Graphene Optical Sensor

Abstract

To provide a graphene optical sensor capable of reducing polarization dependency.SOLUTION: A graphene optical sensor has a graphene layer, a first electrode and a second electrode connected to the graphene layer and arranged in a first direction parallel with a top surface of the graphene layer, and a plurality of plasmon antennas which are provided on the graphene layer between the first electrode and second electrodes, wherein the plasmon antennas each have a first rod part extending in a second direction inclined from the first direction in plan view, and a second rod part extending in a third direction inclined from the first direction to the opposite side of the second direction based upon the first direction, and crossing the first rod part, and the plurality of plasmon antennas are arranged periodically in the second direction and the third direction.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to graphene optical sensors. [Background technology]

[0002] Materials emit thermal radiation according to their temperature. The wavelength of thermal radiation from materials at room temperature is the strongest in the infrared wavelength range. For this reason, infrared sensors that detect thermal radiation are widely used in night vision and thermography.

[0003] In recent years, a method of infrared detection using graphene has been proposed. Graphene is a two-dimensional material in which carbon atoms are arranged in a two-dimensional honeycomb pattern. Graphene has a unique energy band structure, so it can absorb light in a wide wavelength range from the ultraviolet to the terahertz band.

[0004] One example of an optical sensor using graphene is the graphene photoconductor. The photoconductor is composed of graphene and electrodes connected to both ends. When light is incident on the graphene layer, it is absorbed and electron-hole pairs are generated. The generated electrons and holes are collected by the electrodes by applying a voltage between them, forming a photocurrent. By measuring the change in the amount of current, the light incident on the sensor element can be detected.

[0005] However, the light absorption efficiency of graphene is typically a few percent, which is smaller than that of common materials used in optical sensors. In addition, electron-hole pairs generated by light absorption may recombine, becoming inactive and no longer contributing to the photocurrent. To solve these problems, optical sensors with antenna rods arranged on graphene have been proposed. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2015-45629 A [Patent Document 2] JP 2019-36755 A [Patent Document 3] US Patent Application Publication No. 2017 / 0153367 [Non-patent literature]

[0007] [Non-Patent Document 1] Y. Yao et. al, "High-Responsivity Mid-Infrared Graphene Detectors with Antenna-Enhanced Photocarrier Generation and Collection," Nano Letters 14, 3749 (2014) Summary of the Invention [Problem to be solved by the invention]

[0008] Conventional optical sensors with antenna rods arranged on graphene have polarization dependency. Therefore, when the optical signal to be detected is polarized, the optical sensor must be arranged in a direction that allows efficient detection. In addition, when the optical signal to be detected is unpolarized, more than half of the components of the optical signal cannot be detected.

[0009] An object of the present disclosure is to provide a graphene optical sensor capable of reducing polarization dependence. [Means for solving the problem]

[0010] According to one embodiment of the present disclosure, there is provided a graphene optical sensor comprising: a graphene layer; first and second electrodes connected to the graphene layer and aligned in a first direction parallel to a surface of the graphene layer; and a plurality of plasmon antennas provided on the graphene layer between the first and second electrodes, each of the plasmon antennas comprising: a first rod portion extending in a second direction inclined from the first direction in a planar view; and a second rod portion extending from the first direction in a third direction inclined opposite to the second direction with respect to the first direction and intersecting the first rod portion, and the plurality of plasmon antennas are periodically arranged in the second direction and the third direction. Effect of the Invention

[0011] According to the present disclosure, it is possible to reduce the polarization dependence. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a plan view showing a graphene optical sensor according to an embodiment. [Diagram 2] 1 is a cross-sectional view showing a graphene optical sensor according to an embodiment and a mode of use thereof; [Diagram 3] FIG. 2 is a schematic diagram showing a structure to be a target of a simulation. [Figure 4] 13A and 13B are diagrams illustrating the results of a simulation in the case where light polarized in a second direction is incident. [Diagram 5] FIG. 13 is a diagram showing the results of a simulation in the case where light polarized in a third direction is incident. [Figure 6] 3A and 3B are schematic diagrams illustrating the behavior of electrons and holes in the embodiment. [Figure 7] FIG. 11 is a schematic diagram showing the behavior of electrons and holes in a reference example. [Figure 8] FIG. 11 is a plan view showing a graphene optical sensor according to a modified example of the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In this specification and the drawings, components having substantially the same functional configurations are denoted by the same reference numerals, and redundant description may be omitted.

