Metasurface element, electron tube, and method for manufacturing electron tube

Abstract

The metasurface element includes a support and a metasurface formed on a surface of the support, the metasurface having a metal pattern arranged to emit electrons in response to incidence of electromagnetic waves, and a metal layer including an alkali metal formed on the metal pattern, the metal layer extending beyond the metal pattern to areas of the surface of the support where the metal pattern is not formed.

Description

[Technical field]

[0001] One aspect of the present disclosure relates to a metasurface element, an electron tube, and a method for manufacturing an electron tube. [Background technology]

[0002] US 2016 / 0216201 discloses a detection system for detecting terahertz waves. In this detection system, terahertz waves are incident on a metal layer having a metamaterial structure. When the terahertz waves are incident on this metal layer, electrons are emitted. The emitted electrons react with the surrounding gas to generate light. The terahertz waves can be detected by detecting this light. Summary of the Invention [Problem to be solved by the invention]

[0003] A metasurface element that emits electrons in response to the incidence of electromagnetic waves is required to have improved sensitivity (electron emission ability). Therefore, an object of one aspect of the present disclosure is to provide a metasurface element and an electron tube with improved sensitivity, as well as a method for manufacturing such an electron tube.

[0004] A metasurface element according to one aspect of the present disclosure comprises a support and a metasurface formed on a surface of the support, the metasurface having a metal pattern arranged to emit electrons in response to incidence of electromagnetic waves, and a metal layer containing an alkali metal and formed on the metal pattern, the metal layer extending beyond the metal pattern to areas on the surface of the support where the metal pattern is not formed.

[0005] In this metasurface element, a metal layer containing an alkali metal is formed on the metal pattern. This distorts the band structure on the surface of the metal pattern, lowering the work function and thinning the potential barrier. As a result, the probability of tunneling can be increased, and the sensitivity (electron emission ability) can be improved. In addition, in this metasurface element, the metal layer containing an alkali metal is provided not only on the metal pattern, but also in an area on the surface of the support where the metal pattern is not formed. This reduces the resistance value of the surface of the support, and suppresses the occurrence of charge-up on the metal pattern. As a result, it is possible to suppress the difficulty of emitting electrons from the metasurface due to charge-up. As a result, the sensitivity of this metasurface element is increased.

[0006] The metal layer may be a monolayer of an alkali metal, which can effectively reduce the work function at the surface of the metal pattern.

[0007] The metal layer may contain an oxide of an alkali metal, which can more effectively reduce the work function on the surface of the metal pattern.

[0008] The alkali metal may be cesium, which can more effectively reduce the work function on the surface of the metal pattern.

[0009] The material of the metal pattern may include gold, platinum, aluminum, graphene, silver, or copper, in which case the electrical conductivity of the metal pattern can be highly ensured.

[0010] The metasurface may have only metal patterns of the same potential as the metal patterns. In this case, since there is no need to consider insulation between the metal patterns, a metal layer can be easily provided in the area where no metal pattern is formed on the surface of the support.

[0011] The support may be made of silicon, quartz, sapphire, or zinc selenide. In this case, electromagnetic waves in a frequency band ranging from millimeter waves to infrared light can be transmitted through the support, and the electromagnetic waves can be incident on the metasurface through the support.

[0012] An electron tube according to one aspect of the present disclosure includes a metasurface element, a housing having a first window portion through which electromagnetic waves pass and accommodating the metasurface element, and an electron multiplier portion disposed within the housing and multiplying electrons emitted from the metasurface element. This electron tube has an increased sensitivity for the reasons described above.

[0013] The inside of the housing may be a vacuum. In this case, the metal layer can adsorb residual gas inside the housing (getter effect), thereby increasing the degree of vacuum inside the housing.

[0014] The electron tube according to one aspect of the present disclosure may further include an alkali metal dispenser disposed in the housing and configured to release an alkali metal when energized. In this case, the alkali metal dispenser can be energized during manufacturing to release the vaporized alkali metal.

[0015] The electron tube according to one aspect of the present disclosure further includes a monitor member disposed in the housing and emitting electrons in response to incidence of a monitor electromagnetic wave having a wavelength different from the wavelength of the electromagnetic wave, and the housing may have a second window through which the monitor electromagnetic wave passes. In this case, during manufacture, the monitor electromagnetic wave is incident on the monitor member and the amount of electrons emitted from the monitor member is monitored, thereby making it possible to grasp the change in work function on the surface of the metal pattern.

[0016] A method for manufacturing an electron tube according to one aspect of the present disclosure includes a first step of arranging a metasurface element and an alkali supply source in a housing, wherein the metasurface element comprises a support and a metasurface having a metal pattern formed on a surface of the support and arranged to emit electrons in response to incidence of electromagnetic waves, and a second step of causing the alkali supply source to release vaporized alkali metal and heating the housing to disperse the alkali metal within the housing and form a metal layer containing the alkali metal on the metal pattern.

[0017] In this method for manufacturing an electron tube, vaporized alkali metal is released into an alkali source, and the housing is heated to disperse the alkali metal within the housing, forming a metal layer containing the alkali metal on the metal pattern. By heating the housing to disperse the alkali metal within the housing, the metal layer can be reliably formed on the metal pattern. Thus, according to this method for manufacturing an electron tube, the metal layer containing the alkali metal can be suitably formed on the metal pattern. As a result, an electron tube with enhanced sensitivity can be manufactured as described above.

