Optical Modulators and Optical Integrated Circuits

Abstract

The optical modulator (100) includes a photoelectric conversion unit (1), a two-dimensional material layer (2), and an optical waveguide unit (4). A control light (CL) is incident on the photoelectric conversion unit (1). The two-dimensional material layer (2) faces at least a portion of the photoelectric conversion unit (1). When viewed from the direction in which the control light (CL) is incident, at least a portion of the optical waveguide unit (4) is covered by the two-dimensional material layer (2).

Description

[Technical Field]

[0001] The present disclosure relates to optical modulators and optical integrated circuits. [Background technology]

[0002] Conventionally, an optical modulator that modulates signal light by applying a voltage to graphene as a light absorption layer has been known (see, for example, JP 2014-164195 A). In order to adjust the Fermi level of graphene, the absorption and transmission of signal light can be controlled by applying a voltage to the graphene. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-164195 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the optical absorption rate of graphene per layer is extremely low. When the absorption and transmission of signal light is controlled by applying a voltage to graphene, the modulation efficiency of the optical modulator is low, resulting in high power consumption. Therefore, there is room for improvement in the power consumption of optical modulators.

[0005] The present disclosure has been made to solve the above-mentioned problems, and an object of the present disclosure is to provide an optical modulator with reduced power consumption. [Means for solving the problem]

[0006] An optical modulator according to the present disclosure includes a photoelectric conversion unit, a two-dimensional material layer, and an optical waveguide unit. Control light is incident on the photoelectric conversion unit. The two-dimensional material layer faces at least a portion of the photoelectric conversion unit. When viewed from the direction in which the control light is incident, at least a portion of the optical waveguide unit is covered by the two-dimensional material layer.

[0007] The optical integrated circuit according to the present disclosure includes an optical modulator, which is a Mach-Zehnder modulator or a ring modulator. [Effects of the Invention]

[0008] Based on the above, an optical modulator with reduced power consumption can be obtained. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic perspective view of an optical modulator according to a first embodiment. [Figure 2] 1 is a schematic plan view of an optical modulator according to a first embodiment. [Figure 3] FIG. 3 is a schematic cross-sectional view taken along line III-III in FIG. 2. [Figure 4] FIG. 2 is a schematic diagram showing the range in which graphene absorbs and transmits signal light. [Figure 5] FIG. 10 is a schematic perspective view of an optical modulator according to a modified example of the first embodiment. [Figure 6] FIG. 10 is a schematic perspective view of an optical modulator according to a modified example of the first embodiment. [Figure 7] 3 is a schematic cross-sectional view showing one step of a method for manufacturing the optical modulator according to the first embodiment. FIG. [Figure 8] 3 is a schematic cross-sectional view showing one step of a method for manufacturing the optical modulator according to the first embodiment. FIG. [Figure 9] 3 is a schematic cross-sectional view showing one step of a method for manufacturing the optical modulator according to the first embodiment. FIG. [Figure 10] 3 is a schematic cross-sectional view showing one step of a method for manufacturing the optical modulator according to the first embodiment. FIG. [Figure 11] 3 is a schematic cross-sectional view showing one step of a method for manufacturing the optical modulator according to the first embodiment. FIG. [Figure 12] FIG. 10 is a schematic cross-sectional view of an optical modulator according to a second embodiment. [Figure 13] FIG. 11 is a schematic cross-sectional view of an optical modulator according to a third embodiment. [Figure 14]FIG. 10 is a schematic cross-sectional view of an optical modulator according to a fourth embodiment. [Figure 15] FIG. 10 is a schematic cross-sectional view of an optical modulator according to a fifth embodiment. [Figure 16] FIG. 13 is a schematic cross-sectional view of an optical modulator according to a sixth embodiment. [Figure 17] FIG. 13 is a schematic cross-sectional view of an optical modulator according to a seventh embodiment. [Figure 18] FIG. 13 is a schematic cross-sectional view of an optical modulator according to an eighth embodiment. [Figure 19] FIG. 13 is a schematic perspective view of an optical modulator according to a ninth embodiment. [Figure 20] FIG. 23 is a schematic plan view of a two-dimensional material layer of an optical modulator according to a tenth embodiment. [Figure 21] FIG. 23 is a schematic plan view of a two-dimensional material layer of an optical modulator according to a modification of the tenth embodiment. [Figure 22] FIG. 23 is a schematic plan view of a two-dimensional material layer of an optical modulator according to a modification of the tenth embodiment. [Figure 23] FIG. 23 is a schematic plan view of a two-dimensional material layer of an optical modulator according to a modification of the tenth embodiment. [Figure 24] FIG. 22 is a schematic diagram of an optical integrated circuit according to an eleventh embodiment. [Figure 25] FIG. 22 is a schematic diagram of an optical integrated circuit according to a twelfth embodiment. [Figure 26] FIG. 23 is a schematic perspective view of an optical modulator according to a thirteenth embodiment. [Figure 27] FIG. 23 is a schematic perspective view of an optical modulator according to a fourteenth embodiment. [Figure 28] FIG. 22 is a schematic cross-sectional view of an optical modulator according to a fourteenth embodiment. [Figure 29] FIG. 23 is a schematic perspective view of an optical modulator according to a modification of the fourteenth embodiment. [Figure 30] FIG. 23 is a schematic cross-sectional view of an optical modulator according to a modification of the fourteenth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present disclosure will be described. Unless otherwise specified, the same or corresponding parts in the following drawings will be denoted by the same reference numerals, and the description thereof will not be repeated.

[0011] Embodiment 1 <Configuration of optical modulator> Fig. 1 is a schematic perspective view of an optical modulator 100 according to embodiment 1. Fig. 2 is a schematic plan view of the optical modulator 100 according to embodiment 1. Fig. 3 is a schematic cross-sectional view taken along line III-III in Fig. 2.

[0012] The optical modulator 100 shown in Figures 1 to 3 is, for example, an optical modulator 100 used in optical communications, and includes a substrate 6, an insulating layer 5, an optical waveguide section 4, a photoelectric conversion section 1, a two-dimensional material layer 2, a first electrode 3a, and a second electrode 3b.

[0013] The light modulated by the optical modulator 100 according to the first embodiment is, for example, an electromagnetic wave such as near-infrared light. Note that the electromagnetic wave is not limited to near-infrared light. The electromagnetic wave may be, for example, visible light, mid-wavelength infrared light, long-wavelength infrared light, ultraviolet light, terahertz (THz) waves, or other light and radio waves.

[0014] The material constituting the substrate 6 may be, for example, an elemental semiconductor such as silicon (Si) or germanium (Ge). The material constituting the substrate 6 may be, for example, a compound semiconductor such as a III-V semiconductor or a II-V semiconductor. Examples of compound semiconductors include cadmium mercury telluride (HgCdTe), indium antimonide (InSb), lead selenium (PbSe), lead sulfide (PbS), cadmium sulfur (CdS), gallium nitride (GaN), silicon carbide (SiC), gallium phosphide (GaP), indium gallium arsenide (InGaAs), indium arsenide (InAs), gallium antimony (GaSb), and indium gallium (InGa).

[0015] The substrate 6 may include a quantum well structure in which the direction of electron movement is constrained. The substrate 6 may include quantum dots in which the direction of electron movement is constrained in all three dimensions. The substrate 6 may have a band structure called a Type-II superlattice. The material constituting the substrate 6 may be any of the above materials alone or a combination of the above materials.

[0016] The insulating layer 5 is formed on the substrate 6. The material constituting the insulating layer 5 is an electrically insulating material, such as an oxide. The material constituting the insulating layer 5 may be, for example, silicon oxide (SiO), tetraethyl orthosilicate (Si(OCH)), silicon nitride (SiN), silicon nitride (SiN), hafnium oxide (HfO), aluminum oxide (AlO), nickel oxide (NiO), boron nitride (BN), or a siloxane-based polymer material.

[0017] 3, the optical waveguide portion 4 and the photoelectric conversion portion 1 are each formed inside an insulating layer 5. As shown in FIGS. 1 to 3, the direction in which the optical waveguide portion 4 extends is defined as the Y direction. The signal light SL propagates through the optical waveguide portion 4 along the Y direction.

[0018] As will be described later, the direction in which the control light CL is incident is defined as the Z direction. The Z direction is perpendicular to the Y direction. The direction perpendicular to the Y and Z directions is defined as the X direction.

[0019] The photoelectric conversion unit 1 is formed of a photoelectric conversion material that generates photoelectric conversion when control light CL is incident on it. The photoelectric conversion material may be a two-dimensional material such as graphene. The photoelectric conversion material is not limited to a two-dimensional material and may be a semiconductor material such as silicon. If the material constituting the photoelectric conversion unit 1 is graphene, the modulation speed in the optical modulator 100 is improved. Furthermore, since the Fermi level in the photoelectric conversion unit 1 changes significantly, the modulation efficiency in the optical modulator 100 is improved.

