrode portion using a plating method. Alternatively, a similar first electrode can be fabricated by performing either the plating method or the vapor deposition method alone multiple times, or by performing the plating method and the vapor deposition method together three or more times. In these cases, different masks can be used for each treatment, thereby achieving different thicknesses depending on the position within the linear electrode portion. In addition to adjusting the width of the linear electrode portion in the present invention, forming the linear electrode portion with different thicknesses depending on the position allows for more flexible adjustment of the electrical resistance of the current passing through a cross section perpendicular to the line for each position of the linear electrode portion, thereby further reducing the effects of voltage drop.

[0024] Incidentally, making the thickness of the linear electrode portion thicker than the thickness of the window frame portion in this manner is not limited to the two-dimensional photonic crystal laser of the present invention, in which the width of the linear electrode portion varies from the window frame portion toward the terminal end, but also has the same effect in conventional two-dimensional photonic crystal lasers (such as those described in non-patent document 1) that have linear electrode portions with a uniform width. Thus, a two-dimensional photonic crystal laser characterized by the thickness of the linear electrode portion and the window frame portion, regardless of the width of the linear electrode portion, comprises: a) an active layer; b) a two-dimensional photonic crystal layer provided parallel to the active layer, in which modified refractive index areas having a refractive index different from that of the base material are periodically arranged in a two-dimensional pattern within a plate-shaped base material; and c) a first electrode and a second electrode provided to sandwich the active layer and the two-dimensional photonic crystal layer in the thickness direction, wherein the first electrode has a window frame portion made of a conductive plate material, a window portion provided inside the window frame portion, and a plurality of linear electrode portions made of conductive linear members extending radially from the window frame portion toward a predetermined focal point within the window portion, between the window frame portion and the focal point or to an end portion located at the focal point, and the thickness of the linear electrode portions is thicker than the thickness of the window frame portion.

[0025] According to the two-dimensional photonic crystal laser of the present invention, the current density distribution can be set without the current density changing suddenly depending on the position in the active layer.

[0026] 7A is a perspective view showing one embodiment of a two-dimensional photonic crystal laser according to the present invention. It is a plan view showing the configuration of a two-dimensional photonic crystal layer in the two-dimensional photonic crystal laser of this embodiment. It is a plan view showing the entire first electrode in the two-dimensional photonic crystal laser of this embodiment and a partially enlarged view thereof. It is a partial longitudinal cross-sectional view of the first electrode. It is a plan view showing a modified example of the first electrode. It is a graph showing the occupancy rate of the linear electrode portion within the window portion as a function of the distance from the center of the window portion in the two-dimensional photonic crystal laser of this embodiment. It is a graph showing the results of calculating the function η(r) of the electrode occupancy rate for three cases where the number N of linear electrodes is 80, the intercept b of the voltage of the linear electrode portion approximated by a linear function of the distance r from the focal point is 0.4, and the slope m of the linear function is -0.02, -0.04, and -0.06. It is a graph showing the results of calculating the function W(r) of width for the example of FIG. 7A. 7C 。 Graph showing the results of calculating η(r) for three cases where N=80, m=0.04, and b is 0.3, 0.4, and 0.5. Graph showing the results of calculating W(r) for the example of FIG. 7C . Graph showing the voltage drop in the linear electrode portion in the two-dimensional photonic crystal laser of this embodiment. Plan view showing the entire first electrode in the two-dimensional photonic crystal laser of the comparative example. Graph showing the occupancy rate of the linear electrode portion in the window portion as a function of the distance from the center of the window portion in the two-dimensional photonic crystal laser of the comparative example. Graph showing the results of calculating the current density distribution in the current injection region of the active layer for the two-dimensional photonic crystal laser of this embodiment. Graph showing the results of calculating the current density distribution in the current injection region of the active layer for the two-dimensional photonic crystal laser of the comparative example. Photographs of a manufacturing example of the two-dimensional photonic crystal laser of this embodiment. The photonic crystal laser of the present embodiment is fabricated by injecting a current into the active layer from the first electrode and the second electrode, and the photonic crystal laser is fabricated in a state of light emission before laser oscillation. The photo shows cracks that occur when the window frame portion of the first electrode is made to have the same thickness as the linear electrode portion.The diagrams show the calculated distribution of stress magnitude generated in the first electrode during manufacturing (cooling), expressed as shades of color. (a) The diagrams show the case where the window frame portion and the linear electrode portion have the same thickness (80 μm), (b) an enlarged view of (a), (c) the case where the thickness of the linear electrode portion (80 μm) is thicker than the thickness of the window frame portion (20 μm), and (d) an enlarged view of (c).

