Light detection device

Abstract

The present invention provides a photodetector that can accurately and easily position a sample at the focal point of a predetermined measurement light. [Solution] The photodetector 100 includes a photodetector having an electron emission unit 3 including a metasurface 32 that emits electrons in response to the incidence of electromagnetic waves W, and an electron multiplication unit 4 that multiplies the electrons emitted from the electron emission unit 3, and a voltage application unit 2 that applies a voltage to an object to be applied to, which includes at least one of the metasurface 32 and the electron multiplication unit 4. The voltage application unit 2 applies a voltage to the object to be applied to such that the potential on the metasurface side of the electron multiplication unit 4 is less than or equal to the potential of the metasurface 32, so as to allow first electrons, which are field-emitted in response to the incidence of electromagnetic waves W to the metasurface 32, to reach the electron multiplication unit 4, while preventing second electrons, which are generated due to factors other than the incidence of electromagnetic waves W to the metasurface 32, from reaching the electron multiplication unit 4.

Description

Technical Field

[0006]

[0001] The present disclosure relates to a photodetector device.

Background Art

[0002] There is known a photodetector device (electron tube) including an electron emission part including a metasurface that emits electrons in response to the incidence of electromagnetic waves, and an electron multiplier part that multiplies the electrons emitted from the electron emission part (for example, see Patent Document 1). In such a photodetector device, field emission occurs at the metasurface in response to the incidence of electromagnetic waves, and electrons are emitted from the metasurface.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the photodetector device as described above, not only electrons field-emitted from the metasurface in response to the incidence of electromagnetic waves (hereinafter referred to as "FE electrons"), but also electrons other than FE electrons such as photoelectrons caused by the incidence of visible light and thermoelectrons caused by an increase in temperature may be emitted. When detecting electromagnetic waves (that is, when detecting only electrons derived from FE electrons caused by electromagnetic waves), if electrons other than FE electrons (such as photoelectrons and thermoelectrons) are emitted and multiplied in the electron multiplier part, the background noise may increase and the signal-to-noise ratio may decrease.

[0005] Therefore, an object of the present disclosure is to provide a photodetector device capable of suitably improving the signal-to-noise ratio in the detection of electromagnetic waves.

Means for Solving the Problems

[0006] This disclosure includes the following photodetectors [1] to [9].

[0007] [1] A photodetector having an electron emission unit including a metasurface that emits electrons in response to the incidence of electromagnetic waves, and an electron multiplier unit that multiplies the electrons emitted from the electron emission unit, The system includes a voltage application unit that applies a voltage to an object to which a voltage is applied, which includes at least one of the metasurface and the electron multiplier unit, The voltage application unit applies a voltage to the target object such that the potential of the electron multiplication unit on the metasurface side becomes less than or equal to the potential of the metasurface, so as to allow first electrons, which are field-emitted in response to the incidence of electromagnetic waves onto the metasurface, to reach the electron multiplication unit, while preventing second electrons, which are generated due to factors other than the incidence of electromagnetic waves onto the metasurface, from reaching the electron multiplication unit.

[0008] In the photodetector described in [1] above, a voltage is applied from the voltage application unit to at least one of the metasurface and the electron multiplier so that first electrons emitted from the metasurface in response to the incidence of electromagnetic waves reach the electron multiplier, while second electrons generated due to factors other than those mentioned above do not reach the electron multiplier, and the potential on the metasurface side of the electron multiplier is set to be less than or equal to the potential of the metasurface. This suppresses the arrival of second electrons, such as photoelectrons generated due to the incidence of visible light and thermionic electrons generated due to the rise in temperature, to the electron multiplier, and makes it possible to selectively allow only the first electrons emitted from the metasurface to reach the electron multiplier. As a result, background noise (noise caused by second electrons multiplied in the electron multiplier) can be reduced, and the signal-to-noise ratio in the detection of electromagnetic waves (i.e., detection of first electrons multiplied in the electron multiplier) can be suitably improved.

[0009] [2] The electron emission portion further includes a metal layer covering the metasurface, The metasurface is formed of a first metal, The aforementioned metal layer is formed of a second metal, The work function of the second metal is lower than that of the first metal in the photodetector [1].

[0010] According to the configuration described in [2] above, the sensitivity (electron emission ability) of the metasurface is improved, so that even when the intensity of electromagnetic waves is low, field emission can be generated in the metasurface and first electrons can be emitted from the metasurface. As described above, when the sensitivity of the metasurface is improved, second electrons such as photoelectrons and thermionic electrons are also more easily emitted from the electron emission part, but for the reasons described in [1] above, the arrival of such second electrons to the electron multiplication part is suppressed. Therefore, according to the above configuration, it is expected that the amount of electrons detected from the first electrons will be relatively increased relative to the amount of electrons detected from the second electrons, so the signal-to-noise ratio can be further improved.

[0011] [3] The second metal is an alkali metal, the photodetector of [2].

[0012] According to the configuration described in [3] above, the sensitivity of the metasurface can be suitably improved.

[0013] [4] The voltage application unit applies a voltage to the target object such that the potential of the electron multiplier unit on the metasurface side is lower than the potential of the metasurface, any of the photodetectors [1] to [3].