[0020] 1 to 3, the photoelectric conversion unit 1 may be disposed adjacent to the optical waveguide unit 4 in the X direction. The photoelectric conversion unit 1 is disposed spaced apart from the optical waveguide unit 4 in the X direction so as not to come into contact with the optical waveguide unit 4.

[0021] The two-dimensional material layer 2 functions as a light absorption layer by absorbing and transmitting the signal light SL propagating through the optical waveguide portion 4. In other words, the optical modulator 100 can modulate the signal light SL by controlling the absorption and transmission of the signal light SL by the two-dimensional material layer 2. The material that constitutes the two-dimensional material layer 2 is a two-dimensional material such as graphene.

[0022] The two-dimensional material layer 2 is formed on the surface of the insulating layer 5. That is, the two-dimensional material layer 2 is disposed at a position farther from the substrate 6 in the Z direction than the optical waveguide portion 4 and the photoelectric conversion portion 1. As shown in FIG. 2, when viewed from the direction in which the control light CL is incident (the direction along the Z direction), at least a portion of the optical waveguide portion 4 and at least a portion of the photoelectric conversion portion 1 are covered with the two-dimensional material layer 2. That is, the two-dimensional material layer 2 is disposed at a position facing at least a portion of the optical waveguide portion 4 and at least a portion of the photoelectric conversion portion 1. Note that in FIG. 2, the optical waveguide portion 4 and the photoelectric conversion portion 1 are indicated by dotted lines.

[0023] 3, the distance L1 in the Z direction from the two-dimensional material layer 2 to the optical waveguide portion 4 may be, for example, 1 nm or more and 100 nm or less. Thus, the modulation efficiency of the optical modulator 100 improves as the distance L1 in the Z direction from the two-dimensional material layer 2 to the optical waveguide portion 4 decreases.

[0024] The insulating layer 5 includes a first insulating region 5a. The first insulating region 5a is a region disposed between the photoelectric conversion unit 1 and the two-dimensional material layer 2. In other words, the photoelectric conversion unit 1 is disposed apart from the two-dimensional material layer 2 in the Z direction via the first insulating region 5a.

[0025] As will be described later, an optical gate effect occurs in the first insulating region 5a. As shown in Fig. 3, the thickness of the first insulating region 5a (the distance L2 in the Z direction from the two-dimensional material layer 2 to the photoelectric conversion unit 1) may be, for example, 1 nm or more and 100 nm or less. As such, the smaller the distance L2 in the Z direction from the two-dimensional material layer 2 to the photoelectric conversion unit 1, the larger the Fermi level shift in the two-dimensional material layer 2, and therefore the modulation efficiency of the optical modulator 100 improves.

[0026] The thickness of the first insulating region 5a may be equal to or greater than the distance (thickness) through which a tunnel current flows, or may be small enough to allow a tunnel current to flow, as will be described later.

[0027] The first electrode 3a is electrically connected to the photoelectric conversion unit 1. As shown in FIGS. 1 and 3, a portion of the first electrode 3a is disposed inside the insulating layer 5, and the remaining first electrode 3a protrudes from the surface of the insulating layer 5. Note that in FIG. 1, the first electrode 3a disposed inside the insulating layer 5 is illustrated by a dotted line. The second electrode 3b is electrically connected to the two-dimensional material layer 2. The second electrode 3b is connected to one end of the two-dimensional material layer 2 in the X direction. As shown in FIGS. 1 and 2, the left side surface of the first electrode 3a is continuous with the left side surface of the photoelectric conversion unit 1, but the left side surface of the first electrode 3a does not have to be continuous with the left side surface of the photoelectric conversion unit 1. Specifically, the first electrode 3a may be connected to one end (left end) of the photoelectric conversion unit 1 in the x direction, or may be connected to the center of the photoelectric conversion unit 1 in the x direction.

[0028] The first electrode 3 a and the second electrode 3 b are made of a conductive material, and may include at least one of gold (Au), silver (Ag), copper (Cu), aluminum (Al), nickel (Ni), chromium (Cr), and palladium (Pd).

[0029] A bias voltage is applied between the first electrode 3a and the second electrode 3b. Specifically, a back gate voltage is applied to the two-dimensional material layer 2. In this way, a potential difference is generated between the photoelectric conversion unit 1 and the two-dimensional material layer 2.

[0030] Conventionally, the absorption and transmission of signal light SL have been controlled by electrically controlling the voltage in the photoelectric conversion unit 1 and the two-dimensional material layer 2. The light absorption rate of graphene per layer is extremely low, at approximately 2.3 percent. Therefore, when the material constituting the photoelectric conversion unit 1 and the two-dimensional material layer 2 is graphene, controlling the absorption and transmission of signal light SL by applying a voltage to the graphene results in high power consumption due to the low modulation efficiency of the optical modulator 100.

[0031] Here, the optical modulator 100 according to the first embodiment is characterized in that the control light CL is irradiated onto the photoelectric conversion unit 1, thereby causing photoelectric conversion in the photoelectric conversion unit 1 and modulating the signal light SL. In this way, the modulation efficiency of the optical modulator 100 is improved and power consumption is significantly reduced.

[0032] <Operation> Next, the operation of the optical modulator 100 according to the first embodiment will be described.

[0033] Assume that the material forming the substrate 6 and the optical waveguide portion 4 is, for example, silicon (Si). Assume that the material forming the insulating layer 5 is, for example, silicon oxide (SiO2). Assume that the material forming the photoelectric conversion portion 1 and the two-dimensional material layer 2 is, for example, graphene. Graphene is a semimetallic material and has a Dirac cone-shaped band gap. Therefore, when the two-dimensional material forming the photoelectric conversion portion 1 is graphene, the photoelectric conversion portion 1 can detect electromagnetic waves containing a wide range of wavelength components, from ultraviolet to terahertz waves. In other words, any electromagnetic wave from ultraviolet to terahertz waves can be selected as the control light CL to be irradiated to the photoelectric conversion portion 1.

[0034] FIG. 4 is a schematic diagram showing the range A3 in which graphene absorbs the signal light SL and the ranges A1 and A2 in which it transmits the signal light SL. As described above, the signal light SL propagates through the optical waveguide portion 4 along the Y direction. The signal light SL has energy hf, which is the product of the frequency f of the signal light SL and Planck's constant h. When the Fermi level Ef of graphene in the two-dimensional material layer 2 is controlled to be within the range A1 or the range A2 shown in FIG. 4, the signal light SL is transmitted without being absorbed by the two-dimensional material layer 2. The range A1 is a range greater than +hf / 2 with respect to the energy hf of the signal light SL. The range A2 is a range less than −hf / 2 with respect to the energy hf of the signal light SL.

[0035] When visible light is incident on the photoelectric conversion unit 1 as control light CL, photoelectric conversion occurs in the graphene in the photoelectric conversion unit 1, causing a change in the voltage in the photoelectric conversion unit 1. In other words, the back gate voltage applied to the two-dimensional material layer 2 changes. As a result, the Fermi level Ef of the graphene in the two-dimensional material layer 2 changes significantly. This phenomenon is called the photo-gating effect.

[0036] The smaller the thickness of the first insulating region 5a (the distance L2 in the Z direction from the two-dimensional material layer 2 to the photoelectric conversion unit 1), the larger the amount of change in the bias voltage. Therefore, in order to significantly change the Fermi level Ef of graphene in the two-dimensional material layer 2, the thickness of the first insulating region 5a may be reduced.

[0037] 4, the signal light SL is absorbed by the two-dimensional material layer 2. The range A3 is a range smaller than +hf / 2 and larger than −hf / 2 with respect to the energy hf of the signal light SL.

[0038] In this manner, by irradiating the control light CL to the photoelectric conversion unit 1, the Fermi level Ef of the graphene in the two-dimensional material layer 2 can be changed. As a result, it is possible to control the signal light SL propagating through the optical waveguide unit 4 to be absorbed. In other words, by turning the control light CL on and off, it is possible to switch between absorbing and not absorbing the signal light SL, thereby modulating the signal light SL. Note that although the cases where the Fermi level Ef of the graphene in the two-dimensional material layer 2 is initially in the ranges A1 and A2 have been described, the initial state of the Fermi level Ef of the graphene in the two-dimensional material layer 2 may also be in the range A3. In this manner, it is possible to control the signal light SL to be absorbed and not absorbed by turning the control light CL on and off.