[0018] The alkali supply source may be an alkali metal dispenser that releases an alkali metal when energized. In this case, the amount of alkali metal released can be controlled by adjusting the amount of electricity or the time of electricity application to the alkali metal dispenser, and the metal layer can be formed with good reproducibility.

[0019] In the second step, oxygen may be supplied into the housing, and a metal layer containing an oxide of an alkali metal may be formed on the metal pattern. In this case, a metal layer containing an oxide of an alkali metal may be formed on the metal pattern.

[0020] In the second step, the housing may be heated by passing current through a coil disposed outside the housing so as to surround the housing. In this case, the housing can be effectively heated.

[0021] In the second step, the inside of the housing may be a vacuum. In this case, the metal layer can adsorb residual gas in the housing (getter effect), and the degree of vacuum in the housing can be increased.

[0022] In the first step, a monitor member that emits electrons in response to the incidence of a monitor electromagnetic wave having a wavelength different from the wavelength of the electromagnetic wave is further disposed in the housing, and in the second step, an alkali metal may be emitted into the alkali supply source while the monitor electromagnetic wave is incident on the monitor member and the amount of electrons emitted from the monitor member is monitored. In this case, by monitoring the amount of electrons emitted from the monitor member, the change in the work function on the surface of the metal pattern can be grasped. As a result, the metal layer can be formed with even better reproducibility.

[0023] In the first step, the metasurface element and the alkali supply source may be disposed in the housing so that the metasurface element is spaced from the alkali supply source, and in the second step, a part of the housing surrounding the alkali supply source may be heated. In this case, since the part of the housing surrounding the alkali supply source is heated, heating of the metal pattern can be suppressed. As a result, alloying between the metal pattern and the metal layer can be suppressed, and the metal layer can be formed more suitably. Effect of the Invention

[0024] According to one aspect of the present disclosure, it is possible to provide a metasurface element and an electron tube with enhanced sensitivity, as well as a method for manufacturing such an electron tube. [Brief description of the drawings]

[0025] [Figure 1] FIG. 1 is a cross-sectional view of an electron tube according to an embodiment. [Diagram 2] FIG. 2 is a side view of the electron tube. [Diagram 3] FIG. 3 is a partial side view of an electron tube. [Figure 4] FIG. 4 is a plan view of the holding portion. [Diagram 5] FIG. 5 is a perspective view of the holding portion. [Figure 6] FIG. 6 is a cross-sectional view of a metasurface element. [Figure 7] FIG. 7 is a diagram for explaining how electrons are emitted in the comparative example. [Figure 8] FIG. 8 is a diagram for explaining how electrons are emitted in the embodiment. [Figure 9] FIG. 9 is a graph showing the results of the effect confirmation experiment. [Figure 10] FIG. 10 is a diagram for explaining a method of manufacturing an electron tube. [Figure 11] FIG. 11 is a graph for explaining a method for manufacturing an electron tube. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the following description, the same or corresponding elements are designated by the same reference numerals, and duplicated description will be omitted. [Overall configuration of electron tube]

[0027] Electron tube 1 shown in Figs. 1 to 3 is configured as a photomultiplier tube (PMT) that outputs an electric signal in response to incidence of electromagnetic waves. Electron tube 1 is sensitive to electromagnetic waves in the frequency band from millimeter waves to infrared light, for example. When electromagnetic waves are incident on electron tube 1, electrons are emitted inside, and the emitted electrons are multiplied. In electron tube 1, electromagnetic waves are incident on a photocathode, and electrons are emitted from the photocathode due to the tunnel effect (field emission) caused by a strong electric field.

[0028] The electron tube 1 includes a housing 10, a metasurface element 20, a holding section 30, an electron multiplier section 40, and an electron collector section 50. The metasurface element 20, the holding section 30, the electron multiplier section 40, and the electron collector section 50 are disposed within the housing 10.

[0029] The housing 10 has a valve 11 and a stem 12. The housing 10 is formed, for example, in a cylindrical shape. The housing 10 is airtightly sealed by the valve 11 and the stem 12, and a vacuum is maintained inside the housing 10. The vacuum inside the housing 10 may be an absolute vacuum, or the housing 10 may be filled with a gas at a pressure lower than atmospheric pressure. For example, the pressure inside the housing 10 may be 1×10 -4 ~1×10 -7 The pressure may be maintained at 1 Pa. In FIG.

[0030] A part of the bulb 11 constitutes a window portion (first window portion) 11a through which electromagnetic waves pass. The window portion 11a constitutes the top surface of the housing 10 (one end surface of the housing 10 in the axial direction A). The window portion 11a is formed, for example, in a circular flat plate shape perpendicular to the axial direction A. The window portion 11a is transparent to electromagnetic waves and transmits at least a part of the frequency band of the electromagnetic waves incident on the window portion 11a. The stem 12 constitutes the bottom surface of the housing 10 (the other end surface of the housing 10 in the axial direction A). The stem 12 is formed, for example, in a circular flat plate shape perpendicular to the axial direction A, and is arranged so as to overlap with the window portion 11a when viewed from the axial direction A.

