Light receiving element for optical communication, and optical communication device

Abstract

To provide a light receiving element for optical communication superior in polarization distinctiveness, wavelength separability, high-speed responsiveness, and thinning.SOLUTION: A light receiving element for optical communication comprises: a semiconductor layer or a dielectric layer; a semiconductor light absorption layer that is formed on the semiconductor layer or the dielectric layer; a short wavelength infrared transparent layer that is formed on the semiconductor light absorption layer; and a metal diffraction grating that is formed on the short wavelength infrared transparent layer. Short wavelength infrared light incident from an upper direction of the metal diffraction grating made incident, a period p of the metal diffraction grating is set so that a surface plasmon resonance wave is excited on a boundary surface between the metal diffraction grating and the short wavelength infrared transparent layer by the short wavelength infrared light. A thickness of the semiconductor light absorption layer is set so that the period p of the metal diffraction grating and a waveguide mode transmission wavelength β in which the three layers (the semiconductor layer or the dielectric layer, the semiconductor light absorption layer, and the short wavelength infrared transparent layer) are regarded as a clad core light waveguide path satisfy a condition of β=p*0.9 to 1.1.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a light receiving element for optical communication and an optical communication device. [Background technology]

[0002] The progress of optical communications in recent years has been remarkable, and information traffic is increasing year by year. The introduction of Beyond5G is being considered as the next-generation information and communications infrastructure for 5G communications. Beyond5G aims to transmit information capacity on the petabit level, so there is a demand for the development of optical communication photodetectors with high sensitivity and fast response. In addition, with the advancement of space-time division multiplexing technology, the acquisition of phase information is required in multi-level modulation methods. Conventional in-plane signal input / output methods using waveguide branching require the use of optical couplers, splitters, polarizers, receivers, and optical waveguides to connect them, etc. This requires an area of ​​several square millimeters per channel, making it difficult to integrate and parallelize multiple channels.

[0003] As a method for solving the above-mentioned problems, a normal incidence type coherent receiver has been proposed in Non-Patent Document 1, and in recent years, the reception of high-speed optical signals using a multilevel modulation method has been demonstrated. This normal incidence type coherent receiver improves the transmittance for specific polarized light by utilizing the surface plasmon resonance of a metal diffraction grating. For example, in order to improve the polarization discrimination, the period of the metal diffraction grating is designed to be 1,100 nm, and the thickness of InGaAs is designed to be 800 nm in order to increase the optical absorption rate of InGaAs and achieve high sensitivity.

[0004] Patent Document 1 proposes a semiconductor light receiving element including a semiconductor layer including a light absorbing layer, a metal periodic structure provided on the semiconductor layer, forming a Schottky junction with the semiconductor layer, having a slit-shaped opening, and having a period capable of inducing surface plasmons for incident light polarized perpendicular to the long axis direction of the slit-shaped opening, and a thickness capable of obtaining sufficient transmittance for incident light polarized parallel to the long axis direction of the slit-shaped opening, an anti-reflection film formed on the semiconductor layer and the metal periodic structure, and a semiconductor multilayer reflective film provided below the semiconductor layer. The semiconductor light receiving element proposed in Patent Document 1 has high transmittance for both TM (Transverse Magnetic) polarized light, which is polarized perpendicular to the long axis direction of the slit-shaped opening, and TE (Transverse Electric) polarized light, which is polarized parallel to the long axis direction of the slit-shaped opening. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication 2010 / 021073

[0006] [Non-Patent Document 1] Takuo Tanemura et.al, “Ultra-Broadband Surface-Normal Coherent Optical Receiver with Nanometallic Polarizers”, ACS Photonics 2022 9, 2842-2849 Summary of the Invention [Problem to be solved by the invention]

[0007] The semiconductor photodetector proposed in Patent Document 1 has high transmittance for both TM polarized light and TE polarized light, and is intended to reduce dependency on the polarization direction of incident light. Meanwhile, in a multilevel modulation method, which is one of the multiplexing techniques for optical signals, it is required to detect the phase, and from the viewpoint of enabling phase detection, there is a demand for a semiconductor photodetector with excellent polarization discrimination ability that improves the transmittance for a specific polarized light.