[0039] In this way, the Fermi level Ef of the graphene in the two-dimensional material layer 2 can be controlled by irradiating the control light CL to the photoelectric conversion unit 1 without electrically controlling the Fermi level Ef of the graphene in the two-dimensional material layer 2. As a result, the power consumption in the optical modulator 100 can be significantly reduced compared to electrically controlling the Fermi level Ef of the graphene in the two-dimensional material layer 2. Furthermore, since the signal light SL can be modulated using the control light CL, the modulation speed in the optical modulator 100 is improved. Furthermore, since it is not necessary to simultaneously control the Fermi levels Ef of both the graphene in the two-dimensional material layer 2 and the photoelectric conversion unit 1, the absorption and transmission of the signal light SL can be easily controlled.

[0040] As described above, the materials constituting the photoelectric conversion unit 1 and the two-dimensional material layer 2 may be, for example, two-dimensional materials. The two-dimensional materials constituting the photoelectric conversion unit 1 and the two-dimensional material layer 2 may be, for example, single-layer graphene. Graphene is a monoatomic layer of two-dimensional carbon crystals. The carbon atom arrangement structure of graphene is a hexagonal honeycomb structure formed by bonding between carbon atoms. The single layer is a layer extending along the X and Y directions.

[0041] Graphene with more holes than intrinsic graphene is called p-type graphene. Graphene with more electrons than intrinsic graphene is called n-type graphene. That is, a material with n-type conductivity has electron-donating properties, and a material with p-type conductivity has electron-withdrawing properties. The material constituting the photoelectric conversion unit 1 and the two-dimensional material layer 2 may be impurity-doped graphene with p-type or n-type conductivity.

[0042] When there is a bias in the charge throughout the molecule, the conductivity type in which electrons are dominant may be called n-type. When there is a bias in the charge throughout the molecule, the conductivity type in which holes are dominant may be called p-type. The material constituting the member in contact with graphene, which is an example of the two-dimensional material layer 2, may be either an organic material or an inorganic material, or a mixture of an organic material and an inorganic material.

[0043] The optical absorption rate of graphene per layer is extremely low, at about 2.3 percent. However, single-layer graphene has linear band dispersion, making it sensitive to electromagnetic waves (light) over a wide wavelength range.

[0044] The two-dimensional material is not limited to single-layer graphene. The two-dimensional material constituting the photoelectric conversion unit 1 and the two-dimensional material layer 2 may be, for example, multi-layer graphene in which multiple graphene layers are stacked. When graphene has a stacked structure of two or more stacked multi-layer graphene layers, the directions of the hexagonal lattice vectors of any two graphene layers included in the stacked structure do not have to match. In other words, there may be a misalignment between the directions of the hexagonal lattice vectors of any two graphene layers. Furthermore, the directions of the hexagonal lattice vectors of any two graphene layers may match.

[0045] In particular, if the two-dimensional material layer 2 has a stacked structure of multilayer graphene consisting of two or more layers, a band gap is formed in the two-dimensional material layer 2. This allows the two-dimensional material layer 2 to selectively detect the wavelength of the electromagnetic wave that is photoelectrically converted in the photoelectric conversion unit 1. In other words, increasing the number of stacked graphene layers increases the light absorption rate in the two-dimensional material layer 2. Therefore, the modulation efficiency of the optical modulator 100 is improved.

[0046] Multilayer graphene may be formed by transferring single-layer graphene prepared by chemical vapor deposition (CVD) multiple times. Multilayer graphene may also be formed by placing ethanol or methane as a carbon source on graphene and growing the graphene by CVD.

[0047] When the two-dimensional material is multilayer graphene, the multilayer graphene may be turbostratic stacked graphene. Turbostratic stacked graphene is multilayer graphene having a turbostratic structure in which stacking orientation angles are randomly arranged. The turbostratic structure is a stacked structure in which single layers in which atoms are arranged in a two-dimensional plane are stacked on top of each other. Turbostratic stacked graphene has a structure in which multiple graphenes are stacked in a state in which their lattices are mismatched.

[0048] Ordinary multilayer graphene has an AB stacking pattern of carbon atom layers. AB stacking is a stacking pattern in which the lattices of the graphene are matched. However, graphene formed using the CVD method is polycrystalline. Therefore, when graphene is transferred onto graphene multiple times, or when graphene is grown using the CVD method using the underlying graphene as a nucleus, turbostratic stacked graphene is formed, which has a turbostratic structure in which the lattices of the graphene are mismatched.

[0049] Multi-layering of graphene reduces the charge mobility, which reduces the amount of change in the Fermi level Ef of graphene and reduces the modulation efficiency of the optical modulator 100. Furthermore, if the graphene is a single-layer graphene, the charge mobility of the graphene decreases if the surface of the insulating layer 5 in contact with the graphene has large irregularities or if residual charges exist on the surface of the insulating layer 5.

[0050] On the other hand, in turbostratic-stacked graphene, the interlayer interaction between multiple graphene layers is weak. Therefore, the electron mobility in the turbostratic-stacked graphene is comparable to that in single-layer graphene. Furthermore, because electrical disturbances in graphene (such as irregularities formed on the surface of the insulating layer 5 or residual charges present on the surface) are suppressed, turbostratic-stacked graphene has better electron mobility than normal single-layer graphene. Therefore, the optical absorption rate in the two-dimensional material layer 2 increases. As a result, the modulation efficiency and modulation speed of the optical modulator 100 are improved.

[0051] Turbostratically stacked graphene has linear band dispersion, similar to single-layer graphene. Therefore, turbostratically stacked graphene is sensitive to electromagnetic waves (light) in a wide wavelength range. As a result, the optical modulator 100 can modulate signal light SL in a wide wavelength range.

[0052] The stacking angle of the graphene in turbostratic stacked graphene may be an angle at which moire occurs. In this case, the energy band of the graphene becomes flat. Furthermore, by controlling the stacking angle in the multi-layer graphene so that the angle at which moire occurs, a moire potential can be formed. As a result, the two-dimensional material layer 2 can selectively detect electromagnetic waves (light) in a specific wavelength range. As a result, the light absorption rate of electromagnetic waves in a wide wavelength range is improved compared to single-layer graphene, and the modulation efficiency of the optical modulator 100 is improved.

[0053] Graphene may be a graphene nanoribbon, which is a strip-shaped piece cut into nanometer-sized pieces. The graphene structure may be a structure of a single graphene nanoribbon, a structure in which multiple graphene nanoribbons are stacked, or a structure in which graphene nanoribbons are periodically arranged on a plane. A structure in which graphene nanoribbons are periodically arranged is called a graphene metamaterial. When the two-dimensional material layer 2 has a structure in which graphene nanoribbons are periodically arranged, plasmon resonance occurs in the two-dimensional material layer 2. As a result, the optical absorption rate of electromagnetic waves (light) in a specific wavelength range is improved in the two-dimensional material layer 2.

[0054] In particular, if the photoelectric conversion unit 1 is a graphene nanoribbon, the width of the graphene nanoribbon will form a band gap in the photoelectric conversion unit 1. If the wavelength of the control light CL matches the band gap wavelength of the graphene nanoribbon, the light absorption rate in the photoelectric conversion unit 1 will increase, and the amount of change in voltage due to photoelectric conversion will increase. In other words, the Fermi level Ef in the two-dimensional material layer 2 will change significantly, and the modulation efficiency of the optical modulator 100 will improve.

[0055] Furthermore, if the two-dimensional material layer 2 is a graphene nanoribbon, the optical absorption rate in the two-dimensional material layer 2 increases, and the amount of change in voltage due to photoelectric conversion increases. In other words, the Fermi level Ef in the two-dimensional material layer 2 changes significantly, and the modulation efficiency of the optical modulator 100 improves.

[0056] Here, the surface plasmon resonance phenomenon that occurs due to the interaction between a metal surface and light, the phenomenon called pseudo-surface plasmon resonance that occurs on metal surfaces outside the visible light or near-infrared light range, and the resonance phenomenon that occurs in metamaterials (plasmonic metamaterials) or metasurfaces that artificially manipulate wavelengths using structures smaller than the wavelength of electromagnetic waves are referred to as surface plasmon resonance, plasmon resonance, or simply resonance, without distinguishing between the names.

[0057] The material constituting the two-dimensional material layer 2 may be a two-dimensional semiconductor material having a band gap other than graphene. Examples of the material constituting the two-dimensional material layer 2 include Weyl semimetals such as transition metal dichalcogenides, borophene, black phosphorus, arsenic phosphide (AsP), silicene, germanene, aluminene, lead sulfide (PbS), gallium selenide (GaSe), indium selenide (In2Se3), zirconium disulfide (ZrS2), zirconium selenide (ZrSe2), tantalum sulfide (TaS), and tantalum arsenide (TaAs). Examples of transition metal dichalcogenides include molybdenum disulfide (MoS2), molybdenum selenide (MoSe2), molybdenum telluride (MoTe2), tungsten disulfide (WS2), tungsten selenide (WSe2), tungsten telluride (WTe2), hafnium disulfide (HfS2), tin sulfide (SnS2), tin selenide (SnSe2), rhenium sulfide (ReS2), and rhenium selenide (ReSe2).

