Photocathode and evaluation method thereof

Abstract

The photocathode has a substrate, a plurality of structures arranged to form a nano-order periodic array structure on the substrate, and a photoelectron emitting portion arranged in at least one of the plurality of structures, and emits photoelectrons from the photoelectron emitting portion in response to incidence of excitation light.

Description

[Technical field]

[0001] The present disclosure relates to photocathodes and methods for evaluating the same. [Background technology]

[0002] Known photocathodes that emit photoelectrons in response to light irradiation include Au film photocathodes having a gold (Au) film and NEA semiconductor photocathodes using a semiconductor with a surface in a negative electron affinity state (NEA). The Au film photocathode is irradiated with ultraviolet light having a wavelength of 266 nm, which is the third harmonic of a laser beam having a wavelength of 800 nm emitted from a titanium sapphire laser, or the fourth harmonic of a laser beam having a wavelength of 1064 nm emitted from a YAG laser. The Au film photocathode emits photoelectrons in response to irradiation with laser light having a wavelength of 266 nm and a photon energy of 4.66 eV, which is greater than the work function of Au, 4.5 eV. However, the Au film photocathode does not emit photoelectrons as efficiently as the NEA semiconductor photocathode, because the photon density attenuates to about 10% in the process of generating harmonics, and the quantum efficiency is low. In addition, the nonlinear crystal used to convert the wavelength of the laser light into ultraviolet light is easily damaged, and the Au film is damaged by the ultraviolet light whose wavelength is converted by the nonlinear crystal. Au film photocathode devices using Au film photocathodes use nonlinear crystals that are easily damaged, and the Au film is damaged by ultraviolet light, so there is a risk that the life span will be relatively short.

[0003] On the other hand, in the NEA semiconductor photocathode, the surface is brought into the NEA state by supplying a low electronegativity material, for example cesium (Cs), to the surface of, for example, a gallium arsenide (GaAs) semiconductor. The NEA semiconductor photocathode has a higher quantum efficiency than the Au film photocathode, but it needs to be used in an ultra-high vacuum state to maintain the surface in the NEA state, and it is not easy to maintain the surface in the NEA state, so that the life span is short. Furthermore, the NEA semiconductor photocathode needs to be periodically subjected to surface treatment, including removal by heating and deposition of the low electronegativity material, in order to maintain the surface in the NEA state, so that maintenance and management are not easy.

[0004] Furthermore, Patent Documents 1 and 2 describe photoelectric conversion elements that utilize surface plasmon resonance. In the technology described in Patent Document 1, a semiconductor thin film is disposed between a metal thin film that absorbs light by surface plasmon resonance and an electrolyte solution, thereby suppressing corrosion of the metal thin film by the electrolyte solution and dissolution of the metal thin film into the electrolyte solution. In addition, in the technology described in Patent Document 2, a light-transmitting electrode formed of indium tin oxide (ITO) is replaced with a metal anode layer equipped with a nanostructure that generates localized surface plasmon resonance, thereby solving various problems caused by using ITO materials.

[0005] Furthermore, Patent Document 3 describes a photocathode device that generates surface plasmon resonance light by visible light having a wavelength of 400 nm to 600 nm and emits photoelectrons from the photocathode of a metal coating layer by the photoelectric effect of the second harmonic of the surface plasmon resonance. The photocathode device described in Patent Document 3 generates surface plasmon resonance by visible light, not ultraviolet light, so there is little risk of the metal coating layer being damaged and its lifespan being shortened, but since it utilizes the photoelectric effect of the second harmonic, photoelectron emission is not efficient.

[0006] Non-Patent Documents 1 and 2 describe a technique for emitting photoelectrons from a laser beam having a wavelength of 660 nm emitted from a titanium sapphire laser by utilizing localized surface plasmon resonance at the protrusions of star-shaped Au nanoparticles. The techniques described in Non-Patent Documents 1 and 2 can emit photoelectrons using an inexpensive titanium sapphire laser, making it possible to reduce the cost of the photocathode device. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2009-71147 A [Patent Document 2] Special Publication No. 2015-507351 [Patent Document 3] JP 2009-277515 A [Non-patent literature]

[0008] [Non-Patent Document 1] “Continuous-wave multiphoton photoemission from plasmonic nanostars”, Murat Sivisl et al., COMMUNICATIONS PHYSICS, 2018 [Non-Patent Document 2] "Plasmonic nanostar photocathodes for optically-controlled directional currents", Jacob Pettine et al., COMMUNICATIONS PHYSICS, 2020 Summary of the Invention

[0009] However, in the techniques described in Non-Patent Documents 1 and 2, the shape of the star-shaped Au nanoparticles used varies, so the spectral width of the optical resonance spectrum tends to be broad. In addition, since observation with a transmission electron microscope is required when selecting the star-shaped Au nanoparticles to be used, there is a risk of increasing costs. Furthermore, since the optical resonance wavelength depends on the fine structure of the protrusions of the star-shaped Au nanoparticles, if the protrusions of the star-shaped Au nanoparticles are deformed or damaged, the photoelectron emission characteristics may be reduced.

[0010] The present disclosure is intended to solve these problems, and has an object to provide a photocathode that can be manufactured at low cost. [Means for solving the problem]

[0011] The photocathode of the present disclosure has a substrate, a plurality of structures arranged to form a nano-order periodic array structure on the substrate, and a photoelectron emitter arranged in at least one of the plurality of structures, and emits photoelectrons from the photoelectron emitter in response to incidence of excitation light.

[0012] Furthermore, in the photocathode according to the present invention, each of the plurality of structures preferably has a polygonal planar shape having a plurality of vertices, and the photoelectron emitting portion is preferably disposed in the vicinity of any one of the plurality of vertices.

