Photonic crystal and method for manufacturing photonic crystal waveguide

Abstract

A photonic crystal (10) according to the present invention, in which holes having a lower refractive index than a dielectric are disposed in a dielectric thin film, comprises: a region in which holes (1) are disposed at periodic positions; and a region in which holes (2) are disposed shifted from periodic positions, and which constitutes a photonic crystal waveguide (11) composed of a dielectric. In a portion of a prescribed surface (3) among surfaces present between two adjacent rows of holes, the holes disposed within a prescribed distance from the prescribed surface are shifted from the periodic positions in mirror symmetry with respect to the prescribed surface, and the amount by which the holes are shifted from the periodic positions decreases with increased distance from the prescribed surface.　Accordingly, the present invention can provide a photonic crystal that can improve the coupling efficiency between a photonic crystal waveguide and a photonic crystal optical device.

Description

Photonic crystal and method for manufacturing photonic crystal waveguide

[0001] The present invention relates to a photonic crystal and a method for manufacturing a photonic crystal waveguide that can improve coupling efficiency with a photonic crystal optical device.

[0002] With the recent advances in AI and IoT, the increasing power consumption of information and communication devices has become a problem. To reduce this power consumption, studies are underway to replace the electrical wiring between and within LSI chips with optical circuits. Photonic crystal lasers, which consume little power, are attracting attention for their use in the integration of optical circuits and LSIs and in future optical information processing devices.

[0003] Two-dimensional photonic crystal optical devices strongly confine light in the in-plane direction due to a photonic band gap caused by periodic refractive index modulation, and in the out-of-plane direction due to the refractive index difference between the upper and lower claddings. If SiO2 is used as the cladding material, heat dissipation and mechanical stability are improved compared to air, but the refractive index difference is smaller, so out-of-plane radiation is increased.

[0004] A photonic crystal line-defect waveguide is a basic photonic crystal optical device formed by removing a row of holes (e.g., holes) in a photonic crystal. A photonic crystal line-defect waveguide with glide symmetry (glide-symmetry waveguide, hereinafter referred to as a "glide-symmetric waveguide") has been reported (Non-Patent Document 1). In glide symmetry, as shown in FIG. 12, a combination of a mirror operation (arrow 51 in the figure) about a given plane 3 and a translation operation (arrow 52 in the figure) parallel to that plane 3 is called a glide operation, and the plane on which the operation is performed is called a glide plane (glide-plane) 3. A glide-symmetric waveguide is formed by removing a row of holes (dotted circle in the figure) in a photonic crystal to form a line-defect waveguide (FIG. 13A), and then translating one side of the photonic crystal by half a period (arrow in the figure) (FIG. 13B). In a glide-symmetric waveguide, the modes are out of phase on either side of the waveguide, resulting in destructive interference in the glide plane, which can reduce the out-of-plane radiation loss of photonic crystal waveguides on high-index claddings such as SiO2.

[0005] A. Mock, L. Lu, and J. O'Brien, “Space group theory and Fourier space analysis of two-dimensional photonic crystal waveguides,” Phys. Rev. B 81(15), 155115 (2010).

[0006] However, when a glide-symmetric waveguide is coupled not only by itself but also with other photonic crystal optical devices such as a photonic crystal optical resonator, a problem of reduced coupling efficiency has been posed. For example, when a glide plane is introduced into a system in which a photonic crystal optical resonator and a photonic crystal waveguide are coupled, a method of translating one photonic crystal by half a period generates a guided mode due to a disruption of the periodicity (defect) caused by a half-period shift in the positions of the holes forming the photonic crystal. As a result, the coupling efficiency between the photonic crystal optical resonator and the photonic crystal waveguide decreases.