[0014] According to the configuration described in [4] above, an electric field is generated that accelerates electrons from the electron multiplication section towards the metasurface, thereby more effectively suppressing the arrival of second electrons at the electron multiplication section. Although such an electric field also acts on the first electrons, the initial velocity and energy of the first electrons emitted by field emission are significantly greater than those of second electrons such as photoelectrons and thermionic electrons, so only the first electrons can be selectively brought to the electron multiplication section.

[0015] [5] The voltage application unit is configured to switch between a first state in which the potential on the metasurface side of the electron multiplication unit is less than or equal to the potential of the metasurface, and a second state in which the potential on the metasurface side of the electron multiplication unit is higher than the potential of the metasurface, as any of the photodetectors in [1] to [4].

[0016] According to the configuration described in [5] above, a first state in which only the first electron can reach the electron multiplication unit and a second state in which both the first and second electrons can reach the electron multiplication unit can be appropriately switched depending on the operating environment and purpose of the photodetector. This improves the convenience for users who perform measurements using the photodetector.

[0017] [6] The photodetector is an image intensifier comprising a microchannel plate as the electron multiplier and a fluorescent film that emits fluorescence in response to the incidence of electrons multiplied by the electron multiplier, any of the photodetectors in [1] to [5].

[0018] According to the configuration described in [6] above, the image intensifier can be used to obtain a high-quality electromagnetic wave image (an image based on the electron-multiplied first electron) in which background noise caused by the second electron is reduced.

[0019] [7] The photodetector according to [6], wherein the voltage application unit applies a voltage to the target such that the potential on the metasurface side of the electron multiplier unit is lower than the potential of the metasurface, and the difference between the potential of the metasurface and the potential on the metasurface side of the electron multiplier unit is 3V or more.

[0020] According to the configuration described in [7] above, a configuration is achieved in which only the first electron is selectively delivered to the electron multiplication unit, as described above, while suitably reducing the influence of ion feedback caused by positive ions generated between the metasurface and the electron multiplication unit.

[0021] [8] The voltage application unit applies a voltage to the application target such that the potential on the metasurface side of the electron multiplication unit is lower than the potential of the metasurface, and the difference between the potential of the metasurface and the potential on the metasurface side of the electron multiplication unit is 100 V or more. The photodetector according to [6].

[0022] According to the configuration of [8] above, while realizing the configuration that selectively allows only the first electrons to reach the electron multiplication unit as described above, not only the influence of ion feedback by positive ions generated between the metasurface and the electron multiplication unit, but also the influence of ion feedback by positive ions generated between the electron multiplication unit and the fluorescent film can be reduced.

[0023] [9] The photodetection unit is a photomultiplier tube including a plurality of dynodes as the electron multiplication unit. The voltage application unit applies a voltage to the application target such that the potential of the first-stage dynode is equal to or lower than the potential of the metasurface while allowing the first electrons to reach the first-stage dynode among the plurality of dynodes and preventing the second electrons from reaching the first-stage dynode. The photodetector according to any one of [1] to [5].

[0024] According to the configuration of [9] above, in a photomultiplier tube including a plurality of dynodes configured in multiple stages, a configuration that exhibits the effect described in [1] above can be realized.

Effects of the Invention

[0025] According to the present disclosure, it is possible to provide a photodetector that can suitably improve the signal-to-noise ratio in the detection of electromagnetic waves.

Brief Description of the Drawings

[0026] [Figure 1] FIG. 1 is a diagram showing a configuration example of a photodetector according to the first embodiment. [Figure 2] FIG. 2 is an enlarged cross-sectional view of a main part of the image intensifier shown in FIG. 1. [Figure 3]Figure 3 is a rear view of the electron emission section. [Figure 4] Figure 4 is a schematic diagram showing the potential control of the comparative example. [Figure 5] Figure 5 is a schematic diagram showing the potential control in the first embodiment. [Figure 6] Figure 6 is a schematic diagram showing the potential control in the second embodiment. [Figure 7] Figure 7 shows an example of the configuration of a photodetector according to the second embodiment. [Modes for carrying out the invention]

[0027] Embodiments of this disclosure will be described in detail below with reference to the drawings. In the description of the drawings, the same elements will be denoted by the same reference numerals, and redundant descriptions will be omitted.

[0028] [First Embodiment] As shown in Figures 1 and 2, the photodetector 100 according to the first embodiment comprises an image intensifier 1 (photodetector) and a voltage application unit 2. The image intensifier 1 has a housing 10. Inside the housing 10, the image intensifier 1 has an electron emission unit 3, an electron multiplication unit 4, and a fluorescent film 5.

[0029] The interior of the image intensifier 1 is maintained in a high vacuum state by hermetically sealing both ends of a housing 10, which is roughly hollow cylindrical with axis A as its central axis, with roughly disc-shaped light incidence windows 11 and light emission windows 12. The housing 10 is composed of, for example, a roughly hollow cylindrical ceramic side tube 13, a roughly hollow cylindrical silicone rubber mold member 14 that covers the sides of the side tube 13, and a roughly hollow cylindrical polyoxymethylene case member 15 that covers the sides and bottom of the mold member 14.

[0030] For example, two through holes are formed at both ends of the mold member 14. One end of the case member 15 is open, and the other end of the case member 15 has a through hole formed that aligns with one of the through holes in the mold member 14 and its periphery.

