Optical waveguide and method of manufacturing optical waveguide

Abstract

An optical waveguide 100 includes a substrate 30, and a waveguide portion 10 provided on the substrate 30, having a color center 40, and through which light propagates, wherein the waveguide portion has a recess 18 on at least one of a surface 14 and a surface 15 facing each other of the waveguide portion such that an absolute value of a difference between a shortest distance L1 between the surface 14 and the color center 40 and a shortest distance L2 between the surface 15 and the color center 40 becomes small. Thereby, the symmetry of the magnetic field distribution generated by the propagation of the light emitted from the color center 40 through the waveguide portion 10 is improved, so that the propagation loss of the light can be reduced.　

Description

OPTICAL WAVEGUIDE AND METHOD OF MANUFACTURING OPTICAL WAVEGUIDE

[0001] A certain aspect of the embodiments is related to an optical waveguide and a method of manufacturing an optical waveguide, and more particularly to the optical waveguide and the method for manufacturing the optical waveguide which are usable in a quantum computer using a photon as a quantum bit.

[0002] The quantum computer is studied as a next generation computer. As one of the techniques of the quantum computer, there is a method of processing photons as quantum bits. For example, there is known a quantum computer in which a color center is formed by ion-implanting silicon into a waveguide portion made of diamond, and fluorescent photons emitted from the color center are used (for example, NPL 1).

[0003] Further, as a source for generating a single photon, there is known a single photon generation source in which a deep localized level is formed by doping the surface of a carbon tube with oxygen or the like so that an exciton is localized at a temperature higher than 50K (for example, PTL 1). Further, there is known a single photon generation source using a nitrogen defect color center of a diamond crystal (for example, PTL 2).

[0004] PTL 1: Japanese Laid-open Patent Publication No. 2017-175096 PTL 2: Japanese Laid-open Patent Publication No. 2007-274108

[0005] NPL 1: A. Sipahigil et al., “An integrated diamond nanophotonics platform for quantum-optical networks”, Scienc, November 18, 2016, vol. 354, pp. 847 to 850

[0006] When the color center is formed in the waveguide portion, the formation position of the color center varies, and this may increase propagation loss when the light emitted from the color center propagates in the waveguide portion.

[0007] Accordingly, it is an object of one aspect to reduce the propagation loss of the light.

[0008] In one aspect of embodiments, there is provided an optical waveguide including a substrate; and a waveguide portion provided on the substrate, having a color center, and through which light propagates; wherein the waveguide portion has a recess on at least one of a first surface and a second surface facing each other of the waveguide portion such that an absolute value of a difference between a first shortest distance between the first surface and the color center and a second shortest distance between the second surface and the color center becomes small.

[0009] In one aspect of embodiments, there is provided a method of manufacturing an optical waveguide comprising: forming a waveguide portion through which light propagates on a substrate; forming a color center in the waveguide portion; and forming a recess on at least one of a first surface and a second surface facing each other of the waveguide portion so that an absolute value of a difference between a first shortest distance between the first surface and the color center and a second shortest distance between the second surface and the color center becomes small.

[0010] As one aspect, the propagation loss of the light can be reduced.

[0011] FIG. 1A is a plan view of an optical waveguide according to a first embodiment, and FIG. 1B is a cross-sectional view taken along line A-A of FIG. 1A.FIGs. 2A to 2F are views illustrating a method of manufacturing the optical waveguide according to the first embodiment (part 1).FIGs. 3A to 3D are views illustrating a method of manufacturing the optical waveguide according to the first embodiment (part 2).FIG. 4A is a plan view of an optical waveguide according to a comparative example, and FIG. 4B is a cross-sectional view taken along the line A-A in FIG. 4A.FIG. 5 is a plan view of a model 1 used in a simulation.FIG. 6 is a plan view of a model 2 used in the simulation.FIG. 7 is a view illustrating a simulation result of a magnetic field distribution in the model 1.FIG. 8 is a view illustrating a simulation result of a magnetic field distribution in the model 2.FIGs. 9A to 9D are plan views of the optical waveguide according to first to fourth modifications of the first embodiment.FIG. 10A is a plan view of an optical waveguide according to a second embodiment, FIG. 10B is a cross-sectional view taken along line A-A in FIG. 10A, and FIG. 10C is a cross-sectional view taken along line B-B in FIG. 10A.FIG. 11 is a plan view of a model 3 used in the simulation.FIG. 12 is a view illustrating a simulation result of a magnetic field distribution in the model 3.FIG. 13A is a plan view of an optical waveguide according to a third embodiment, and FIG. 13B is a cross-sectional view taken along line A-A of FIG. 13A;FIGs. 14A to 14F are views illustrating a method of manufacturing an optical waveguide according to a third embodiment.FIG. 15A is a plan view of an optical waveguide according to a modification of the third embodiment, and FIG. 15B is a cross-sectional view taken along line A-A of FIG. 15A.FIG. 16A is a plan view of an optical waveguide according to a fourth embodiment, and FIG. 16B is a cross-sectional view taken along line A-A of FIG. 16A.