[0013] Furthermore, in the photocathode according to the present disclosure, the diameter of the photoelectron emitting portion is preferably not less than 5 nm and not more than 100 nm.

[0014] Furthermore, in the photocathode according to the present disclosure, the lattice constant of the periodic arrangement structure of the plurality of structures is preferably not less than 400 nm and not more than 1000 nm.

[0015] Furthermore, in the photocathode according to the present disclosure, the wavelength of the excitation light is preferably not less than 650 nm and not more than 1200 nm.

[0016] Furthermore, the photocathode according to the present disclosure preferably further comprises a conductive thin film disposed over the entire surface of the substrate, with a plurality of structures disposed on the surface opposite the surface facing the substrate.

[0017] Furthermore, in the photocathode according to the present disclosure, each of the plurality of structures preferably has a base formed from the same material as the substrate, and a conductive thin film disposed so as to cover the base.

[0018] Furthermore, in the photocathode according to the present disclosure, the substrate is preferably made of a conductor, and the plurality of structures are preferably made of the same material as the substrate.

[0019] In addition, a photocathode evaluation method according to the present disclosure includes determining an emission position on the surface of the photocathode from which photoelectrons are emitted, irradiating excitation light to the emission position, obtaining drain current information indicating the drain current supplied from the photocathode, estimating a photoelectron quantity, which is the amount of photoelectrons emitted from the photocathode, based on the drain current information, and outputting a photoelectron quantity signal indicating the estimated photoelectron quantity.

[0020] In addition, the manufacturing method of a photocathode according to the present disclosure includes a step of arranging a plurality of structures on a substrate so as to form a nano-order periodic array structure, and arranging a photoelectron emitting portion that emits electrons in response to incidence of excitation light in at least one of the plurality of structures.

[0021] The photocathode according to the present disclosure has a simple structure, is less susceptible to damage during use, and can be manufactured at low cost. [Brief description of the drawings]

[0022] [Figure 1] FIG. 1A is a photoelectron microscope image of a wafer on which a photocathode according to an embodiment is formed, and FIG. 1B is an SEM image of the photocathode shown in FIG. [Diagram 2] FIG. 2 is a perspective view of a photocathode according to the first embodiment. [Diagram 3] FIG. 3 is a partial plan view of the photocathode shown in FIG. [Figure 4] 3 is a flowchart showing a method for manufacturing the photocathode shown in FIG. 2. [Diagram 5] 5(a) is a photoelectron microscope image of a wafer on which a photoelectron emitting portion is formed in the photoelectron emitting portion forming step indicated as S104 in FIG. 4, and (b) is an enlarged view of the area indicated by dashed line D in (a). [Figure 6] FIG. 3 is a block diagram of a photocathode evaluation device for evaluating the photocathode shown in FIG. [Figure 7] FIG. 7 is a block diagram of the control device shown in FIG. [Figure 8] 7 is a flowchart of a photocathode evaluation process executed by the photocathode evaluation device shown in FIG. 6. [Figure 9] 7 is a planar image of a photocathode irradiated with excitation light by the photocathode evaluation device shown in FIG. 6. [Figure 10] 1 is a diagram showing the relationship between the wavelength of excitation light incident on a photocathode, the amount of photoelectrons measured by a photoelectron microscope, and the drain current detected by a current sensor. [Figure 11] FIG. 13 is a diagram showing the relationship between the wavelength of excitation light incident on a photocathode and the lattice constant of a periodic array structure formed by a plurality of nanoprotrusions in which surface plasmon resonance occurs. [Figure 12] FIG. 11 is a perspective view of a photocathode according to a second embodiment. [Figure 13] FIG. 13 is a perspective view of the base material shown in FIG. [Figure 14] 13 is a flowchart showing a method for manufacturing the photocathode shown in FIG. 12. [Figure 15] FIG. 11 is a perspective view of a photocathode according to a third embodiment. [Figure 16] 16 is a flowchart showing a method for manufacturing the photocathode shown in FIG. 15. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] The photocathode and its evaluation method according to the present disclosure will be described below with reference to the drawings. However, it should be noted that the technical scope of the present invention is not limited to the embodiments, but extends to the inventions described in the claims and their equivalents.

[0024] (Outline of photocathode according to embodiment) The photocathode of the embodiment can emit photoelectrons without utilizing the photoelectric effect of high-order harmonics by forming a photoelectron emitting portion, which is an even finer structure than the structures, in any of a plurality of structures arranged to form a nano-order periodic array structure.

[0025] Fig. 1(a) is a photoelectron microscope image of a wafer on which a photocathode according to an embodiment is formed, and Fig. 1(b) is an SEM image of the photocathode shown in Fig. 1(a). Fig. 1(a) shows the state in which the photocathode is irradiated with a UV lamp and an fs laser, and Fig. 1(b) shows the state in which a photoelectron emitting portion is formed near one vertex of the structure.

[0026] The photocathode according to the embodiment does not utilize the photoelectric effect of high-order harmonics, so that photoelectrons can be emitted by irradiation with visible light or infrared light, not ultraviolet light, and there is no risk of the structure being deteriorated by irradiation with ultraviolet light. In addition, the photocathode according to the embodiment can emit photoelectrons from the photoelectron emitting portion formed in the structure without using star-shaped nanoparticles, so there is no increase in the spectral width due to the variation of star-shaped nanoparticles as in the techniques described in Non-Patent Documents 1 and 2. In addition, the photocathode according to the embodiment does not require the selection of star-shaped nanoparticles as in the techniques described in Non-Patent Documents 1 and 2, so it has a simple configuration, has few damaged parts due to use, and can be manufactured at low cost.

