Light detection device

Abstract

This light detection device comprises: a light detection unit that is provided with an electron emission unit that includes a metasurface which emits electrons in response to the incidence of electromagnetic waves, and an electron multiplication unit that multiplies the electrons emitted from the electron emission unit; and a voltage application unit that applies a voltage to an application target including at least one among the metasurface and the electron multiplication unit. In order to allow first electrons, which are field-emitted in response to the incidence of electromagnetic waves on the metasurface, to reach the electron multiplication unit while preventing second electrons, which are generated due to a factor other than the incidence of electromagnetic waves on the metasurface, from reaching the electron multiplication unit, the voltage application unit applies a voltage to the application target such that the electric potential on the metasurface side of the electron multiplication unit is equal to or lower than the electric potential of the metasurface.

Description

Photodetector

[0001] The present disclosure relates to a light detection device.

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

[0003] Special Publication No. 2023-512566

[0004] In the photodetector device described above, not only electrons (hereinafter referred to as "FE electrons") that are field-emitted from the metasurface in response to the incidence of electromagnetic waves, 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 it is desired to detect electromagnetic waves (i.e., when it is desired to detect only electrons derived from FE electrons caused by electromagnetic waves), if electrons other than FE electrons (photoelectrons, thermoelectrons, etc.) are emitted and multiplied by the electron multiplier section, 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 that can suitably improve the signal-to-noise ratio in detecting electromagnetic waves.

[0006] The present disclosure includes the following photodetection devices [1] to [9].

[0007] [1] A photodetector comprising: an electron emitting section including a metasurface that emits electrons in response to incidence of electromagnetic waves; and an electron multiplier section that multiplies the electrons emitted from the electron emitting section; and a voltage application section that applies a voltage to an object to be applied that includes at least one of the metasurface and the electron multiplier section, wherein the voltage application section applies a voltage to the object to be applied so that the potential of the metasurface side of the electron multiplier section is equal to or lower than the potential of the metasurface, so as to allow first electrons that are field-emitted in response to incidence of the electromagnetic waves on the metasurface to reach the electron multiplier section, while preventing second electrons that are generated due to factors other than incidence of the electromagnetic waves on the metasurface from reaching the electron multiplier section.

[0008] In the photodetector device of [1] described above, a voltage is applied from the voltage application unit to at least one of the metasurface and the electron multiplier unit so that first electrons emitted from the metasurface in response to the incidence of electromagnetic waves reach the electron multiplier unit, while second electrons generated due to factors other than the above do not reach the electron multiplier unit. The voltage application unit is set so that the potential of the metasurface side of the electron multiplier unit is equal to or lower than the potential of the metasurface. This prevents second electrons, such as photoelectrons generated due to the incidence of visible light and thermoelectrons generated due to a rise in temperature, from reaching the electron multiplier unit, and allows only the first electrons emitted from the metasurface to selectively reach the electron multiplier unit. As a result, background noise (noise caused by the second electrons multiplied by the electron multiplier unit) can be reduced, and the signal-to-noise ratio in detecting electromagnetic waves (i.e., detecting the first electrons multiplied by the electron multiplier unit) can be favorably improved.

[0009] [2] The photodetector device of [1], wherein the electron emission portion further includes a metal layer covering the metasurface, the metasurface being formed of a first metal, the metal layer being formed of a second metal, and the work function of the second metal being lower than the work function of the first metal.

[0010] According to the configuration [2] above, the sensitivity (electron emission ability) of the metasurface is improved, so that even when the electromagnetic wave intensity is low, field emission can be generated in the metasurface, allowing primary electrons to be emitted from the metasurface. Note that when the sensitivity of the metasurface is improved as described above, secondary electrons such as photoelectrons and thermoelectrons are also more likely to be emitted from the electron emission section. However, for the reasons described in [1] above, these secondary electrons are prevented from reaching the electron multiplier section. Therefore, according to the configuration described above, it is expected that the amount of detected electrons derived from primary electrons can be increased relative to the amount of detected electrons derived from secondary electrons, thereby further improving the signal-to-noise ratio.