[0058] In the first embodiment, the optical modulator 100 has been described as having a structure in which the optical waveguide portion 4 is embedded inside the insulating layer 5 as shown in FIG. 1, but the optical modulator 100 may have other structures.

[0059] 5 and 6 are schematic perspective views of an optical modulator 100 according to a modification of the first embodiment. FIG. 5 illustrates a channel-type optical modulator 100. Specifically, the optical waveguide portion 4 may be provided on the surface of the insulating layer 5. FIG. 6 illustrates a rib-type optical modulator 100. Specifically, the optical waveguide portion 4 may be provided on the surface of the insulating layer 5. The optical waveguide portion 4 includes a first region 4a and a second region 4b. The first region 4a is formed so as to cover the entire surface of the insulating layer 5. The second region 4b is provided in a portion of the surface of the first region 4a. The second region 4b is provided in the center of the first region 4a in the X direction. Note that FIGS. 5 and 6 illustrate the substrate 6, the insulating layer 5, and the optical waveguide portion 4. The other components are basically similar in configuration to the optical modulator 100 illustrated in FIGS. 1 to 3 and are therefore not illustrated.

[0060] <Method for manufacturing optical modulator> The following describes a method for manufacturing the optical modulator 100 according to the first embodiment. Figures 7 to 11 are schematic cross-sectional views showing steps in the method for manufacturing the optical modulator 100 according to the first embodiment.

[0061] First, a step (S1) of preparing a substrate 6 is performed. In this step (S1), a substrate 6 having an insulating layer 5 formed thereon is prepared. The insulating layer 5 may be a buried oxide. The material constituting the substrate 6 is, for example, silicon. When the material constituting the insulating layer 5 is silicon oxide, the optical waveguide section 4 may be formed using an SOI (Silicon On Insulator) substrate.

[0062] Next, a step (S2) of forming the optical waveguide portion 4 is carried out. In this step (S2), as shown in Fig. 7, a resist mask formed on the insulating layer 5 is patterned using electron beam exposure lithography to form the optical waveguide portion 4. After the optical waveguide portion 4 is formed, the remaining resist mask is removed by dry etching using an ICP (Inductive Coupled Plasma) device or the like.

[0063] Next, a step (S3) of depositing an insulating layer 5 is performed. In this step (S3), an insulating layer 5 made of silicon oxide or the like is deposited using a plasma CVD method, as shown in Fig. 8. The surface of the deposited insulating layer 5 may be planarized using chemical mechanical polishing (CMP) or the like.

[0064] Next, a step (S4) of forming the photoelectric conversion unit 1 is performed. In this step (S4), as shown in FIG. 9, the photoelectric conversion unit 1 is formed on the insulating layer 5. When the material constituting the photoelectric conversion unit 1 is graphene, the photoelectric conversion unit 1 may be formed by depositing a film of graphene on the insulating layer 5 using a CVD method, or by transferring graphene that has been mechanically peeled off from graphite onto the insulating layer 5. Thereafter, the photoelectric conversion unit 1 is formed by patterning the graphene coated with a resist mask using a photolithography method or the like. After the photoelectric conversion unit 1 is formed, the remaining resist mask is removed by dry etching.

[0065] Next, the first electrode 3a is formed on the photoelectric conversion part 1. The first electrode 3a may be formed using a lift-off method. Alternatively, the first electrode 3a may be formed by depositing a metal that constitutes the first electrode 3a and then removing unnecessary metal using a dry etching method or a wet etching method.

[0066] Next, a step (S5) of forming an insulating layer 5 is performed. In this step (S5), as shown in FIG. 10, an insulating layer 5 made of silicon oxide or the like is formed using a plasma CVD method. A thin insulating layer 5 may be formed using an atomic layer deposition (ALD) apparatus. In order to maintain the inherent electron mobility of graphene, hexagonal boron nitride may be formed as the material constituting the insulating layer 5. This improves the electron mobility of graphene compared to forming an insulating layer 5 using other materials, thereby improving the modulation efficiency of the optical modulator 100. The surface of the formed insulating layer 5 may be planarized using chemical mechanical polishing (CMP) or the like.

[0067] Next, a step (S6) of forming a two-dimensional material layer 2 is performed. In this step (S6), as shown in FIG. 11, the two-dimensional material layer 2 is formed on an insulating layer 5. This step (S6) may be performed in the same manner as the step (S4) of forming a photoelectric conversion unit 1, thereby forming the two-dimensional material layer 2 and the second electrode 3b. A thin insulating layer (not shown) may be further formed on the two-dimensional material layer 2 as a protective film using an ALD apparatus. If the first electrode 3a is covered with the insulating layer 5, an opening may be provided in the insulating layer 5 to expose the first electrode 3a. Note that, as shown in FIG. 3, the first electrode 3a may be formed to have a sufficient height so that the top surface of the first electrode 3a protrudes from the surface of the insulating layer 5. In this manner, the optical modulator 100 shown in FIGS. 1 to 3 can be obtained.

[0068] Although graphene has been mentioned as a material for forming the photoelectric conversion unit 1 and the two-dimensional material layer 2 in the optical modulator 100 according to the first embodiment, it may also be a semiconductor material such as silicon.

[0069] <Action and effect> The optical modulator 100 according to the present disclosure includes a photoelectric conversion unit 1, a two-dimensional material layer 2, and an optical waveguide unit 4. Control light CL is incident on the photoelectric conversion unit 1. The two-dimensional material layer 2 faces at least a portion of the photoelectric conversion unit 1. When viewed from the direction in which the control light CL is incident, at least a portion of the optical waveguide unit 4 is covered by the two-dimensional material layer 2.

[0070] In this way, photoelectric conversion can be caused to occur in the photoelectric conversion unit 1 by irradiating the photoelectric conversion unit 1 with control light CL. The occurrence of photoelectric conversion in the photoelectric conversion unit 1 can significantly change the Fermi level Ef of the graphene in the two-dimensional material layer 2. As a result, power consumption in the optical modulator 100 can be reduced compared to electrically controlling the Fermi level Ef of the graphene in the two-dimensional material layer 2. Furthermore, since the signal light SL can be modulated using the control light CL, the modulation speed in the optical modulator 100 is improved. Furthermore, since it is not necessary to simultaneously control the Fermi levels Ef of both the graphene in the two-dimensional material layer 2 and the photoelectric conversion unit 1, the absorption and transmission of the signal light SL can be easily controlled.

[0071] The optical modulator 100 includes a first electrode 3a and a second electrode 3b. The first electrode 3a is connected to the photoelectric conversion unit 1. The second electrode 3b is connected to the two-dimensional material layer 2.

[0072] In this way, a potential difference occurs between the photoelectric conversion section 1 and the two-dimensional material layer 2 by applying a bias voltage between the first electrode 3a and the second electrode 3b.

[0073] According to the optical modulator 100, at least one of the photoelectric conversion section 1 and the two-dimensional material layer 2 is formed of single-layer graphene or multi-layer graphene.

[0074] In this way, by using graphene as the material constituting the photoelectric conversion unit 1, the photoelectric conversion unit 1 can detect electromagnetic waves containing wavelength components in a wide band from ultraviolet to terahertz as the control light CL. Furthermore, by using graphene as the material constituting the two-dimensional material layer 2, the optical absorptance in the two-dimensional material layer 2 increases due to the optical gate effect. As a result, the modulation efficiency and modulation speed of the optical modulator 100 are improved.

[0075] According to the optical modulator 100, the multilayer graphene is turbostratic stacked graphene. This increases the light absorption rate in the two-dimensional material layer 2. As a result, the modulation efficiency and modulation speed of the optical modulator 100 are improved.

[0076] According to the optical modulator 100, the stacking angle in the multi-layer graphene is an angle that generates moire.

[0077] In this way, the optical absorption rate of electromagnetic waves in a wide wavelength range is improved compared to that of single-layer graphene, and the modulation efficiency of the optical modulator 100 is improved.

[0078] According to the optical modulator 100, at least one of the photoelectric conversion section 1 and the two-dimensional material layer 2 is made of graphene nanoribbon.

[0079] In this way, the two-dimensional material layer 2 improves the light absorption rate of electromagnetic waves (light) in a specific wavelength range.