[0012] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. (First Embodiment)

[0013] FIG. 1A is a plan view of an optical waveguide 100 according to a first embodiment, and FIG. 1B is a cross-sectional view taken along line A-A of FIG. 1A. In FIG. 1A, for the sake of clarity of the drawing, hatching is given to a portion other than holes 12 provided in a waveguide portion 10 (the same applies to the following similar drawings). As illustrated in FIGs. 1A and 1B, the waveguide portion 10 is provided on a substrate 30 via an intermediate layer 32. The substrate 30 is a silicon substrate, for example, and the intermediate layer 32 is a silicon oxide (SiO2) layer, for example. The substrate 30 may be a substrate other than a silicon substrate, for example, a sapphire substrate, an alumina substrate, a quartz substrate, a silicon carbide substrate, or a diamond substrate, for example, an inorganic material substrate or an organic material substrate. The intermediate layer 32 may be a layer other than the silicon oxide layer, for example, a silicon nitride layer, an aluminum oxide layer, a silicon layer, or a diamond layer.

[0014] A longitudinal direction of the waveguide portion 10 is defined as an X direction, a lateral direction (width direction) of the waveguide portion 10 is defined as a Y direction, and a thickness direction of the waveguide portion 10 is defined as a Z direction. The length of the waveguide portion 10 in the X direction is, for example, about 1 μm to 50 μm, the length of the waveguide portion 10 in the Y direction is, for example, about 100 nm to 500 nm, and the thickness of the waveguide portion 10 in the Z direction is, for example, about 100 nm to 1000 nm. A gap 34 is formed between the waveguide portion 10 and the substrate 30. Therefore, the waveguide portion 10 is surrounded by air. The waveguide portion 10 is made of single-crystal diamond, for example. Since the waveguide portion 10 is a core whose periphery is covered with air serving as a clad, light propagates in the waveguide portion 10. A film made of a material having a refractive index lower than that of the waveguide portion 10, such as a silicon oxide film, may be provided in the gap 34.

[0015] The waveguide portion 10 includes the plurality of holes 12 spaced apart in the X direction. The holes 12 penetrate the waveguide portion 10 and are connected to the gap 34. By providing the holes 12 in the waveguide portion 10, the waveguide portion 10 becomes a photonic crystal waveguide portion in which the refractive index repeatedly changes in the X direction at a period approximately equal to the wavelength of light propagating inside the waveguide portion. In other words, the optical waveguide 100 is a hollow nanobeam resonator. The holes 12 may be filled with a material having a refractive index different from that of the diamond forming the waveguide portion 10 and having a lower refractive index than that of the diamond.

[0016] In the waveguide portion 10, a color center 40 is formed near the center in a plan view. The color center 40 is formed by lattice defects (crystal defects) formed by ion implantation of atoms of nitrogen, silicon, tin, germanium, or the like into the diamond forming the waveguide portion 10. The color center 40 is formed by, for example, an NV center which is a complex defect of nitrogen (N) substituted for carbon and vacancy (V) at adjacent positions, or an SiV center which is a complex defect of silicon (Si) substituted for carbon and vacancy (V) at the adjacent positions. Alternatively, the color center 40 is formed by, for example, an SnV center which is a composite defect of tin (Sn) substituted for carbon and vacancy (V) at the adjacent positions, or a GeV center which is a composite defect of germanium (Ge) substituted for carbon and vacancy (V) at the adjacent positions. A space between the holes 12 at the position where the color center 40 is formed is wider than a space between the holes 12 at the position where the color center 40 is not formed from the viewpoint of characteristics, and is, for example, twice as wide.

[0017] Although it is preferable that the color center 40 is formed at the center of the waveguide portion 10 in a plan view, when the color center 40 is formed by ion implantation, the formation position thereof varies within a range of several tens of nm. If the color center 40 is deviated from the center of the waveguide portion 10 in the Y direction, the propagation loss of the light emitted from the color center 40 becomes large. Therefore, a recess 18 is formed on at least a side surface 15 farthest from the color center 40 among the side surfaces 14 and 15 facing each other in the Y direction of the waveguide section 10, such that the absolute value of the difference between a shortest distance L1 between a side surface 14 and the color center 40 and a shortest distance L2 between the side surface 15 and the color center 40 becomes small. By forming the recess 18, the shortest distance L2 is 0.8 times or more and 1.2 times or less the shortest distance L1.

[0018] The recess 18 is formed so as to overlap the color center 40 in the Y direction, and has a shape of an elliptical arc or an arc in a plan view, for example. A portion 20 closest to the color center 40 of the recess 18 is located on a straight line 42 which passes through the color center 40 and is parallel to the Y direction, for example. The recess 18 is formed symmetrically with respect to the straight line 42, for example.

[0019] (Manufacturing Method of First Embodiment) FIGs. 2A to 3D are views illustrating a method of manufacturing the optical waveguide 100 according to the first embodiment. FIGs. 2A, 2C, 2E, 3A and 3C are plan views illustrating a method of manufacturing the optical waveguide 100 according to the first embodiment. FIGs. 2B, 2D, 2F, 3 B and 3D are cross-sectional views taken along line A-A of FIGs. 2A, 2C, 2E, 3A and 3C. In the plan view, a diamond substrate 50, a mask layer 56, the waveguide portion 10 and the substrate 30 are hatched for clarity of the drawings.