[0031] At one end of the molded member 14, a glass light-incident window 11 is joined to the surface around one of the through-holes of the molded member 14. An electron-emitting section 3 is provided in the approximate center of the vacuum-side surface of the light-incident window 11. The light-incident window 11 is formed of, for example, quartz glass.

[0032] At the other end of the molded member 14, a light-emitting window 12 is fitted into the other through-hole of the molded member 14. A thin fluorescent film 5 is provided in the approximate center of the vacuum-side surface of the light-emitting window 12. The light-emitting window 12 is, for example, a fiber optic plate (FOP) composed of bundled optical fibers. Each optical fiber of the light-emitting window 12 has its optical axis perpendicular to the electron-emitting section 3, and its vacuum-side end face is flush with the surface. The fluorescent film 5 is provided on the vacuum-side surface of this light-emitting window 12.

[0033] An electron multiplier unit 4, which is a roughly disc-shaped microchannel plate, is positioned between the electron emission unit 3 and the fluorescent film 5. The electron multiplier unit 4 is supported by the inner edges of mounting members 21 and 22 fixed to the inner wall of the side tube 13, and is positioned opposite the electron emission unit 3 and the fluorescent film 5 at a predetermined distance. The electron multiplier unit 4 multiplies the electrons emitted from the electron emission unit 3 and outputs the multiplied electrons toward the fluorescent film 5.

[0034] In the peripheral region of the vacuum-side surface of the light incidence window 11, a metallic wiring layer (not shown) is electrically connected to the electron emission section 3. For this connection between the wiring layer and the electron emission section 3, a mounting member 23, sandwiched between the side tube 13 and the light incidence window 11, extends into the molded member 14 and is fixed. In addition, in the peripheral region of the vacuum-side surface of the light emission window 12, another metallic wiring layer (not shown) is electrically connected to the fluorescent film 5. For this connection between the wiring layer and the fluorescent film 5, a mounting member 24, sandwiched between the side tube 13 and the molded member 14, extends into the molded member 14 and is fixed.

[0035] One end of each of the mounting members 21-24 is connected to a lead wire 25-28, which is made of, for example, Kovar metal. The other ends of the lead wires 25-28 hermetically penetrate the mold member 14 and the case member 15 and protrude to the outside, and are electrically connected to the voltage application unit 2, which is an external voltage source. As a result, a predetermined voltage from the voltage application unit 2 is applied to the input surface 4a and output surface 4b of the electron emission unit 3 and the electron multiplication unit 4, as well as the fluorescent film 5. The voltage application unit 2 has a circuit for generating voltage and a control circuit including a processor, memory, etc. for controlling the magnitude of the voltage supplied to each of the lead wires 25-28.

[0036] The electron emission unit 3 emits electrons in response to the incidence of light (in this embodiment, electromagnetic wave W) from the light incidence window 11. The electromagnetic wave W is an electromagnetic wave having a predetermined electric field oscillation direction perpendicular to the direction of propagation of the electromagnetic wave W (i.e., an electromagnetic wave having a predetermined polarization (polarization) component). The electromagnetic wave W is, for example, an electromagnetic wave in a predetermined band included in the frequency band from millimeter waves to infrared light, and one example is a terahertz wave. Hereinafter, the direction parallel to the direction of propagation of the electromagnetic wave W will be represented as the X direction, the direction perpendicular to the X direction and parallel to the electric field oscillation direction of the electromagnetic wave W will be represented as the Z direction, and the direction perpendicular to both the X direction and the Z direction will be represented as the Y direction. Furthermore, in the X direction, the direction from the light emission window 12 toward the light incidence window 11 will be represented as the front, and the opposite direction will be represented as the rear. As shown in Figures 1 and 2, the X direction is the direction in which the light incidence window 11, electron emission section 3, electron multiplication section 4, fluorescent film 5, and light emission window 12 are aligned, and is also the direction in which axis A extends. Each of the electron emission section 3, electron multiplication section 4, and fluorescent film 5 is aligned along the YZ plane perpendicular to the X direction.

[0037] As shown in Figure 3, the electron emission unit 3 comprises a substrate 31, a metasurface 32, a pair of electrodes 33, and a metal layer 34. The substrate 31 is formed in a plate shape (for example, a rectangular plate shape) from a material that is transparent to electromagnetic waves W. The material of the substrate 31 is an electrically insulating material (for example, silicon, quartz, sapphire, zinc selenide, etc.). The metasurface 32 and the pair of electrodes 33 are formed on the surface 31a of the substrate 31. The metal layer 34 is formed on the surface 31a of the substrate 31, covering the metasurface 32 and the pair of electrodes 33. Surface 31a is the surface of the substrate 31 on the side of the electron multiplication unit 4. That is, the electron emission unit 3 is positioned such that the side of the substrate 31 opposite to surface 31a faces the rear surface of the light incidence window 11 (the side opposite to the light incidence side). In Figure 4, the metal layer 34 is shown by a dashed line.