[0031] The window 11a is made of a material corresponding to the frequency band of the electromagnetic wave incident on the electron tube 1. For example, the material of the window 11a includes at least one selected from quartz, silicon, germanium, sapphire, zinc selenide, zinc sulfide, magnesium fluoride, lithium fluoride, barium fluoride, calcium fluoride, magnesium oxide, calcium carbonate, and chalcogenide glass. This allows electromagnetic waves of a certain band selected from the frequency band from millimeter waves to infrared light to be guided into the housing 10. For example, quartz is suitable for transmitting frequency bands of 0.1 THz to 5 THz, silicon is suitable for transmitting frequency bands of 0.04 THz to 11 THz and 46 THz or more, magnesium fluoride is suitable for transmitting frequency bands of 40 THz or more, germanium is suitable for transmitting frequency bands of 13 THz or more, and zinc selenide is suitable for transmitting frequency bands of 14 THz or more.

[0032] The housing 10 further has a plurality of wires 13 for electrical connection between the inside and outside of the housing 10. Each wire 13 is, for example, a lead wire or a pin. In this example, each wire 13 is a pin that penetrates the stem 12 along the axial direction A. Each wire 13 is electrically connected to various members provided in the housing 10.

[0033] The metasurface element 20 emits electrons in response to incidence of an electromagnetic wave. The metasurface element 20 includes a support 21 and a metasurface 22. The metasurface element 20 is formed in, for example, a substantially rectangular plate shape.

[0034] The support 21 is, for example, a rectangular plate-shaped substrate. The support 21 has a first surface 21a and a second surface 21b opposite to the first surface 21a. In this example, each of the first surface 21a and the second surface 21b is a flat surface parallel to the window portion 11a. The first surface 21a faces the window portion 11a with a gap therebetween. Electromagnetic waves that have passed through the window portion 11a are incident on the first surface 21a. The support 21 is transparent to the electromagnetic waves that pass through the window portion 11a, and transmits at least a part of the frequency band of the electromagnetic waves that have passed through the window portion 11a.

[0035] The support 21 is formed of, for example, silicon, quartz, sapphire, or zinc selenide (ZnSe). The support 21 is disposed so as to be spaced apart from the window portion 11a and the electron multiplier portion 40 in the axial direction A. The metasurface 22 is provided on a second surface 21b of the support 21. An electromagnetic wave that has passed through the window portion 11a and the support 21 is incident on the metasurface 22. The metasurface 22 will be described in detail later.

[0036] As shown in Figs. 1 to 5, the holding portion 30 holds the metasurface element 20. The holding portion 30 has a disk-shaped bottom wall portion 31 and a cylindrical side wall portion 32 extending from the outer edge of the bottom wall portion 31. An arrangement portion 31a having a shape corresponding to the outer shape of the metasurface element 20 is formed in the center of the bottom wall portion 31, and the metasurface element 20 is arranged in the arrangement portion 31a. In this example, the arrangement portion 31a is configured by an opening formed in the bottom wall portion 31.

[0037] The holding part 30 is fixed to the inner circumferential surface of the bulb 11, and positions the metasurface element 20 with respect to the housing 10. The holding part 30 is positioned with respect to the housing 10, for example, so that the optical axis of the electromagnetic wave passing through the arrangement part 31a is parallel to the tube axis (axial direction A) of the housing 10. The holding part 30 has a plurality of terminals 33 (four in this example). At least one of the plurality of terminals 33 is electrically connected to the wiring 13 via the conductive layer 14, and a voltage is applied to the metasurface element 20 via the wiring 13, the conductive layer 14, and the terminal 33. The conductive layer 14 is formed on the inner circumferential surface of the bulb 11.

[0038] A monitor member 34 is provided on the bottom wall 31 of the holding part 30. When the electron tube 1 is manufactured, a monitor electromagnetic wave having a wavelength different from the wavelength of the electromagnetic wave incident on the metasurface element 20 is made incident on the monitor member 34. A manufacturing method of the electron tube 1 will be described later.

[0039] The monitor member 34 is disposed adjacent to the metasurface element 20 on the surface of the bottom wall 31 on the electron multiplier section 40 side (opposite the window section 11a and the side wall section 32). The monitor member 34 is, for example, an electrode formed by forming a metal film on a base material. In this example, the metal film is formed of the same metal material as the metal material constituting the metal pattern 24 of the metasurface 22 described later. As an example, the material of the metal film is gold, and the material of the base material is beryllium copper (CuBe).

[0040] Further, a through hole 31b is formed in the bottom wall portion 31. The through hole 31b penetrates the bottom wall portion 31 along the axial direction A. The monitor member 34 is arranged so that the monitor electromagnetic wave passing through the through hole 31b is incident on the monitor member 34. The monitor electromagnetic wave passes through the window portion 11a and enters the housing 10, and passes through the through hole 31b and is incident on the monitor member 34. That is, the window portion 11a also functions as a second window portion through which the monitor electromagnetic wave passes. The monitor member 34 is electrically connected to the electrode 23 of the metasurface 22 described later, and has the same potential as the electrode 23.