[0007] In order to solve the above-mentioned problems, the photonic crystal of the present invention is a photonic crystal in which holes having a refractive index lower than that of the dielectric are arranged in a dielectric thin film, and comprises a region in which the holes are arranged at periodic positions and a region in which the holes are arranged shifted from the periodic positions and form a photonic crystal waveguide made of the dielectric, and in which, in a part of a predetermined surface among surfaces present between two adjacent rows of the holes, the holes arranged within a predetermined distance from the predetermined surface are shifted from the periodic positions in a mirror-symmetric manner with respect to the predetermined surface, and the amount by which the holes are shifted from the periodic positions decreases with increasing distance from the predetermined surface.

[0008] Furthermore, a method for manufacturing a photonic crystal waveguide according to the present invention comprises the steps of: determining a portion of a predetermined plane between adjacent rows of holes in a photonic crystal in which holes are arranged at periodic positions; determining the positions of the holes by shifting the holes arranged within a predetermined distance from the portion of the predetermined plane from the periodic positions so that the holes are on both sides of the portion of the predetermined plane, with the portion of the predetermined plane as a boundary, and are symmetrical with respect to the portion of the predetermined plane; and processing the material of the photonic crystal based on the positions of the holes, wherein the amount by which the holes are shifted from the periodic positions decreases as they move away from the predetermined plane.

[0009] According to the present invention, it is possible to provide a photonic crystal and a method for manufacturing a photonic crystal waveguide that can improve the coupling efficiency between the photonic crystal waveguide and a photonic crystal optical device.

[0010] FIG. 1 is a schematic diagram showing the configuration of a photonic crystal according to a first embodiment of the present invention. FIG. 2 is a schematic diagram showing the configuration of a photonic crystal waveguide according to the first embodiment of the present invention. FIG. 3 is a diagram for explaining the configuration of a photonic crystal waveguide according to the first embodiment of the present invention. FIG. 4A is a diagram for explaining a method for manufacturing a photonic crystal waveguide according to the first embodiment of the present invention. FIG. 4B is a diagram for explaining a method for manufacturing a photonic crystal waveguide according to the first embodiment of the present invention. FIG. 4C is a diagram for explaining a method for manufacturing a photonic crystal waveguide according to the first embodiment of the present invention. FIG. 5A is a diagram for explaining the effect of a photonic crystal waveguide according to the first embodiment of the present invention. FIG. 5B is a diagram for explaining the effect of a photonic crystal waveguide according to the first embodiment of the present invention. FIG. 5C is a diagram for explaining the effect of a photonic crystal waveguide according to the first embodiment of the present invention. FIG. 6A is a diagram for explaining the effect of a photonic crystal waveguide according to the first embodiment of the present invention. FIG. 6B is a diagram for explaining the effect of a photonic crystal waveguide according to the first embodiment of the present invention. FIG. 6C is a diagram for explaining the effect of a photonic crystal waveguide according to the first embodiment of the present invention. Fig. 7 is a schematic diagram showing the configuration of a photonic crystal according to a first embodiment of the present invention. Fig. 8 is a schematic diagram showing an example of the configuration of a photonic crystal according to the first embodiment of the present invention. Fig. 9 is a schematic diagram showing an example of the configuration of a photonic crystal according to the first embodiment of the present invention. Fig. 10 is a schematic diagram showing an example of the configuration of a photonic crystal according to the first embodiment of the present invention. Fig. 11 is a schematic diagram showing an example of the configuration of a photonic crystal according to the first embodiment of the present invention. Fig. 12 is a diagram for explaining a conventional glide operation. Fig. 13A is a diagram for explaining a conventional method for manufacturing a waveguide having glide symmetry. Fig. 13B is a diagram for explaining a conventional method for manufacturing a waveguide having glide symmetry.

[0011] First Embodiment A photonic crystal and a method for manufacturing a photonic crystal waveguide according to a first embodiment of the present invention will be described with reference to FIGS. 1 to 6C.

[0012] <Configuration of Photonic Crystal Waveguide> As shown in FIG. 1, a photonic crystal 10 according to this embodiment includes a photonic crystal waveguide 11 and a photonic crystal optical resonator 12.