[0027] (Configuration and Function of Photocathode According to First Embodiment) FIG. 2 is a perspective view of the photocathode according to the first embodiment, and FIG. 3 is a partial plan view of the photocathode shown in FIG.

[0028] The photocathode 1 has a substrate 10, a thin film 11, a plurality of nanoprotrusions 12, and a photoelectron emitter 13, and emits photoelectrons from the photoelectron emitter 13 by generating surface plasmon resonance in response to incidence of excitation light having a wavelength of 350 nm or more and 2000 nm or less. The wavelength of the excitation light incident on the photocathode 1 is preferably 650 nm or more and 1200 nm or less.

[0029] The substrate 10 is, for example, a silicon (Si) substrate. The substrate 10 may be a quartz (SiO2) substrate, a sapphire (Al2O3) substrate, an aluminum (Al) substrate, or a semiconductor substrate such as a SiC substrate, a gallium arsenide (GaAs) substrate, or a gallium nitride (GaN) substrate. The thin film 11 is a gold (Au) film and is disposed over the entire surface of one surface of the substrate 10. The thin film 11 may be a conductive thin film other than an Au film, such as a silver (Ag) film, an Al film, a copper (Cu) film, a titanium (Ti) film, or a chromium (Cr) film. The thin film 11 is grounded, so that a drain current is supplied from the photocathode 1 to supply electrons to the photocathode 1 in response to the emission of photoelectrons. The thin film 11 may be omitted. When the thin film 11 is omitted, the drain current is supplied via the substrate 10.

[0030] Each of the nanoprotrusions 12 is also called a structure or a triangular nanodisk, and is a flat nano-order protrusion having a regular triangular planar shape. The length of one side of each of the nanoprotrusions 12 is longer than the thickness of each of the nanoprotrusions 12. Each of the nanoprotrusions 12 is made of the same material as the material forming the thin film 11, and is arranged on the surface of the thin film 11 opposite to the surface facing the substrate 10. The nanoprotrusions 12 are arranged so that the sides of adjacent nanoprotrusions 12 are aligned on the same line, that is, so as to form a periodic arrangement structure having a constant lattice constant on the surface of the thin film 11 in the form of a triangular lattice.

[0031] The lattice constant P of the periodic arrangement structure formed by the plurality of nanoprotrusions 12 is the distance between the centers of gravity of the triangles that are the planar shapes when each of the adjacent nanoprotrusions 12 is viewed in plan. The lattice constant P of the nanoprotrusions 12 is 500 nm in one example, and is preferably 400 nm or more and 1000 nm or less. By setting the lattice constant P to 400 nm or more and 1000 nm or less, the photocathode 1 generates surface plasmon resonance in response to the incidence of light having a wavelength of 650 nm or more and 1200 nm. The length L of one side of the nanoprotrusions 12 is 400 nm in one example, and is preferably 300 nm or more and 800 nm or less. The thickness of the nanoprotrusions 12 is 60 nm in one example, and is preferably 20 nm or more and 2000 nm or less. When the thickness of the nanoprotrusions 12 is less than 20 nm, the photoelectron emission efficiency decreases, which is not preferable. When the thickness of the nanoprotrusions 12 is greater than 2000 nm, the stability of the surface structure decreases, which is not preferable.

[0032] In the photocathode 1, the fill factor, which is the ratio of the total surface area of ​​the nanoprotrusions 12 arranged in a given target area to the surface area of ​​the substrate 10 in the given target area, is preferably 10% or more and 50% or less. When the fill factor is less than 10%, the photoelectron emission efficiency decreases, which is not preferable. When the fill factor is more than 50%, the nanoprotrusions are fused to each other, which is not preferable.

[0033] The photoelectron emitting portion 13 is a convex or concave portion disposed near at least one apex of the nanoprotrusion 12, and emits photoelectrons in response to incidence of light having a wavelength of 350 nm or more and 2000 nm or less. The vicinity of the apex is a range from one apex of the nanoprotrusion 12 to a length equivalent to 20% of the length of one side of the nanoprotrusion 12. The diameter of the photoelectron emitting portion is 5 nm or more and 100 nm or less, and is determined according to the wavelength of the incident light.

[0034] (Method of Manufacturing Photocathode According to the First Embodiment) FIG. 4 is a flowchart showing a method for manufacturing the photocathode 1.

[0035] First, in a wafer preparation step, a silicon wafer is prepared (S101). Next, in a thin film deposition step, a thin film 11 is deposited on the silicon wafer (S102). The thin film 11 is deposited on the silicon wafer by, for example, sputtering. Next, in a nanoprotrusion formation step, a plurality of nanoprotrusions 12 are formed on the thin film 11 (S103). The plurality of nanoprotrusions 12 are formed by nano-fabrication techniques such as electron beam lithography and focused ion beam lithography. Next, in a photoelectron emission portion formation step, a photoelectron emission portion 13 is formed on any one of the surfaces of the plurality of nanoprotrusions 12. The photoelectron emission portion 13 is formed by irradiating the surface of the nanoprotrusions 12 with, for example, an electron beam emitted from an electron gun or an ion beam (S104). The photoelectron emission portion 13 is formed by nano-fabrication techniques such as electron beam lithography and focused ion beam lithography, like the plurality of nanoprotrusions 12. The photoelectron emitting portion 13 may be formed by nanoimprinting. Then, in a cutting step, the wafer is cut to a desired size to form the photocathode 1 (S105). The cutting step shown in S105 may be performed before the wafer preparation step. By performing the cutting step before the wafer preparation step, it is possible to reduce the possibility that fine particles generated when cutting the wafer will adhere to the surface and deteriorate the performance of the photocathode structure.