[0008] The normal incidence type coherent receiver proposed in Non-Patent Document 1 has excellent polarization discrimination that improves the transmittance for specific polarized light, but is intended to absorb TM incident light in a wide wavelength range. On the other hand, there is also a demand to improve the wavelength discrimination that selectively absorbs light of specific wavelengths, and to selectively absorb light of specific wavelengths that differ for each receiver.

[0009] The present disclosure has been made in consideration of the above, and aims to provide a light receiving element for optical communications that is excellent in polarization discrimination, wavelength discrimination, high-speed response, and thinness, and an optical communication device equipped with the same. [Means for solving the problem]

[0010] Specific means for achieving the above object are as follows. <1> a semiconductor or dielectric layer; a semiconductor light absorbing layer formed on the semiconductor layer or the dielectric layer; a short-wave infrared transparent layer formed on the semiconductor light absorbing layer; a metal diffraction grating formed on the short-wave infrared transparent layer; a period p of the metal diffraction grating is set so that a surface plasmon resonance wave is excited at an interface between the metal diffraction grating and the short-wave infrared transparent layer by short-wave infrared light incident from above the metal diffraction grating; A light-receiving element for optical communications, in which the thickness of the semiconductor light absorption layer is set so that the period p of the metal diffraction grating and the waveguide mode propagation wavelength β, in which the three layers are regarded as a clad-core optical waveguide, satisfy the condition β=p×0.9 to 1.1. <2> The period p is 200 nm to 1000 nm. <1> 2. The light receiving element for optical communication according to claim 1 . <3> The thickness of the semiconductor light absorbing layer is 100 nm to 600 nm. <1> or <2> 2. The light receiving element for optical communication according to claim 1 . <4> The thickness of the short-wave infrared transparent layer is 600 nm or less. <1> ~ <3> 13. A light-receiving element for optical communications according to claim 12. <5> The metal diffraction grating has a duty ratio of 0.5 to 0.9 and a height of 100 nm to 500 nm. <1> ~ <4> 13. A light-receiving element for optical communications according to claim 12. <6> the semiconductor layer or dielectric layer and the short-wave infrared transparent layer each independently contain at least one selected from the group consisting of InP, InAlAs, Si, and SiO2; The semiconductor light absorbing layer is made of InGaAs, Si x Ge 1-x , Ge, GaN, GaAs, and Si. <1> ~ <5> 13. A light receiving element for optical communication according to claim 12. <7> The metal diffraction grating contains at least one metal selected from the group consisting of Al, Ag, Au, and Cu. <1> ~ <6> 13. A light receiving element for optical communication according to claim 12. <8> <1> ~ <7> 13. An optical communication device comprising a light receiving section in which the light receiving element for optical communication according to any one of claims 1 to 12 is disposed. Effect of the Invention

[0011] The present disclosure can provide a light-receiving element for optical communications that is excellent in polarization discrimination, wavelength discrimination, high-speed response, and thinness, and an optical communication device including the same. [Brief description of the drawings]

[0012] [Figure 1] 1 is a schematic diagram illustrating an example of a light receiving element for optical communication according to the present disclosure. [Diagram 2] 1 is a graph showing the relationship between the thickness t1 of an InGaAs layer and the propagation wavelength β for a vacuum wavelength of 1550 nm. [Diagram 3]13 is a graph showing the analysis results of the reflection and absorption spectra of TM incident light and TE incident light when m=1. [Figure 4] FIG. 13 is a diagram showing an analysis result of the electric field intensity distribution for TM incident light with a wavelength of 1550 nm when m=1. [Diagram 5] FIG. 13 is a diagram showing an analysis result of the electric field intensity distribution for TE incident light with a wavelength of 1550 nm when m=1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, embodiments of the present disclosure will be described in detail. However, the present disclosure is not limited to the following embodiments. In the present disclosure, the components (including element steps, etc.) are not essential unless otherwise specified. The same applies to numerical values ​​and their ranges, and do not limit the present disclosure. In the present disclosure, a numerical range indicated using "~" includes the numerical values ​​before and after "~" as the minimum and maximum values, respectively. In the numerical ranges described in the present disclosure in stages, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. In addition, in the numerical ranges described in the present disclosure, the upper or lower limit value of the numerical range may be replaced with a value shown in the examples. In the present disclosure, the terms "layer" and "film" include cases where the layer or film is formed over the entire area when the area in which the layer or film is present is observed, as well as cases where the layer or film is formed over only a portion of the area. In the present disclosure, the thickness of a layer or film is determined as the arithmetic mean value of thicknesses measured at five points on the layer or film of interest. The thickness of the layer or film may be measured by observing a cross section of the measurement target using an electron microscope. The short-wave infrared light incident from above the metal diffraction grating 4 is, for example, light in a wavelength range of 700 nm to 2500 nm. In the embodiment described later, light in a wavelength range of 1550 nm is used as an example, but the present disclosure is not limited to this.