[0038] The metasurface 32 includes a plurality of antenna structures 35 arranged in two dimensions along the surface 31a. As an example, each antenna structure 35 is configured to include a pair of ends facing each other in the Z direction. Each antenna structure 35 is sensitive in the direction in which the pair of ends face each other (the Z direction). More specifically, each antenna structure 35 emits electrons by field emission (field electron emission) in response to the incidence of an electromagnetic wave W whose electric field oscillation direction is in the Z direction. In other words, the metasurface 32 has polarization characteristics that cause it to emit electrons in response to the incidence of an electromagnetic wave W having a predetermined electric field oscillation direction (in this embodiment, an oscillation direction parallel to the Z direction). To put it another way, the metasurface 32 emits electrons when the direction in which the straight line connecting the pair of ends of each antenna structure 35 extends (in this embodiment, the Z direction) coincides with the electric field oscillation direction of the electromagnetic wave W. Each antenna structure 35 may be configured as a bowtie antenna, for example. However, the antenna structure 35 is not limited to the above example, and may be composed of, for example, a dipole antenna, a split-ring antenna, a double split-ring antenna, etc.

[0039] As shown in Figure 3, the metasurface 32 is positioned on axis A. A pair of electrodes 33 are positioned on both sides of the metasurface 32. Each electrode 33 is electrically connected to each antenna structure 35 via wiring (not shown). A predetermined potential difference is applied between a pair of opposing ends in each antenna structure 35 via the pair of electrodes 33. The metasurface 32 is formed of a conductive material (for example, metallic materials such as gold, platinum, aluminum, silver, and copper, or electrically conductive inorganic carbon materials such as graphene and graphite). The metasurface 32 is formed by patterning the conductive material. Each electrode 33 is formed, for example, of the same material as the metasurface 32. A metal layer 34 covers the entire metasurface 32 and the pair of electrodes 33 and is in contact with the metasurface 32 and the pair of electrodes 33. The metal layer 34 is deposited with a metal having a lower work function than the material of the metasurface 32 (for example, an alkali metal such as cesium) to a thickness of about 1 to several atomic layers. The metal layer 34, by adhering to the surface of each antenna structure 35, has the effect of lowering the work function of the surface material constituting each antenna structure 35, thereby promoting field electron emission from the ends of each antenna structure 35. In other words, the metal layer 34 has the function of improving the sensitivity (electron emission ability) of the metasurface 32. In this example, different potentials are applied to each of the pair of electrodes 33 so that a predetermined potential difference is given between the pair of ends of the antenna structure 35. However, instead of the pair of electrodes 33, a frame-shaped electrode 33 may be formed on the surface 31a. In that case, a uniform potential is applied to the entire metasurface 32.

[0040] As shown in Figure 2, when electromagnetic waves W, which are incident light, enter the electron emission unit 3 through the light incidence window 11, the electron emission unit 3 (metasurface 32) emits electrons E1 in response to the electromagnetic waves W. When electrons E1 enter the input surface 4a of the electron multiplication unit 4, the electron multiplication unit 4 multiplies the electrons E1 and emits the multiplied electrons E2 from the output surface 4b to the fluorescent film 5. When electrons E2 enter the fluorescent film 5, the fluorescent film 5 emits fluorescence L in response to the electrons E2. The fluorescence L (light image) emitted from the fluorescent film 5 passes through the light emission window 12 and is then acquired by an imaging means such as a CCD camera.

[0041] Next, the potential control performed by the voltage application unit 2 will be described. The voltage application unit 2 applies a voltage to the application target, which includes at least one of the metasurface 32 (electron emission unit 3) and the electron multiplication unit 4. As described above, in this embodiment, the voltage application unit 2 is electrically connected to the input surface 4a and output surface 4b of the electron emission unit 3 and the electron multiplication unit 4, as well as to each part of the fluorescent film 5, via each lead wire 25 to 28, and is configured to apply a predetermined voltage to each of these parts. That is, the application target includes these parts. In this embodiment, as an example, the distance between the metasurface 32 and the input surface 4a in the X direction is 0.2 mm, and the distance between the output surface 4b and the fluorescent film 5 in the X direction is 0.6 mm.

[0042] As described above, when electromagnetic waves W are incident on the electron emission unit 3 (metasurface 32), electrons (hereinafter referred to as "FE electrons") are emitted by field emission at the metasurface 32. In addition, photoelectrons may be emitted due to the incidence of visible light on the metasurface 32. Furthermore, thermionic electrons may be emitted due to an increase in temperature. In this embodiment, when the electron emission unit 3 has a metal layer 34 made of alkali metal for the purpose of improving sensitivity, photoelectrons and thermionic electrons are more easily emitted along with FE electrons. If electrons other than FE electrons (photoelectrons, thermionic electrons, etc.) caused by electromagnetic waves W are multiplied by the electron multiplication unit 4, the multiplied photoelectrons, etc. become background noise, which may reduce the signal-to-noise ratio in the detection of electromagnetic waves W (i.e., detection of electrons originating from FE electrons).

[0043] The above problem will be explained in detail with reference to the comparative example in Figure 4. In this comparative example, the voltage application unit 2 applies voltage to each part such that the potential of the electron emission unit 3 becomes -200V, the potential of the input surface 4a of the electron multiplication unit 4 becomes 0V (GND potential), the potential of the output surface 4b of the electron multiplication unit 4 becomes +1kV, and the potential of the fluorescent film 5 becomes +6kV. In other words, the potential of the input surface 4a of the electron multiplication unit 4 is higher than the potential of the electron emission unit 3, the potential of the output surface 4b of the electron multiplication unit 4 is higher than the potential of the input surface 4a, and the potential of the fluorescent film 5 is higher than the potential of the output surface 4b. That is, by sequentially increasing the potential from the electron emission unit 3 to the input surface 4a, the output surface 4b, and the fluorescent film 5, electrons emitted from the electron emission unit 3 can be appropriately guided to the electron multiplication unit 4, and electrons multiplied in the electron multiplication unit 4 can be appropriately guided to the fluorescent film 5. Hereinafter, an electric field in which the potential of the electron multiplier unit 4 is higher than the potential of the electron emission unit 3 is called a "forward bias electric field".