[0020] As illustrated in FIGs. 2A and 2B, after a first layer 52 is formed on one surface of the single-crystal diamond substrate 50, the first layer 52 is bonded to a second layer 54 provided on the substrate 30. As a result, the diamond substrate 50 is formed on the substrate 30 via the intermediate layer 32 including the first layer 52 and the second layer 54. The first layer 52 and the second layer 54 may be bonded by, for example, a direct bonding method, or may be bonded via a metal layer, an alloy layer, or an insulating layer. Thereafter, the diamond substrate 50 is thinned to a desired thickness by dry etching and chemical mechanical polishing. Mechanical polishing, laser cutting, ion cutting, wet etching, or cleavage processing other than the dry etching. may be used.

[0021] As illustrated in FIGs. 2C and 2D, the mask layer 56 is formed on the diamond substrate 50 to cover a region where the waveguide portion 10 is to be formed. In the mask layer 56, openings 58 corresponding to the positions where the holes 12 are formed are formed.

[0022] As illustrated in FIGs. 2E and 2F, the diamond substrate 50 is etched using, for example, inductively coupled plasma reactive ion etching (ICP-RIE) using the mask layer 56 as a mask. Thereafter, the intermediate layer 32 is etched by using, for example, ICP-RIE using the mask layer 56 as the mask while the substrate 30 is tilted obliquely. Thereafter, the mask layer 56 is removed. Thereby, the waveguide portion 10 composed of the diamond substrate 50 and having the plurality of holes 12 formed therein, and the gap 34 between the substrate 30 and the waveguide portion 10 are formed.

[0023] As illustrated in FIGs. 3A and 3B, atoms of nitrogen, silicon, tin, germanium, or the like are ion-implanted from above the waveguide portion 10 toward the center of the waveguide portion 10. As a method of ion implantation, for example, a focused ion beam (FIB) method is used. Thereby, the color center 40 is formed near the center of the waveguide portion 10. The color center (40) is, for example, the NV center which is the complex defect of nitrogen (N) substituted for carbon and vacancy (V) at the adjacent positions, the SiV center which is the complex defect of silicon (Si) substituted for carbon and vacancy (V) at the adjacent positions, the SnV center which is the composite defect of tin (Sn) substituted for carbon and vacancy (V) at the adjacent positions, or the GeV center which is the composite defect of germanium (Ge) substituted for carbon and vacancy (V) at the adjacent positions. When the color center 40 is formed by ion implantation of atoms into the waveguide portion 10, the formation position of the color center 40 may vary within a range of several tens of nm and may be formed out of alignment with the center of the waveguide portion 10 in the X direction and / or the Y direction.

[0024] As illustrated in FIGs. 3C and 3D, an optical technique such as a confocal laser microscope is used to specify the position of the color center 40 in the waveguide portion 10. The recess 18 is formed at a position corresponding to the color center 40 in the X direction in the side surface 15 of the waveguide portion 10. Thereby, the recess 18 overlapping with the color center 40 in the Y direction is formed. The formation of the recess 18 reduces the absolute value of the difference between the shortest distance L1 between the color center 40 and the side surface 14 of the waveguide portion 10 and the shortest distance L2 between the color center 40 and the side surface 15 of the waveguide portion 10. The recess 18 is formed, for example, by FIB processing.

[0025] (Comparative Example) FIG. 4A is a plan view of an optical waveguide 500 according to the comparative example, and FIG. 4B is a cross-sectional view taken along line A-A of FIG. 4A. As illustrated in FIGs. 4A and 4B, in the optical waveguide 500 according to the comparative example, the recess 18 is not formed on the side surface 15 of the waveguide portion 10. Since other configurations are the same as those of the first embodiment, description thereof will be omitted.

[0026] (Simulation) In the optical waveguides according to the first embodiment and the comparative example, a magnetic field distribution generated by the propagation of the light emitted from the color center 40 through the waveguide portion 10 was simulated. FIG. 5 is a plan view of the model 1 used in the simulation. FIG. 6 is a plan view of the model 2 used in the simulation. As illustrated in FIG. 5, the model 1 is a model corresponding to the optical waveguide 500 according to the comparative example, and no recess is provided on the side surface 15 of the waveguide portion 10. The color center 40 in the waveguide portion 10 is deviated from the center O of the waveguide portion 10 by 50 nm in the X direction and by 50 nm in the Y direction. As illustrated in FIG. 6, the model 2 is a model corresponding to the optical waveguide 100 according to the first embodiment, and the recess 18 is provided on the side surface 15 of the waveguide portion 10. The recess 18 is formed so as to overlap with the color center 40 in the Y direction and has an elliptic arc shape in the plan view. The recess 18 has a symmetrical shape with respect to the straight line 42 that passes through the color center 40 and is parallel to the Y direction. A depth D of the recess 18 is 40 nm, and a width W is 200 nm. Since the other configuration is the same as that of the model 1 illustrated in FIG. 5, description thereof is omitted.

[0027] In both the model 1 of FIG. 5 and the model 2 of FIG. 6, it is assumed that the waveguide portion 10 is made of diamond and is surrounded by air. The holes 12 are assumed to be also filled with air. The wavelength of the light emitted from the color center 40 is assumed to be 637 nm.

[0028] FIG. 7 is a view illustrating a simulation result of the magnetic field distribution in the model 1. FIG. 8 is a view illustrating a simulation result of the magnetic field distribution in the model 2. In FIGs. 7 and 8, a maximum intensity of the magnetic field is normalized to 1.0 and a minimum intensity of the magnetic field is normalized to -1.0.