[0036] Figure 5(a) is a photoelectron microscope image of a wafer on which a photoelectron emitting portion 13 is formed in the photoelectron emitting portion formation process indicated as S104 in Figure 4, and Figure 5(b) is an enlarged view of the area indicated by dashed line D in Figure 5(a).

[0037] In the wafer shown in Figures 5(a) and 5(b), each of the nanoprotrusions 12 has a planar shape of an equilateral triangle with a side length of 400 nm and is arranged in a triangular lattice with a lattice constant of 500 nm.

[0038] (Configuration and Functions of the Photocathode Evaluation Apparatus According to the Embodiment) FIG. 6 is a block diagram of a photocathode evaluation device for evaluating the photocathode 1. As shown in FIG.

[0039] The photocathode evaluation device 2 has an optical unit 21, a photoelectron microscope device 22, a current sensor 23, and a control device 24, and evaluates the amount of photoelectrons emitted from the photocathode 1 by irradiating the photocathode 1 with excitation light.

[0040] The optical unit 21 has a laser light source 30, a wavelength converter 31, a plurality of optical mirrors 32, and a mirror driving device 33. The optical unit 21 converts the wavelength of the laser light emitted from the laser light source 30 to generate excitation light, and irradiates the surface of the photocathode 1 with the generated excitation light.

[0041] The laser light source 30 outputs a pulsed laser light having a pulse width on the order of femtoseconds and a variable repetition frequency. The laser light source 30 has, for example, an oscillation wavelength of 1028 nm (corresponding to a photon energy of 1.2 eV), a pulse width of 180 fs, and a variable repetition frequency in the range of single shot to 1 MHz. The laser light source 30 may be, for example, an ultrashort pulse laser (PHAROS) manufactured by LightConversion. The laser light source 30 may be an attosecond pulse laser light source that outputs a pulse having a pulse width on the order of attoseconds and a variable repetition frequency.

[0042] The wavelength converter 31 is an optical parametric amplifier (OPA) capable of changing the photon energy of the output laser light within a predetermined range, for example, from 0.5 eV to 2.4 eV. The multiple optical mirrors 32 are controlled by a mirror driver 33, and the laser light emitted from the wavelength converter 31 is irradiated as excitation light onto a desired region on the surface of the photocathode 1. In response to an emission position signal indicating an emission position for emitting photoelectrons being input from the control device 24, the mirror driver 33 controls the multiple optical mirrors 32 to irradiate the excitation light onto a position corresponding to the emission position signal.

[0043] The photon energy of the excitation light irradiated from the optical unit 21 to the surface of the photocathode 1 is, for example, 0.5 eV or more and 5.9 eV or less. The wavelength of the excitation light with a photon energy of 0.5 eV is 2500 nm, and the wavelength of the excitation light with a photon energy of 5.9 eV is 210 nm. The optical unit 21 irradiates the surface of the photocathode 1 with excitation light having a wavelength in the range of 210 nm or more and 2500 nm or less.

[0044] The photoelectron microscope device 22 has a photoemission electron microscope (PEEM) 41, a fluorescent screen 42, and an imaging device 43. The photoelectron microscope device 22 outputs spatial distribution information of the amount of photoelectrons emitted from a sample. The PEEM 41 is, for example, a PEEM manufactured by Omicron Corporation. The PEEM 41 has multiple electrostatic lenses, and controls the multiple electrostatic lenses to enlarge and project the spatial distribution of photoelectrons emitted from the photocathode 1, converts it into an optical signal by the fluorescent screen 42, and outputs it to the imaging device 43. The imaging device 43 has, for example, a charge coupled device (CCD).

[0045] The current sensor 23 is electrically connected to the thin film 11 and ground via wiring. The current sensor 23 detects a drain current Id that flows from the nanoprotrusions 12 to ground via the thin film 11 as the photocathode 1 emits photoelectrons, and outputs a drain current signal indicating the detected drain current Id to the control device 24.

[0046] FIG. 7 is a block diagram of the control device 24.

[0047] The control device 24 has a communication unit 51, a storage unit 52, an operation unit 53, a display unit 54, and a processing unit 60.

[0048] The communication unit 51 is configured to enable the control device 24 to communicate with the optical unit 21, the photoelectron microscope device 22, and the current sensor 23, and includes a communication interface circuit. The communication interface circuit is, for example, a communication interface circuit for wired LAN, wireless LAN, LTE (Long Term Evolution), etc. The communication unit 51 supplies data received from the optical unit 21, the photoelectron microscope device 22, and the current sensor 23 to the processing unit 60, and transmits data supplied from the processing unit 60 to the optical unit 21, the photoelectron microscope device 22, and the current sensor 23.

[0049] The storage unit 52 is configured to store data and programs, and includes, for example, a semiconductor memory. The storage unit 52 stores an operating system program, a driver program, an application program, data, and the like used in processing by the processing unit 60. The storage unit 52 stores, for example, a photocathode evaluation program for causing the processing unit 60 to execute a photocathode evaluation process for evaluating the characteristics of the photocathode 1. The photocathode evaluation program is installed in the storage unit 52 from a computer-readable, non-transitory, portable storage medium such as a CD (Compact Disc)-ROM (Read Only Memory) or a DVD (Digital Versatile Disc)-ROM.

[0050] The storage unit 52 also stores various data and information used when executing the photocathode evaluation process. For example, the storage unit 52 stores a photoelectron emission amount table indicating the relationship between the drain current detected by the current sensor 23 and the amount of photoelectrons emitted from the photocathode 1.

[0051] The operation unit 53 is configured to receive operations by the user to the control device 24, and includes, for example, a keypad. The operation unit 53 generates a signal according to the operation by the user and supplies the signal to the processing unit 60.