[0027] 1 to 16, an embodiment of a two-dimensional photonic crystal laser according to the present invention will be described.

[0028] (1) Configuration of the Two-Dimensional Photonic Crystal Laser of the Present Embodiment As shown in FIG. 1 , the two-dimensional photonic crystal laser of the present embodiment has a configuration in which a first electrode 171, a substrate 16, a first cladding layer 141, an active layer 11, a spacer layer 13, a two-dimensional photonic crystal layer 12, a second cladding layer 142, and a second electrode 172 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 shows the first electrode 171 on the upper side and the second electrode 172 on the lower side, but the orientation of the two-dimensional photonic crystal laser 10 during use is not limited to that shown in this figure. The configuration of each layer and electrode will be described below.

[0029] The active layer 11 emits light having a predetermined wavelength band when electrons injected from the first electrode 171 and holes injected from the second electrode 172 combine in the layer. The active layer 11 can be made of, for example, an InGaAs / AlGaAs multiple quantum well (emission wavelength band: 935 to 945 nm).

[0030] As shown in FIG. 2 , the two-dimensional photonic crystal layer 12 is formed by arranging modified refractive index areas 122, each having a refractive index different from that of a plate-shaped base material 121, at each lattice point of the two-dimensional lattice. Note that FIG. 2 shows an enlarged view of only a portion of the two-dimensional photonic crystal layer 12; in an actual two-dimensional photonic crystal layer 12, multiple modified refractive index areas 122 are arranged over an area roughly equivalent to the outer edge of the first electrode 171. While the two-dimensional lattice shown in FIG. 2 is a square lattice, other two-dimensional lattices, such as a rectangular lattice or a triangular lattice, may also be used. The lattice point period (length) a of the square lattice is determined appropriately depending on the material of the base material 121 and the emission wavelength band of the active layer 11. A p-type semiconductor is used as the material of the base material 121 so that holes injected from the second electrode 172 can pass through. 2, the modified refractive index area 122 is a combination of two holes, the first modified refractive index area 1221 and the second modified refractive index area 1222, but a modified refractive index area 122 consisting of one hole or a combination of three or more holes may also be used. Furthermore, instead of the holes, a material with a refractive index different from that of the base material 121 may be used. The shape and size of each hole (or material with a refractive index different from that of the base material 121) are not limited to the example shown in FIG. 2.

[0031] The first cladding layer 141 not only serves to inject electrons from the first electrode 171 into the active layer 11 but also serves to prevent in-plane guided light, which is guided parallel to the two-dimensional photonic crystal layer 12, from leaking from the layer. Similarly, the second cladding layer 142 not only serves to inject holes from the second electrode 172 into the active layer but also serves to prevent in-plane guided light from leaking from the layer in the two-dimensional photonic crystal layer 12. To fulfill the former role, an n-type semiconductor is used as the material for the first cladding layer 141, and a p-type semiconductor is used as the material for the second cladding layer 142.

[0032] The spacer layer 13 is provided to allow holes injected from the second electrode 172 to pass through and be introduced into the active layer 11, while preventing electrons injected from the first electrode 171 from passing through the active layer 11 (and thereby combining with holes on the second electrode 172 side of the active layer 11). A p-type semiconductor is used as the material for the spacer layer 13.

[0033] 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 and to make the distance between first electrode 171 and active layer 11 sufficiently greater than the distance between second electrode 172 and active layer 11. For the same reason as for first cladding layer 141, an n-type semiconductor is used as the material for substrate 16.