[0029] As illustrated in FIG. 7, in the model 1 corresponding to the comparative example, the magnetic field generated in the waveguide portion 10 has a shape inclined with respect to the Y direction, resulting in poor symmetry of the magnetic field distribution with respect to the center of the waveguide portion 10 in the X direction. This is probably because, as illustrated in FIG. 5, the color center 40 is formed to be deviated from the center O of the waveguide portion 10, and hence the difference between the shortest distance between the color center 40 and the side surface 14 of the waveguide portion 10 and the shortest distance between the color center 40 and the side surface 15 of the waveguide portion 10 becomes large. When the symmetry of the electromagnetic field distribution is poor, an unnecessary propagation mode occurs and the propagation loss of the light becomes large, so that in the model 1, the propagation loss when the light emitted from the color center 40 propagates through the waveguide portion 10 becomes large.

[0030] As illustrated in FIG. 8, in the model 2 corresponding to the first embodiment, the inclination of the magnetic field generated in the waveguide portion 10 with respect to the Y direction is smaller than that in the model 1 of FIG. 7, resulting in good symmetry of the magnetic field distribution with respect to the center of the waveguide portion 10 in the X direction. This is probably because, in the model 2, as illustrated in FIG. 6, the recess 18 is formed on the side surface 15 of the waveguide portion 10, so that the absolute value of the difference between the shortest distance between the color center 40 and the side surface 14 and the shortest distance between the color center 40 and the side surface 15 becomes small. That is, it is considered that although the light emitted from the color center 40 spreads concentrically, when the absolute value of the difference in the shortest distance between the color center 40 and the side surfaces 14 and 15 becomes small, the light is similarly reflected by the side surfaces 14 and 15, so that the symmetry of the magnetic field distribution is improved. Since the generation of the unnecessary propagation mode is suppressed by improving the symmetry of the magnetic field distribution, the propagation loss when the light emitted from the color center 40 propagates through the waveguide portion 10 is reduced in the model 2.

[0031] According to the first embodiment, as illustrated in FIGs. 1A and 1B, the waveguide portion 10 has the color center 40. The recess 18 is provided on the side surface 15 of the waveguide portion 10 such that the absolute value of the difference between the shortest distance L1 between the side surface 14 of the waveguide portion 10 and the color center 40 and the shortest distance L2 between the side surface 15 and the color center 40 becomes small. Thereby, as illustrated in FIG. 8, the symmetry of the magnetic field distribution generated by the propagation of the light emitted from the color center 40 through the waveguide portion 10 is improved, so that the propagation loss of the light can be reduced.

[0032] In addition, according to the first embodiment, as illustrated in FIGs. 2E and 2F, the waveguide portion 10 for propagating the light is formed on the substrate 30. As illustrated in FIGs. 3A and 3B, the color center 40 is formed in the waveguide portion 10. As illustrated in FIGs. 3C and 3D, the recess 18 is formed on the side surface 15 of the waveguide portion 10 such that the absolute value of the difference between the shortest distance L1 between the side surface 14 of the waveguide portion 10 and the color center 40 and the shortest distance L2 between the side surface 15 and the color center 40 becomes small. Thereby, the symmetry of the magnetic field distribution generated by the propagation of the light emitted from the color center 40 through the waveguide portion 10 is improved, so that the propagation loss of the light can be reduced.

[0033] Further, in the first embodiment, as illustrated in FIGs. 3A and 3B, the color center 40 is formed in the waveguide portion 10 by ion implantation of atoms into the waveguide portion 10. Thereby, the color center 40 can be easily formed in the waveguide portion 10. When the color center 40 is formed by ion implantation, since the formation position of the color center 40 varies within a range of several tens nm, it is preferable to form the recess 18 on the side surface 15 of the waveguide portion 10 in order to improve the symmetry of the magnetic field distribution and reduce the propagation loss.

[0034] As one method for reducing the propagation loss of the light, it is conceivable to form the waveguide portion by specifying the position of the color center after forming the color center on the diamond substrate and processing the diamond substrate so that the color center is positioned at the center. However, in this case, since the position of the waveguide differs from one optical waveguide chip to another optical waveguide chip and it becomes difficult to connect the waveguides of a plurality of optical waveguide chips, quantum computation using photons, for example, may not be performed well.

[0035] In the first embodiment, the waveguide portion 10 is made of diamond. The color center 40 is formed by the complex defect of nitrogen (N) - vacancy (V), a complex defect of silicon (Si) - vacancy (V), the complex defect of tin (Sn) - vacancy (V), or the complex defect of germanium (Ge) - vacancy (V) formed by ion implantation of nitrogen, silicon, tin, or germanium into the waveguide portion 10. Thereby, the color center 40 can be easily formed in the waveguide portion 10. The waveguide portion 10 may be formed of a material other than diamond as long as it is a material in which the color center 40 is formed, and the atoms to be ion-implanted may be other than nitrogen, silicon, tin, and germanium.

[0036] Further, in the first embodiment, as illustrated in FIG. 1A, the recess 18 is provided on one side surface 15 of the side surfaces 14 and 15 facing each other in the Y direction of the waveguide portion 10. In the plan view of the waveguide portion 10, the portion 20 of the recess 18 closest to the color center 40 is located on the straight line 42 passing through the color center 40 and parallel to the Y direction. Thereby, the symmetry of the magnetic field distribution generated by the propagation of the light emitted from the color center 40 through the waveguide portion 10 is improved, and the propagation loss of the light can be reduced.