[0011] [3] The photodetector according to [2], wherein the second metal is an alkali metal.

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

[0013] [4] The voltage application unit applies a voltage to the object to be applied so that the potential on the metasurface side of the electron multiplier unit is lower than the potential of the metasurface. Any of the optical detection devices of [1] to [3].

[0014] According to the configuration of [4] above, by generating an electric field in a direction that accelerates electrons from the electron multiplier section side to the metasurface side, it is possible to more effectively prevent the second electrons from reaching the electron multiplier section. Note that 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 the initial velocity and energy of the second electrons such as photoelectrons and thermoelectrons, so that only the first electrons can selectively reach the electron multiplier section.

[0015] [5] The voltage application unit is configured to be switchable between a first state in which the potential on the metasurface side of the electron multiplier unit is lower than 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, any of the optical detection devices [1] to [4].

[0016] According to the configuration [5] above, it is possible to appropriately switch between a first state in which only the first electrons can reach the electron multiplier section and a second state in which both the first electrons and the second electrons can reach the electron multiplier section depending on the usage environment, purpose of use, etc. of the photodetector, thereby improving the convenience for a user who performs measurements using the photodetector.

[0017] [6] The photodetector device according to any one of [1] to [5], wherein the photodetector is an image intensifier including a microchannel plate as the electron multiplier and a fluorescent film that outputs fluorescent light in response to incidence of electrons multiplied by the electron multiplier.

[0018] According to the configuration [6] above, the image intensifier can provide a high-quality electromagnetic wave image (an image based on electron-multiplied primary electrons) in which background noise caused by secondary electrons is reduced.

[0019] [7] The optical detection device of [6], wherein the voltage application unit applies a voltage to the object so 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 3 V or more.

[0020] According to the configuration [7] above, it is possible to realize a configuration that selectively allows only the first electrons to reach the electron multiplier section as described above, while suitably reducing the influence of ion feedback caused by positive ions generated between the metasurface and the electron multiplier section.

[0021] [8] The optical detection device of [6], wherein the voltage application unit applies a voltage to the object to be applied so 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 100 V or more.

[0022] According to the configuration [8] above, while realizing a configuration that selectively allows only the first electrons to reach the electron multiplier section as described above, it is possible to reduce not only the influence of ion feedback due to positive ions generated between the metasurface and the electron multiplier section, but also the influence of ion feedback due to positive ions generated between the electron multiplier section and the fluorescent film.

[0023] [9] The photodetector unit is a photomultiplier tube including a plurality of dynodes as the electron multiplier unit, and the voltage application unit applies a voltage to the target so 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 a first-stage dynode among the plurality of dynodes, while preventing the second electrons from reaching the first-stage dynode.

[0024] According to the configuration of [9] above, in a photomultiplier tube including a plurality of dynodes arranged in multiple stages, it is possible to realize a configuration that provides the effects described in [1] above.

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

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

[0027] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and duplicated description will be omitted.

[0028] 1 and 2 , a photodetector 100 according to the first embodiment includes 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 emitter 3, an electron multiplier 4, and a fluorescent film 5.

[0029] The interior of the image intensifier 1 is maintained in a high vacuum state by airtightly sealing both ends of a substantially hollow cylindrical housing 10 having a central axis on an axis A with a substantially disk-shaped light entrance window 11 and a light exit window 12. The housing 10 is composed of, for example, a substantially hollow cylindrical ceramic side tube 13, a substantially hollow cylindrical silicone rubber molded member 14 that covers the sides of the side tube 13, and a substantially hollow cylindrical polyoxymethylene case member 15 that covers the sides and bottom of the molded member 14.

[0030] For example, two through holes are formed at each end of the molded 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 therein whose periphery is aligned with one of the through holes in the molded member 14.

[0031] At one end of the molding member 14, a glass light entrance window 11 is bonded to the surface around one of the through holes of the molding member 14. An electron emitter 3 is provided in the approximate center of the vacuum side surface of the light entrance window 11. The light entrance window 11 is made of, for example, quartz glass.