[0052] The display unit 54 is configured to display an image, and includes a display such as a liquid crystal display or an organic EL (Electro Luminescence) display. The display unit 54 generates and displays an image based on a signal supplied from the processing unit 60.

[0053] The processing unit 60 has one or more processors and their peripheral circuits. The processing unit 60 centrally controls the overall operation of the control device 24, and is, for example, a CPU. The processing unit 60 executes processing based on programs (driver programs, operating system programs, application programs, etc.) stored in the storage unit 52. The processing unit 60 can also execute multiple programs (application programs, etc.) in parallel.

[0054] The processing unit 60 has a control signal output unit 61, an information acquisition unit 62, a photoelectron amount estimation unit 63, and a signal output unit 64. The control signal output unit 61, the information acquisition unit 62, the photoelectron amount estimation unit 63, and the signal output unit 64 are functional modules realized by a program executed by a processor included in the processing unit 60. Alternatively, the control signal output unit 61, the information acquisition unit 62, the photoelectron amount estimation unit 63, and the signal output unit 64 may be implemented in the control device 24 as firmware.

[0055] (Photocathode evaluation process executed by the photocathode evaluation device according to the embodiment) 8 is a flowchart of a photocathode evaluation process executed by the photocathode evaluation device 2. The photocathode evaluation process is executed mainly by the processing unit 60 in cooperation with each element of the photocathode evaluation device 2, based on a program stored in advance in the storage unit 52.

[0056] First, the control signal output unit 61 outputs a repetition frequency signal indicating the repetition frequency of the laser light output by the laser light source 30 to the laser light source 30 (S201). The laser light source 30 sets the repetition frequency corresponding to the input repetition frequency signal as the repetition frequency of the pulsed laser light.

[0057] Next, the control signal output unit 61 outputs an excitation light energy signal indicating the photon energy of the excitation light output by the optical unit 21 to the wavelength converter 31 (S202). The photon energy corresponding to the excitation light energy signal corresponds to the wavelength of the excitation light irradiated to the photocathode 1 to emit photoelectrons.

[0058] Next, the control signal output unit 61 outputs an emission position signal indicating an emission position where the excitation light is irradiated and photoelectrons are emitted to the mirror drive device 33 and the PEEM 41 (S203). In response to the emission position signal input from the control device 24, the mirror drive device 33 controls the multiple optical mirrors 32 so as to irradiate the excitation light to a position corresponding to the emission position signal. In response to the emission position signal input from the control device 24, the PEEM 41 controls the multiple mirrors so as to detect the spatial distribution of the amount of photoelectron emission at the position corresponding to the emission position signal. The control signal output unit 61 outputs the emission position signal to the mirror drive device 33 and the PEEM 41, thereby specifying the emission position where photoelectrons are emitted on the surface of the photocathode 1.

[0059] Next, the control signal output unit 61 outputs an excitation light irradiation signal indicating that excitation light is to be irradiated to the laser light source 30 (S204). In response to the input of the excitation light irradiation signal, the laser light source 30 outputs a pulsed laser light to the optical unit 21. Thereafter, the laser light source 30 continues to output the pulsed laser light to the optical unit 21. When the control signal output unit 61 outputs the excitation light irradiation signal to the laser light source 30, the emission position that emits photoelectrons is irradiated with excitation light.

[0060] The optical unit 21 generates excitation light having the photon energy set in the process of S202 from the laser light input from the laser light source 30, and irradiates the generated excitation light onto the photocathode 1. When the optical unit 21 irradiates the photocathode 1 with the excitation light, the photocathode 1 emits photoelectrons from the photoelectron emitting unit 13, as shown in FIG.

[0061] Next, the information acquisition unit 62 acquires spatial distribution information indicating the spatial distribution of the amount of photoelectron emission detected by the photoelectron microscope device 22 (S205). The photoelectron microscope device 22 detects the spatial distribution of the amount of photoelectron emission emitted from the sample. The photoelectron microscope device 22 outputs spatial distribution information indicating the spatial distribution of the photoelectrons emitted from the photoelectron emitter 13 to the control device 24. The information acquisition unit 62 acquires the spatial distribution information input from the photoelectron microscope device 22, and stores the acquired spatial distribution information in the memory unit 52.

[0062] Next, the information acquiring unit 62 acquires drain current information indicating the drain current Id supplied from the photocathode 1 to ground from the current sensor 23 (S206). The current sensor 23 detects the drain current Id supplied from the photocathode 1, and outputs drain current information indicating the detected drain current to the control device 24. The information acquiring unit 62 acquires the drain current information input from the current sensor 23, and stores the acquired drain current information in the memory unit 52.

[0063] Next, the photoelectron amount estimation unit 63 estimates the amount of photoelectrons emitted from the photocathode 1 based on the drain current information acquired in the process of S206 (S207). The photoelectron amount estimation unit 63 refers to the photoelectron emission amount table stored in the storage unit 52 and extracts the photoelectron emission amount equivalent to the drain current corresponding to the drain current information acquired in the process of S206. The photoelectron amount estimation unit 63 estimates the photoelectron emission amount extracted from the photoelectron emission amount table as the emission amount of photoelectrons emitted from the photocathode 1.

[0064] Next, the signal output unit 64 outputs a spatial information signal indicating spatial information corresponding to the spatial distribution information acquired in the process of S205 to the display unit 54 (S208). The display unit 54 displays the spatial information corresponding to the input spatial information signal.

[0065] Then, the signal output unit 64 outputs a photoelectron amount signal indicating the amount of photoelectrons estimated in the process of S207 to the display unit 54 (S209). The display unit 54 displays the amount of photoelectrons corresponding to the input photoelectron amount signal.