[0014] The light receiving element for optical communication according to the present disclosure includes a semiconductor layer or a dielectric layer, a semiconductor light absorbing layer formed on the semiconductor layer or the dielectric layer, a short-wave infrared transparent layer formed on the semiconductor light absorbing layer, and a metal diffraction grating formed on the short-wave infrared transparent layer, in which a period p of the metal diffraction grating is set so that a surface plasmon resonance wave is excited at the interface between the metal diffraction grating and the short-wave infrared transparent layer by short-wave infrared light incident from above the metal diffraction grating, and a thickness of the semiconductor light absorbing layer is set so that the period p of the metal diffraction grating and a waveguide mode propagation wavelength β, in which the three layers (semiconductor layer or dielectric layer, semiconductor light absorbing layer, and short-wave infrared transparent layer) are regarded as a clad core optical waveguide, satisfy the condition that β=p×0.9 to 1.1.

[0015] In the light receiving element for optical communication disclosed herein, the period p of the metal diffraction grating is set so that the surface plasmon resonance wave is excited, and the thickness t1 of the semiconductor light absorbing layer is set so that the period p of the metal diffraction grating and the waveguide mode propagation wavelength β are equal or close to each other. As a result, a light receiving element for optical communication having high light absorption rate and having both high sensitivity and high speed response can be obtained by coupling the surface plasmon resonance with respect to the TM incident light to the semiconductor light absorbing layer. Furthermore, by adopting the above configuration, it is possible to selectively improve the transmittance for TM polarized light of a specific wavelength, and the light receiving element has excellent polarization discrimination and wavelength discrimination. Even if the thickness t1 of the semiconductor light absorbing layer is made thinner than that of Non-Patent Document 1, a high light absorption rate of the same level can be obtained, and the light receiving element for optical communication can be made thinner and have a higher response speed.

[0016] The light receiving element for optical communication according to the present disclosure is used as a photodiode or the like in an optical communication device such as a photodetector.

[0017] An example of a light receiving element for optical communications according to the present disclosure will be described below with reference to Fig. 1. The light receiving element for optical communications 10 includes a semiconductor layer or dielectric layer 1, a semiconductor light absorbing layer 2, a short-wave infrared transparent layer 3, and a metal diffraction grating 4, in this order.

[0018] The semiconductor layer or dielectric layer 1 is a layer provided on the side opposite to the short-wave infrared transparent layer 3 side of the semiconductor light absorbing layer 2, and is a semiconductor layer made of a semiconductor or a dielectric layer made of a dielectric. From the viewpoint of exerting a light trapping effect in the semiconductor light absorbing layer 2, the semiconductor layer or dielectric layer 1 is preferably a layer having a smaller refractive index than the semiconductor light absorbing layer 2.

[0019] The semiconductor layer or dielectric layer 1 preferably contains at least one material selected from the group consisting of InP, InAlAs, Si, and SiO2. The semiconductor layer or dielectric layer 1 may be made of one of the above materials, or may be made of two or more of the above materials.

[0020] The semiconductor layer or dielectric layer 1 may also serve as the substrate, or a substrate may be disposed on the side of the semiconductor layer or dielectric layer 1 opposite to the semiconductor light absorbing layer 2. The material of the substrate is not particularly limited, and examples thereof include InP.