[0041] The electron multiplier section 40 multiplies the electrons emitted from the metasurface element 20. The electron multiplier section 40 has an incident surface 40a that faces the second surface 21b of the support 21 of the metasurface element 20 with a gap therebetween. The electron multiplier section 40 multiplies the electrons emitted from the metasurface element 20 and incident on the incident surface 40a. The electron multiplier section 40 is configured, for example, as a so-called line focus type, and has multiple stages (for example, 10 stages) of dynodes. The uppermost dynode includes the incident surface 40a described above. Wiring 13 is electrically connected to each dynode, and a predetermined potential is applied via the wiring 13. The dynodes multiply electrons in accordance with the applied potential.

[0042] The electron collector 50 collects the electrons multiplied by the electron multiplier 40. The electron collector 50 has, for example, an anode arranged to receive electrons from the final stage dynode of the electron multiplier 40. The anode is electrically connected to a wiring 13, and a predetermined potential is applied via the wiring 13. The anode captures the electrons multiplied by the dynode. The electron collector 50 may have a diode (such as an avalanche diode) instead of the anode.

[0043] The electron tube 1 further includes an alkali metal dispenser (AMD) 60 arranged in the housing 10. The alkali metal dispenser 60 is an alkali supply source that releases an alkali metal when energized. As described later, the alkali metal dispenser 60 is used during the manufacture of the electron tube 1. A manufacturing method of the electron tube 1 will be described later. The alkali metal dispenser 60 is formed, for example, in a rod shape and extends along the axial direction A. The alkali metal dispenser 60 is arranged in the space between the electron multiplier section 40 and the inner surface of the housing 10 (the inner peripheral surface of the bulb 11), and extends toward the stem 12. The alkali metal dispenser 60 is spaced apart from the metasurface element 20 in the axial direction A.

[0044] The alkali metal dispenser 60 includes a storage member 61 formed of a metal material, and an alkali metal is stored in the storage member 61. As an example, the storage member 61 is made of nickel and chromium, and the stored alkali metal is cesium. The storage member 61 is electrically connected to the wiring 13. When a current flows through the storage member 61 via the wiring 13, the storage member 61 is resistively heated, and the vaporized alkali metal is released from the storage member 61. The amount of alkali metal released can be controlled by adjusting the amount of electricity passed through the storage member 61 or the time for which electricity is passed through the storage member 61.

[0045] The housing member 61 has an outlet through which the alkali metal is released. The outlet faces the inner circumferential surface of the bulb 11. Therefore, the alkali metal is released from the housing member 61 toward the inner circumferential surface of the bulb 11. This makes it possible to suppress the alkali metal from being adsorbed to the electron multiplier section 40 or the electron collector section 50 in the manufacturing process described below, and to increase the probability that the alkali metal is adsorbed onto the metal pattern 24. [Configuration of metasurface element]

[0046] 4 to 6, the metasurface element 20 includes the support 21 described above and a metasurface 22 formed on the second surface 21b of the support 21. The metasurface 22 has an electrode 23, a metal pattern 24, and a metal layer 25. The metasurface 22 faces the window portion 11a via the support 21 on one side in the axial direction A, and faces the incident surface 40a of the electron multiplier section 40 on the other side in the axial direction A.

[0047] The electrode 23 is formed on the second surface 21b of the support 21, and has a generally rectangular frame shape in a plan view (when viewed in a direction perpendicular to the second surface 21b (a direction parallel to the axial direction A)). A voltage is applied to the electrode 23 via a terminal 33.

[0048] The metal pattern 24 is formed in a layer on the second surface 21b so as to be located inside the electrode 23 in a plan view. The metal pattern 24 is arranged so as to emit electrons in response to the incidence of electromagnetic waves. For example, the pattern described in US 2016 / 0216201 may be used as the arrangement of the metal pattern 24. In this example, the metal pattern 24 has a plurality of conductive lines 24a arranged so as to emit electrons when electromagnetic waves (e.g., terahertz waves) in a certain band selected from the frequency band from millimeter waves to infrared light are incident. The plurality of conductive lines 24a have, for example, the same shape as each other and are arranged along a predetermined direction. Each conductive line 24a is electrically connected to the electrode 23. As a result, the plurality of conductive lines 24a have the same potential as each other. That is, in this example, the metal pattern 24 has only metal patterns of the same potential.

[0049] The metal pattern 24 includes an oxide layer formed on the second surface 21b of the support 21 and a metal layer formed on the oxide layer. The oxide layer includes a silicon dioxide layer formed on the second surface 21b and a titanium oxide layer formed on the silicon dioxide layer. As an example, the silicon dioxide layer has a thickness of about 1 μm, and the titanium oxide layer has a thickness of about 10 nm. As an example, the material of the metal layer is gold, and the thickness of the metal layer is about 200 nm. The material of the metal pattern 24 does not contain an alkali metal. The electrode 23 has, for example, the same layer structure as the metal pattern 24.

[0050] The metal layer 25 is formed on the electrode 23 and the metal pattern 24, and covers the electrode 23 and the metal pattern 24. The metal layer 25 extends beyond the metal pattern 24 to reach a region R on the second surface 21b of the support 21 where the electrode 23 and the metal pattern 24 are not formed. That is, the metal layer 25 is provided so as to straddle the metal pattern 24 and the region R on the second surface 21b. In this example, the metal layer 25 is provided over the entire surface of the electrode 23, the entire surface of the metal pattern 24, and the entire region R. In other words, the metal layer 25 is provided over the entire region inside the outer edge of the electrode 23 on the second surface 21b.