[0032] At the other end of the molded member 14, a light exit window 12 is fitted into the other through-hole of the molded member 14. A thin-film fluorescent film 5 is provided in approximately the center of the vacuum-side surface of the light exit window 12. The light exit window 12 is, for example, a fiber optic plate (FOP) formed by bundling multiple optical fibers. The optical axes of the optical fibers of the light exit window 12 are perpendicular to the electron emitter 3, and the vacuum-side end faces are aligned flush. The fluorescent film 5 is provided on the vacuum-side surface of this light exit window 12.

[0033] An electron multiplier 4, which is a substantially circular microchannel plate, is disposed between the electron emitter 3 and the fluorescent screen 5. The electron multiplier 4 is supported by the inner edges of mounting members 21 and 22 fixed to the inner wall of the side tube 13, and faces the electron emitter 3 and the fluorescent screen 5 at a predetermined distance. The electron multiplier 4 multiplies the electrons emitted from the electron emitter 3 and outputs the multiplied electrons toward the fluorescent screen 5.

[0034] In the peripheral region of the vacuum-side surface of the light entrance window 11, a metal wiring layer (not shown) is electrically connected to the electron emission portion 3. To connect this wiring layer to the electron emission portion 3, an attachment member 23 sandwiched between the side tube 13 and the light entrance window 11 is extended into and fixed to the molded member 14. In addition, in the peripheral region of the vacuum-side surface of the light exit window 12, another metal wiring layer (not shown) is electrically connected to the fluorescent film 5. To connect this wiring layer to the fluorescent film 5, an attachment member 24 sandwiched between the side tube 13 and the molded member 14 is extended into and fixed to the molded member 14.

[0035] One end of lead wires 25-28 made of, for example, Kovar metal is connected to the end of each of the mounting members 21-24. The other ends of the lead wires 25-28 airtightly penetrate the mold member 14 and the case member 15, 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 is applied from the voltage application unit 2 to the electron emission unit 3, the input surface 4a and output surface 4b of the electron multiplier unit 4, and the fluorescent film 5. The voltage application unit 2 has a circuit that generates a 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 emitter 3 emits electrons in response to light (electromagnetic wave W in this embodiment) incident through the light entrance window 11. The electromagnetic wave W is an electromagnetic wave having a predetermined electric field oscillation direction perpendicular to the propagation direction 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 propagation direction of the electromagnetic wave W will be referred to 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 referred to as the Z direction, and the direction perpendicular to both the X direction and the Z direction will be referred to as the Y direction. Furthermore, in the X direction, the direction from the light exit window 12 toward the light entrance window 11 will be referred to as the forward direction, and the opposite direction will be referred to as the backward direction. 1 and 2 , the X direction is the direction in which the light entrance window 11, the electron emitting section 3, the electron multiplier section 4, the fluorescent film 5, and the light exit window 12 are aligned, and is also the extension direction of the axis A. The electron emitting section 3, the electron multiplier section 4, and the fluorescent film 5 each lie along the YZ plane that is perpendicular to the X direction.

[0037] As shown in FIG. 3 , the electron emitter 3 includes 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 (e.g., a rectangular plate shape) from a material that is transparent to electromagnetic waves W. The substrate 31 is made of an electrically insulating material (e.g., silicon, quartz, sapphire, zinc selenide, etc.). The metasurface 32 and the pair of electrodes 33 are formed on a 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. The surface 31a is the surface of the substrate 31 facing the electron multiplier 4. That is, the electron emitter 3 is disposed such that the surface of the substrate 31 opposite the surface 31a faces the rear surface (the surface opposite the light incident side) of the light incident window 11. Note that in FIG. 3 , the metal layer 34 is indicated by a two-dot chain line.