[0044] In the comparative example in Figure 4, electrons E11, which are FE electrons, and electrons E12, which are not FE electrons, are emitted from the electron emission unit 3 (metasurface 32). Electrons E11 are electrons field-emitted in response to electromagnetic waves W incident on the electron emission unit 3 (metasurface 32). Electrons E12 are, for example, photoelectrons and thermionic electrons emitted in response to visible light VL incident on the electron emission unit 3 (metasurface 32).

[0045] As described above, in this comparative example, a forward bias electric field is maintained between the electron emission unit 3 and the electron multiplication unit 4, so both electrons E11 and E12 are accelerated toward the input surface 4a of the electron multiplication unit 4. As a result, both electrons E11 and E12 are multiplied in the electron multiplication unit 4, and both the multiplied electrons E21 (E11) and E22 (E12) reach the fluorescent film 5 from the output surface 4b. Consequently, not only electrons E21 originating from FE electrons but also electrons E22 originating from electrons other than FE electrons (photoelectrons, thermionic electrons, etc.) are detected as background noise, which reduces the signal-to-noise ratio in the detection of electromagnetic waves W.

[0046] Furthermore, separate from the decrease in the signal-to-noise ratio mentioned above, there is a risk of ion feedback occurring in the vacuum space in which the electron emission unit 3, the electron multiplication unit 4, and the fluorescent film 5 are enclosed. Ion feedback is a phenomenon in which, in a state where the electron density is high and accelerated electrons with high energy are present, ions (positive ions) are generated by collisions between residual gas in the vacuum space and electrons, and these ions collide with the photocathode (in this embodiment, the metasurface 32 of the electron emission unit 3), degrading the photocathode. Ions that cause such ion feedback are generally more likely to occur in the space after electron multiplication (i.e., the space between the output surface 4b of the electron multiplication unit 4 and the fluorescent film 5). However, in cases such as the photodetector 100, where the metasurface 32 emits a large amount of FE electrons with high initial velocity and energy by field emission, ions that cause ion feedback are also relatively likely to occur in the space between the metasurface 32 and the electron multiplication unit 4. In Figure 4, ion F1 represents a positive ion generated between the metasurface 32 and the electron multiplier unit 4, and ion F2 represents a positive ion generated between the electron multiplier unit 4 and the fluorescent film 5. In the comparative example, both ions F1 and F2 are accelerated towards the metasurface 32, and there is a risk of ion feedback (i.e., collisions of ions F1 and F2 with the metasurface 32) caused by both ions F1 and F2.

[0047] As described above, in the comparative example shown in Figure 4, the potential control may result in two problems: a decrease in the signal-to-noise ratio in the detection of electromagnetic waves W due to the detection of electrons E22 originating from electrons other than FE electrons as background noise, and the occurrence of ion feedback caused by ions F1 and F2. In order to avoid or mitigate these problems, the voltage application unit 2 is configured to apply a voltage to the target such that the potential of the input surface 4a of the electron multiplication unit 4 (the potential on the metasurface side of the electron multiplication unit) is less than or equal to the potential of the metasurface 32, while allowing electrons E11 (first electrons) that are field-emitted in response to the incidence of electromagnetic waves W on the metasurface 32 to reach the electron multiplication unit 4, while preventing electrons E12 (second electrons) generated due to factors other than the incidence of electromagnetic waves W on the metasurface 32 from reaching the electron multiplication unit 4.

[0048] The following describes the first and second embodiments as specific examples of potential control of the voltage application unit 2.

[0049] The first embodiment in Figure 5 differs from the comparative example (Figure 4) in that the potential of the electron emission unit 3 is set to 3V. That is, in the first embodiment, the voltage application unit 2 applies a voltage to the target such that the potential of the input surface 4a of the electron multiplication unit 4 is lower than the potential of the metasurface 32. Hereinafter, an electric field in which the potential of the electron multiplication unit 4 is lower than the potential of the electron emission unit 3 is called a "reverse bias electric field". With the above configuration, an electric field (reverse bias electric field) can be generated that accelerates electrons from the electron multiplication unit 4 side to the metasurface 32 side. This makes it possible to more effectively suppress electrons E12 from reaching the electron multiplication unit. Although such a reverse bias electric field also acts on electrons E11, which are FE electrons, the initial velocity and energy of electrons E11 emitted by field emission are significantly larger than the initial velocity and energy of electrons E12 such as photoelectrons and thermionic electrons, so only electrons E11 can be selectively made to reach the electron multiplication unit 4.