[0037] Further, in the first embodiment, as illustrated in FIG. 1A, the recesses 18 are provided symmetrically with respect to the straight line 42 in the plan view of the waveguide portion 10. Thereby, when the light emitted from the color center 40 is reflected by the recess 18, the light is reflected with good symmetry in the X direction. Therefore, the symmetry of the magnetic field distribution is improved, and the propagation loss of the light can be reduced.

[0038] Further, in the first embodiment, since the recess 18 is provided on the side surface 15 of the waveguide portion 10, the shortest distance L1 between the side surface 14 of the waveguide portion 10 and the color center 40 is 0.8 times or more and 1.2 times or less the shortest distance L2 between the side surface 15 and the color center 40. Thereby, the symmetry of the magnetic field distribution generated by the propagation of the light emitted from the color center 40 through the waveguide portion 10 is improved, and the propagation loss of the light can be reduced. From the viewpoint of reducing the propagation loss of the light by improving the symmetry of the magnetic field distribution, the shortest distance L1 is preferably 0.85 times or more and 1.15 times or less, more preferably 0.9 times or more and 1.1 times or less, and even more preferably 0.95 times or more and 1.05 times or less the shortest distance L2.

[0039] Further, in the first embodiment, the waveguide portion 10 is the photonic crystal waveguide portion in which the refractive index repeatedly changes in the X direction by repeatedly providing the plurality of holes 12 at intervals in the X direction. Thereby, the output characteristic of the light emitted from the color center 40 can be improved.

[0040] (Modification of First Embodiment) FIGs. 9A to 9D are plan views of optical waveguides 110 to 140 according to first to fourth modifications of the first embodiment. As illustrated in FIG. 9A, in the optical waveguide 110 according to the first modification of the first embodiment, a recess 18a has a vertex 22, which is a portion closest to the color center 40, on the straight line 42 in the plan view, and has a shape curved toward the vertex 22. The recess 18a is formed symmetrically with respect to the straight line 42, for example.

[0041] As illustrated in FIG. 9B, in the optical waveguide 120 according to the second modification of the first embodiment, a recess 18b has a rectangular shape in the plan view. The recess 18b is formed symmetrically with respect to the straight line 42, for example. As illustrated in FIG. 9C, in the optical waveguide 130 according to the third modification of the first embodiment, a recess 18c has a triangular shape in the plan view. The recess 18c is formed symmetrically with respect to the straight line 42, for example. In the first embodiment and the first modification of the first embodiment, the recess is provided in a curved shape on the side surface 15, but as in the second modification and the third modification of the first embodiment, the recess may be provided in a rectangular or triangular shape on the side surface 15.

[0042] As illustrated in FIG. 9D, in the optical waveguide 140 according to the fourth modification of the first embodiment, a recess 19 is formed on the side surface 14 in addition to forming the recess 18 on the side surface 15 of the waveguide portion 10. The recess 19 is formed to overlap with the color center 40 in the Y direction as in the recess 18, and has a shape of an elliptical arc or an arc in the plan view, for example. For example, a portion of the recess 19 closest to the color center 40 is located on the straight line 42, and the recess 19 is formed symmetrically with respect to the straight line 42. The recess 19 may have the same size as the recess 18, but is preferably smaller than the recess 18. In this manner, the recesses 18 and 19 may be provided on both of the side surfaces 14 and 15 of the waveguide portion 10, respectively. The recess 19 may have the same shape as the recesses 18a to 18c illustrated in FIGs. 9A to 9C. Since other configurations of the first to fourth modifications of the first embodiment are the same as those of the first embodiment, description thereof will be omitted. (Second Embodiment)

[0043] FIG. 10A is a plan view of an optical waveguide 200 according to the second embodiment, FIG. 10B is a cross-sectional view taken along the line A-A in FIG. 10A, and FIG. 10C is a cross-sectional view taken along the line B-B in FIG. 10A. As illustrated in FIGs. 10A to 10C, in the optical waveguide 200 according to the second embodiment, a low refractive index film 24 is provided in contact with the side surface 15 so as to fill the recess 18 formed in the side surface 15 of the waveguide portion 10. The low refractive index film 24 is provided in the recess 18 of the side surface 15 or in the vicinity of the recess 18, and is not provided at a position away from the recess 18. The low refractive index film 24 has a symmetrical shape with respect to the straight line 42, for example. The low refractive index film 24 is formed of a material having a refractive index lower than that of the waveguide portion 10, and is, for example, a silicon oxide (SiO2) film. The low refractive index film 24 may be formed of a material having a refractive index lower than that of the waveguide portion 10 and a refractive index higher than that of air, and may be formed of an organic material such as aluminum oxide, plastic, or resin. Since other configurations are the same as those of the first embodiment, description thereof will be omitted.

[0044] (Manufacturing Method of Second Embodiment) First, the manufacturing steps illustrated in FIGs. 2A to 3D of the first embodiment are performed. Thereafter, as illustrated in FIGs. 10A to 10C, the low refractive index film 24 is formed on the side surface 15 of the waveguide portion 10 so as to fill the recess 18 formed on the side surface 15. The low refractive index film 24 is formed by, for example, the FIB method.