[0051] The material of the metal layer 25 contains an alkali metal. The metal layer 25 is, for example, an oxide layer of an alkali metal. In this example, the alkali metal is cesium, and the metal layer 25 is a cesium oxide layer. The thickness of the metal layer 25 is, for example, about several angstroms. The work function of the material of the metal layer 25 is lower than the work function of the metal material (gold in this example) constituting the metal pattern 24. [Action and Effects]

[0052] In the metasurface element 20, a metal layer 25 containing an alkali metal is formed on the metal pattern 24. This distorts the band structure on the surface of the metal pattern 24 to reduce the work function, and the potential barrier can be made thinner. As a result, the probability of tunneling can be increased, and the sensitivity (electron emission ability) can be improved. In addition, in the metasurface element 20, the metal layer 25 containing an alkali metal is provided not only on the metal pattern 24, but also in the region R on the second surface 21b of the support 21 where the metal pattern 24 is not formed. This reduces the resistance value of the second surface 21b of the support 21, and suppresses the occurrence of charge-up on the metal pattern 24. As a result, it is possible to suppress the difficulty of emitting electrons from the metasurface 22 due to charge-up. As a result, the sensitivity of the metasurface element 20 is increased. The electron tube 1 equipped with such a metasurface element 20 can be used in various fields as an uncooled, highly sensitive detector capable of wide-ranging detection from the mid-infrared region to the terahertz region. The charge-up is a phenomenon in which emitted electrons collide with a region R where the metal pattern 24 is not formed, and the region R becomes positively or negatively charged. When the charge-up occurs, it becomes difficult for electrons to be emitted from the metasurface 22.

[0053] The above points will be further described with reference to Fig. 7 and Fig. 8. Fig. 7 is a diagram for explaining how electrons are emitted from the metasurface 22 of the comparative example, and Fig. 8 is a diagram for explaining how electrons are emitted from the metasurface 22 of the embodiment. The comparative example corresponds to a configuration in which the metal layer 25 is not provided in the metasurface 22 of the embodiment. In Fig. 7 and Fig. 8, the vertical axis represents potential energy U, and the horizontal axis represents distance x from the metal pattern 24.

[0054] In both the comparative example and the embodiment, when an electromagnetic wave is incident on the metasurface 22, the potential in the vicinity of the metal pattern 24 is inclined, as indicated by the arrow and the dashed line B. This increases the number of electrons e passing through the potential barrier by the tunnel effect, compared to a state in which no voltage is applied.

[0055] As shown in Figures 7 and 8, when the metal layer 25 containing an alkali metal is formed on the metal pattern 24 (Figure 8), it is expected that the potential barrier PB will be thinner than when the metal layer 25 is not formed on the metal pattern 24 (Figure 7). The reason for this is believed to be that the band structure on the surface of the metal pattern 24 is distorted by the metal layer 25, causing the work function to decrease, as shown in Figure 8. That is, when the metal layer 25 is provided, it is believed that the work function on the surface of the metal pattern 24 is smaller than when the metal layer 25 is not provided. As a result, it is expected that the probability of tunneling can be increased, and the sensitivity (electron emission ability) can be improved.

[0056] In the metasurface element 20 of the embodiment, secondary electrons are emitted when primary electrons emitted from the metasurface 22 collide with the surrounding metal pattern 24, etc. It is believed that the metal layer 25 formed on the metal pattern 24 increases the amount of emitted secondary electrons as well as primary electrons.

[0057] FIG. 9 is a graph showing the results of an effect confirmation experiment. In FIG. 9, the horizontal axis represents the intensity of the incident electromagnetic wave, and the vertical axis represents the magnitude of the output voltage. In this experiment, five electron tubes corresponding to the electron tube 1 of the embodiment were created as examples, and the sensitivity of each was measured. Data S1 to S5 show the measurement data of each example. The two-dot chain line L shows the maximum sensitivity in the comparative example. The comparative example corresponds to a configuration in which the metal layer 25 is not provided in the metasurface 22 of the embodiment.

[0058] As shown in Fig. 9, all of the Examples had higher sensitivity than the Comparative Example. Moreover, the Examples were able to achieve a sensitivity up to 36 times higher than the Comparative Example. Specifically, the threshold value of the Comparative Example was 20 kV / cm, whereas the threshold value of the Example of Data S1 was 0.553 kV / cm.

[0059] Next, the action and effect of the metasurface element 20 will be described. In the metasurface element 20, the metal layer 25 contains an oxide of an alkali metal. The alkali metal constituting the metal layer 25 is cesium. This makes it possible to more effectively reduce the work function on the surface of the metal pattern 24.

[0060] The material of the metal pattern 24 contains gold, which can ensure high electrical conductivity of the metal pattern 24.

[0061] The metasurface 22 has only the metal patterns 24 of the same potential as the metal patterns. This eliminates the need to consider insulation between the metal patterns 24, and allows the metal layer 25 to be easily provided in the region R on the second surface 21b of the support 21 where the metal patterns 24 are not formed.

[0062] The support 21 is made of silicon, quartz, sapphire, or zinc selenide. This allows electromagnetic waves in a frequency band ranging from millimeter waves to infrared light to pass through the support 21, and allows the electromagnetic waves to be incident on the metasurface 22 via the support 21.