[0021] The semiconductor light absorbing layer 2 is formed on the semiconductor layer or dielectric layer 1, and is a layer located between the semiconductor layer or dielectric layer 1 and the short-wave infrared transparent layer 3. The thickness of the semiconductor light absorbing layer 2 is set so that the period p of the metal diffraction grating 4 and the waveguide mode propagation wavelength β satisfy the condition β=p×0.9 to 1.1. As a result, surface plasmon resonance is coupled to the semiconductor light absorbing layer 2 for TM incident light, thereby obtaining a high light absorptance.

[0022] The semiconductor light absorbing layer 2 is preferably a layer having a higher refractive index than the semiconductor layer or dielectric layer 1 and the short-wave infrared transparent layer 3. This tends to provide a strong light confinement effect in the semiconductor light absorbing layer 2 and to provide a higher light absorptance.

[0023] The semiconductor light absorbing layer 2 is made of InGaAs, Si x Ge 1-xIt is preferable that the semiconductor light absorbing layer 2 contains at least one material selected from the group consisting of Ge, GaN, GaAs, and Si. The semiconductor light absorbing layer 2 may be made of one of the above-mentioned materials, or may be made of two or more of the above-mentioned materials.

[0024] The thickness of the semiconductor light absorbing layer 2 may be set so as to satisfy the condition β=p×0.9 to 1.1, and from the viewpoint of obtaining a higher light absorptance, may be set so as to satisfy preferably the condition β=p×0.95 to 1.05, more preferably the condition β=p.

[0025] The thickness of the semiconductor light absorbing layer 2 is not particularly limited as long as it satisfies the above-mentioned conditions, and is a value that is appropriately set according to the value of the period p or the propagation wavelength β. From the viewpoint of high-speed response, the thinner the thickness of the semiconductor light absorbing layer 2, the more preferable. For example, the thickness of the semiconductor light absorbing layer 2 is preferably 100 nm to 600 nm, more preferably 150 nm to 500 nm, and further preferably 200 nm to 400 nm.

[0026] The short-wave infrared transparent layer 3 is a layer formed on the semiconductor light absorbing layer 2 and located between the metal diffraction grating 4 and the semiconductor light absorbing layer 2. The short-wave infrared transparent layer 3 is made of a material that is transparent to the short-wave infrared light incident from above the metal diffraction grating 4.

[0027] The shortwave infrared transparent layer 3 includes at least one selected from the group consisting of InP, InAlAs, Si, and SiO2. The shortwave infrared transparent layer 3 may be made of one of the above-mentioned materials, or may be made of two or more of the above-mentioned materials.

[0028] For example, the materials of each of the three layers (semiconductor layer or dielectric layer 1 / semiconductor light absorption layer 2 / shortwave infrared transparent layer 3) may be InP / InGaAs / InP, InAlAs / InGaAs / InAlAs, SiO2 / Si / SiO2, etc., but the present disclosure is not limited thereto.

[0029] The above-mentioned three layers can be formed by, for example, metal organic chemical vapor deposition, molecular beam epitaxy, or the like.

[0030] The thickness of the shortwave infrared transparent layer 3 must be equal to or less than the wavelength of the shortwave infrared light, because the shortwave infrared light converted into an evanescent field by surface plasmon resonance is coupled to the waveguide mode of the semiconductor light absorption layer 2. From the viewpoint of light absorptance, the thickness of the shortwave infrared transparent layer 3 is preferably equal to or less than 600 nm, more preferably 10 nm to 600 nm, and even more preferably 50 nm to 500 nm.

[0031] The metal diffraction grating 4 is formed on the shortwave infrared transparent layer 3. Furthermore, the period p of the metal diffraction grating is set so that shortwave infrared light incident from above the metal diffraction grating 4 (the direction of the arrow A in FIG. 1) excites a surface plasmon resonance wave at the interface between the metal diffraction grating 4 and the shortwave infrared transparent layer 3. The metal diffraction grating 4 is arranged in a slit shape in a plan view.

[0032] The material of the metal diffraction grating 4 is not particularly limited as long as it is a conductive metal. Specifically, the metal diffraction grating 4 preferably contains at least one metal selected from the group consisting of Al, Ag, Au, and Cu. Among them, Ag or Au is preferable from the viewpoint of low electrical resistance and small surface plasmon loss. The metal diffraction grating 4 may be made of one type of metal, or may be made of two or more types of metals.