[0050] Furthermore, in the first embodiment, the voltage application unit 2 applies a voltage to the target such that the potential of the input surface 4a of the electron multiplication unit 4 is lower than the potential of the metasurface 32, and the difference between the potential of the metasurface 32 and the potential of the input surface 4a of the electron multiplication unit 4 is 3V. With the above configuration, as described above, it is possible to achieve a configuration in which only electrons E11 selectively reach the electron multiplication unit 4, while suitably reducing the influence of ion feedback caused by positive ions (ions F1) generated between the metasurface 32 and the electron multiplication unit 4. More specifically, by generating a reverse bias electric field between the potential of the metasurface 32 and the input surface 4a of the electron multiplication unit 4 as described above, even if ions F1 are generated between the metasurface 32 and the input surface 4a, it is possible to suppress the acceleration of the ions F1 towards the metasurface 32 side. While a similar effect can be achieved even when the metasurface 32 and the input surface 4a are at the same potential, setting a potential difference of approximately 3V or more, as in the first embodiment, allows for more favorable suppression of ions F1 colliding with the metasurface 32 (ion feedback by ions F1).

[0051] On the other hand, ions F2 generated between the output surface 4b of the electron multiplier unit 4 and the fluorescent film 5 are accelerated relatively significantly from the fluorescent film 5 side to the output surface 4b side by the relatively large potential difference (5kV in this embodiment) between the output surface 4b and the fluorescent film 5. For this reason, as in the first embodiment, when the potential difference between the metasurface 32 and the input surface 4a is relatively small and the reverse bias electric field is relatively weak, it is difficult to suppress the ions F2 that are accelerated as described above, pass through the electron multiplier unit 4 from the fluorescent film 5 side to the metasurface 32 side, and prevent them from colliding with the metasurface 32. In other words, when the reverse bias electric field is relatively weak, ion feedback by ions F1 can be suppressed, but ion feedback by ions F2 may not be sufficiently suppressed. Therefore, the voltage application unit 2 may perform the potential control shown in the second embodiment of Figure 6 in order to sufficiently suppress not only ion feedback by ions F1 but also ion feedback by ions F2.

[0052] The second embodiment shown in Figure 6 differs from the comparative example (Figure 4) and the first embodiment (Figure 5) in that the potential of the electron emission unit 3 is set to 100V. That is, in the second embodiment, the voltage application unit 2 applies a voltage to the target such that the potential of the input surface 4a of the electron multiplication unit 4 is lower than the potential of the metasurface 32, and the difference between the potential of the metasurface 32 and the potential of the input surface 4a of the electron multiplication unit 4 is 100V. With the above configuration, it is possible to achieve a configuration in which only electrons E11 selectively reach the electron multiplication unit 4, while reducing not only the effect of ion feedback by ions F1 generated between the metasurface 32 and the electron multiplication unit 4, but also the effect of ion feedback by ions F2 generated between the electron multiplication unit 4 and the fluorescent film 5. More specifically, by providing a sufficient potential difference (a potential difference of 100V or more) between the metasurface 32 and the input surface 4a, a relatively strong reverse bias electric field is formed, making it possible to suppress ions F2 from colliding with the metasurface 32. In other words, as shown in Figure 6, the ions F2 that pass through the electron multiplication unit 4 and fly towards the metasurface 32 can be decelerated, thereby suppressing collisions between the ions F2 and the metasurface 32. As mentioned above, since the initial velocity and energy of the FE electrons E11 are sufficiently high, even if a potential difference of about 100V is provided between the metasurface 32 and the input surface 4a, the electrons E11 can reach the electron multiplication unit 4.

[0053] [Effects and Effects] In the photodetector 100, a voltage is applied from the voltage application unit 2 to at least one of the metasurface 32 and the electron multiplier 4 so that electrons E11 emitted from the metasurface 32 in response to the incidence of electromagnetic waves W reach the electron multiplier unit 4, while electrons E12 generated due to factors other than those mentioned above do not reach the electron multiplier unit 4. The potential of the input surface 4a of the electron multiplier unit 4 is set to be less than or equal to the potential of the metasurface 32. This suppresses electrons E12, such as photoelectrons generated due to the incidence of visible light VL and thermionic electrons generated due to the rise in temperature, from reaching the electron multiplier unit 4, and makes it possible to selectively allow only electrons E11 emitted from the metasurface 32 to reach the electron multiplier unit 4. As a result, background noise (noise caused by electrons E12 multiplied by the electron multiplier unit 4) is reduced, and the signal-to-noise ratio in the detection of electromagnetic waves W (i.e., detection of electrons E11 multiplied by the electron multiplier unit 4) can be suitably improved. In this embodiment (the first embodiment (Figure 5) and the second embodiment (Figure 6)), the voltage application unit 2 applied a reverse bias electric field so that the potential of the input surface 4a of the electron multiplier unit 4 was less than the potential of the metasurface 32. However, the input surface 4a of the electron multiplier unit 4 and the metasurface 32 may be at the same potential. Even in this case, it is possible to suppress electrons E12 from reaching the electron multiplier unit 4 compared to the comparative example (Figure 4) in which a forward bias electric field is applied. However, as described above, applying a reverse bias electric field can more effectively suppress electrons E12 from reaching the electron multiplier unit 4.