[0014] The present embodiment relates to an optical sensor using graphene (graphene optical sensor). Fig. 1 is a plan view showing a graphene optical sensor according to an embodiment. Fig. 2 is a cross-sectional view showing the graphene optical sensor according to an embodiment and its usage mode.

[0015] The graphene optical sensor 1 according to the embodiment mainly includes a substrate 10, a graphene layer 20, a first electrode 31, a second electrode 32, and a plurality of plasmon antennas 40, as shown in FIGS.

[0016] The substrate 10 includes a conductive substrate 11 and an insulating layer 12 formed on the conductive substrate 11. For example, the conductive substrate 11 is a Si substrate, and the insulating layer 12 is a SiO2 layer. The conductive substrate 11 is used as a gate electrode.

[0017] A graphene layer 20 is provided on the insulating layer 12. The graphene layer 20 includes one or more graphenes stacked on each other. When the graphene layer 20 includes more than one graphene, it is preferable that the more than one graphenes are randomly (rotated) stacked. This is because the random (rotated) stacking provides a relatively high carrier mobility.

[0018] A first electrode 31 and a second electrode 32 are provided on the graphene layer 20. The first electrode 31 and the second electrode 32 are electrically connected to the graphene layer 20. The first electrode 31 and the second electrode 32 are arranged in a first direction parallel to the surface of the graphene layer 20. The material of the first electrode 31 and the second electrode 32 is not particularly limited, and for example, Au, Pd, Ni, Cr, or Ti may be used. The first electrode 31 and the second electrode 32 may include a laminate of these metals. For example, the first electrode 31 and the second electrode 32 may include a laminate of a Ti film and an Au film formed on the Ti film, or may include a laminate of a Cr film and an Au film formed on the Cr film. The materials of the first electrode 31 and the second electrode 32 may be the same or different.

[0019] A plurality of plasmon antennas 40 are provided on the graphene layer 20 between the first electrode 31 and the second electrode 32. The material of the plasmon antennas 40 is not particularly limited. For example, the plasmon antenna 40 may include a laminate of a Pd film and an Au film formed on the Pd film. For example, the thickness of the Pd film is about 10 nm, and the thickness of the Au film is about 30 nm.

[0020] The plasmon antenna 40 has a first rod portion 41 and a second rod portion 42. The first rod portion 41 extends in a second direction inclined at 45° from the first direction, and the second rod portion 42 extends in a third direction inclined at 45° from the first direction to the opposite side of the second direction with respect to the first direction. Therefore, the angle between the second direction and the third direction is 90°. The first rod portion 41 and the second rod portion 42 intersect with each other. The first rod portion 41 and the second rod portion 42 have, for example, a rectangular planar shape with a longitudinal length of about 1.14 μm and a lateral length of about 0.24 μm, and intersect with each other at the center in the longitudinal direction. The plasmon antenna 40 has an X-shaped planar shape.

[0021] The multiple plasmon antennas 40 are arranged at a constant period in the second direction and the third direction. In the second direction, the multiple first rod portions 41 are arranged linearly at a constant period, and in the third direction, the multiple second rod portions 42 are arranged linearly at a constant period. For example, the period of the plasmon antenna 40 arrangement is 1.20 μm. In the second direction, there is a gap of about 60 μm between adjacent first rod portions 41, and in the third direction, there is a gap of about 60 μm between adjacent second rod portions 42.

[0022] The graphene optical sensor 1 has such a configuration.