[0013] The photonic crystal 10 has a region in which holes 1 are periodically arranged in a dielectric thin film slab and a region in which holes 2 are arranged at positions shifted from the periodic positions. The holes 1 and 2 are cylindrical or polygonal prism-shaped and made of a material (e.g., air) with a lower refractive index than the dielectric thin film slab.

[0014] In a region of photonic crystal 10 where holes 1 are periodically arranged, a photonic crystal optical resonator 12 is formed by a line (line defect) where some holes are not formed.

[0015] In a region where holes 2 in photonic crystal 10 are shifted from their periodic positions, photonic crystal waveguide 11 is formed by shifting holes 2. Photonic crystal waveguide 11 is arranged in the Γ-M direction (dotted arrow in the figure) of photonic crystal optical resonator 12, and is optically coupled to photonic crystal optical resonator 12.

[0016] FIG. 2 shows an enlarged view of the periphery of the photonic crystal waveguide 11.

[0017] The photonic crystal waveguide 11 is formed by shifting the holes 2 from their periodic positions.

[0018] In detail, the photonic crystal waveguide 11 is formed by shifting the holes 2 in a direction perpendicular to the plane (glide plane, hereinafter referred to as the "glide plane") 3 formed between the periodically arranged holes 2 (arrow in the figure), to a position that is mirror-symmetrical with respect to the glide plane 3.

[0019] The amount of shift of holes 2 from the periodic arrangement positions in photonic crystal waveguide 11 decreases with increasing distance from glide plane 3. The holes are arranged at a predetermined distance from glide plane 3 in the periodic arrangement positions.

[0020] An example of the arrangement of holes 2 that constitute photonic crystal waveguide 11 will be described below.

[0021] In the rows of holes 2 that are shifted when forming the photonic crystal waveguide 11, as shown in Figure 3, the row C1 of holes that is farthest from the glide plane 3 is designated as the first row, and the row Cn of holes that is closest to the glide plane 3 is designated as the nth row.

[0022] In photonic crystal 10, the distance between rows of periodically arranged holes 1 is defined as W / 2. The width of photonic crystal waveguide 11 (the length perpendicular to glide plane 3), i.e., the distance between two rows of holes 2 adjacent to photonic crystal waveguide 11, is defined as tW. Of the holes 2 that make up photonic crystal waveguide 11, the distance between the hole 2 closest to glide plane 3 and glide plane 3 is tW / 2.

[0023] If the spacing between rows of holes 2 in photonic crystal waveguide 11 decreases linearly with increasing distance from glide plane 3, the row of holes arranged farthest from glide plane 3 within the shift range is defined as the first row, and the row of holes closest to glide plane 3 is defined as the nth row, then the shift amount S of the holes from the first row to the mth row is ｍ The initial value is W / 2, and the shift amount Sm of the holes in the m-th column is expressed by the formula (1).

[0024]

[0025] In this manner, holes 2 that constitute photonic crystal waveguide 11 are arranged in photonic crystal 10 .

[0026] In this embodiment, an example has been shown in which the interval between rows of holes decreases linearly as the distance from the glide plane increases, but this is not limiting. The interval between rows of holes may decrease using a quadratic function or an exponential function as the distance from the glide plane increases. Conversely, the interval between rows of holes may be arranged so that it decreases as the distance from the glide plane increases.

[0027] <Method for Manufacturing Photonic Crystal Waveguide> A method for manufacturing the photonic crystal waveguide 11 according to this embodiment will be described with reference to FIGS. 4A to 4C.

[0028] First, the photonic crystal waveguide 11 is designed. First, the glide planes 3 are determined in the photonic crystal as shown in Fig. 4A. As shown in Fig. 4B, many glide planes 3 exist between adjacent rows of holes (indicated by dotted lines in the figure). Some of these many glide planes 3 are selected.