[0054] In the photodetector 100, the metasurface 32 in the electron emission section 3 is covered by a metal layer 34. The metasurface 32 is made of a first metal. The metal layer 34 of the electron emission section 3 is made of a second metal. The work function of the second metal is lower than that of the first metal. In the photodetector 100, the second metal is an alkali metal. For example, the first metal is gold and the second metal is cesium. With the above configuration, the sensitivity (electron emission ability) of the metasurface 32 is improved, so that even when the intensity of the electromagnetic wave W is low, field emission can be generated in the metasurface 32 and electrons E11 can be emitted from the metasurface 32. When the sensitivity of the metasurface 32 is improved as described above, electrons E12 such as photoelectrons and thermionic electrons are also more easily emitted from the electron emission section 3, but for the reasons described above, the arrival of such electrons E12 to the electron multiplication section 4 is suppressed. Therefore, with the above configuration, it is expected that the amount of electrons detected from electron E11 will be relatively increased compared to the amount of electrons detected from electron E12, thus further improving the signal-to-noise ratio.

[0055] In the photodetector 100, the metal layer 34 is formed of an alkali metal. This allows for a favorable improvement in the sensitivity of the metasurface 32.

[0056] The photodetector 100 has an image intensifier 1 as its photodetector, which includes a microchannel plate as an electron multiplier 4 and a fluorescent film 5 that emits fluorescence in response to the incidence of electrons multiplied by the electron multiplier 4. With the above configuration, the image intensifier 1 can obtain a high-quality electromagnetic wave image (an image based on electrons E21 originating from field-emitted FE electrons in response to electromagnetic waves W) with reduced background noise caused by electrons E22 (see Figure 4).

[0057] In the photodetector 100, the voltage application unit 2 may be configured to switch between a first state in which the potential of the input surface 4a of the electron multiplier unit 4 is less than or equal to the potential of the metasurface 32 (for example, a reverse bias electric field in the first embodiment (Figure 5) or the second embodiment (Figure 6)) and a second state in which the potential of the input surface 4a of the electron multiplier unit 4 is higher than the potential of the metasurface 32 (for example, a forward bias electric field in the comparative example (Figure 4)).

[0058] For example, in environments where there is no risk of strong visible light VL being incident on the metasurface 32, and under conditions where electrons E12 such as photoelectrons are unlikely to be generated, it is not necessary to control the potential so that the potential of the input surface 4a of the electron multiplier unit 4 is below the potential of the metasurface 32 in order to prevent electrons E12 from reaching the electron multiplier unit 4. Furthermore, in order to more reliably ensure that field-emitted electrons E11 reach the electron multiplier unit 4, it is preferable to make the potential of the input surface 4a of the electron multiplier unit 4 higher than the potential of the metasurface 32 (i.e., to create a forward bias electric field). By creating a forward bias electric field, it becomes possible to appropriately guide electrons with lower energy than electrons E11 (these electrons can also be said to be electrons derived from FE electrons and are therefore preferably detected) to the electron multiplier unit 4, such as secondary electrons generated on the metasurface 32 due to electrons E11. In addition, if it is desired to detect visible light VL simultaneously with electromagnetic waves W (e.g., terahertz waves), it is preferable to allow electrons E12 to reach the electron multiplier unit 4 along with electrons E11.

[0059] As described above, by configuring the voltage application unit 2 to be switchable between a first state and a second state, the first state, in which only electron E11 can reach the electron multiplication unit 4, and the second state, in which both electron E11 and electron E12 can reach the electron multiplication unit 4, can be appropriately switched according to the operating environment and purpose of use of the photodetector 100. This improves the convenience for users who perform measurements using the photodetector 100.

[0060] [Second Embodiment] As shown in Figure 7, the photodetector 100A according to the second embodiment comprises a photomultiplier tube 1A (photodetector) and a voltage application unit 2. In other words, the photodetector 100A mainly differs from the photodetector 100 in that it includes a photomultiplier tube 1A instead of an image intensifier 1 as the photodetector targeted by potential control by the voltage application unit 2.

[0061] The photomultiplier tube 1A has a housing 10A. Inside the housing 10A, the photomultiplier tube 1A has an electron emission section 3, an electron multiplication section 4A, and an electron collection section 5A. The housing 10A is formed in the shape of a bottomed cylinder from a material that is transparent to electromagnetic waves W (for example, quartz). The part corresponding to the bottom of the housing 10A functions as a window into which electromagnetic waves W are incident. The electron emission section 3 is positioned opposite the window. The electron multiplication section 4A is composed of multiple dynodes arranged in multiple stages. The electron collection section 5A is positioned near the end of the housing 10A opposite to the window and collects electrons multiplied by the electron multiplication section 4A. The photomultiplier tube 1A can detect electromagnetic waves W based on the electrons thus collected.

[0062] The electron multiplier unit 4A includes a first-stage first dynode 41 and a second-stage second dynode 42 positioned after the first dynode 41. The voltage application unit 2 is electrically connected to the first dynode 41, the second dynode 42, and the electron emission unit 3 via connecting members such as lead pins (not shown).

[0063] The voltage application unit 2 applies a voltage to the target so that the potential of the first dynode 41 is less than or equal to the potential of the metasurface 32, while allowing electrons E11, which are field-emitted FE electrons from the metasurface 32, to reach the first dynode 41, while preventing electrons E12, such as photoelectrons or thermionic electrons, from reaching the first dynode 41. As an example, the voltage application unit 2 applies voltage to each part so that the potential of the electron emission unit 3 becomes -1500V, the potential of the first dynode 41 becomes -1510V, and the potential of the second dynode 42 becomes -1400V. In the photodetector 100A, by applying a reverse bias electric field between the metasurface 32 and the first dynode 41 in this way, a configuration that produces the same effect as in the first embodiment can be realized in the photomultiplier tube 1A, which includes multiple dynodes (electron multiplier units 4A) configured in multiple stages.