[0023] When the graphene optical sensor 1 is used, a DC power supply 51 is connected between the conductive substrate 11 and the first electrode 31, as shown in Fig. 2. The positive electrode of the DC power supply 51 is connected to the conductive substrate 11, and the negative electrode is connected to the first electrode 31. In addition, a bias power supply 52 and an ammeter 53 are connected between the first electrode 31 and the second electrode 32. The positive electrode of the bias power supply 52 is connected to the ammeter 53, and the negative electrode is connected to the first electrode 31.

[0024] Then, a gate voltage is applied to the conductive substrate 11 by the DC power supply 51, and the potential of the graphene layer 20 is adjusted. A bias voltage is applied between the first electrode 31 and the second electrode 32 from the bias power supply 52, and the amount of current flowing through the ammeter 53 is detected. In this manner, the amount of light incident on the graphene optical sensor 1 can be measured. For example, the magnitude of the gate voltage is 5V, and the magnitude of the bias voltage is 0.6V.

[0025] Here, the function of the plasmon antenna 40 will be described. The first rod portion 41 extending in the second direction has an electric field in the second direction, and is coupled with light having a wavelength that satisfies the resonance condition to generate plasmon resonance. The second rod portion 42 extending in the third direction has an electric field in the third direction, and is coupled with light having a wavelength that satisfies the resonance condition to generate plasmon resonance. Light is expressed as a combination of orthogonal linearly polarized light. Therefore, the plasmon antenna 40, which has an X-shaped planar shape, can be coupled with light having any polarization to generate plasmon resonance.

[0026] Next, a simulation simulating the embodiment will be described. FIG. 3 is a schematic diagram showing a structure to be simulated. This structure 100 has a graphene layer 120 corresponding to the graphene layer 20, and a plurality of plasmon antennas 140 each corresponding to the plasmon antenna 40. The plasmon antenna 140 has a first rod portion 141 corresponding to the first rod portion 41, and a second rod portion 142 corresponding to the second rod portion 42. Then, the electric field intensity distribution was calculated for the case where light having polarization in the second direction is incident on the structure 100 shown in FIG. 3 and the case where light having polarization in the third direction is incident on the structure 100 shown in FIG. 4 and FIG. 5 show the results of the simulation for the case where light having polarization in the second direction is incident. FIG. 5 shows the results of the simulation for the case where light having polarization in the third direction is incident.

[0027] As shown in FIG. 4, when light polarized in the second direction is incident, an electric field component is localized in the gap between the first rod parts 141 adjacent to each other in the second direction. Therefore, a strong electric field region is formed in this gap, and the light absorption efficiency of the graphene layer 120 is enhanced. Also, as shown in FIG. 5, when light polarized in the third direction is incident, an electric field component is localized in the gap between the second rod parts 142 adjacent to each other in the third direction. Therefore, a strong electric field region is formed in this gap, and the light absorption efficiency of the graphene layer 120 is enhanced. In this way, in both the case where light polarized in the second direction is incident and the case where light polarized in the third direction is incident, the light absorption efficiency of the graphene layer 120 is enhanced. Also, when light polarized in a direction between the second direction and the third direction is incident, an electric field region according to the direction is formed in the gap between the first rod parts 141 and the gap between the second rod parts 142, and the light absorption efficiency of the graphene layer 120 is enhanced.

[0028] In addition, in this embodiment, since the multiple plasmon antennas 40 are periodically arranged in the second and third directions, the electrons and holes generated in the graphene layer 20 can be moved to the first electrode 31 and the second electrode 32 while suppressing deactivation due to recombination. Fig. 6 is a schematic diagram showing the behavior of electrons and holes in this embodiment.