[0029] Next, Hole 2 (the shaded circle in the figure) is shifted to both sides of glide plane 3, with glide plane 3 as the boundary, so that it is symmetrical with respect to glide plane 3. Hole 2 is shifted so that the shift amount of Hole 2 decreases as it moves away from glide plane 3.

[0030] Here, holes 3 (dotted circle in the drawing) that are shifted by a distance smaller than the above-mentioned shift amount may be placed at the boundary between the shifted hole row and the non-shifted hole row.

[0031] Finally, holes 1 to 3 constituting photonic crystal waveguide 11 designed as described above are formed by processing the photonic crystal material using a normal photonic crystal manufacturing process, thereby manufacturing photonic crystal waveguide 11.

[0032] As described above, in this embodiment, rather than defining a glide plane for a line-defect waveguide, attention is focused on the inherent periodicity of the photonic crystal, and a waveguide with glide symmetry (glide symmetric waveguide) is formed by cutting and widening along the glide planes that exist between holes.

[0033] <Effects> The effects of the photonic crystal 10 according to this embodiment will be described with reference to FIGS. 5A to 6C.

[0034] The magnetic field strength distribution was calculated for the photonic crystal 10 having the photonic crystal waveguide 11. Fig. 5A shows the calculation results of the magnetic field strength distribution in the photonic crystal having the photonic crystal waveguide 11. Fig. 5B shows the magnetic field strength on the X1-X2 axis and the Y1-Y2 axis in Fig. 5A. Fig. 5C shows an enlarged view of the periphery of the photonic crystal waveguide 11 in the calculation results of the magnetic field strength distribution.

[0035] The calculation was performed using the software "COMSOL Multiphysics" (COMSOL).

[0036] The photonic crystal 10 was configured with holes (air) periodically arranged in InP. The spacing between holes (hole period) was 425 nm, and the distance W / 2 between rows of periodically arranged holes was 368 nm. In the photonic crystal 10, a photonic crystal waveguide 11 was formed at a position in the Γ-M direction of the photonic crystal optical resonator 12.

[0037] For comparison, similar calculation results are shown for a conventional waveguide with glide symmetry (glide-symmetric waveguide). Fig. 6A shows the calculation results for the magnetic field strength distribution in a photonic crystal with a conventional glide-symmetric waveguide. Fig. 6B shows the magnetic field strength along the X1-X2 axis and the Y1-Y2 axis in Fig. 6A. Fig. 6C shows an enlarged view of the periphery of the conventional glide-symmetric waveguide in the calculation results for the magnetic field strength distribution.

[0038] In a configuration having a conventional translation-symmetric waveguide, as shown in FIG. 6C, the hole in the dotted line region is shifted by half a period (translation, indicated by the black arrow) to the right in the figure.

[0039] In this configuration, as shown in Fig. 6A, light propagates from photonic crystal optical resonator 12 in four Γ-M directions, and one of the propagating light beams is optically coupled to glide-symmetric waveguide 110. At this time, as shown in Fig. 6B, light (magnetic) is distributed along the Y1-Y2 axis (glide-symmetric waveguide) (solid lines in the figure), and light (magnetic) is also distributed along the X1-X2 axis (dotted lines in the figure).

[0040] In this way, light from the photonic crystal optical resonator 12 is optically coupled to the glide-symmetric waveguide 110 and propagates therethrough (indicated by the white dotted arrow 5 in the figure), and due to the deviation (defect) in the periodicity caused by the half-period shift of the holes, the light also propagates downward in the photonic crystal optical resonator (indicated by the white dotted arrow 6 in the figure).

[0041] 5C , in the configuration including photonic crystal waveguide 11, the holes in the dotted-line region are shifted vertically (indicated by the black arrows in the figure). The width tW of photonic crystal waveguide 11 (the length perpendicular to glide plane 3) was set to 736 nm. In forming photonic crystal waveguide 11, the range of shifted hole rows (n rows) was limited to the sixth row, and the amount of hole shift was assumed to decrease linearly with increasing distance from glide plane 3, and the amount of shift was determined using Equation (1).