[0066] (Functions and Effects of the Photocathode According to the First Embodiment) The photocathode 1 emits photoelectrons from a photoelectron emitter 13 formed on any of a plurality of nanoprotrusions 12 arranged to form a nano-order periodic array structure, and can emit photoelectrons without using the high-order harmonic photoelectric effect. Since the photocathode 1 does not use the high-order harmonic photoelectric effect, photoelectrons can be emitted by irradiation with visible light or infrared light rather than ultraviolet light, and there is no risk of the nanoprotrusions 12 deteriorating due to irradiation with ultraviolet light.

[0067] Moreover, since the photocathode 1 emits photoelectrons from the photoelectron emitting portion 13 formed on any one of the multiple nanoprotrusions 12, there is no increase in the spectral width due to the variation in the star-shaped nanoparticles as in the techniques described in Non-Patent Documents 1 and 2. Furthermore, since the photocathode 1 does not require the selection of star-shaped nanoparticles as in the techniques described in Non-Patent Documents 1 and 2, it has a simple configuration, is less susceptible to damage during use, and can be manufactured at low cost.

[0068] FIG. 10 is a diagram showing the relationship between the wavelength of the excitation light incident on the photocathode 1, the amount of photoelectrons measured by the PEEM 41, and the drain current Id detected by the current sensor 23. In FIG. 10, the horizontal axis indicates the wavelength of the excitation light incident on the photocathode 1, the left vertical axis indicates the amount of photoelectrons measured by the PEEM 41 when the excitation light is incident on the photocathode 1, and the right vertical axis indicates the drain current Id detected by the current sensor 23. In FIG. 10, the solid line waveform W101 indicates the amount of photoelectrons measured by the PEEM 41, and the dashed line waveform W102 indicates the drain current Id detected by the current sensor 23. FIG. 10 is created by sequentially changing the excitation light energy corresponding to the output excitation light energy signal indicated by S202 in FIG. 8 and sweeping the wavelength of the excitation light. The wavelength of the excitation light is swept in increments of 2 nm from 730 nm to 850 nm.

[0069] In Figure 10, the change in the amount of photoelectrons measured by PEEM 41 roughly coincides with the change in the drain current Id detected by the current sensor 23, and the amount of photoelectrons emitted from the photocathode 1 can be estimated from the drain current Id detected by the current sensor 23.

[0070] In addition, the waveform indicating the amount of photoelectrons measured by the PEEM 41 and the drain current Id detected by the current sensor 23 has a full width at half maximum of approximately 20 nm, and the photocathode 1 can have wavelength characteristics with a steep spectral waveform whose peak value is the wavelength of the excitation light.

[0071] Furthermore, in the photocathode 1, the photoelectron emitting portion 13 is disposed in the vicinity of the apex of the nanoprotrusion 12 where the electric field generated by surface plasmon resonance is large, and therefore photoelectrons can be emitted efficiently.

[0072] Furthermore, in photocathode 1, the diameter of the photoelectron emitting portion is not less than 5 nm and not more than 100 nm, so it can be formed by processing such as electron beam lithography, focused ion beam, nanoimprinting, etc., and there is little risk of an increase in manufacturing costs.

[0073] Furthermore, in the photocathode 1, the length of one side of the multiple nanoprotrusions 12 is 300 nm or more and 800 nm or less, and the lattice constant of the periodic arrangement structure formed by the multiple nanoprotrusions 12 is 400 nm or more and 1000 nm or less, so that it can be formed at low cost by electron beam lithography, nanoimprinting, etc.

[0074] In photocathode 1, the lattice constant of the periodic arrangement structure formed by the multiple nanoprotrusions 12 is not less than 400 nm and not more than 1000 nm, so the wavelength of the excitation light incident on photocathode 1 is in the range of 650 nm to 1200 nm, which is visible light or infrared light.

[0075] Fig. 11 is a diagram showing the relationship between the wavelength of excitation light incident on the photocathode 1 and the lattice constant of the periodic arrangement structure formed by the multiple nanoprotrusions 12 in which surface plasmon resonance occurs. In Fig. 11, the horizontal axis represents the wavelength of excitation light incident on the photocathode 1 and the corresponding photon energy, and the vertical axis represents the lattice constant of the periodic arrangement structure formed by the multiple nanoprotrusions 12 in which surface plasmon resonance occurs.

[0076] When the wavelength of the excitation light incident on the photocathode 1 is 775 nm, the lattice constant of the periodic arrangement structure formed by the multiple nanoprotrusions 12 in which surface plasmon resonance occurs is 500 nm. When the wavelength of the excitation light incident on the photocathode 1 is 1025 nm, the lattice constant of the periodic arrangement structure formed by the multiple nanoprotrusions 12 in which surface plasmon resonance occurs is 680 nm. The lattice constant of the periodic arrangement structure formed by the multiple nanoprotrusions 12 in which surface plasmon resonance occurs increases in proportion to the increase in the wavelength of the excitation light incident on the photocathode 1.

[0077] Furthermore, since the wavelength of the excitation light incident on the photocathode 1 is in the range of 650 nm to 1200 nm, which is visible light or infrared light, there is little risk that the nanoprotrusions 12 will be deteriorated by the incidence of the excitation light.

[0078] Furthermore, since photocathode 1 has thin film 11 disposed over the entire surface of substrate 10, by grounding via thin film 11, the ground resistance between photocathode 1 and the ground can be reduced.

[0079] (Configuration and Function of Photocathode According to Second Embodiment) 12 is a perspective view of a photocathode according to the second embodiment. In the photocathode 1a according to the second embodiment, each of the nanoprotrusions has a base made of the same material as the substrate, and a conductive thin film disposed to cover the base.