[0033] The period p of the metal diffraction grating 4 is not particularly limited as long as it is set to a value at which surface plasmon resonance waves are excited by shortwave infrared light at the interface between the metal diffraction grating 4 and the shortwave infrared transparent layer 3. The period p of the metal diffraction grating described above is a value that varies when the wavelength of the incident light, the angle of incidence, the material of the metal diffraction grating 4 or the shortwave infrared transparent layer 3, etc. are changed, as shown in the examples described later.

[0034] The period p of the metal diffraction grating 4 is preferably equal to or less than the wavelength of the short-wave infrared light that is the incident light. For example, it must be equal to or less than 2500 nm in order to excite surface plasmon resonance waves, and is preferably 200 nm to 1000 nm in consideration of the refractive index of the short-wave infrared transparent layer.

[0035] The width w of the metal diffraction grating 4 is, for example, preferably 100 nm to 900 nm, and more preferably 120 nm to 850 nm.

[0036] The duty ratio of the metal diffraction grating 4 is a value determined by the period p and width w, and from the viewpoint of improving polarization discrimination and surface plasmon resonance excitation efficiency, is preferably 0.5 to 0.9, and more preferably 0.6 to 0.85. The duty ratio refers to w / p, which is the ratio of the width w of the metal diffraction grating 4 to the period p of the metal diffraction grating 4.

[0037] The ratio of the total area of ​​the openings in a plan view of the metal diffraction grating 4 to the total area (the total area of ​​the openings and the metal parts) in a plan view of the metal diffraction grating 4 may be, for example, 0.1 to 0.5 or 0.15 to 0.4. This reduces the transmittance for TE polarized light that does not excite surface plasmons, and provides excellent polarization discrimination.

[0038] The height h of the metal diffraction grating 4 is preferably from 100 nm to 500 nm, and more preferably from 200 nm to 350 nm.

[0039] The light receiving element for optical communication of the present disclosure may or may not have a configuration other than the semiconductor layer or dielectric layer, the semiconductor light absorbing layer, the short-wave infrared transparent layer, or the metal diffraction grating. For example, a substrate may be provided separately from the semiconductor layer or the dielectric layer. Alternatively, an anti-reflection film, a protective film, or the like may be provided on the metal diffraction grating, or may not be provided. The light receiving element for optical communication of the present disclosure selectively improves the transmittance for TM polarized light and has excellent polarization discrimination. Therefore, a configuration for increasing the light absorption of TE polarized light, for example, a configuration in which a Bragg reflection film is disposed on the opposite side of the semiconductor light absorbing layer of the semiconductor layer or the dielectric layer, is not necessary.

[0040] <Optical communication devices> The optical communication device according to the present disclosure includes a light receiving unit in which the light receiving element for optical communication according to the present disclosure is disposed. The optical communication device may include, for example, a module housing for storing the light receiving element for optical communication according to the present disclosure, a light emitting unit for emitting shortwave infrared light to the metal diffraction grating of the light receiving element for optical communication, a preamplifier for amplifying an electrical signal converted by the light receiving element for optical communication, and the like. EXAMPLES

[0041] The present disclosure will be described in more detail below with reference to examples. Note that the present disclosure is not limited to the following examples.

[0042] [Example] A study was conducted on the structure of a photodiode having a semiconductor layer or a dielectric layer, a semiconductor light absorbing layer, a short-wave infrared transparent layer, and a metal diffraction grating in this order, as shown in FIG. The semiconductor or dielectric layer / semiconductor light absorption layer / shortwave infrared transparent layer were made of InP / InGaAs / InP, and the metal diffraction grating was made of gold and had a slit shape in plan view.

[0043] (Determination of period p of metal diffraction grating) We investigated the matching of the wave number of the surface plasmon with the wave number of the period p of the metal diffraction grating for TM light with a wavelength of 1550 nm incident from above the metal diffraction grating. Specifically, we investigated the period p of the metal diffraction grating such that the surface plasmon resonance wave is excited at the interface between the metal diffraction grating and the shortwave infrared transparent layer. The conditional formula for surface plasmon resonance at the interface between the metal diffraction grating and the short-wave infrared transparent layer (also called the Au / InP interface) is given by the following formula (1).