[0029] When a bias electric field 60 is formed from the first electrode 31 toward the second electrode 32, a local electric field 61 associated with the bias electric field 60 is formed near the first rod parts 41 adjacent to each other in the second direction, as shown in FIG. 6. This local electric field 61 is oriented in a direction parallel to the second direction. Therefore, electrons generated in the graphene layer 20 between the first rod parts 41 adjacent to each other in the second direction are collected by the local electric field 61 in the first rod part 41 on the second electrode 32 side (high potential side), and holes are collected by the local electric field 61 in the first rod part 41 on the first electrode 31 side (low potential side). Then, the electrons collected in the first rod part 41 reach the second electrode 32 via the other plasmon antennas 40 and the graphene layer 20 on the second electrode 32 side (high potential side). Moreover, the holes collected in the first rod portion 41 reach the first electrode 31 via the other plasmon antenna 40 and the graphene layer 20 on the first electrode 31 side (low potential side).

[0030] Although not shown in the figure, a local electric field caused by the bias electric field 60 is also formed near the second rod parts 42 adjacent to each other in the second direction. This local electric field is oriented in a direction parallel to the third direction. Therefore, electrons generated in the graphene layer 20 between the second rod parts 42 adjacent to each other in the third direction are collected by the second rod part 42 on the second electrode 32 side (high potential side) by the local electric field, and holes are collected by the second rod part 42 on the first electrode 31 side (low potential side) by the local electric field. Then, the electrons collected by the second rod part 42 reach the second electrode 32 while passing through the other plasmon antenna 40 and the graphene layer 20 on the second electrode 32 side (high potential side). Also, the holes collected by the second rod part 42 reach the first electrode 31 while passing through the other plasmon antenna 40 and the graphene layer 20 on the first electrode 31 side (low potential side).

[0031] As described above, in this embodiment, since the multiple plasmon antennas 40 are periodically arranged in the second and third directions, the electrons and holes generated in the graphene layer 20 can be moved to the first electrode 31 and the second electrode 32 while suppressing deactivation due to recombination. Therefore, according to this embodiment, the polarization dependency can be reduced.

[0032] Note that even if the first rod parts 41 extend in the first direction and the second rod parts 42 extend in a direction inclined by 90° from the first direction, the effect of enhancing the light absorption efficiency of the graphene layer 120 described above can be obtained. However, in this case, as in the following reference example, even if electrons and holes are generated in the graphene layer 20, recombination is likely to occur. Fig. 7 is a schematic diagram showing the behavior of electrons and holes in the reference example.

[0033] In the reference example shown in FIG. 7, even if a bias electric field 60 is formed from the first electrode 31 toward the second electrode 32, a local electric field is not formed near the second rod portions 42 adjacent to each other in a direction perpendicular to the first direction. Therefore, even if electrons and holes are generated in the graphene layer 20 between the adjacent second rod portions 42, the electrons and holes are not collected by the second rod portions 42, and are deactivated by recombination before reaching the first electrode 31 or the second electrode 32. Even if the electrons and holes are collected by the second rod portions 42, no charge bias occurs, and therefore no photocurrent is generated. Therefore, in the reference example shown in FIG. 7, although the effect of enhancing the light absorption efficiency of the graphene layer 120 is obtained, a photocurrent corresponding to the enhanced light absorption efficiency is not obtained, and it is difficult to reduce the polarization dependency.

[0034] The angle between the first direction and the second direction is not limited to 45°, and the angle between the first direction and the third direction is not limited to 45°. The angle between the first direction and the second direction is preferably 40° or more and 50° or less, more preferably 42° or more and 48° or less, and even more preferably 45°. The angle between the first direction and the third direction is preferably 40° or more and 50° or less, more preferably 42° or more and 48° or less, and even more preferably 45°.

[0035] Furthermore, the angle between the second direction and the third direction is not limited to 90°. The angle between the second direction and the third direction is preferably 80° or more and 90° or less, more preferably 85° or more and 90° or less, and even more preferably 90°.

[0036] The distance between adjacent first rod portions in the second direction is preferably 20 nm or more and 100 nm or less, more preferably 30 nm or more and 90 nm or less, and further preferably 40 nm or more and 80 nm or less.