[0080] The photocathode 1a has a base material 10a, a thin film 11a, a plurality of nanoprotrusions 12a, and a photoelectron emitter 13a. Like the photocathode 1, the photocathode 1a emits photoelectrons from the photoelectron emitter 13a by generating surface plasmon resonance in response to the incidence of excitation light having a wavelength of 350 nm or more and 2000 nm or less. The wavelength of the excitation light incident on the photocathode 1a is preferably 650 nm or more and 1200 nm or less.

[0081] FIG. 13 is a perspective view of the base material 10a.

[0082] The base material 10a is a Si substrate like the substrate 10, and includes a substrate 15 and a plurality of bases 16. The base material 10a may be a quartz substrate, a sapphire substrate, an aluminum substrate, or a semiconductor substrate such as a SiC substrate, a gallium arsenide substrate, or a gallium nitride substrate, as long as it is made of a material that can be processed on the nano-order.

[0083] The substrate 15 is a plate-like member having a rectangular planar shape, and a plurality of bases 16 are arranged on one surface of the substrate 15. Each of the plurality of bases 16 is a base for the nanoprotrusions 12a formed of the same material as the material forming the base material 10a and the substrate 15. The plurality of bases 16 are arranged on the surface of the substrate 15 so as to form a periodic array structure having a constant lattice constant.

[0084] The thin film 11a is an Au film, and is disposed so as to cover the entire surface of the substrate 15 on which the plurality of bases 16 are disposed, and forms the nanoprotrusions 12a together with the plurality of bases 16. Note that, like the thin film 11, the thin film 11a may be a metal film other than an Au film, such as an Ag film, an Al film, a Cu film, a Ti film, or a Cr film.

[0085] Each of the nanoprotrusions 12a is also called a structure, and is a flat nano-order protrusion having an equilateral triangular planar shape. The length of one side of each of the nanoprotrusions 12a is longer than the thickness of each of the nanoprotrusions 12a. The nanoprotrusions 12a are arranged, like the nanoprotrusions 12, so that the sides of adjacent nanoprotrusions 12a are aligned on the same line, i.e., so as to form a periodic arrangement structure having a constant lattice constant on the surface of the substrate 15 in the form of a triangular lattice.

[0086] The arrangement parameters, such as the lattice constant and fill factor, of the periodic arrangement structure formed by the plurality of nanoprotrusions 12a are similar to those of the plurality of nanoprotrusions 12, and therefore a detailed description thereof will be omitted here.

[0087] The photoelectron emitting portion 13a is a convex or concave portion disposed near at least one apex of the nanoprotrusion portion 12a, and emits photoelectrons in response to incidence of light having a wavelength of 350 nm or more and 2000 nm or less. The diameter of the photoelectron emitting portion is 5 nm or more and 100 nm or less, and is determined according to the wavelength of the incident light.

[0088] (Method of Manufacturing Photocathode According to Second Embodiment) FIG. 14 is a flowchart showing a method for manufacturing the photocathode 1a.

[0089] First, in a wafer preparation step, a silicon wafer is prepared (S301). Next, in a base formation step, a plurality of bases 16 are formed on one side of the silicon wafer prepared in the wafer preparation step shown in S301 (S302). The plurality of bases 16 are formed by nano-fabrication techniques such as electron beam lithography and focused ion beam lithography. Next, in a thin film formation step, a thin film 11a is formed on the silicon wafer on which the plurality of bases 16 have been formed (S303). The thin film 11a is formed on the silicon wafer by, for example, sputtering. The thin film 11a is formed so as to cover the base 16, thereby forming a plurality of nano-protrusions 12a. Next, in a photoelectron emission portion formation step, a photoelectron emission portion 13a is formed on one surface of the plurality of nano-protrusions 12a (S304). The photoelectron emission portion 13a is formed by irradiating the surface of the nano-protrusions 12a with, for example, an electron beam emitted from an electron gun or an ion beam. The photoelectron emitting portion 13a is formed by nano-fabrication techniques such as electron beam lithography and focused ion beam lithography, similar to the multiple nano-protrusions 12a. The photoelectron emitting portion 13a may be formed using a mold for forming the photoelectron emitting portion 13a. Then, in a cutting step, the wafer is cut to a desired size to form the photocathode 1a (S305). The cutting step indicated in S305 may be performed before the wafer preparation step.

[0090] (Configuration and Function of Photocathode According to Third Embodiment) 15 is a perspective view of a photocathode according to the third embodiment. In the photocathode 1b according to the third embodiment, the substrate is made of a conductor, and the nanoprotrusions are made of the same material as the substrate.

[0091] Photocathode 1b has a substrate 10b, a plurality of nanoprotrusions 12b, and a photoelectron emitter 13b. Like photocathode 1, photocathode 1b emits photoelectrons from photoelectron emitter 13b by generating surface plasmon resonance in response to incidence of excitation light having a wavelength of 350 nm or more and 2000 nm or less. The wavelength of the excitation light incident on photocathode 1b is preferably 650 nm or more and 1200 nm or less.

[0092] The substrate 10b is an Al substrate having a rectangular planar shape, and a plurality of nano-projections 12b are integrally formed on one surface of the substrate 10b. The substrate 10b may be a conductive substrate other than an aluminum substrate, as long as it is made of a conductive material that can be processed on a nano-order basis.

[0093] Each of the nanoprotrusions 12b is made of the same material as the material forming the substrate 10b. The nanoprotrusions 12b are arranged on the surface of the substrate 10b so as to form a periodic array structure having a constant lattice constant.