[0044]

number

[0045] In the above formula (1), k sp is the wave number of the surface plasmon, k0 is the wave number of the incident light, θ is the angle of incidence, m is the diffraction order, p is the period of the metal diffraction grating, and ε Au is the relative dielectric constant of the gold that constitutes the metal diffraction grating, and ε InP is the relative dielectric constant of the InP that constitutes the shortwave infrared transparent layer. As shown in equation (2), for TM light (θ=0) with a wavelength of 1550 nm incident from above the metal diffraction grating, the wave number K of the structural period vector and the wave number k of the first-order (m=1) surface plasmon are SP We investigated ways to improve the light absorption rate by matching the above.

[0046]

number

[0047] The period p of the metal diffraction grating was calculated based on equation (2) to be 470 nm. The period p of the metal diffraction grating that satisfies the surface plasmon resonance condition is a value that varies when the wavelength of the incident light, the incident angle, the diffraction order, the material of the metal diffraction grating or the short-wave infrared transparent layer, etc. are changed, and is not limited to the above value in the present invention. Furthermore, the wave number K of the structural period vector and the wave number k of the surface plasmon other than the first order (e.g., second order, third order) SPEven when the above is matched, the light absorption rate can be improved.

[0048] (Determination of the relationship between the propagation wavelength β and the thickness t1 of the semiconductor light absorption layer) The relationship between the waveguide mode propagation wavelength β, which is assumed to be a clad-core optical waveguide consisting of three layers (InP / InGaAs / InP), and the thickness of the semiconductor light absorption layer (InGaAs layer) was investigated. Considering the TM incident light with a wavelength of 1550 nm, the TM waveguide mode was examined. The validity of each of the above decisions for m=1 was examined under the consideration that the waveguide mode propagation wavelength β and the period p of the metal diffraction grating are matched, and the surface plasmon resonance is coupled to the InGaAs layer, and the light propagates in the InGaAs layer. From the equations expressing the TM and TE waveguide modes of the waveguide, the relationship between the thickness t1 of the InGaAs layer and the propagation wavelength β was obtained for a vacuum wavelength of 1550 nm. The results are shown in Figure 2.

[0049] Figure 2 shows the relationship between the period p (470 nm, arrow (1) in Figure 2) that satisfies the surface plasmon resonance condition when m = 1, and the thickness t1 of the InGaAs layer that matches the propagation wavelength β in the InGaAs layer. Of the InGaAs layer thicknesses t1 that result in a propagation wavelength β of 470 nm, we focused on the TM0 mode, where t1 is the smallest. t1 in the TM0 mode was approximately 300 nm.

[0050] (Analysis of optical absorption rate of InGaAs layer) Based on the analysis results of the propagation characteristics of the waveguide, we analyzed the optical absorption rate when the InGaAs layer thickness t1 was 300 nm for perpendicularly incident light (θ=0) with a wavelength of 1.55 μm at a period p (470 nm) that satisfies the surface plasmon resonance condition at m=1. In this case, the width w and height h of the metal diffraction grating were set to 420 nm and 280 nm, respectively. The InP layer thickness t2 was set to 300 nm.

[0051] (Analysis of reflection and absorption spectra of TM incident light and TE incident light) The reflection and absorption spectra of TM incident light and TE incident light were analyzed for each structural parameter determined as described above. The results are shown in Figure 3. The figure shows the spectral characteristics of reflectance and absorption for TM incident light and TE incident light of each wavelength (1.0 μm to 1.7 μm). For comparison, the absorption spectrum of TM incident light in the InGaAs layer (w / o Au in the figure) was analyzed for a three-layer structure (InP / InGaAs / InP) without a metal diffraction grating.

[0052] As shown in FIG. 3, the photodiode with the Au / InP / InGaAs / InP layer structure had a 6 to 7 times higher optical absorption rate than the photodiode without the metal diffraction grating for the TM incident light with a wavelength of 1.55 μm. Furthermore, the intensity of the reflection spectrum of the TM incident light was significantly reduced around the wavelength of 1.55 μm, and the reflectance was almost 100% and the absorption was almost zero for the TE incident light regardless of the wavelength, confirming excellent wavelength discrimination and polarization discrimination. In addition, the polarization extinction ratio (PER) at a wavelength of 1.55 μm was calculated using the following formula (3). The polarization extinction ratio (PER) is an index of polarization discrimination, and the higher the value, the better the polarization discrimination. The results are shown in Table 1.