[0038] The metasurface 32 includes multiple antenna structures 35 arranged two-dimensionally along the surface 31a. As an example, each antenna structure 35 includes 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 faces each other (the Z direction). More specifically, each antenna structure 35 emits electrons by field emission (field electron emission) in response to incidence of electromagnetic waves W whose electric field oscillation direction is the Z direction. That is, the metasurface 32 has polarization characteristics that emit electrons in response to incidence of electromagnetic waves W having a predetermined electric field oscillation direction (in this embodiment, an oscillation direction parallel to the Z direction). In other words, the metasurface 32 emits electrons when the direction in which a straight line connecting the pair of ends of each antenna structure 35 extends (the Z direction in this embodiment) coincides with the electric field oscillation direction of the electromagnetic waves W. As an example, each antenna structure 35 may be configured as a bowtie antenna. However, the antenna structure 35 is not limited to the above example, and may be configured by, for example, a dipole antenna, a split ring antenna, a double split ring antenna, or the like.

[0039] As shown in FIG. 3 , the metasurface 32 is disposed on an axis A. A pair of electrodes 33 are disposed 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 of each antenna structure 35 via the pair of electrodes 33. The metasurface 32 is formed of a conductive material (e.g., a metal material such as gold, platinum, aluminum, silver, or copper, or an electrically conductive inorganic carbon material such as graphene or graphite). The metasurface 32 is formed by patterning a conductive material. Each electrode 33 is formed, for example, of the same material as the metasurface 32. The metal layer 34 entirely covers the 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 to a thickness of approximately one to several atomic layers using a metal (e.g., an alkali metal such as cesium) with a lower work function than the material of the metasurface 32. The metal layer 34, when attached to the surface of each antenna structure 35, has the effect of lowering the work function of the surface of the material constituting each antenna structure 35, thereby promoting field electron emission from the end of each antenna structure 35. In other words, the metal layer 34 functions to improve 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 applied 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 31 a. In this 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 emitter 3 through the light entrance window 11, the electron emitter 3 (metasurface 32) emits electrons E1 in response to the electromagnetic waves W. When the electrons E1 enter the input surface 4a of the electron multiplier 4, the electron multiplier 4 multiplies the electrons E1 and emits the multiplied electrons E2 from the output surface 4b to the fluorescent film 5. When the 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 exit window 12 and is then captured by 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 an application target including at least one of the metasurface 32 (electron emitting unit 3) and the electron multiplier unit 4. As described above, in this embodiment, the voltage application unit 2 is electrically connected to the electron emitting unit 3, the input surface 4a and output surface 4b of the electron multiplier unit 4, and each of the fluorescent film 5 via the respective lead wires 25 to 28, and is configured to be able to apply a predetermined voltage to each of these parts. In other words, the application target includes each of these parts. Note that, as an example, in this embodiment, 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 emitter 3 (metasurface 32), electrons (hereinafter referred to as "FE electrons") are emitted by field emission in the metasurface 32. In addition, photoelectrons may be emitted due to the incidence of visible light on the metasurface 32. Thermoelectrons may also be emitted due to an increase in temperature. When the electron emitter 3 has a metal layer 34 formed of an alkali metal for the purpose of improving sensitivity, as in this embodiment, photoelectrons and thermoelectrons are more likely to be emitted along with FE electrons. If electrons other than FE electrons (photoelectrons, thermoelectrons, etc.) caused by the electromagnetic waves W are multiplied by the electron multiplier 4, the multiplied photoelectrons, etc., become background noise, which may reduce the signal-to-noise ratio in detecting the electromagnetic waves W (i.e., detecting electrons derived from FE electrons).