[0063] In the electron tube 1, the inside of the housing 10 is a vacuum. This allows the metal layer 25 to adsorb residual gas within the housing 10 (getter effect), and the degree of vacuum within the housing 10 can be increased.

[0064] The electron tube 1 includes an alkali metal dispenser 60 that is disposed in a housing 10 and releases an alkali metal when energized. This allows the alkali metal dispenser 60 to be energized during manufacturing to release vaporized alkali metal.

[0065] The electron tube 1 includes a monitor member 34 that emits electrons in response to the incidence of a monitor electromagnetic wave, and the housing 10 has a window 11a through which the monitor electromagnetic wave passes. This makes it possible to grasp the change in work function on the surface of the metal pattern 24 by making the monitor electromagnetic wave incident on the monitor member 34 and monitoring the amount of electrons emitted from the monitor member 34 during manufacturing. [Electron tube manufacturing method]

[0066] An example of a manufacturing method of the above-mentioned electron tube 1 will be described. First, as shown in FIG. 10, the metasurface element 20 and the alkali metal dispenser 60 are arranged in the housing 10 (first step). This metasurface element 20 is a metasurface element before the metal layer 25 is formed on the metal pattern 24. In the first step, the holding section 30, the electron multiplier section 40, the electron collector section 50, etc., which are other components of the electron tube 1, are also arranged in the housing 10. The monitor member 34 provided in the holding section 30 is also arranged in the housing 10. In the first step, the metasurface element 20 and the alkali metal dispenser 60 are arranged in the housing 10 so that the metasurface element 20 is separated from the alkali metal dispenser 60 in the axial direction A.

[0067] Next, the alkali metal dispenser 60 is energized to release the vaporized alkali metal, and the housing 10 is heated to disperse the alkali metal within the housing 10, thereby forming a metal layer 25 containing the alkali metal on the metal pattern 24 (second step). Before the start of the second step, the housing 10 is sealed, and a vacuum is maintained within the housing 10.

[0068] As shown in FIG. 10, in the second step, a part 10a of the housing 10 is heated by passing electricity through a coil 70 arranged outside the housing 10 so as to surround the housing 10. The part 10a of the housing 10 is a part of the housing 10 in the axial direction A, and is a part surrounding the alkali metal dispenser 60. The coil 70 is wound in a spiral shape multiple times so as to surround the part 10a of the housing 10. When electricity is passed through the coil 70, only the part 10a of the housing 10 is heated. As a result, the metasurface element 20 can be prevented from being heated. In the second step, oxygen is supplied into the housing 10, and a metal layer 25 containing an oxide of an alkali metal is formed on the metal pattern 24. For example, oxygen is supplied from an oxygen cylinder provided in an exhaust system connected to the space inside the housing 10. The oxygen cylinder is provided with a valve, which allows fine adjustment of the amount of oxygen supplied.

[0069] In the second step, the monitoring electromagnetic wave is incident on the monitoring member 34 and the alkali metal dispenser 60 is energized while monitoring the amount of electrons emitted from the monitoring member 34. The monitoring electromagnetic wave is an electromagnetic wave having a wavelength different from that of the electromagnetic wave incident on the metasurface element 20. In this example, as described above, a metal film made of gold is formed on the surface of the monitoring member 34. The work function of gold is 5.47 eV, which corresponds to a wavelength of 227 nm when converted. Therefore, for example, when ultraviolet light having a wavelength shorter than 227 nm is irradiated on the monitoring member 34 as the monitoring electromagnetic wave, photoelectrons are emitted from the monitoring member 34 due to the photoelectric effect. The electrons emitted from the monitoring member 34 are, for example, multiplied by the electron multiplier unit 40 and collected by the electron collector unit 50. Alternatively, the electron tube 1 may include an element for multiplying and collecting the electrons emitted from the monitoring member 34, separate from the electron multiplier unit 40 and / or the electron collector unit 50.

[0070] The amount of electrons emitted from the monitoring member 34 corresponds to the amount of electrons emitted from the metal pattern 24. This is because the alkali metal emitted from the alkali metal dispenser 60 is deposited not only on the metal pattern 24 but also on the monitoring member 34. Therefore, when the work function on the surface of the monitoring member 34 decreases, it can be considered that the work function on the surface of the metal pattern 24 also decreases. In other words, the monitoring member 34 can be considered to be a virtual metasurface. For example, a D2 (deuterium) lamp can be used as a light source that outputs the monitoring electromagnetic wave. A light source 80 is shown in FIG. 10.

[0071] The second step will be further described with reference to FIG. 11. In FIG. 11, the horizontal axis represents the elapsed time from the start of current flow, and the vertical axis represents the magnitude of the output current. As shown in FIG. 11, in a first section T1 immediately after the start of current flow, a predetermined current is applied to the alkali metal dispenser 60, and the alkali metal is introduced into the housing 10. In the subsequent second section T2, a predetermined current is applied to the alkali metal dispenser 60, and oxygen is supplied into the housing 10 at a predetermined pressure, and the alkali metal and oxide are introduced into the housing 10. Through such a process, the metal layer 25 can be formed on the metal pattern 24. The amount of current applied to the alkali metal dispenser 60 and / or the supply pressure of oxygen may change according to the elapsed time.