[0053]

number

[0054] [Table 1]

[0055] (Analysis of electric field, magnetic field, optical power and absorption distribution at a wavelength of 1550 nm) We analyzed the electric field intensity distribution for TM incident light and TE incident light with a wavelength of 1.55 μm for a photodiode with a layer structure of Au / InP / InGaAs / InP that satisfies the conditions shown in Table 1. The finite difference time domain method was used for the analysis. The results are shown in Figures 4 and 5.

[0056] As shown in Figure 4, for TM incident light, an enhanced electric field was observed near the metal diffraction grating, and a standing wave distribution was observed in the InGaAs layer. This suggests that surface plasmon resonance was excited and coupled with the waveguide mode in the InGaAs layer, and it is considered that the absorption rate was improved by light confinement in the InGaAs layer. On the other hand, no standing wave distribution was observed in the InGaAs layer for the TE incident light, as shown in Figure 5. As corresponds to the spectrum results, it was confirmed that the TE incident light was almost entirely reflected by the Au top surface.

[0057] Furthermore, the optical absorptance was 42% when the thickness t1 of the InGaAs layer was 300 nm, which was comparable to the optical absorptance (40%) when the thickness of the InGaAs layer was 800 nm in Non-Patent Document 1. Therefore, in this example, even when the thickness of the InGaAs layer was thinned, it was possible to obtain an optical absorptance equivalent to that in Non-Patent Document 1, and it was confirmed that it is possible to thin the light receiving element for optical communication. [Explanation of symbols]

[0058] 1 Semiconductor layer or dielectric layer 2. Semiconductor light absorbing layer 3. Short-wave infrared transparent layer 4 Metallic Diffraction Grating 10. Photodetectors for optical communications

Claims

1. A semiconductor layer or a dielectric layer made of a semiconductor or a dielectric; a semiconductor light absorbing layer formed on the semiconductor layer or the dielectric layer; a short-wave infrared transparent layer formed on the semiconductor light absorbing layer; a metal diffraction grating formed on the short-wave infrared transparent layer; a period p of the metal diffraction grating is set so that a surface plasmon resonance wave is excited at an interface between the metal diffraction grating and the short-wave infrared transparent layer by short-wave infrared light incident from above the metal diffraction grating; A light receiving element for optical communication, in which the thickness of the semiconductor light absorption layer is set so that the period p of the metal diffraction grating and the waveguide mode propagation wavelength β, in which the three layers are regarded as a clad-core optical waveguide, satisfy the condition β = p × 0.9 to 1.

1.

2. 2. The light-receiving element for optical communications according to claim 1, wherein the period p is in the range of 200 nm to 1000 nm.

3. 2. The light receiving element for optical communications according to claim 1, wherein the semiconductor light absorbing layer has a thickness of 100 nm to 600 nm.

4. 2. The light-receiving element for optical communications according to claim 1, wherein the short-wave infrared transparent layer has a thickness of 600 nm or less.

5. 2. The light receiving element for optical communications according to claim 1, wherein the metal diffraction grating has a duty ratio of 0.5 to 0.9 and a height of the metal diffraction grating of 100 nm to 500 nm.

6. The semiconductor or dielectric layer and the short-wave infrared transparent layer are each independently selected from InP, InAlAs, Si, and SiO ２ At least one selected from the group consisting of: The semiconductor light absorbing layer is made of InGaAs, Si ｘ Ge １－ｘ 2. The light-receiving element for optical communications according to claim 1, comprising at least one selected from the group consisting of Ge, GaN, GaAs and Si.

7. 2. The light-receiving element for optical communication according to claim 1, wherein the metal diffraction grating contains at least one metal selected from the group consisting of Al, Ag, Au, and Cu.

8. An optical communication device comprising a light receiving section in which the light receiving element for optical communication according to any one of claims 1 to 7 is arranged.

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