[0042] In this configuration, as shown in Fig. 5A, light propagates from photonic crystal optical resonator 12 in four Γ-M directions, and one of the propagating light beams is optically coupled to photonic crystal waveguide 11. At this time, as shown in Fig. 5B, light (magnetic) is distributed only along the Y1-Y2 axis (gliding symmetric waveguide) (solid line in the figure), and light (magnetic) is not distributed along the X1-X2 axis (dotted line in the figure).

[0043] In this way, light from the photonic crystal optical resonator 12 is optically coupled only to the photonic crystal waveguide 11 and propagates therethrough (in the figure, the white dotted arrow 5).

[0044] According to the photonic crystal of this embodiment, it is possible to improve the coupling efficiency between the photonic crystal waveguide arranged in the photonic crystal and the photonic crystal optical device.

[0045] First Example A photonic crystal according to a first example of the present invention will be described with reference to FIGS.

[0046] <Configuration of Photonic Crystal> As shown in the figure, the photonic crystal according to this example includes the photonic crystal waveguide 11 according to the first embodiment and a photonic crystal optical device 12.

[0047] An L3 resonator 12 is used as the photonic crystal optical device.

[0048] The photonic crystal waveguide 11 is disposed in the Γ-M direction of the L3 resonator 12 so as to be optically coupled to the L3 resonator 12 .

[0049] 8, a photonic crystal optical device may be used in which an L3 resonator 12 including a buried heterojunction (BH) active layer 221 is included. In this configuration, light oscillated in the L3 resonator 12 including the BH active layer 221 by optical excitation or electric field excitation is optically coupled to the photonic crystal waveguide 11 and propagates therethrough.

[0050] 9, a photonic crystal optical device may be a line-defect waveguide 321 including a buried heterojunction (BH) active layer 221. In this configuration, light emitted in the BH active layer 221 by optical excitation or electric field excitation is optically coupled to the photonic crystal waveguide 11 and propagates therethrough.

[0051] 10 , a photonic crystal optical device may use an Ln resonator 421 such as an L3 resonator including a buried heterojunction (BH) active layer 221 and electrodes. In this configuration, light oscillated in the Ln resonator 421 including the BH active layer 221 by a current injected via a p-type semiconductor (e.g., InP) 422, an n-type semiconductor (e.g., InP) 423, a p-type electrode 424, and an n-type electrode 425 is optically coupled to the photonic crystal waveguide 11 and propagates therethrough.

[0052] 11, a photonic crystal optical device may be used in which a line-defect waveguide 321 including a buried heterojunction (BH) active layer 221 and electrodes is included. In this configuration, light emitted from the BH active layer 221 by a current injected via a p-type semiconductor (e.g., InP) 422, an n-type semiconductor (e.g., InP) 423, a p-type electrode 424, and an n-type electrode 425 is optically coupled to the photonic crystal waveguide 11 and propagates therethrough.

[0053] In the embodiments of the present invention, examples of the structure, dimensions, materials, etc. of each component in the configuration and manufacturing method of the photonic crystal and the photonic crystal waveguide are shown, but the present invention is not limited to these. Any method may be used as long as it can demonstrate the function and effect of the method for manufacturing the photonic crystal and the photonic crystal waveguide.

[0054] It should be noted that the present invention is not limited to the above-described embodiments, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0055] A part or all of the above-described embodiment or an example thereof can be described as, but is not limited to, the following supplementary notes.