[0045] (Simulation) In the optical waveguide according to the second embodiment, the magnetic field distribution generated by the propagation of the light emitted from the color center 40 through the waveguide portion 10 was simulated. FIG. 11 is a plan view of the model 3 used in the simulation. As illustrated in FIG. 11, the model 3 is a model corresponding to the optical waveguide 200 according to the second embodiment, and the same recess 18 as that of the model 2 of FIG. 6 is provided on the side surface 15 of the waveguide portion 10, and the low refractive index film 24 is provided in contact with the side surface 15 so as to fill the recess 18. The low refractive index film 24 has a symmetrical shape with respect to the straight line 42. The low refractive index film 24 is made of silicon oxide (SiO2), and has a length L of 83 nm in the Y direction and a width W of 300 nm. Since other configurations are the same as those of the models 1 and 2 illustrated in FIGs. 5 and 6, description thereof is omitted. Also in the model 3, as in the models 1 and 2, the waveguide portion 10 is made of diamond, and the wavelength of the light emitted from the color center 40 is 637 nm.

[0046] FIG. 12 is a view illustrating a simulation result of a magnetic field distribution in the model 3. In FIG. 12, as in FIGs. 7 and 8, the maximum intensity of the magnetic field is normalized to 1.0 and the minimum intensity of the magnetic field is normalized to -1.0.

[0047] As illustrated in FIG. 12, in the model 3 corresponding to the second embodiment, the inclination of the magnetic field generated in the waveguide portion 10 with respect to the Y direction is smaller than that in the model 1 of FIG. 7, resulting in good symmetry of the magnetic field distribution with respect to the center of the waveguide portion 10 in the X direction. This is probably because the recess 18 is formed on the side surface 15 of the waveguide portion 10 as in the model 2 so that the absolute value of the difference between the shortest distance between the color center 40 and the side surface 14 and the shortest distance between the color center 40 and the side surface 15 becomes small. Since the generation of the necessary propagation mode is suppressed by improving the symmetry of the magnetic field distribution, the propagation loss when the light emitted from the color center 40 propagates through the waveguide portion 10 is reduced in the model 3.

[0048] Further, in the model 3, the leakage of the magnetic field to the outside of the waveguide portion 10 is reduced as compared with the model 2 in FIG. 8. That is, this is a result of suppressing the leakage of light to the outside of the waveguide portion 10. Thus, it is found that the leakage of light to the outside of the waveguide portion 10 is reduced by providing the low refractive index film 24 having a refractive index lower than that of the waveguide portion 10 in the recess 18. Thereby, the propagation loss of the light propagating through the waveguide portion 10 is further reduced.

[0049] According to the second embodiment, as illustrated in FIGs. 10A to 10C, the low refractive index film 24 formed of the material having the lower refractive index than that of the waveguide portion 10 is provided in contact with the side surface 15 of the waveguide portion 10 to fill the recess 18. Thereby, as illustrated in FIG. 12, in addition to improving the symmetry of the magnetic field distribution generated by the propagation of the light emitted from the color center 40 through the waveguide portion 10, leakage of the magnetic field to the outside of the waveguide portion 10 is suppressed. Therefore, the propagation loss of the light can be further reduced. (Third Embodiment)

[0050] FIG. 13A is a plan view of an optical waveguide 300 according to a third embodiment, and FIG. 13B is a cross-sectional view taken along line A-A of FIG. 13A. As illustrated in FIGs. 13A and 13B, in the optical waveguide 300 according to the third embodiment, the recess 18 is not formed on the side surface 15 of the waveguide portion 10, but a recess 28 which overlaps with the color center 40 in the Z direction is formed on an upper surface 17. The color center 40 may be formed to be offset from the center of the waveguide portion 10 in the Z direction, and even in this case, the propagation loss of the light propagating through the waveguide portion 10 becomes large. Therefore, the recess 28 is formed in the upper surface 17 to reduce an absolute value of a difference between a shortest distance L3 between a lower surface 16 and the color center 40 and a shortest distance L4 between the upper surface 17 and the color center 40. By forming the recess 28, the shortest distance L4 is 0.8 times or more and 1.2 times or less the shortest distance L3. The recess 28 has a vertex 23, which is a portion closest to the color center 40, on a straight line 44 parallel to the Z direction and passing through the color center 40, for example, and is curved toward the vertex 23 in cross-sectional view. Preferably, the recess 28 is provided symmetrically with respect to the straight line 44. Since other configurations are the same as those of the first embodiment, description thereof will be omitted.

[0051] (Manufacturing Method of Third Embodiment) FIGs. 14A to 14F are views illustrating a method of manufacturing the optical waveguide 300 according to the third embodiment. FIGs. 14A, 14C and 14E are plan views illustrating the method of manufacturing the optical waveguide 300 according to the third embodiment. FIGs. 14B, 14D and 14F are cross-sectional views taken along line A-A of FIGs. 14A, 14C and 14E.

[0052] As illustrated in FIGs. 14A and 14B, first, the manufacturing process illustrated in FIGs. 2A to 2F of the first embodiment is performed to form the waveguide portion 10 provided with the plurality of holes 12 on the substrate 30 via the gap 34.