[0034] The first electrode 171 is made of a conductive material, and in this embodiment, gold, a material with low electrical resistivity, is used. As shown in FIG. 3 , the first electrode 171 has a window frame 1711 formed by shaping a conductive plate into a ring, a window 1713 formed inside the ring and having a circular space in plan view, and a plurality of linear electrodes 1712 made of conductive linear members and extending radially from the window frame 1711 toward the center of the circle of the window 1713 (a focal point 1714). In this embodiment, the number of linear electrodes 1712 is 80. The radius of the window 1713 is 5 mm, and the length of the linear electrodes 1712 is 4.9 mm, slightly shorter than the radius. The width of each linear electrode portion 1712 is 40 μm immediately adjacent to the window frame portion 1711, and gradually (continuously) changes (decreases overall) from there toward the terminal portion 1715 near (slightly before) the focal point 1714. The ratio of the maximum width to the length of the linear electrode portion 1712 (in this embodiment, at a position approximately 3.5 mm from the focal point 1714) is 46 / 4900 (approximately 0.0094). The radius of the window portion 1713 and the number, length, and width of the linear electrode portions 1712 are merely examples, and the present invention is not limited to these examples.

[0035] In this embodiment, two adjacent linear electrode portions 1712 join at a position (position marked with reference numeral 1716 in FIG. 3 ) where the width of the two joined linear electrode portions 1712 is 20 μm (half the width immediately adjacent to the window frame portion 1711). The combined width of the two joined linear electrode portions 1712 is 40 μm, the same as the width of one linear electrode portion 1712 immediately adjacent to the window frame portion 1711, but the width per linear electrode portion is 20 μm, the same as the width immediately before the joining. The width of the two joined linear electrode portions 1712 further decreases toward the terminal end 1715, and they further join the joined two adjacent linear electrode portions 1712 at a position (position marked with reference numeral 1717 in FIG. 3 ) where the combined width of the two linear electrode portions 1712 is 20 μm (10 μm per linear electrode portion). That is, the four linear electrode portions 1712 join together to form a total width of 40 μm, but each electrode has a width of 10 μm, as described above. Furthermore, they join together in a similar manner at the position indicated by reference numeral 1718 in FIG. 3 , and reach the terminal end 1715 in a state where eight linear electrode portions 1712 join together. At the terminal end 1715, the combined width of the eight electrodes is 20 μm, and the width of each electrode is 2.5 μm.

[0036] The thickness of the first electrode 171 is 20 μm at the window frame portion 1711 and 100 μm at the linear electrode portion 1712 ( FIG. 4 ). By making the linear electrode portion 1712 thicker than the window frame portion 1711 in this way, when the first electrode 171 is fabricated by a method involving heating, such as vapor deposition or plating, peeling of the first electrode 171 from the substrate 16 due to stress generated during cooling due to the difference in thermal expansion coefficients between the first electrode 171 and the substrate 16 and internal stress in the first electrode 171 can be prevented, while also suppressing the effect of voltage drop in the linear electrode portion 1712. This first electrode 171 can be easily fabricated by first forming the entire shape of the first electrode 171 by the thickness of the window frame portion 1711 using vapor deposition, and then forming only the linear electrode portion 1712 using plating.

[0037] A portion 1719 of the linear electrode portion 1712 near the tip on the window frame portion 1711 side may be formed so as to overlap with the window frame portion 1711 (see FIG. 4 ). In other words, a linear portion having a thickness greater than that of the window frame portion 1711 and equal to that of the linear electrode portion 1712 may be formed on the extension of the linear electrode portion 1712 on the window frame portion 1711 side. This prevents the cross-sectional area of ​​the linear electrode portion 1712 from becoming smaller at the boundary with the window frame portion 1711, making it possible to increase the current flowing through the linear electrode portion 1712 and to prevent heat generation due to increased electrical resistance at the boundary.

[0038] The outer edge of the window frame 1711 may have a shape other than a circle (non-circular), such as a square or other polygon, as shown in Fig. 5. In this case, the window 1713 may have a circular shape (Fig. 5) or a non-circular shape.