[0080] Embodiment 2 <Configuration of optical modulator> FIG. 12 is a schematic cross-sectional view of an optical modulator 100 according to the second embodiment. FIG. 12 corresponds to FIG. 3. The optical modulator 100 shown in FIG. 12 basically has the same configuration as the optical modulator 100 shown in FIGS. 1 to 3 and can achieve the same effects, but differs in that the photoelectric conversion material constituting the photoelectric conversion unit 1 is a ferroelectric. In this way, when control light CL is incident on the photoelectric conversion unit 1, the potential in the photoelectric conversion unit 1 changes due to the pyroelectric effect. Therefore, an optical gate effect occurs in the first insulating region 5a, and the optical modulator 100 according to the second embodiment operates in the same way as the optical modulator 100 according to the first embodiment.

[0081] As described above, the second electrode 3b is connected to one end of the two-dimensional material layer 2. As shown in FIG. 12, a third electrode 3c may be connected to the other end of the two-dimensional material layer 2. The other end is located opposite the one end in the X direction. In this manner, a built-in potential is generated inside the two-dimensional material layer 2 due to the pyroelectric effect. Therefore, it is not necessary to apply a bias voltage between the first electrode 3a (see FIG. 3) and the second electrode 3b, and it is not necessary to provide the first electrode 3a connected to the photoelectric conversion unit 1. As a result, the power consumption in the optical modulator 100 can be further reduced.

[0082] The ferroelectric may be a material that generates polarization when electromagnetic waves having a wavelength to be detected are incident on the ferroelectric. The ferroelectric may include, for example, any material selected from the group consisting of barium titanate (BaTiO), lithium niobate (LiNbO), lithium tantalate (LiTaO), strontium titanate (SrTiO), lead zirconate titanate (PZT), strontium tantalate bismuthate (SBT), bismuth ferrite (BFO), zinc oxide (ZnO), hafnium oxide (HfO), and organic polymer polyvinylidene fluoride ferroelectrics (PVDF, P(VDF-TrFE), P(VDF-TrFE-CTFE), etc.). The ferroelectric may also be a material in which multiple layers of the above different materials are stacked or mixed.

[0083] As the ferroelectric, any material other than the above-mentioned materials can be used as long as it is a pyroelectric that exhibits the pyroelectric effect. Specifically, the pyroelectric that constitutes the ferroelectric may be any material that undergoes a polarization change in response to a change in thermal energy inside the pyroelectric. Regarding the pyroelectric effect described above, electromagnetic waves simply act as a heat source. Therefore, the magnitude of the pyroelectric effect does not fundamentally depend on the wavelength components contained in the electromagnetic waves. In other words, ferroelectrics are sensitive to electromagnetic waves that have a wide range of wavelength components.

[0084] The ferroelectric material constituting the photoelectric conversion section 1 can be formed by using an ALD apparatus in the step (S4) of forming the photoelectric conversion section 1.

[0085] <Action and effect> In the optical modulator 100, the photoelectric conversion section 1 is a ferroelectric material.

[0086] In this way, a built-in potential is generated inside the two-dimensional material layer 2 due to the pyroelectric effect. Therefore, it is no longer necessary to apply a bias voltage between the photoelectric conversion unit 1 and the two-dimensional material layer 2. As a result, the power consumption in the optical modulator 100 can be further reduced.

[0087] Embodiment 3 <Configuration of optical modulator> Fig. 13 is a schematic cross-sectional view of an optical modulator 100 according to the third embodiment. Fig. 13 corresponds to Fig. 3. The optical modulator 100 shown in Fig. 13 basically has the same configuration as the optical modulator 100 shown in Figs. 1 to 3 and can obtain the same effects, but differs in that a second electrode 3b is connected to one end of the two-dimensional material layer 2 and a third electrode 3c is connected to the other end.

[0088] For example, the second electrode 3b may be a source electrode, and the third electrode 3c may be a drain electrode. In this way, the two-dimensional material layer 2 becomes a channel, and the optical modulator 100 operates as a transistor. This makes it easier to control the Fermi level Ef in the two-dimensional material layer 2. Furthermore, the Fermi level Ef can be significantly changed by the optical gating effect. In other words, the optical absorption rate in the two-dimensional material layer 2 increases. As a result, the modulation efficiency and modulation speed of the optical modulator 100 are improved.

[0089] <Action and effect> The optical modulator 100 includes a third electrode 3c. The second electrode 3b is connected to one end of the two-dimensional material layer 2. The other end is located opposite the second electrode 3b. The third electrode 3c is connected to the other end.

[0090] In this way, the two-dimensional material layer 2 becomes a channel, and the optical modulator 100 operates as a transistor. This makes it easier to control the Fermi level Ef in the two-dimensional material layer 2. Furthermore, the Fermi level Ef can be significantly changed by the optical gating effect. In other words, the optical absorption rate in the two-dimensional material layer 2 increases. As a result, the modulation efficiency and modulation speed of the optical modulator 100 are improved.

[0091] Embodiment 4 <Configuration of optical modulator> Fig. 14 is a schematic cross-sectional view of an optical modulator 100 according to embodiment 4. Fig. 14 corresponds to Fig. 13. The optical modulator 100 shown in Fig. 14 basically has the same configuration as the optical modulator 100 shown in Fig. 13 and can obtain the same effects, but differs in that the materials constituting the second electrode 3b and the third electrode 3c are different from each other.

[0092] The two-dimensional material layer 2 receives different amounts of doping from the contact points of the second electrode 3b and the third electrode 3c. As a result, the two-dimensional material layer 2 serves as a channel, and a built-in potential is generated within the two-dimensional material layer 2. This eliminates the need to apply a bias voltage between the first electrode 3a and the second electrode 3b. As a result, the power consumption of the optical modulator 100 can be further reduced. As shown in FIG. 14, the second electrode 3b is electrically grounded, but the optical modulator 100 according to the fourth embodiment can operate even if the second electrode 3b is not electrically grounded.

[0093] <Action and effect> In the optical modulator 100, the second electrode 3b and the third electrode 3c are made of different materials.

[0094] In this way, the two-dimensional material layer 2 becomes a channel, and a built-in potential is generated inside the two-dimensional material layer 2. Therefore, it is no longer necessary to apply a bias voltage between the first electrode 3 a and the second electrode 3 b. As a result, the power consumption of the optical modulator 100 can be further reduced.

[0095] Embodiment 5 <Configuration of optical modulator> FIG. 15 is a schematic cross-sectional view of an optical modulator 100 according to a fifth embodiment. FIG. 15 corresponds to FIG. 3. The optical modulator 100 shown in FIG. 15 basically has the same configuration and can achieve the same effects as the optical modulator 100 shown in FIGS. 1 to 3, but differs in that the distance L2 between the photoelectric conversion unit 1 and the two-dimensional material layer 2 in the first insulating region 5a is the distance (thickness) through which a tunnel current flows. Specifically, the thickness of the first insulating region 5a (the distance L2 between the photoelectric conversion unit 1 and the two-dimensional material layer 2 in the first insulating region 5a) varies depending on the material of the insulating layer 5 and the bias voltage. For example, when the insulating layer is made of silicon oxide (SiO2), the thickness is approximately 3 nm or less. In particular, when the insulating layer 5 in the first insulating region 5a is formed from a single layer of hexagonal boron nitride, the thickness of the first insulating region 5a is 1 nm or less.

[0096] In this way, carriers generated in the photoelectric conversion unit 1 are injected into the two-dimensional material layer 2. As a result, the Fermi level Ef in the two-dimensional material layer 2 changes. That is, by setting the distance L2 between the photoelectric conversion unit 1 and the two-dimensional material layer 2 in the first insulating region 5a to a thickness that allows a tunnel current to flow, the signal light SL can be modulated. Since carriers are directly injected into the two-dimensional material layer 2, the Fermi level Ef in the two-dimensional material layer 2 changes significantly, and the modulation efficiency of the optical modulator 100 improves.

[0097] <Action and effect> The optical modulator 100 includes an insulating layer 5. The insulating layer 5 is connected to the two-dimensional material layer 2. The insulating layer 5 includes a first insulating region 5a. The first insulating region 5a is disposed between the photoelectric conversion unit 1 and the two-dimensional material layer 2. A distance L2 between the photoelectric conversion unit 1 and the two-dimensional material layer 2 in the first insulating region 5a is the distance through which a tunnel current flows.

[0098] In this way, carriers are injected directly into the two-dimensional material layer 2, so that the Fermi level Ef in the two-dimensional material layer 2 changes significantly, and the modulation efficiency in the optical modulator 100 improves.