[0043] The above problem will be described in detail with reference to the comparative example of FIG. 4 . In this comparative example, the voltage application unit 2 applies voltages to each unit so that the potential of the electron emitter 3 is −200 V, the potential of the input surface 4 a of the electron multiplier unit 4 is 0 V (GND potential), the potential of the output surface 4 b of the electron multiplier unit 4 is +1 kV, and the potential of the fluorescent film 5 is +6 kV. That is, the potential of the input surface 4 a of the electron multiplier unit 4 is made higher than the potential of the electron emitter 3, the potential of the output surface 4 b of the electron multiplier unit 4 is made higher than the potential of the input surface 4 a, and the potential of the fluorescent film 5 is made higher than the potential of the output surface 4 b. In other words, by increasing the potential from the electron emitter 3 to the input surface 4 a, the output surface 4 b, and the fluorescent film 5 in this order, electrons emitted from the electron emitter 3 can be properly guided to the electron multiplier unit 4, and electrons multiplied by the electron multiplier unit 4 can be properly guided to the fluorescent film 5. Hereinafter, such an electric field in which the potential of the electron multiplier section 4 is higher than the potential of the electron emitting section 3 will be referred to as a "forward bias electric field."

[0044] 4 , electrons E11, which are FE electrons, and electrons E12, which are not FE electrons, are emitted from the electron emitter 3 (metasurface 32). The electrons E11 are electrons field-emitted in response to electromagnetic waves W being incident on the electron emitter 3 (metasurface 32). The electrons E12 are, for example, photoelectrons and thermoelectrons emitted in response to visible light VL being incident on the electron emitter 3 (metasurface 32).

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

[0046] In addition to the above-mentioned problem of a decrease in the S / N ratio, ion feedback may occur within the vacuum space containing the electron emitter 3, electron multiplier 4, and fluorescent film 5. Ion feedback occurs when ions (positive ions) are generated by collisions between residual gas and electrons in the vacuum space in a state where electron density is high and accelerated electrons with high energy are present. These ions then collide with the photocathode (in this embodiment, the metasurface 32 of the electron emitter 3), degrading the photocathode. Ions that cause ion feedback are generally likely to occur in the space after electron multiplication (i.e., the space between the output surface 4b of the electron multiplier 4 and the fluorescent film 5). However, in cases where the metasurface 32 emits a large amount of FE electrons with high initial velocity and energy by field emission, as in the photodetector 100, ions that cause ion feedback are also relatively likely to occur in the space between the metasurface 32 and the electron multiplier 4. 4, ion F1 indicates a positive ion generated between the metasurface 32 and the electron multiplier section 4, and ion F2 indicates a positive ion generated between the electron multiplier section 4 and the fluorescent film 5. In the comparative example, both ions F1 and F2 are accelerated toward the metasurface 32, which may cause ion feedback due to both ions F1 and F2 (i.e., collisions of ions F1 and F2 with the metasurface 32).

[0047] 4 , there is a risk that the S / N ratio in detecting the electromagnetic wave W will be reduced due to the electrons E22 derived from electrons other than FE electrons being detected as background noise, and that ion feedback will occur due to the ions F1 and F2. To avoid or mitigate these problems, the voltage application unit 2 is configured to be able to apply a voltage to the target so that the potential of the input surface 4 a of the electron multiplier unit 4 (the potential on the metasurface side of the electron multiplier unit) is equal to or lower than the potential of the metasurface 32, while allowing the electrons E11 (first electrons) field-emitted in response to the electromagnetic wave W being incident on the metasurface 32 to reach the electron multiplier unit 4, while preventing the electrons E12 (second electrons) generated due to factors other than the electromagnetic wave W being incident on the metasurface 32 from reaching the electron multiplier unit 4.

[0048] Hereinafter, a first embodiment and a second embodiment will be described as specific examples of potential control of the voltage application unit 2.

[0049] The first embodiment of FIG. 5 differs from the comparative example ( FIG. 4 ) in that the potential of the electron emitter 3 is set to 3 V. That is, in the first embodiment, the voltage application unit 2 applies a voltage to the application target so that the potential of the input surface 4 a of the electron multiplier unit 4 is lower than the potential of the metasurface 32. Hereinafter, an electric field in which the potential of the electron multiplier unit 4 is lower than the potential of the electron emitter 3 is referred to as a "reverse bias electric field." According to the above configuration, an electric field (reverse bias electric field) in a direction that accelerates electrons from the electron multiplier unit 4 toward the metasurface 32 can be generated. This more effectively prevents electrons E12 from reaching the electron multiplier unit. While this 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 greater than the initial velocity and energy of electrons E12, such as photoelectrons and thermoelectrons, allowing only electrons E11 to selectively reach the electron multiplier unit 4.