[0072] As described above, in the manufacturing method of the electron tube 1, the alkali metal dispenser 60 (alkali supply source) releases the vaporized alkali metal, and the housing 10 is heated to disperse the alkali metal in the housing 10, thereby forming the metal layer 25 containing the alkali metal on the metal pattern 24. By heating the housing 10 to disperse the alkali metal in the housing 10, the metal layer 25 can be reliably formed on the metal pattern 24. That is, if the housing 10 is not heated, the released alkali metal may remain in the vicinity of the alkali metal dispenser 60, but by heating the housing 10, such a situation can be suppressed. Therefore, according to the manufacturing method of the electron tube 1, the metal layer 25 containing the alkali metal can be suitably formed on the metal pattern 24. That is, for the above-mentioned reasons, the electron tube 1 having enhanced sensitivity can be manufactured.

[0073] An alkali metal dispenser 60 is used as an alkali supply source. This makes it possible to control the amount of alkali metal released by adjusting the amount of electricity or the time of electricity supply to the alkali metal dispenser 60, and to form the metal layer 25 with good reproducibility. Furthermore, the control of the amount of alkali metal released using the alkali metal dispenser 60 is unlikely to vary depending on the operator, and can be easily automated.

[0074] In the second step, oxygen is supplied into the housing 10, and a metal layer 25 containing an oxide of an alkali metal is formed on the metal pattern 24. This allows the metal layer 25 containing an oxide of an alkali metal to be formed on the metal pattern 24.

[0075] In the second step, the housing 10 is heated by passing current through the coil 70 that is disposed outside the housing 10 so as to surround the housing 10. This makes it possible to heat the housing 10 effectively.

[0076] In the second step, the inside of the housing 10 is a vacuum. This allows the metal layer 25 to adsorb residual gases in the housing 10 (getter effect), and the degree of vacuum in the housing 10 can be increased. In addition, the alkali metal can be handled in a vacuum.

[0077] In the second step, the monitoring electromagnetic wave is incident on the monitoring member 34 and current is passed through the alkali metal dispenser 60 while monitoring the amount of electrons emitted from the monitoring member 34. This makes it possible to monitor the amount of electrons emitted from the monitoring member 34 and grasp the change in work function on the surface of the metal pattern 24. As a result, the metal layer 25 can be formed with even better reproducibility. Such monitoring using the monitoring member 34 is effective when it is difficult to use a light source that outputs electromagnetic waves to which the electron tube 1 is sensitive (for example, a terahertz light source that outputs terahertz waves). At the time of filing this application, terahertz light sources are extremely expensive and are not easy to use.

[0078] In the first step, the metasurface element 20 and the alkali metal dispenser 60 are arranged in the housing 10 so that the metasurface element 20 is separated from the alkali metal dispenser 60, and in the second step, a part 10a of the housing 10 surrounding the alkali metal dispenser 60 is heated. As a result, the part 10a of the housing 10 surrounding the alkali metal dispenser 60 is heated, so that the metal pattern 24 can be prevented from being heated. In other words, activity at room temperature can be achieved. As a result, alloying between the metal pattern 24 and the metal layer 25 can be prevented, and the metal layer 25 can be formed more suitably.

[0079] For example, in the above-described method for manufacturing the electron tube 1, the temperature of the metasurface 22 is maintained at about 100° C. or lower. The temperature of the metasurface 22 may be maintained at about 60° C. or lower, or may be maintained at about 25° C. Meanwhile, the part 10a of the housing 10 is heated to about 200° C. by the coil 70. That is, in the second step, the temperature of the metasurface 22 is maintained at a temperature lower than the temperature of the part 10a of the housing 10.

[0080] In the above-described manufacturing method of the electron tube 1, the alkali metal is supplied using the alkali metal dispenser 60, but another method using an ampoule or pellet is also conceivable. In the former method, an operator introduces the alkali metal into the housing 10 using an ampoule tube, and after the introduction is completed, the operator pulls out the ampoule tube from the housing 10. This method has the disadvantages of low reproducibility in the formation of the metal layer 25 and the tendency of the metasurface 22 to become hot, as compared with the manufacturing method of the embodiment.

[0081] The method using pellets is a method of supplying alkali metal by high-frequency heating of a lump (pellet) of alkali metal arranged in the housing 10. This method has disadvantages in comparison with the manufacturing method of the embodiment, such as the metasurface 22 easily becoming hot, and the difficulty in controlling the amount of alkali metal supplied. The manufacturing method of the embodiment has advantages in comparison with these methods, such as the metasurface 22 not easily becoming hot, the ease of controlling the amount of alkali metal supplied, and high reproducibility in the formation of the metal layer 25.

[0082] Although one embodiment of the present disclosure has been described above, the present disclosure is not limited to the above embodiment. The materials and shapes of each component are not limited to the above-mentioned materials and shapes, and various materials and shapes can be adopted. For example, the electrode 23 is not limited to a rectangular frame shape, and may be any shape such as a rectangular shape. The metal pattern 24 may have any arrangement as long as it can emit electrons in response to the incidence of electromagnetic waves.

[0083] In the above embodiment, the metal layer 25 is formed on the electrode 23 and the metal pattern 24, but the metal layer 25 may be formed only on the metal pattern 24. In the above embodiment, the metal layer 25 is provided over the entire surface of the metal pattern 24, but it is sufficient that the metal layer 25 is formed on at least a part of the metal pattern 24. In the above embodiment, the metal pattern 24 has only metal patterns of the same potential, but the metal pattern 24 may have two or more metal patterns to which different potentials are applied.