[0099] Embodiment 6 <Configuration of optical modulator> FIG. 16 is a schematic cross-sectional view of an optical modulator 100 according to a sixth embodiment. FIG. 16 corresponds to FIG. 3. The optical modulator 100 shown in FIG. 16 basically has the same configuration as the optical modulator 100 shown in FIGS. 1 to 3 and can achieve the same effects. However, it differs in that the photoelectric conversion unit 1 is directly connected to the two-dimensional material layer 2. As shown in FIG. 16, the two-dimensional material layer 2 is bent inside the insulating layer 5. Specifically, when viewed from the Z direction, the two-dimensional material layer 2 is bent at a first bend toward the photoelectric conversion unit 1 at a position overlapping with an end of the photoelectric conversion unit 1. The portion of the two-dimensional material layer 2 located closer to the photoelectric conversion unit 1 than the first bend includes a first portion extending toward the photoelectric conversion unit 1 and a second portion extending in a direction intersecting the first portion. The second portion is in contact with the photoelectric conversion unit 1. The second portion is connected to the first portion via the second bend. In this way, the two-dimensional material layer 2 is connected to the photoelectric conversion unit 1 inside the insulating layer 5.

[0100] When the material constituting the photoelectric conversion unit 1 is silicon and the material constituting the two-dimensional material layer 2 is graphene, the photoelectric conversion unit 1 and the two-dimensional material layer 2 are connected to form a Schottky junction. When the control light CL is visible light, irradiating the control light CL onto silicon serving as the photoelectric conversion unit 1 causes photoelectric conversion in the silicon. Carriers generated by the photoelectric conversion are injected into graphene serving as the two-dimensional material layer 2. Alternatively, carriers may be injected from graphene into silicon. This significantly changes the Fermi level Ef in the graphene. In this way, the signal light SL can be modulated.

[0101] An energy barrier is formed between silicon and graphene by a Schottky barrier. If carriers can overcome the energy barrier by irradiating the photoelectric conversion unit 1 with control light CL, the Fermi level Ef of graphene can be changed without applying a bias voltage between the first electrode 3a and the second electrode 3b. Therefore, the optical modulator 100 can operate at zero bias, and the power consumption of the optical modulator 100 can be further reduced.

[0102] <Action and effect> According to the optical modulator 100 , the photoelectric conversion section 1 is directly connected to the two-dimensional material layer 2 .

[0103] In this way, the Fermi level Ef of the two-dimensional material layer 2 can be changed without applying a bias voltage between the first electrode 3 a and the second electrode 3 b. Therefore, the optical modulator 100 can operate at zero bias, and the power consumption of the optical modulator 100 can be further reduced.

[0104] Embodiment 7 <Configuration of optical modulator> FIG. 17 is a schematic cross-sectional view of an optical modulator 100 according to a seventh embodiment. FIG. 17 corresponds to FIG. 16. The optical modulator 100 shown in FIG. 17 basically has the same configuration as the optical modulator 100 shown in FIG. 16 and can achieve the same effects. However, it differs in that a barrier layer 7 is disposed between the photoelectric conversion unit 1 and the two-dimensional material layer 2. That is, the photoelectric conversion unit 1 and the two-dimensional material layer 2 may be connected via the barrier layer 7. The material constituting the barrier layer 7 may be, for example, an insulating material similar to the insulating layer 5. The thickness of the barrier layer 7 in the Z direction is the thickness through which a tunneling current flows. The thickness of the barrier layer 7 in the Z direction may be, for example, 1 nm or more and 10 nm or less.

[0105] This makes it possible to suppress dark current generated between the photoelectric conversion section 1 and the two-dimensional material layer 2. That is, the signal-to-noise ratio (S / N ratio) of the optical modulator 100 is improved. As a result, the modulation efficiency of the optical modulator 100 is improved.

[0106] <Action and effect> The optical modulator 100 includes a barrier layer 7. The barrier layer 7 is disposed between the photoelectric conversion section 1 and the two-dimensional material layer 2.

[0107] This makes it possible to suppress dark current generated between the photoelectric conversion section 1 and the two-dimensional material layer 2. That is, the signal-to-noise ratio (S / N ratio) of the optical modulator 100 is improved. As a result, the modulation efficiency of the optical modulator 100 is improved.

[0108] Embodiment 8 <Configuration of optical modulator> FIG. 18 is a schematic cross-sectional view of an optical modulator 100 according to the eighth embodiment. FIG. 18 corresponds to FIG. 3. The optical modulator 100 shown in FIG. 18 basically has the same configuration as the optical modulator 100 shown in FIGS. 1 to 3 and can achieve the same effects, but differs in that it includes a reflective layer 8. At least a portion of the reflective layer 8 is covered by the photoelectric conversion unit 1 when viewed from the direction in which the control light CL is incident. In other words, the reflective layer 8 is disposed between the photoelectric conversion unit 1 and the substrate 6 in the Z direction.

[0109] A portion of the control light CL incident on the photoelectric conversion unit 1 is transmitted through the photoelectric conversion unit 1. The control light CL that has transmitted through the photoelectric conversion unit 1 is reflected by the reflective layer 8. The reflected control light CL is incident on the photoelectric conversion unit 1 again. In this way, the amount of light incident on the photoelectric conversion unit 1 increases, and the amount of change in voltage due to photoelectric conversion increases. In other words, the Fermi level Ef in the two-dimensional material layer 2 changes significantly, and the modulation efficiency of the optical modulator 100 improves.

[0110] When the control light CL is visible light, the material constituting the reflective layer 8 may be thin-film aluminum. The thickness of the reflective layer 8 in the Z direction may be changed appropriately depending on the wavelength of the control light CL, as long as it is thick enough to reflect all of the control light CL. In other words, the thickness of the reflective layer 8 in the Z direction may be determined by the skin depth (the thickness at which evanescent light seeps out), and is, for example, equal to or greater than the skin depth. The material constituting the reflective layer 8 may also be a multilayer ferroelectric.

[0111] The reflective layer 8 may have a photonic crystal structure, and a plurality of holes may be periodically formed in the reflective layer 8. When the reflective layer 8 has a photonic crystal structure, the reflective layer 8 reflects the control light CL. The material and structure constituting the reflective layer 8 may be set arbitrarily.

[0112] <Action and effect> The optical modulator 100 includes a reflective layer 8. At least a part of the reflective layer 8 is covered by the photoelectric conversion section 1 when viewed from the direction in which the control light CL is incident.

[0113] In this way, the control light CL that has passed through the photoelectric conversion unit 1 is reflected by the reflective layer 8. The reflected control light CL is incident on the photoelectric conversion unit 1 again. That is, since the amount of light incident on the photoelectric conversion unit 1 increases, the amount of change in voltage due to photoelectric conversion increases. As a result, the Fermi level Ef in the two-dimensional material layer 2 changes significantly, and the modulation efficiency of the optical modulator 100 improves.

[0114] Embodiment 9 <Configuration of optical modulator> Fig. 19 is a schematic perspective view of an optical modulator 100 according to the ninth embodiment. Fig. 19 corresponds to Fig. 1. The optical modulator 100 shown in Fig. 19 basically has the same configuration as the optical modulator 100 shown in Figs. 1 to 3 and can obtain the same effects, but differs in that it is a rib-type optical modulator 100.

[0115] Specifically, as shown in FIG. 19 , the insulating layer 5 includes a flat portion 5b and a convex portion 5c. The flat portion 5b is disposed on the substrate 6. The convex portion 5c is disposed at the center of the flat portion 5b in the X direction. In this manner, the insulating layer 5 has a rib shape. The photoelectric conversion portion 1 is disposed inside the flat portion 5b. The optical waveguide portion 4 is disposed inside the convex portion 5c. The two-dimensional material layer 2 is formed on the surfaces of the flat portion 5b and the convex portion 5c. Although not shown, an insulating layer may be further formed on the two-dimensional material layer 2 as a protective film. In this case, the first electrode 3a and the second electrode 3b may be exposed from the insulating layer as a protective film. The first electrode 3a is disposed at a position that does not overlap with the two-dimensional material layer 2 in the Y direction. Therefore, when viewed from the Z direction, the two-dimensional material layer 2 is formed so as to overlap the entire width of the photoelectric conversion portion 1 in the X direction. The photoelectric conversion portion 1 is formed so as to be longer than the length of the two-dimensional material layer 2 in the Y direction. The first electrode 3a is disposed on a portion of the photoelectric conversion unit 1 that extends to a region that does not overlap with the two-dimensional material layer 2 in the Y direction.

[0116] Embodiment 10 <Configuration of optical modulator> Fig. 20 is a schematic plan view of a two-dimensional material layer 2 of an optical modulator 100 according to embodiment 10. Figs. 21 to 23 are schematic plan views of a two-dimensional material layer 2 of an optical modulator 100 according to a modification of embodiment 10. The optical modulator 100 shown in Figs. 20 to 23 basically has the same configuration as the optical modulator 100 shown in Figs. 1 to 3 and can obtain the same effects, but differs in that the material constituting the photoelectric conversion unit 1 is a two-dimensional material in which a pattern that generates plasmon resonance is formed.