[0050] Furthermore, in the first embodiment, the voltage application unit 2 applies a voltage to the application target so that the potential of the input surface 4a of the electron multiplier 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 multiplier unit 4 is 3 V. With the above configuration, it is possible to realize a configuration that selectively allows only electrons E11 to reach the electron multiplier unit 4 as described above, while suitably reducing the influence of ion feedback due to positive ions (ions F1) generated between the metasurface 32 and the electron multiplier unit 4. More specifically, by generating a reverse bias electric field between the metasurface 32 and the potential of the input surface 4a of the electron multiplier 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 toward the metasurface 32. Although a similar effect can be achieved by making the metasurface 32 and the input surface 4a equipotential, by setting a potential difference of approximately 3 V or more as in the first embodiment, it is possible to more effectively suppress ions F1 from 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 rapidly from the fluorescent film 5 side toward the output surface 4b side due to the relatively large potential difference (5 kV in this embodiment) between the output surface 4b and the fluorescent film 5. Therefore, 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, as in the first embodiment, it is difficult to prevent ions F2, which are accelerated as described above and fly from the fluorescent film 5 side to the metasurface 32 side through the electron multiplier unit 4, from colliding with the metasurface 32. In other words, when the reverse bias electric field is relatively weak, while ion feedback by ion F1 can be suppressed, ion feedback by ion F2 may not be sufficiently suppressed. Therefore, the voltage application unit 2 may perform the potential control of the second embodiment of FIG. 6 to sufficiently suppress not only ion feedback by ion F1 but also ion feedback by ion F2.

[0052] The second embodiment of FIG. 6 differs from the comparative example ( FIG. 4 ) and the first embodiment ( FIG. 5 ) in that the potential of the electron emitter 3 is set to 100 V. That is, in the second embodiment, the voltage application unit 2 applies a voltage to the application target so that the potential of the input surface 4 a of the electron multiplier 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 4 a of the electron multiplier 4 is 100 V. This configuration selectively allows only electrons E11 to reach the electron multiplier 4 while reducing not only the influence of ion feedback due to ions F1 generated between the metasurface 32 and the electron multiplier 4 but also the influence of ion feedback due to ions F2 generated between the electron multiplier 4 and the fluorescent film 5. More specifically, by providing a sufficient potential difference (a potential difference of 100 V or more) between the metasurface 32 and the input surface 4 a, a relatively strong reverse bias electric field is formed, making it possible to suppress ions F2 from colliding with the metasurface 32. 6, ions F2 passing through the electron multiplier unit 4 and flying toward the metasurface 32 can be decelerated to prevent the ions F2 from colliding with the metasurface 32. As described above, the initial velocity and energy of the electrons E11, which are FE electrons, are sufficiently high, so that the electrons E11 can reach the electron multiplier unit 4 even if a potential difference of about 100 V is provided between the metasurface 32 and the input surface 4a.

[0053] [Effects] In the photodetector 100, electrons E11 field-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 described above do not reach the electron multiplier unit 4. A voltage is applied from the voltage application unit 2 to at least one of the metasurface 32 and the electron multiplier unit 4, and the potential of the input surface 4 a of the electron multiplier unit 4 is set to be equal to or lower than the potential of the metasurface 32. This prevents electrons E12, such as photoelectrons generated due to the incidence of visible light VL and thermoelectrons generated due to a temperature rise, from reaching the electron multiplier unit 4, and allows only electrons E11 field-emitted from the metasurface 32 to selectively 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 detecting the electromagnetic waves W (i.e., detecting electrons E11 multiplied by the electron multiplier unit 4) can be favorably improved. In this embodiment (first example (FIG. 5) and second example (FIG. 6)), the voltage application unit 2 applied a reverse bias electric field in which the potential of the input surface 4a of the electron multiplier unit 4 is smaller than the potential of the metasurface 32, but the input surface 4a of the electron multiplier unit 4 and the metasurface 32 may be at the same potential. Even in this case, electrons E12 can be prevented from reaching the electron multiplier unit 4, compared to the comparative example (FIG. 4) in which a forward bias electric field is applied. However, as described above, applying a reverse bias electric field can more effectively prevent electrons E12 from reaching the electron multiplier unit 4.