[0039] The second electrode 172 is made of a conductive material and is a circular electrode with a diameter smaller than the outer edge of the window frame portion 1711. The thickness of the second electrode 172 is 10 μm. By making the thickness of the second electrode 172 as thin as possible, peeling due to the effects of thermal expansion during manufacturing is also prevented.

[0040] (2) Operation of the Two-Dimensional Photonic Crystal Laser of This Embodiment Next, the operation of the two-dimensional photonic crystal laser 10 of this embodiment will be described. A voltage is applied between the first electrode 171 and the second electrode 172, with the first electrode 171 being negative. This causes a current to flow between the two electrodes, and electrons are injected into the active layer 11 from the first electrode 171, and holes are injected into the active layer 11 from the second electrode 172. Electrons are injected into the active layer 11 from the first electrode 171 not only through the window frame 1711 but also through the linear electrode portion 1712 within the window 1713. Because the substrate 16 is sufficiently thicker than the other layers, and the distance between the first electrode 171 and the active layer 11 is therefore sufficiently greater than the distance between the second electrode 172 and the active layer 11, the current (electrons and holes) is injected into a region (current injection region) within the active layer 11 that has approximately the same shape and size as the second electrode 172.

[0041] In this way, current is injected into the current injection region of the active layer 11, causing electrons and holes to combine, and light having a wavelength within a predetermined wavelength band that corresponds to the material of the active layer 11 is emitted from the current injection region. The emitted light thus generated is selectively amplified by the resonance of light with a resonance wavelength that corresponds to the periodic length of the square lattice within the two-dimensional photonic crystal layer 12, resulting in laser oscillation.

[0042] The oscillated laser beams are emitted from both surfaces of the two-dimensional photonic crystal layer 12 in directions perpendicular to the two-dimensional photonic crystal layer 12. Of these, the laser beams emitted toward the second electrode 172 are reflected by the second electrode 172 and directed toward the first electrode 171. The laser beams emitted directly from the two-dimensional photonic crystal layer 12 or directed toward the first electrode 171 after being reflected by the second electrode 172 pass through the window 1713 and are extracted to the outside of the two-dimensional photonic crystal laser 10. At this time, part of the laser beam is blocked by the linear electrode 1712, but the influence is suppressed because the ratio of the width of the linear electrode 1712 to the length of the linear electrode 1712 is sufficiently small even at the position where the width of the linear electrode 1712 is greatest.

[0043] In two-dimensional photonic crystal laser 10 of this embodiment, multiple linear electrode portions 1712 are provided within window portion 1713, and therefore the current density can be made more uniform within the current injection region of active layer 11 than in the case where a first electrode consisting only of a window frame portion and a window portion is used without such linear electrode portions 1712. Furthermore, because the width of linear electrode portion 1712 decreases from window frame portion 1711 toward terminal portion 1715, the current density can be made more uniform than in the case where multiple linear electrode portions of the same length and with the same width are provided.

[0044] Furthermore, in the two-dimensional photonic crystal laser described in Non-Patent Document 1, linear electrode portions of the same width are used, but linear electrode portions of different lengths are combined to make the current density nearly uniform, which causes the current density to change suddenly at the position within the current injection region corresponding to the position where the linear electrode portion ends.However, in this embodiment, the current density can be made nearly uniform without causing such a sudden change in current density depending on the position.

[0045] 6 is a graph showing the ratio of the area occupied by the linear electrode portion 1712 within the window portion 1713 (electrode occupancy rate) as a function of distance from the center (focus point 1714) of the window portion 1713 for this embodiment. The origin of the horizontal axis of this graph is the focus point 1714, and the right end (5 mm on the horizontal axis) is the end of the linear electrode portion 1712 on the window frame portion 1711 side. The electrode occupancy rate gradually increases from the window frame portion 1711 toward the end portion 1715, at which point it suddenly drops to zero.