[0117] 20 to 23 are schematic plan views of the photoelectric conversion unit 1 as viewed from the Z direction. The photoelectric conversion unit 1 may have any shape as long as plasmon resonance occurs. For example, as shown in FIGS. 20 and 21, the photoelectric conversion unit 1 may have a plurality of through holes 21h. The plurality of through holes 21h may be periodically arranged in each of the X direction and the Y direction. In other words, the plurality of through holes 21h may be arranged in a matrix. The planar shape of the plurality of through holes 21h may be any shape, and may be, for example, a circular shape as shown in FIG. 20. The planar shape of the plurality of through holes 21h may be, for example, an elliptical shape. Note that the planar shape of the photoelectric conversion unit 1 is, for example, a rectangular shape, but may be any other shape.

[0118] 21, the planar shape of the plurality of through holes 21h may be quadrangular. The planar shape of the plurality of through holes 21h may be square or rectangular. The planar shape of the plurality of through holes 21h may be polygonal, such as triangular or pentagonal. The planar shapes of the plurality of through holes 21h may be different from each other.

[0119] As shown in FIGS. 22 and 23, the photoelectric conversion unit 1 may have a rectangular planar shape. As shown in FIG. 22, the photoelectric conversion unit 1 may be composed of a plurality of photoelectric conversion portions 21a arranged side by side at intervals in the Y direction. Each of the plurality of photoelectric conversion portions 21a has a linear shape extending along the X direction. As shown in FIG. 22, the widths of the plurality of photoelectric conversion portions 21a in the Y direction may be the same or different. Furthermore, the distance in the Y direction between two adjacent photoelectric conversion portions 21a may vary periodically or non-periodically depending on the position in the Y direction.

[0120] As shown in FIG. 23, the photoelectric conversion unit 1 may be composed of a plurality of photoelectric conversion portions 21a arranged at intervals in the Y direction and a plurality of photoelectric conversion portions 21b arranged in a matrix in each of the X and Y directions. The photoelectric conversion portions 21a are arranged so as to connect two adjacent photoelectric conversion portions 21b. In FIG. 23, the photoelectric conversion portions 21a are arranged so as to connect two adjacent photoelectric conversion portions 21b in the X direction. The plurality of photoelectric conversion portions 21a and 21b may be formed in the same layer, or the photoelectric conversion portion 21b may be stacked on a portion of the photoelectric conversion portion 21a. Alternatively, the photoelectric conversion portion 21a may be stacked on a portion of the photoelectric conversion portion 21b.

[0121] As described above, the shape of the photoelectric conversion unit 1 has a periodic or aperiodic pattern, which causes plasmon resonance in the photoelectric conversion unit 1. The generation of plasmon resonance improves the light absorption rate of electromagnetic waves in the photoelectric conversion unit 1. As a result, the voltage in the two-dimensional material layer 2 changes significantly, improving the modulation efficiency of the optical modulator 100. Furthermore, the bias voltage applied between the first electrode 3a and the second electrode 3b can be suppressed, which further reduces the power consumption of the optical modulator 100.

[0122] Furthermore, when the photoelectric conversion unit 1 has a rectangular planar shape, it can absorb light having an electric field component in a direction perpendicular to the longitudinal direction of the photoelectric conversion portion 21a. In other words, the graphene in the photoelectric conversion unit 1 has polarization selectivity for the control light CL. In this way, when the optical modulator 100 is applied to a large-scale optical circuit and multiple control lights CL are present, the amount of information obtained by polarization can be increased.

[0123] <Action and effect> According to the optical modulator 100, the material constituting the photoelectric conversion section 1 is a two-dimensional material in which a pattern that generates plasmon resonance is formed.

[0124] In this way, plasmon resonance occurs in the photoelectric conversion unit 1. The generation of plasmon resonance improves the light absorption rate of electromagnetic waves in the photoelectric conversion unit 1. As a result, the voltage in the two-dimensional material layer 2 changes significantly, improving the modulation efficiency of the optical modulator 100. In addition, since the bias voltage applied between the first electrode 3a and the second electrode 3b can be suppressed, the power consumption of the optical modulator 100 can be further reduced.

[0125] Embodiment 11 <Configuration of optical integrated circuit> FIG. 24 is a schematic diagram of an optical integrated circuit 200 according to the eleventh embodiment. The optical integrated circuit 200 shown in FIG. 24 includes an optical modulator 100, which is a Mach-Zehnder modulator. Specifically, the Mach-Zehnder modulator includes a substrate (not shown) and an optical waveguide section 4. The substrate is the same as the substrate 6 of the first embodiment. The optical waveguide section 4 is provided on the substrate. The optical waveguide section 4 includes an input section 41, a lower arm section 42, an upper arm section 43, and an output section 44.

[0126] The input section 41 branches into an upper arm section 43 and a lower arm section 42. The branched upper arm section 43 and lower arm section 42 join together at the output section 44. The optical modulator 100 according to any one of the first to tenth embodiments is applied to the upper arm section 43. The optical modulator 100 may also be applied to the lower arm section 42.

[0127] The signal light SL is input to the input unit 41. The signal light SL propagating through the input unit 41 is split into an upper arm unit 43 and a lower arm unit 42. The signal light SL propagating through the upper arm unit 43 and the lower arm unit 42 is combined and output to the output unit 44. Using the optical modulator 100 provided in the upper arm unit 43, the signal light SL is modulated, for example, so that the phase of the signal light SL is adjusted.

[0128] Embodiment 12 <Configuration of optical integrated circuit> FIG. 25 is a schematic diagram of an optical integrated circuit 200 according to a twelfth embodiment. The optical integrated circuit 200 shown in FIG. 25 includes an optical modulator 100 that is a ring modulator. Specifically, the ring modulator includes an input waveguide portion 45 and a ring waveguide portion 46 as the optical waveguide portion 4. The input waveguide portion 45 is, for example, linear. The ring waveguide portion 46 is, for example, annular. The ring waveguide portion 46 is disposed near the input waveguide portion 45. The optical modulator 100 according to any one of the first to tenth embodiments is applied to the ring waveguide portion 46.

[0129] The signal light SL is input from one end of the input waveguide 45. A portion of the signal light SL is coupled into the ring waveguide 46. The signal light SL propagating through the ring waveguide 46 is modulated using an optical modulator 100. The signal light SL propagating through the ring waveguide 46 is coupled into the input waveguide 45 and output from the other end of the input waveguide 45.

[0130] <Action and effect> The optical integrated circuit 200 according to the present disclosure includes an optical modulator 100. The optical modulator 100 is a Mach-Zehnder modulator or a ring modulator.

[0131] In this way, it is possible to obtain an optical integrated circuit 200 that can modulate the signal light SL using a Mach-Zehnder modulator or a ring modulator and has reduced power consumption.

[0132] Embodiment 13 <Configuration of optical modulator> Fig. 26 is a schematic perspective view of an optical modulator 100 according to embodiment 13. Fig. 26 corresponds to Fig. 1. The optical modulator 100 shown in Fig. 26 basically has the same configuration as the optical modulator 100 shown in Figs. 1 to 3 and can obtain the same effects, but differs in that the optical waveguide portion 4 is a plasmonic waveguide 40 in which surface plasmon resonance occurs.

[0133] The conventional optical waveguide unit 4 uses the difference in refractive index to confine the signal light SL propagating through the optical waveguide unit 4 inside the optical waveguide unit 4. On the other hand, the optical waveguide unit 4 as the plasmonic waveguide 40 according to the thirteenth embodiment confines the signal light SL inside the optical waveguide unit 4 by utilizing surface plasmon polaritons.

[0134] The surface plasmon polaritons are localized on the surface of the optical waveguide unit 4 serving as the plasmonic waveguide 40. Therefore, some of the surface plasmon polaritons seep out from the optical waveguide unit 4 as evanescent light. The seeped-out surface plasmon polaritons are absorbed by the two-dimensional material layer 2, thereby enabling the optical modulator 100 to modulate the signal light SL. The modulation principle by the two-dimensional material layer 2 according to the thirteenth embodiment is basically the same as the modulation principle by the two-dimensional material layer 2 according to the first embodiment.

[0135] The material constituting the optical waveguide portion 4 (plasmonic waveguide 40) in the present embodiment 13, which generates surface plasmon polaritons, may be changed depending on the wavelength of the target signal light SL. Specifically, the wavelength range for optical communication is 1.3 μm or 1.5 μm. When the wavelength of the signal light SL propagating through the optical waveguide portion 4 is in the wavelength range for optical communication, the material constituting the optical waveguide portion 4 may be a noble metal such as gold (Au) or silver (Ag), or may be titanium nitride (TiN).