[0054] In the photodetector 100, the metasurface 32 of the electron emitter 3 is covered with a metal layer 34. The metasurface 32 is formed of a first metal. The metal layer 34 of the electron emitter 3 is formed of a second metal. The work function of the second metal is lower than the work function of the first metal. In the photodetector 100, the second metal is an alkali metal. As an example, the first metal is gold, and the second metal is cesium. According to 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. Note that when the sensitivity of the metasurface 32 is improved as described above, electrons E12 such as photoelectrons and thermoelectrons are also more likely to be emitted from the electron emitter 3. However, for the reasons described above, such electrons E12 are prevented from reaching the electron multiplier 4. Therefore, with the above configuration, it is expected that the amount of detected electrons originating from electrons E11 can be increased relative to the amount of detected electrons originating from electrons E12, thereby making it possible to further improve the signal-to-noise ratio.

[0055] In the photodetector 100, the metal layer 34 is formed of an alkali metal, which can suitably improve the sensitivity of the metasurface 32.

[0056] The photodetector 100 has, as a photodetector, an image intensifier 1 including a microchannel plate as an electron multiplier 4 and a fluorescent film 5 that outputs fluorescent light in response to the incidence of electrons multiplied by the electron multiplier 4. According to the above configuration, the image intensifier 1 can obtain a high-quality electromagnetic wave image (an image based on electrons E21 derived from FE electrons field-emitted in response to the electromagnetic wave W) in which background noise caused by electrons E22 (see FIG. 4) is reduced.

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

[0058] For example, in a situation where electrons E12 such as photoelectrons are unlikely to be generated, such as in an environment where strong visible light VL is unlikely to be incident on the metasurface 32, there is no need to control the potential of the input surface 4a of the electron multiplier unit 4 so that it is equal to or lower than the potential of the metasurface 32 in order to prevent the electrons E12 from reaching the electron multiplier unit 4. Furthermore, in order to more reliably ensure that the 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 use a forward bias electric field). By using a forward bias electric field, it is possible to properly guide electrons with lower energy than the electrons E11, such as secondary electrons generated on the metasurface 32 due to the electrons E11 (these electrons can also be considered electrons derived from FE electrons and are therefore preferably detected), to the electron multiplier unit 4. Furthermore, when it is desired to simultaneously detect visible light VL and electromagnetic waves W (e.g., terahertz waves), it is preferable to allow the electrons E12 to reach the electron multiplier unit 4 along with the electrons E11.

[0059] As described above, by configuring the voltage application unit 2 to be switchable between the first state and the second state, it is possible to appropriately switch between the first state, in which only the electron E11 can reach the electron multiplier unit 4, and the second state, in which both the electrons E11 and E12 can reach the electron multiplier unit 4, depending on the usage environment, purpose of use, etc. of the photodetector 100. This improves the convenience for users who perform measurements using the photodetector 100.

[0060] 7 , a photodetector 100A according to the second embodiment includes a photomultiplier tube 1A (photodetector) and a voltage application unit 2. That is, the photodetector 100A differs from the photodetector 100 mainly in that the photodetector 100A includes a photomultiplier tube 1A instead of an image intensifier 1 as a photodetector whose potential is controlled by the voltage application unit 2.