[0046] When the linear electrode portion has a uniform width, as in the conventional two-dimensional photonic crystal laser described in Non-Patent Document 1, the voltage between each point in the linear electrode portion and the second electrode generally decreases from the window frame portion to the terminal end due to the electrical resistance of the linear electrode portion. Therefore, in this embodiment, the influence of the voltage change is compensated for by changing the electrode occupancy rate of the linear electrode portion 1712 from the window frame portion 1711 to the terminal end 1715 at a rate of change having an opposite sign (positive / negative) to the rate of change of the voltage, as shown in FIG. 6 . This makes it possible to more uniformly distribute the current density in the current injection region of the active layer 11. Note that the electrode occupancy rate may be uniform between the window frame portion 1711 and the terminal end 1715 if the voltage drop is made negligibly small by using a conductor with sufficiently low electrical resistivity as the material for the linear electrode portion 1712 or by making the linear electrode portion 1712 sufficiently thick.

[0047] Below, an example of a method for compensating for voltage drop will be described, in which the structure of the first electrode is more generalized than that of this embodiment. If the distance from the focal point is r, the number of linear electrode portions is N, and the width of the linear electrode portions is a function w(r) of the distance r from the focal point, then the electrode occupancy function η(r) with the distance r as a variable is given by η(r)=NW(r) / (2πr) (1). Solving this for W(r), we get W(r)=2πrη(r) / N (2). This gives the width function W(r) when a certain electrode occupancy function η(r) is set.

[0048] As an example of voltage drop, consider the case where the voltage between each position within the linear electrode and the second electrode decreases linearly from the window frame to the end. This voltage drop is compensated for by the change in electrode occupancy rate with position (distance r from the focal point). To do this, the electrode occupancy rate function η(r) is increased linearly from the window frame to the end. That is, η(r) can be expressed as η(r)=mr+b …(3). Here, because r decreases from the window frame to the end, the slope m of the linear function in equation (3) becomes negative. b is the intercept of the linear function. The values ​​of m and b can be set based on the magnitude of the voltage between the first and second electrodes, the thickness of the linear electrode, and other factors. From equations (2) and (3), we obtain W(r)=2πr(mr+b) / N …(4).

[0049] Based on equations (3) and (4), the electrode occupancy function η(r) and the width function W(r) are plotted in Figures 7A and 7B for three cases where m is -0.02, -0.04, and -0.06, with N = 80 and b = 0.4. The graph in Figure 7B shows that when m = -0.02 and -0.04, where the absolute value of m is relatively small and the voltage drop is relatively gradual, the width of the linear electrode portion can be monotonically decreased as r decreases, i.e., from the window frame to the terminal end. On the other hand, the graph also shows that when m = -0.06, where the absolute value of m is larger and the voltage drop is more rapid, the width of the linear electrode portion can be first increased and then decreased from the window frame to the terminal end. 7C and 7D show graphs of the electrode occupancy function η(r) and the width function W(r) for three cases where N = 80, m = -0.04, and b is 0.3, 0.4, and 0.5. In this case, too, depending on the value of b, it may be better to monotonically decrease the width of the linear electrode portion toward the terminal end (b = 0.4, 0.5), or it may be better to first increase it and then decrease it (b = 0.3).

[0050] As described above, the width of the linear electrode may be monotonically decreased from the window frame to the terminal end when the voltage drop is relatively gradual, but when the effect of the voltage drop is large, the width does not necessarily have to be monotonically decreased and may be increased from the window frame to the terminal end as necessary. Also, in the above example, the electrode occupancy rate is expressed as a linear function of the distance r, but it may be expressed as another function depending on the thickness of the linear electrode, etc.

[0051] Next, for two-dimensional photonic crystal laser 10 of this embodiment, the distribution of current density in the current injection region of active layer 11 was calculated, taking into account the effect of voltage drop shown in the graph of Fig. 8. For comparison, a similar calculation was also performed on a two-dimensional photonic crystal laser of a comparative example (the effect of voltage drop is different from that shown in Fig. 8) in which, instead of first electrode 171 in this embodiment, first electrode 971 has five types of linear electrode portions 97121 to 97125 with a uniform width but different lengths, located within window portion 9713 inside window frame portion 9711, as shown in Fig. 9.