[0094] Each of the nanoprotrusions 12b is also called a structure, and is a flat nano-order protrusion having an equilateral triangular planar shape. The length of one side of each of the nanoprotrusions 12b is longer than the thickness of each of the nanoprotrusions 12b. The nanoprotrusions 12b are arranged, like the nanoprotrusions 12, so that the sides of adjacent nanoprotrusions 12b are aligned on the same line, i.e., so as to form a periodic arrangement structure having a constant lattice constant on the surface of the substrate 15b in the form of a triangular lattice.

[0095] The arrangement parameters, such as the lattice constant and fill factor, of the periodic arrangement structure formed by the plurality of nanoprotrusions 12b are similar to the arrangement parameters of the plurality of nanoprotrusions 12, and therefore a detailed description thereof will be omitted here.

[0096] The photoelectron emitting portion 13b is a convex or concave portion disposed near at least one apex of the nanoprotrusion portion 12b, and emits photoelectrons in response to incidence of light having a wavelength of 350 nm or more and 2000 nm or less. The diameter of the photoelectron emitting portion is 5 nm or more and 100 nm or less, and is determined according to the wavelength of the incident light.

[0097] (Method of Manufacturing Photocathode According to Third Embodiment) FIG. 16 is a flowchart showing a method for manufacturing the photocathode 1a.

[0098] First, in a substrate preparation step, an aluminum plate is prepared (S401). Next, in a nano-protrusion formation step, a plurality of nano-protrusions 12b are formed on one surface of the aluminum plate prepared in the substrate preparation step shown in S401 (S402). The plurality of nano-protrusions 12b are formed by nano-processing techniques such as electron beam lithography and focused ion beam lithography, as in the case of the base 16. Next, in a photoelectron emission portion formation step, a photoelectron emission portion 13b is formed on one surface of the plurality of nano-protrusions 12b (S403). The photoelectron emission portion 13b is formed by irradiating the surface of the nano-protrusions 12b with, for example, an electron beam emitted from an electron gun or an ion beam. The photoelectron emission portion 13b is formed by nano-processing techniques such as electron beam lithography and focused ion beam lithography, as in the case of the plurality of nano-protrusions 12b. The photoelectron emission portion 13b may be formed by using a mold for forming the photoelectron emission portion 13b. Then, in a cutting step, the wafer is cut to a desired size to form the photocathode 1b (S404). Note that the cutting step indicated in S404 may be performed before the substrate preparation step.

[0099] (Modification of the photocathode according to the embodiment) In the photocathodes 1, 1a, and 1b, the nanoprotrusions 12 are structures having a planar shape of an equilateral triangle. However, in the photocathodes according to the embodiment, the nanoprotrusions may be structures having a planar shape of a polygon other than a triangle such as a square or pentagon, and a planar shape other than a polygon such as a perfect circle or an ellipse. When the nanoprotrusions have a polygonal planar shape, the photoelectron emitting portion is preferably disposed near the vertex. In the photocathodes according to the embodiment, the diameter of the nanoprotrusions is preferably 300 nm or more and 800 nm or less. Here, the diameter of the nanoprotrusions is the distance between the longest ends when the nanoprotrusions are viewed in plan. For example, when the nanoprotrusions have a square planar shape, the diameter of the nanoprotrusions is the length of the diagonal line. When the nanoprotrusions have a pentagonal planar shape, the diameter of the nanoprotrusions is the length between the vertex and the vertex of the end of the side opposite the vertex. When the nanoprotrusions have an elliptical planar shape, the diameter of the nanoprotrusions is the length of the major axis. The lattice constant of the periodic arrangement structure formed by the nanoprotrusions is the distance between the centers of gravity of the planar shapes of adjacent nanoprotrusions when viewed from above.

Claims

1. A substrate; A plurality of structures arranged on the substrate so as to form a nano-order periodic array structure; a photoelectron emitting portion disposed in at least one of the plurality of structures; Each of the plurality of structures has a polygonal planar shape having a plurality of vertices, the photoelectron emitting portion is disposed near any one of the plurality of vertices, In response to the incidence of excitation light, photoelectrons are emitted from the photoelectron emitting portion.

1. A photocathode comprising:

2. 2. The photocathode according to claim 1, wherein the photoelectron emitting portion has a diameter of 5 nm or more and 100 nm or less.

3. The photocathode according to claim 1 , wherein the lattice constant of the periodic arrangement of the plurality of structures is not less than 400 nm and not more than 1000 nm.

4. 4. The photocathode according to claim 3, wherein the excitation light has a wavelength of 650 nm or more and 1200 nm or less.

5. 2. The photocathode according to claim 1, further comprising a conductive thin film disposed over the entire surface of the substrate, the plurality of structures being disposed on a surface opposite to the surface facing the substrate.

6. Each of the plurality of structures is a base formed of the same material as the substrate; A conductive thin film disposed so as to cover the base; 2. The photocathode of claim 1 having the following structure:

7. the substrate is formed of a conductor; The photocathode of claim 1 , wherein the plurality of structures are formed from the same material as the substrate.

8. defining an emission position for emitting photoelectrons on a surface of the photocathode according to any one of claims 1 to 7; Irradiating the emission position with excitation light; obtaining drain current information indicating a drain current supplied from the photocathode; estimating a photoelectron amount, which is an amount of photoelectrons emitted from the photocathode, based on the drain current information; outputting a photoelectron amount signal indicating the estimated photoelectron amount; A method for evaluating a photocathode, comprising:

9. A plurality of structures are arranged on the substrate so as to form a nano-order periodic array structure; a step of disposing a photoelectron emitting portion that emits electrons in response to excitation light being incident on at least one of the plurality of structures; Each of the plurality of structures has a polygonal planar shape having a plurality of vertices, The photoelectron emitting portion is disposed in the vicinity of any one of the plurality of vertices. A method for manufacturing a photocathode.

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