[0084] The housing 10 may have a second window portion through which the monitoring electromagnetic wave passes, separate from the window portion 11a. The metasurface 22 may be formed on the first surface 21a of the support 21. In this case, the support 21 does not necessarily have to be transparent to the electromagnetic wave passing through the window portion 11a. The electron multiplier section 40 and the electron collector section 50 are not limited to the above-mentioned configuration, and may have any configuration. The monitoring member 34 and / or the alkali metal dispenser 60 may not be disposed within the housing 10.

[0085] The metal layer 25 may be a monoatomic layer of an alkali metal. In this case, the work function on the surface of the metal pattern 24 can be effectively reduced. For example, the metal layer 25 may be a monoatomic layer of cesium. The material of the metal pattern 24 is not limited to gold, but may be platinum, aluminum, graphene, silver, or copper. Even in this case, the conductivity of the metal pattern 24 can be largely ensured. The material of the metal layer 25 may be an alkali metal other than cesium. Another alkali supply source may be used instead of the alkali metal dispenser 60.

[0086] In the above embodiment, the metasurface element 20 is configured as a transmission type in which the electromagnetic wave transmitted through the support 21 is incident on the metasurface 22, but the metasurface element 20 may be configured as a reflection type in which the electromagnetic wave is incident on the metasurface 22 from the side opposite the support 21, and electrons are emitted from the metasurface 22 to the side opposite the support 21. In this case, the support 21 does not necessarily have to be formed of a material that is transparent to the electromagnetic wave.

Claims

1. A support; A metasurface formed on a surface of the support, The metasurface comprises: a metal pattern arranged to emit electrons in response to incidence of electromagnetic waves by field emission; a metal layer including an alkali metal and formed on the metal pattern; A metasurface element, wherein the metal layer extends beyond the metal pattern to an area of ​​the surface of the support where the metal pattern is not formed.

2. The metasurface element of claim 1 , wherein the metal layer is a monolayer of the alkali metal.

3. The metasurface element of claim 1 , wherein the metal layer comprises an oxide of the alkali metal.

4. The metasurface element according to any one of claims 1 to 3, wherein the alkali metal is cesium.

5. The metasurface element according to any one of claims 1 to 4, wherein the material of the metal pattern comprises gold, platinum, aluminum, graphene, silver or copper.

6. The metasurface element according to any one of claims 1 to 5, wherein the metasurface has only a metal pattern of the same potential as the metal pattern.

7. The metasurface element according to any one of claims 1 to 6, wherein the support is formed of silicon, quartz, sapphire or zinc selenide.

8. The metasurface element according to any one of claims 1 to 7, A housing having a first window through which the electromagnetic wave passes and accommodating the metasurface element; An electron tube comprising: an electron multiplier section disposed within the housing and configured to multiply electrons emitted from the metasurface element.

9. The electron tube according to claim 8 , wherein the inside of the housing is a vacuum.

10. 10. The electron tube according to claim 8, further comprising an alkali metal dispenser disposed within the housing and configured to release the alkali metal when energized.

11. a monitoring member disposed within the housing and configured to emit electrons in response to incidence of a monitoring electromagnetic wave having a wavelength different from that of the electromagnetic wave; 11. The electron tube according to claim 8, wherein the housing has a second window portion through which the monitor electromagnetic wave passes.

12. A first step of disposing a metasurface element and an alkali source in a housing, the metasurface element comprising a support and a metasurface having a metal pattern formed on a surface of the support and arranged to emit electrons in response to incidence of an electromagnetic wave by field emission; a second step of releasing vaporized alkali metal into the alkali source and heating the housing to disperse the alkali metal within the housing, and forming a metal layer containing the alkali metal on the metal pattern.

13. The method for manufacturing an electron tube according to claim 12, wherein the alkali supply source is an alkali metal dispenser that releases the alkali metal when energized.

14. 14. The method for manufacturing an electron tube according to claim 12, wherein in the second step, oxygen is supplied into the housing, and the metal layer containing the oxide of the alkali metal is formed on the metal pattern.

15. The method for manufacturing an electron tube according to any one of claims 12 to 14, wherein in the second step, the housing is heated by passing current through a coil that is arranged outside the housing so as to surround the housing.

16. The method for manufacturing an electron tube according to any one of claims 12 to 15, wherein in the second step, the inside of the housing is a vacuum.

17. In the first step, a monitor member that emits electrons in response to incidence of a monitor electromagnetic wave having a wavelength different from that of the electromagnetic wave is further disposed in the housing; The method for manufacturing an electron tube according to any one of claims 12 to 16, wherein in the second step, the alkali metal is released to the alkali supply source while the monitoring electromagnetic wave is incident on the monitoring member and the amount of electrons released from the monitoring member is monitored.

18. In the first step, the metasurface element and the alkali supply source are disposed in the housing such that the metasurface element is spaced from the alkali supply source; The method for manufacturing an electron tube according to any one of claims 12 to 17, wherein in the second step, a part of the housing surrounding the alkali source is heated.

19. A metasurface element described in any one of claims 1 to 7, wherein the metal pattern is arranged to emit electrons by field emission in response to incidence of terahertz waves.

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