[0052] Here, the two-dimensional photonic crystal laser of the comparative example will be further described. In this comparative example, the radius of the window portion 9713 is 5 mm. The following linear electrodes 97121 are arranged radially from the window frame portion 9711 toward the center of the window portion 9713: 10 linear electrodes 97121 each 4.9 mm long and 17 μm wide; 10 linear electrodes 97122 each 4.7 mm long and 17 μm wide; 20 linear electrodes 97123 each 4.0 mm long and 17 μm wide; 40 linear electrodes 97124 each 2.5 mm long and 17 μm wide; and 80 linear electrodes 97125 each 1.0 mm long and 12 μm wide. 9, linear electrode portion 97122 is shown by a dashed line so that it can be easily distinguished from the other linear electrode portions, but in reality, linear electrode portion 97122 has a linear shape that is continuously connected from window frame portion 9711 to its terminal end. When the electrode occupancy rate in this comparative example two-dimensional photonic crystal laser is expressed as a function of the distance from the center of window portion 9713, discontinuities occur at positions corresponding to the terminal ends of linear electrode portions 97121 to 97125, as shown in the graph in FIG.

[0053] The calculation results of the current density distribution in the current injection region are shown in FIG. 11 for the two-dimensional photonic crystal laser 10 of this embodiment and in FIG. 12 for the comparative example. In the comparative example, discontinuities in the current density also occur at positions corresponding to the terminal ends of each of the linear electrode portions 97121 to 97125. In contrast, in this embodiment, no positional discontinuities in the current density are observed, and a nearly uniform current density distribution is obtained that is almost independent of position. The calculation results of this embodiment show that discontinuities in the current density can be prevented by changing the width rather than the length of the linear electrode portions, and that a nearly uniform current density distribution can be obtained by gradually increasing the electrode occupancy rate from the window frame portion 1711 toward the terminal end 1715, thereby compensating for the effects of voltage drop.

[0054] (3) Example of Fabrication of a Two-Dimensional Photonic Crystal Laser According to the Present Embodiment Next, an example of fabricating a two-dimensional photonic crystal laser 10 according to the present embodiment will be described. FIG. 13 shows a photograph of the fabricated two-dimensional photonic crystal laser 10 taken from the first electrode 171 side. For size comparison, the two-dimensional photonic crystal laser 10 was photographed next to a 1-yen coin (20 mm diameter). The diameter of the window 1713 is 10 mm (radius 5 mm). FIG. 14 shows a photograph of this two-dimensional photonic crystal laser 10 taken from the window frame 1711 side, showing a state in which current is injected into the active layer 11 by passing a current between the first electrode 171 and the second electrode 172, and the active layer 11 emitting light before laser oscillation. This photograph shows that light is emitted with a nearly uniform intensity across the entire area visible from the window frame 1711 side. This demonstrates that current can be supplied to the current injection region of the active layer 11 with a nearly uniform current density distribution.

[0055] (4) Consideration of peeling of first electrode In this embodiment, as described above, the thickness of the window frame portion 1711 of the first electrode 171 was set to 20 μm, and the thickness of the linear electrode portion 1712 was set to 80 μm. However, when the thicknesses of the window frame portion 1711 and the linear electrode portion 1712 were both set to 80 μm, cracks occurred in the substrate 16 outside the outer edge of the window frame portion 1711 and at positions within the window portion 1713 close to the window frame portion 1711, as shown in Figure 15. This is thought to be because the first electrode 171 (particularly the window frame portion 1711) peeled off from the substrate 16 due to thermal expansion (contraction), and at that time, the surface vicinity of the substrate 16 was pulled up together with the first electrode 171, causing the cracks.