[0064] More specifically, electrons E11 (FE electrons) emitted from the metasurface 32 and heading toward the first dynode 41 have high initial velocity and energy, so even when the reverse bias electric field described above is applied, they can reach the first dynode 41. As a result, secondary electrons generated by the collision between the first dynode 41 and electrons E11, and electrons E11 that recoil from the first dynode 41 and head toward the second dynode 42, can be made to reach the second dynode 42. In this way, only electrons with high initial velocity and energy (electrons originating from electrons E11) can be selectively guided toward the second dynode 42.

[0065] While a similar effect can be achieved even when the metasurface 32 and the first dynode 41 are at the same potential (for example, when the potential of the first dynode 41 is -1500V), generating a reverse bias electric field as described above makes it possible to more effectively suppress electrons E12 from reaching the first dynode 41, and selectively allow only electrons E11 to reach the first dynode 41.

[0066] [Differentiation] Although one embodiment of the present disclosure has been described above, the present disclosure is not limited to the embodiment described above. The materials and shapes of each component are not limited to the specific materials and shapes described above, but a variety of other materials and shapes can be used. Furthermore, some of the components included in the above embodiment may be omitted or modified as appropriate, or can be arbitrarily combined with other additional components.

[0067] For example, the voltage application unit 2 may be configured to apply a voltage to at least one of the electron emission unit 3 and the electron multiplication units 4, 4A. In this case as well, the forward bias electric field and the reverse bias electric field described above can be switched. Furthermore, the voltage application unit 2 does not have to be configured to switch between the forward bias electric field and the reverse bias electric field. For example, if it is always desired to detect only electromagnetic waves W, and it is expected that electrons other than FE electrons, such as photoelectrons or thermionic electrons, will be emitted from the metasurface 32, the voltage application unit 2 may apply a voltage to the target so as to be fixed in a reverse bias electric field. [Explanation of symbols]

[0068] 1...Image intensifier (photodetector), 1A...Photomultiplier tube (photodetector), 2...Voltage application unit, 3...Electron emission unit, 4,4A...Electron multiplication unit, 5...Fluorescent film, 32...Metasurface, 34...Metal layer, 41...First dynode, 100,100A...Photodetector, E11...Electron (first electron), E12...Electron (second electron), L...Fluorescence, W...Electromagnetic wave.

Claims

1. A photodetector having an electron emission unit including a metasurface that emits electrons in response to the incidence of electromagnetic waves, and an electron multiplier unit that multiplies the electrons emitted from the electron emission unit, The system includes a voltage application unit that applies a voltage to an object to which a voltage is applied, which includes at least one of the metasurface and the electron multiplier unit, The voltage application unit applies a voltage to the target object such that the potential on the metasurface side of the electron multiplication unit becomes less than or equal to the potential of the metasurface, so as to allow first electrons, which are field-emitted in response to the incidence of electromagnetic waves onto the metasurface, to reach the electron multiplication unit, while preventing second electrons, which are generated due to factors other than the incidence of electromagnetic waves onto the metasurface, to reach the electron multiplication unit.

2. The electron emission portion further includes a metal layer covering the metasurface, The metasurface is formed of a first metal, The aforementioned metal layer is formed of a second metal, The photodetector according to claim 1, wherein the work function of the second metal is lower than the work function of the first metal.

3. The photodetector according to claim 2, wherein the second metal is an alkali metal.

4. The photodetector according to claim 1, wherein the voltage application unit applies a voltage to the target object such that the potential on the metasurface side of the electron multiplier unit is lower than the potential of the metasurface.

5. The photodetector according to claim 1, wherein the voltage application unit is configured to switch between a first state in which the potential on the metasurface side of the electron multiplier unit is less than or equal to the potential of the metasurface, and a second state in which the potential on the metasurface side of the electron multiplier unit is higher than the potential of the metasurface.

6. The photodetector according to claim 1, wherein the photodetector is an image intensifier comprising a microchannel plate as the electron multiplier and a fluorescent film that emits fluorescence in response to the incidence of electrons multiplied by the electron multiplier.

7. The photodetector according to claim 6, wherein the voltage application unit applies a voltage to the target object such that the potential on the metasurface side of the electron multiplier unit is lower than the potential of the metasurface, and the difference between the potential of the metasurface and the potential on the metasurface side of the electron multiplier unit is 3V or more.

8. The photodetector according to claim 6, wherein the voltage application unit applies a voltage to the target object such that the potential on the metasurface side of the electron multiplier unit is lower than the potential of the metasurface, and the difference between the potential of the metasurface and the potential on the metasurface side of the electron multiplier unit is 100V or more.

9. The light detection unit is a photomultiplier tube containing a plurality of dynodes as the electron multiplication unit, The photodetector according to claim 1, wherein the voltage application unit applies a voltage to the target object such that the potential of the first-stage dynode becomes less than or equal to the potential of the metasurface, while causing the first electrons to reach the first-stage dynode among the plurality of dynodes, and preventing the second electrons from reaching the first-stage dynode.

Images