[0056] (Supplementary Note 1) A photonic crystal in which holes having a refractive index lower than that of the dielectric are arranged within a dielectric thin film, the photonic crystal comprising: a region in which the holes are arranged at periodic positions; and a region in which the holes are arranged shifted from the periodic positions and form a photonic crystal waveguide made of the dielectric; wherein, in a part of a predetermined surface among surfaces present between two adjacent rows of the holes, the holes arranged within a predetermined distance from the predetermined surface are shifted from the periodic positions in a mirror-symmetric manner with respect to the predetermined surface, and the amount by which the holes are shifted from the periodic positions decreases with increasing distance from the predetermined surface.

[0057] (Supplementary Note 2) The photonic crystal according to Supplementary Note 1, wherein the interval between the rows of holes arranged at the periodic positions is W / 2, the interval between the rows of holes adjacent to the photonic crystal waveguide is tW, the row of holes arranged at a predetermined distance from the predetermined surface is the first row, and the row of holes closest to the predetermined surface is the nth row, the shift amount Sm of the holes from the first row to the mth row is expressed by formula (A).

[0058] (Supplementary Note 3) The photonic crystal according to Supplementary Note 1 or Supplementary Note 2, comprising a photonic crystal optical device optically coupled to the photonic crystal waveguide, the photonic crystal waveguide being arranged in the Γ-M direction of the photonic crystal optical device.

[0059] (Supplementary Note 4) A method for manufacturing a photonic crystal waveguide, comprising: a step of determining a portion of a predetermined plane between adjacent rows of holes in a photonic crystal in which holes are arranged at periodic positions; a step of shifting the holes, which are arranged within a predetermined distance from the portion of the predetermined plane, from the periodic positions so that the holes are symmetrical with respect to the portion of the predetermined plane, on both sides of the portion of the predetermined plane, with the portion of the predetermined plane as a boundary, to determine the positions of the holes; and a step of processing the material of the photonic crystal based on the positions of the holes, wherein the amount by which the holes are shifted from the periodic positions decreases with increasing distance from the predetermined plane.

[0060] The present invention relates to a photonic crystal, and can be applied to optical information processing devices, information and communication equipment, optical communication systems, and optical computers.

[0061] 1, 2 Hole 3 Glide plane 10 Photonic crystal 11 Photonic crystal waveguide

Claims

1. A photonic crystal in which holes having a lower refractive index than the dielectric are arranged within a dielectric thin film, comprising: a region in which the holes are arranged at periodic positions; and a region in which the holes are arranged shifted from the periodic positions to form a photonic crystal waveguide made of the dielectric; wherein, in a part of a predetermined surface among surfaces lying between two adjacent rows of holes, the holes arranged within a predetermined distance from the predetermined surface are shifted from the periodic positions in a mirror-symmetric manner with respect to the predetermined surface, and the amount by which the holes are shifted from the periodic positions decreases with increasing distance from the predetermined surface.

2. The photonic crystal of claim 1, wherein the interval between the rows of holes arranged at the periodic positions is W / 2, the interval between the rows of holes adjacent to the photonic crystal waveguide is tW, the row of holes arranged at a predetermined distance from the predetermined surface is the first row, and the row of holes closest to the predetermined surface is the nth row, the shift amount Sm of the holes from the first row to the mth row is expressed by formula (A).

3. The photonic crystal according to claim 1 or 2, further comprising a photonic crystal optical device optically coupled to said photonic crystal waveguide, said photonic crystal waveguide being disposed in the Γ-M direction of said photonic crystal optical device.

4. A method for manufacturing a photonic crystal waveguide, comprising: a step of determining a portion of a predetermined plane between adjacent rows of holes in a photonic crystal in which holes are arranged at periodic positions; a step of shifting the holes, which are arranged within a predetermined distance from the portion of the predetermined plane, from the periodic positions so that the holes are on both sides of the portion of the predetermined plane, with the portion of the predetermined plane as a boundary, and are symmetrical with respect to the portion of the predetermined plane, to determine the positions of the holes; and a step of processing the material of the photonic crystal based on the positions of the holes, wherein the amount by which the holes are shifted from the periodic positions decreases with increasing distance from the predetermined plane.

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