[0056] FIG. 16 shows the calculated distribution of stress generated in the first electrode 171 during manufacturing (cooling) for the cases where the window frame 1711 and the linear electrode 1712 are both 80 μm thick ((a) and (b)), and for the cases where the window frame 1711 is 20 μm thick and the linear electrode 1712 is 80 μm thick ((c) and (d)). The results are expressed as shades of color. (b) and (d) in FIG. 16 are enlarged views of portions of (a) and (c) (areas enclosed by rectangles in (a) and (c)). The black (or dark gray) portion represents the substrate 16 (including the portion visible through the window 1713), and the remaining light gray portion represents the window frame 1711 and the linear electrode 1712. The lighter the light gray color, the greater the stress generated within the window frame 1711 and the linear electrode 1712. From these figures, it can be seen that the stress generated in the first electrode 171 is smaller in (c) and (d) than in (a) and (b). In this way, by making the thickness of the window frame portion 1711 thinner than the thickness of the linear electrode portion 1712, the stress can be reduced, which is thought to make the first electrode 171 less likely to peel off.

[0057] In addition to the above-mentioned effect of preventing peeling, making the linear electrode portion 1712 thicker than the window frame portion 1711 also has the effect of reducing the amount of material used for the first electrode 171 because the window frame portion 1711 can be made thinner, thereby reducing material costs.

[0058] Although the 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 the above embodiments and various modifications are possible.

[0059] 10... Two-dimensional photonic crystal laser 11... Active layer 12... Two-dimensional photonic crystal layer 121... Base material 122... Modified refractive index area 1221... First modified refractive index area 1222... Second modified refractive index area 13... Spacer layer 141... First cladding layer 142... Second cladding layer 16... Substrate 171, 971... First electrode 1711, 9711... Window frame portion 1712, 97121 to 97125... Linear electrode portion 1713, 9713... Window portion 1714... Focusing point 1715... End portion 1716 to 1718... Position where two linear electrode portions join 1719... Part of the linear electrode portion near the tip on the window frame portion side 172... Second electrode

Claims

1. A two-dimensional photonic crystal laser comprising: a) an active layer; b) a two-dimensional photonic crystal layer arranged parallel to the active layer, in which modified refractive index areas having a refractive index different from that of the base material are periodically arranged in a two-dimensional pattern within a plate-shaped base material; and c) a first electrode and a second electrode arranged to sandwich the active layer and the two-dimensional photonic crystal layer in the thickness direction, wherein the first electrode has a window frame portion made of a conductive plate material, a window portion provided inside the window frame portion, and a plurality of linear electrode portions made of linear conductive members extending radially from the window frame portion toward a predetermined focal point within the window portion, between the window frame portion and the focal point or to an end portion located at the focal point, and the width of the linear electrode portions varies from the window frame portion toward the end portion.

2. The two-dimensional photonic crystal laser according to claim 1, wherein the width of said linear electrode portion decreases from said window frame portion toward said terminal end portion.

3. A two-dimensional photonic crystal laser as described in claim 1 or 2, characterized in that the electrode occupancy rate, which is expressed as a function of the distance from the focal point and is defined as the proportion of the circumference of a circle centered at the focal point and having the distance as its radius, that is occupied by the linear electrode portion, changes from the window frame side toward the terminal end side so as to have an opposite sign to the positive or negative sign of the rate of change of voltage between the second electrode.

4. A two-dimensional photonic crystal laser as described in claim 1 or 2, characterized in that two adjacent ones of the plurality of linear electrode portions meet at a position between the window frame portion and the terminal portion.

5. The two-dimensional photonic crystal laser according to claim 1 or 2, wherein the thickness of the linear electrode portion is greater than the thickness of the window frame portion.

6. A two-dimensional photonic crystal laser comprising: a) an active layer; b) a two-dimensional photonic crystal layer arranged parallel to the active layer, in a plate-shaped base material, in which modified refractive index areas having a refractive index different from that of the base material are periodically arranged in a two-dimensional pattern; and c) a first electrode and a second electrode arranged to sandwich the active layer and the two-dimensional photonic crystal layer in the thickness direction, wherein the first electrode has a window frame portion made of a conductive plate material, a window portion provided inside the window frame portion, and a plurality of linear electrode portions made of linear conductive members extending radially from the window frame portion toward a predetermined focal point within the window portion, between the window frame portion and the focal point or to an end portion located at the focal point, and the thickness of the linear electrode portions is thicker than the thickness of the window frame portion.

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