[0136] The material forming the optical waveguide unit 4 as the plasmonic waveguide 40 may be graphene. Graphene is a material in which plasmon resonance occurs. When the material forming the optical waveguide unit 4 is graphene, the signal light SL is converted into surface plasmon polaritons. As a result, the signal light SL is tightly confined within the graphene. That is, the volume of the optical waveguide unit 4 as the plasmonic waveguide 40 can be reduced to half or less the volume of an optical waveguide unit 4 made of a normal dielectric. In this way, an optical circuit using the optical modulator 100 according to the thirteenth embodiment can be further miniaturized. Furthermore, when the material forming the optical waveguide unit 4 is graphene, the modulation efficiency of the optical modulator 100 is improved.

[0137] <Action and effect> According to the optical modulator 100, surface plasmon resonance occurs in the optical waveguide section 4.

[0138] In this way, the optical waveguide portion 4 functions as a plasmonic waveguide 40. As a result, it is possible to further miniaturize an optical circuit using the optical modulator 100. Furthermore, when the material forming the optical waveguide portion 4 is graphene, the modulation efficiency of the optical modulator 100 is improved.

[0139] Embodiment 14 <Configuration of optical modulator> Fig. 27 is a schematic perspective view of an optical modulator 100 according to the fourteenth embodiment. Fig. 27 corresponds to Fig. 1. Fig. 28 is a schematic cross-sectional view of the optical modulator 100 according to the fourteenth embodiment. Fig. 28 corresponds to Fig. 3. The optical modulator 100 shown in Figs. 27 and 28 basically has the same configuration as the optical modulator 100 shown in Figs. 1 to 3 and can obtain the same effects, but differs in that the optical waveguide portion 4 is a slot waveguide.

[0140] Specifically, the optical waveguide portion 4, which is a slot waveguide, includes a pair of end portions 47 and a central portion 48. A gap is formed between the pair of end portions 47. The central portion 48 is disposed in the gap. From a different perspective, the central portion 48 is sandwiched between the pair of end portions 47 in a direction (X direction in the fourteenth embodiment) perpendicular to the direction in which the optical waveguide portion 4 extends (Y direction). The refractive index of the central portion 48 is smaller than the refractive index of the end portions 47. The pair of end portions 47 are made of a material such as Si. The material of the central portion 48 may be air or an insulator. In this way, the signal light SL is confined in the gap formed between the pair of end portions 47 and propagates therethrough.

[0141] In a conventional optical waveguide section 4 made of Si, there is a limit to the input power in two-photon absorption in a high power density region. Also, there is a limit to the carrier absorption rate due to free carrier absorption. On the other hand, in the optical waveguide section 4 as a slot waveguide according to the fourteenth embodiment, the input power in two-photon absorption in a high power density region is increased. Also, the carrier absorption rate due to free carrier absorption is increased.

[0142] The material constituting the pair of end portions 47, which generates surface plasmon polaritons, may be a noble metal such as gold (Au) or silver (Ag). In this way, the optical waveguide portion 4 becomes a slot waveguide that functions as the plasmonic waveguide 40. As a result, the optical circuit using the optical modulator 100 can be further miniaturized. Furthermore, the modulation efficiency of the optical modulator 100 is improved.

[0143] Fig. 29 is a schematic perspective view of an optical modulator 100 according to a modification of the fourteenth embodiment. Fig. 29 corresponds to Fig. 27. Fig. 30 is a schematic cross-sectional view of an optical modulator 100 according to a modification of the fourteenth embodiment. Fig. 30 corresponds to Fig. 28. As shown in Figs. 29 and 30, in the modification of the fourteenth embodiment, a central portion 48 may be sandwiched between a pair of end portions 47 in the Y direction.

[0144] 30 , a portion of one end 47 extends in the X direction to both ends of the insulating layer 5. In this manner, the width in the X direction of a portion of one end 47 may be larger than the width in the X direction of the other end 47, and may be larger than the width in the X direction of the central portion 48.

[0145] <Action and effect> According to the optical modulator 100, the optical waveguide portion 4 includes a pair of end portions 47 and a central portion 48. In a direction (X direction or Z direction) perpendicular to the direction in which the optical waveguide portion 4 extends (Y direction), the central portion 48 is sandwiched between the pair of end portions 47. The refractive index of the central portion 48 is smaller than the refractive index of the end portions 47.

[0146] This increases the input power of two-photon absorption in the high power density region, and also increases the rate of carrier absorption due to free carrier absorption.

[0147] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The basic scope of the present disclosure is defined by the claims, not the above description, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0148] 1 photoelectric conversion section, 2 two-dimensional material layer, 3a first electrode, 3b second electrode, 3c third electrode, 4 optical waveguide section, 4a first region, 4b second region, 5 insulating layer, 5a first insulating region, 5b flat section, 5c convex section, 6 substrate, 7 barrier layer, 8 reflective layer, 21a photoelectric conversion section, 21b photoelectric conversion section, 21h through-hole, 40 plasmonic waveguide, 41 input section, 42 lower arm section, 43 upper arm section, 44 output section, 45 input waveguide section, 46 ring waveguide section, 47 end section, 48 central section, 100 optical modulator, 200 optical integrated circuit, A1 range, A2 range, A3 range, CL control light, Ef Fermi level, f frequency, h Planck constant, hf energy, L1 distance, L2 Distance, SL signal light.

Claims

1. a photoelectric conversion material layer to which control light is incident; a two-dimensional material layer facing at least a portion of the photoelectric conversion material layer; an optical waveguide portion at least part of which is covered by the two-dimensional material layer when viewed from a direction in which the control light is incident; the two-dimensional material layer includes, when viewed from a direction in which the control light is incident, a first portion covering the photoelectric conversion material layer and a second portion covering the optical waveguide portion; the control light is incident in a direction perpendicular to the photoelectric conversion material layer; In a plan view of the photoelectric conversion material layer, the photoelectric conversion material layer is spaced apart from the optical waveguide portion.

2. a first electrode connected to the photoelectric conversion material layer; and a second electrode connected to the two-dimensional material layer.

3. 3. The optical modulator according to claim 2, further comprising a third electrode connected to one end of the two-dimensional material layer, the third electrode being located opposite to the second electrode connected to the other end.

4. The optical modulator according to claim 3 , wherein the second electrode and the third electrode are made of different materials.

5. an insulating layer connected to the two-dimensional material layer; the insulating layer includes a first insulating region disposed between the photoelectric conversion material layer and the two-dimensional material layer; The optical modulator according to claim 1 , wherein a distance between the photoelectric conversion material layer and the two-dimensional material layer in the first insulating region is a distance through which a tunnel current flows.

6. The optical modulator of claim 1 , wherein the photoelectric conversion material layer is directly connected to the two-dimensional material layer.

7. The optical modulator of claim 1 , further comprising a barrier layer disposed between the photoelectric conversion material layer and the two-dimensional material layer.

8. The optical modulator according to claim 1 , wherein at least one of the photoelectric conversion material layer and the two-dimensional material layer is formed of single-layer graphene or multi-layer graphene.

9. The optical modulator of claim 8 , wherein the multilayer graphene is turbostratic stacked graphene.

10. The optical modulator according to claim 8 , wherein a stacking angle in the multilayer graphene is an angle at which moire occurs with respect to an incident plane perpendicular to a direction in which the control light is incident.

11. The optical modulator according to claim 1 , wherein at least one of the photoelectric conversion material layer and the two-dimensional material layer is a graphene nanoribbon.

12. 2. The optical modulator according to claim 1, wherein the photoelectric conversion material layer is a ferroelectric material.

13. 2. The optical modulator according to claim 1, wherein the material constituting the photoelectric conversion material layer is a two-dimensional material having a pattern formed thereon that generates plasmon resonance.

14. 2. The optical modulator according to claim 1, further comprising a reflective layer at least part of which is covered by the photoelectric conversion material layer when viewed from the direction in which the control light is incident.

15. The optical modulator according to claim 1 , wherein surface plasmon resonance occurs in the optical waveguide portion.

16. In a direction perpendicular to the direction in which the optical waveguide portion extends, the optical waveguide portion includes a pair of end portions and a central portion sandwiched between the pair of end portions, 2. The optical modulator of claim 1, wherein the refractive index at the center portion is smaller than the refractive index at the end portions.

17. An optical modulator according to any one of claims 1 to 16, The optical integrated circuit, wherein the optical modulator is a Mach-Zehnder modulator or a ring modulator.

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