[0053] As illustrated in FIGs. 14C and 14D, atoms of nitrogen, silicon, tin, germanium, or the like are ion-implanted from above the waveguide portion 10 toward the center of the waveguide portion 10. Thereby, the color center 40 is formed in the vicinity of the center of the waveguide portion 10. In the case where the color center 40 is formed by ion implantation of the atoms into the waveguide portion 10, the formation position of the color center 40 may vary within a range of several tens of nm and may be formed out of alignment with the center of the waveguide portion 10 in the Z direction.

[0054] As illustrated in FIGs. 14E and 14F, the optical technique such as a confocal laser microscope is used to specify the position of the color center 40 in the waveguide portion 10. The recess 28 is formed at a position corresponding to the color center 40 in the X direction in the upper surface 17 of the waveguide portion 10. Thereby, the recess 28 overlapping with the color center 40 in the Z direction is formed. The formation of the recess 28 reduces the absolute value of the difference between the shortest distance L3 between the color center 40 and the lower surface 16 of the waveguide portion 10 and the shortest distance L4 between the color center 40 and the upper surface 17 of the waveguide portion 10. The recess 28 is formed, for example, by FIB processing. The recess 28 may be formed by using a photolithography method and an etching method.

[0055] According to the third embodiment, as illustrated in FIGs. 13A and 13B, the waveguide portion 10 has the color center 40. The recess 28 is provided on the upper surface 17 of the waveguide portion 10 so that the absolute value of the difference between the shortest distance L3 between the lower surface 16 and the color center 40 and the shortest distance L4 between the upper surface 17 and the color center 40 becomes small. Even in this case, the symmetry of the magnetic field distribution generated by the propagation of the light emitted from the color center 40 through the waveguide portion 10 is improved, and the propagation loss of the light can be reduced.

[0056] In the third embodiment, the recess 28 is provided on the upper surface 17 away from the substrate 30 among the lower surface 16 and the upper surface 17 facing each other in the Z direction of the waveguide portion 10. The recess 28 is located on the straight line 44 in which the vertex 23, which is the position closest to the color center 40, passes through the color center 40 and is parallel to the Z direction in the sectional view of the waveguide portion 10. Thereby, the symmetry of the magnetic field distribution generated by the propagation of the light emitted from the color center 40 through the waveguide portion 10 is improved, and the propagation loss of the light can be reduced.

[0057] Further, in the third embodiment, the recess 28 is provided symmetrically with respect to the straight line 44 in a cross-sectional view of the waveguide portion 10. Thereby, when the light emitted from the color center 40 is reflected by the recess 28, the light is reflected with good symmetry in the X direction, so that the propagation loss of the light can be reduced.

[0058] Further, in the third embodiment, since the recess 28 is provided in the upper surface 17 of the waveguide portion 10, the shortest distance L3 between the lower surface 16 of the waveguide portion 10 and the color center 40 is 0.8 times or more and 1.2 times or less the shortest distance L4 between the upper surface 17 and the color center 40. Thereby, the propagation loss of the light can be reduced. From the viewpoint of reducing the propagation loss of the light, the shortest distance L3 is preferably 0.85 times or more and 1.15 times or less, more preferably 0.9 times or more and 1.1 times or less, and even more preferably 0.95 times or more and 1.05 times or less the shortest distance L4.

[0059] In the third embodiment, the recess 28 may have the elliptical arc shape or the arc shape such as the recess 18 in FIG. 1A, the rectangular shape such as the recess 18b in FIG. 9B, or the triangular shape such as the recess 18c in FIG. 9C.

[0060] (Modification of Third Embodiment) FIG. 15A is a plan view of an optical waveguide 310 according to a modification of the third embodiment, and FIG. 15B is a cross-sectional view taken along line A-A of FIG. 15A. As illustrated in FIGs. 15A and 15B, in the optical waveguide 310 according to the modification of the third embodiment, the recess 18 is formed on the side surface 15 in addition to the recess 28 formed on the upper surface 17 of the waveguide portion 10. In this manner, the recess 18 may be formed on one of the surfaces facing each other in the width direction of the waveguide portion 10, and the recess 28 may be formed on one of the surfaces facing each other in the thickness direction of the waveguide portion 10. (Fourth Embodiment)

[0061] FIG. 16A is a plan view of an optical waveguide 400 according to a fourth embodiment, and FIG. 16B is a cross-sectional view taken along line A-A of FIG. 16A. As illustrated in FIGs. 16A and 16B, in the optical waveguide 400 according to the fourth embodiment, a low refractive index film 29 is provided in contact with the upper surface 17 so as to fill the recess 28 formed on the upper surface 17 of the waveguide portion 10. The low refractive index film 29 is provided in the recess 28 and in the vicinity of the recess 28 of the upper surface 17, and is not provided at a position away from the recess 28. The low refractive index film 29 is formed of a material having a refractive index lower than that of the waveguide portion 10, and is, for example, a silicon oxide (SiO2) film. The low refractive index film 29 may be formed of a material having a refractive index lower than that of the waveguide portion 10 and a refractive index higher than that of air, and may be formed of an organic material such as aluminum oxide, plastic, or resin. Since other configurations are the same as those of the third embodiment, description thereof will be omitted.