[0037] The planar shape of the plasmon antenna is not limited to an X-shape. For example, as in the graphene optical sensor 2 according to the modified example shown in Fig. 8, the plasmon antenna 40 may have a third rod portion 43 extending in the first direction in addition to the first rod portion 41 and the second rod portion 42. In this case, for example, the second direction may be a direction inclined at 60° from the first direction, and the third direction may be a direction inclined at 60° from the first direction to the opposite side to the second direction with respect to the first direction.

[0038] Although the preferred embodiments have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications and substitutions can be made to the above-described embodiments without departing from the scope of the claims.

[0039] Various aspects of the present disclosure are summarized below as appendices.

[0040] (Appendix 1) A graphene layer; A first electrode and a second electrode connected to the graphene layer and aligned in a first direction parallel to a surface of the graphene layer; a plurality of plasmon antennas disposed on the graphene layer between the first electrode and the second electrode; having Each of the plasmon antennas has, in a plan view, A first rod portion extending in a second direction inclined from the first direction; a second rod portion extending from the first direction in a third direction inclined toward an opposite side to the second direction with respect to the first direction and intersecting the first rod portion; having The graphene optical sensor is characterized in that the multiple plasmon antennas are periodically arranged in the second direction and the third direction. (Appendix 2) the angle between the first direction and the second direction is greater than or equal to 40° and less than or equal to 50°; The graphene optical sensor described in Appendix 1, characterized in that the angle between the first direction and the third direction is greater than or equal to 40° and less than or equal to 50°. (Appendix 3) The angle between the first direction and the second direction is 45°, The graphene optical sensor described in Appendix 1, characterized in that the angle between the first direction and the third direction is 45°. (Appendix 4) 4. The graphene optical sensor according to claim 1, wherein the angle between the second direction and the third direction is greater than or equal to 80° and less than or equal to 90°. (Appendix 5) 4. The graphene optical sensor according to claim 1, wherein the angle between the second direction and the third direction is 90°. (Appendix 6) 6. The graphene optical sensor according to claim 1, wherein the planar shape of the plasmon antenna is X-shaped. (Appendix 7) the distance between the first rod portions adjacent to each other in the second direction is equal to or greater than 20 nm and equal to or less than 100 nm; The graphene optical sensor according to any one of claims 1 to 5, characterized in that the distance between the second rod portions adjacent to each other in the third direction is greater than or equal to 20 nm and less than or equal to 100 nm. (Appendix 8) A conductive substrate; an insulating layer provided on the conductive substrate; having 8. The graphene optical sensor according to claim 1, wherein the graphene layer is provided on the insulating layer. [Explanation of symbols]

[0041] 1, 2: Graphene optical sensor 10: Substrate 11: Conductive substrate 12: Insulating layer 20: Graphene layer 31: 1st electrode 32:Second electrode 40: Plasmon antenna 41: First rod section 42: Second rod section

Claims

1. A graphene layer; a first electrode and a second electrode connected to the graphene layer and aligned in a first direction parallel to a surface of the graphene layer; a plurality of plasmon antennas disposed on the graphene layer between the first electrode and the second electrode; having Each of the plasmon antennas has, in a plan view, A first rod portion extending in a second direction inclined from the first direction; a second rod portion extending from the first direction in a third direction inclined toward an opposite side to the second direction with respect to the first direction and intersecting the first rod portion; having The graphene optical sensor is characterized in that the multiple plasmon antennas are periodically arranged in the second direction and the third direction.

2. the angle between the first direction and the second direction is greater than or equal to 40° and less than or equal to 50°; The graphene optical sensor of claim 1 , wherein the angle between the first direction and the third direction is greater than or equal to 40° and less than or equal to 50°.

3. the angle between the first direction and the second direction is 45°; The graphene optical sensor of claim 1 , wherein the angle between the first direction and the third direction is 45°.

4. The graphene optical sensor according to claim 1 , wherein the angle between the second direction and the third direction is greater than or equal to 80° and less than or equal to 90°.

5. The graphene optical sensor according to claim 1 , wherein the angle between the second direction and the third direction is 90°.

6. The graphene optical sensor according to claim 1 , wherein the plasmon antenna has an X-shaped planar shape.

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