[0061] The photomultiplier tube 1A has a housing 10A. The housing 10A includes an electron emitter 3, an electron multiplier 4A, and an electron collector 5A. The housing 10A is formed into a cylindrical shape with a bottom and made of, for example, a material (e.g., quartz) that is transparent to electromagnetic waves W. The portion corresponding to the bottom of the housing 10A functions as a window that allows electromagnetic waves W to enter the housing 10A. The electron emitter 3 is located opposite the window. The electron multiplier 4A is composed of multiple dynodes arranged in multiple stages. The electron collector 5A is located near the end of the housing 10A opposite the window and collects electrons multiplied by the electron multiplier 4A. The photomultiplier tube 1A can detect electromagnetic waves W based on the electrons collected in this manner.

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

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

[0064] More specifically, electrons E11 (FE electrons) emitted from the metasurface 32 heading toward the first dynode 41 have a high initial velocity and energy, and are therefore able to reach the first dynode 41 even when the above-described reverse bias electric field is applied. As a result, secondary electrons generated by collisions 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 a high initial velocity and energy (electrons originating from electrons E11) can be selectively guided to the second dynode 42.

[0065] Note that a similar effect can be achieved 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 set to −1500 V), but by generating a reverse bias electric field as described above, it is possible to more effectively prevent the electron E12 from reaching the first dynode 41, and selectively allow only the electron E11 to reach the first dynode 41.

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

[0067] For example, the voltage application unit 2 may be configured to be able to apply a voltage to at least one of the electron emitter 3 and the electron multiplier 4, 4A. Even in this case, 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 be able to switch between the forward bias electric field and the reverse bias electric field. For example, if it is desired to always detect only the electromagnetic wave W and it is expected that electrons other than FE electrons, such as photoelectrons or thermoelectrons, will be emitted from the metasurface 32, the voltage application unit 2 may apply a voltage to the target so that it is fixed to a reverse bias electric field.

[0068] 1...image intensifier (photodetector), 1A...photomultiplier tube (photodetector), 2...voltage application section, 3...electron emission section, 4, 4A...electron multiplier section, 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 comprising: an electron emitting section including a metasurface that emits electrons in response to incidence of electromagnetic waves; and an electron multiplier section that multiplies the electrons emitted from the electron emitting section; and a voltage application section that applies a voltage to an object to be applied that includes at least one of the metasurface and the electron multiplier section, wherein the voltage application section applies a voltage to the object to be applied so that the potential of the metasurface side of the electron multiplier section is equal to or lower than the potential of the metasurface, so as to allow first electrons that are field-emitted in response to incidence of the electromagnetic waves on the metasurface to reach the electron multiplier section, while preventing second electrons that are generated due to factors other than incidence of the electromagnetic waves on the metasurface from reaching the electron multiplier section.

2. The photodetector device of claim 1, wherein the electron emission portion further includes a metal layer covering the metasurface, the metasurface being formed of a first metal, the metal layer being formed of a second metal, and the work function of the second metal being lower than the work function of the first metal.

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

4. An optical detection device described in any one of claims 1 to 3, wherein the voltage application unit applies a voltage to the object to be applied so that the potential on the metasurface side of the electron multiplier unit is lower than the potential of the metasurface.

5. An optical detection device described in any one of claims 1 to 4, wherein the voltage application unit is configured to be switchable between a first state in which the potential on the metasurface side of the electron multiplier unit is lower than 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. A photodetection device according to any one of claims 1 to 5, wherein the photodetection section is an image intensifier including a microchannel plate as the electron multiplier section, and a fluorescent film that outputs fluorescent light in response to the incidence of electrons multiplied by the electron multiplier section.

7. The optical detection device described in claim 6, wherein the voltage application unit applies a voltage to the object so 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 3 V or more.

8. The optical detection device described in claim 6, wherein the voltage application unit applies a voltage to the object so 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 100 V or more.

9. A photodetection device according to any one of claims 1 to 5, wherein the photodetection unit is a photomultiplier tube including a plurality of dynodes as the electron multiplier unit, and the voltage application unit applies a voltage to the object to be applied so 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 a first-stage dynode among the plurality of dynodes and preventing the second electrons from reaching the first-stage dynode.

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