[0062] (Manufacturing Method of Fourth Embodiment) First, the manufacturing steps illustrated in FIGs. 14A to 14F of the third embodiment are performed. Thereafter, as illustrated in FIGs. 16A and 16B, the low refractive index film 29 in contact with the upper surface 17 is formed so as to fill the recess 28 formed in the upper surface 17 of the waveguide portion 10. The low refractive index film 29 is formed by, for example, the FIB method.

[0063] According to the fourth embodiment, the low refractive index film 29 formed of the material having the lower refractive index than that of the waveguide portion 10 is provided in contact with the upper surface 17 of the waveguide portion 10 to fill the recess 28. Thereby, as in the second embodiment, the leakage of the magnetic field to the outside of the waveguide portion 10 is suppressed, so that the propagation loss of the light can be further reduced.

[0064] Although the embodiments of the present disclosure have been described in detail, the present disclosure is not limited to the specific embodiments described above, and various variations and changes are possible within the scope of the gist of the present disclosure as described in the claims.

[0065] 10 waveguide portion 12 hole 14 side surface 15 side surface 16 lower surface 17 upper surface 18, 18a, 18b, 18c recess 19 recess 20 closest portion 23 vertex 24 low refractive index film 28 recess 29 low refractive index film 30 substrate 32 intermediate layer 34 gap 40 color center 42 straight line 44 straight line 50 diamond substrate 52 first layer 54 second layer 56 mask layer 58 opening 100, 110, 120, 130, 140, 200, 300, 310, 400, 500 optical waveguide

Claims

1. An optical waveguide comprising: a substrate; and a waveguide portion provided on the substrate, having a color center, and through which light propagates; wherein the waveguide portion has a recess on at least one of a first surface and a second surface facing each other of the waveguide portion such that an absolute value of a difference between a first shortest distance between the first surface and the color center and a second shortest distance between the second surface and the color center becomes small.

2. The optical waveguide according to claim 1, wherein the waveguide portion is formed of diamond, the color center is formed by any one of a nitrogen-vacancy complex defect, a silicon-vacancy complex defect, a tin-vacancy complex defect, or a germanium-vacancy complex defect.

3. The optical waveguide as claimed in claim 1 or 2, wherein the first surface and the second surface face each other in a short direction of the waveguide portion, and a portion of the recess closest to the color center is located on a straight line passing through the color center and parallel to the short direction of the waveguide portion in a plan view of the waveguide portion.

4. The optical waveguide as claimed in claim 3, wherein the recess is provided symmetrically with respect to the straight line in the plan view of the waveguide portion.

5. The optical waveguide as claimed in claim 1 or 2, wherein the first surface and the second surface face each other in a thickness direction of the waveguide portion, the recess is provided on a surface away from the substrate of the first surface and the second surface, and a portion of the recess closest to the color center is located on a straight line passing through the color center and parallel to the thickness direction of the waveguide portion in a cross-sectional view of the waveguide portion.

6. The optical waveguide as claimed in claim 5, wherein the recess is provided symmetrically with respect to the straight line in the cross-sectional view of the waveguide portion.

7. The optical waveguide as claimed in claim 1 or 2, further comprising a low refractive index film formed of a material having a refractive index lower than that of the waveguide portion, the low refractive index film being provided in contact with the at least one of the first surface and the second surface to fill the recess.

8. The optical waveguide as claimed in claim 1 or 2, wherein the first shortest distance is 0.8 times or more and 1.2 times or less the second shortest distance.

9. The optical waveguide as claimed in claim 1 or 2, wherein the waveguide portion is a photonic crystal waveguide portion whose refractive index repeatedly changes in a longitudinal direction of the waveguide portion.

10. The optical waveguide as claimed in claim 9, wherein the waveguide has a plurality of holes penetrating through the waveguide portion and provided at intervals in the longitudinal direction of the waveguide portion, so that the refractive index repeatedly changes in the longitudinal direction of the waveguide portion.

11. The optical waveguide as claimed in claim 1 or 2, wherein the recess is provided in a curved shape on the at least one of the first surface and the second surface.

12. The optical waveguide as claimed in claim 1 or 2, wherein the recess is formed in a rectangular or triangular shape on the at least one of the first surface and the second surface.

13. A method of manufacturing an optical waveguide comprising: forming a waveguide portion through which light propagates on a substrate; forming a color center in the waveguide portion; and forming a recess on at least one of a first surface and a second surface facing each other of the waveguide portion so that an absolute value of a difference between a first shortest distance between the first surface and the color center and a second shortest distance between the second surface and the color center becomes small.

14. The method of manufacturing the optical waveguide as claimed in claim 13, wherein the forming the color center is forming the color center by ion implantation of atoms into the waveguide portion.

15. The method of manufacturing the optical waveguide as claimed in claim 13, wherein the forming the waveguide portion is forming the waveguide portion formed of diamond, and the forming the color center is forming the color center by ion implantation of nitrogen, silicon, tin, silicon, tin, or germanium into the waveguide portion to form a nitrogen-vacancy complex defect, a silicon-vacancy complex defect, a tin-vacancy complex defect, or a germanium-vacancy complex defect.

16. The method of manufacturing the optical waveguide as claimed in claim 13 or 14, further comprising specifying a position of the color center in the waveguide portion by an optical technique, wherein the forming the recess is forming the recess so that the absolute value of the difference between the first shortest distance and the second shortest distance becomes small based on a result of specifying the position of the color center by the optical technique.

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