Photocathode for vacuum systems

Abstract

The present invention relates to a photocathode for a vacuum system, wherein the photocathode is configured to receive electromagnetic radiation having an incident wavelength and to emit electrons in response thereto. The photocathode comprises a conductive structure having a geometry, the geometry comprising a tip portion. The tip portion is adapted to provide a field enhancement β when the conductive structure is illuminated with electromagnetic radiation, wherein β is greater than about 10 2 . The photocathode further comprises a substrate, the substrate being or comprising a dielectric substrate, the substrate supporting the conductive structure.

Description

Technical Field

[0001] The invention relates to a photocathode for a vacuum system. Background Art

[0002] Vacuum systems such as photomultiplier tubes (PMTs) or multichannel plates are widely known for realizing detectors of light in the visible and ultraviolet (UV) range.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: WO Unexamined Patent Application Publication No. 2015 / 028029 Summary of the Invention

[0006] Technical issues

[0007] Photomultiplier tubes are used for sensitive detection of very small amounts of light, down to the single-photon limit, making them attractive for low-light applications. However, their attractive properties are only useful in the spectral range from red to UV.

[0008] Patent Document 1 discloses a device for detecting terahertz radiation based on the principle that intense THz pulses can induce ultrafast field emission of electrons from the surface of a metal layer through non-perturbative nonlinear interactions. The electrons can be accelerated to kinetic energies of tens of eV by the THz field, which is also enhanced near the metal. This can be used to initiate collision-induced physical processes on ultrafast timescales, such as the formation of nitrogen plasma.

[0009] Thus, improved photocathodes would be advantageous, particularly photocathodes that are sensitive to a wider range of wavelengths would be advantageous.

[0010] It is an object of the present invention to provide an alternative to the prior art.

[0011] In particular, it may be seen as a further object of the present invention to provide a photocathode for vacuum systems which solves the above-mentioned problems of the prior art having a limited wavelength range of operation.

[0012] Technical means

[0013] Thus, the above objects and several other objects are achieved in a first aspect of the present invention by providing a photocathode for a vacuum system, wherein the photocathode is configured to receive electromagnetic radiation having an incident wavelength and to emit electrons in response thereto. The photocathode comprises a conductive structure having a geometry including a tip portion. When the conductive structure is illuminated by electromagnetic radiation, the tip portion is adapted to provide a field enhancement β, wherein β is greater than about 10 2. The photocathode further comprises a substrate, which is or comprises a dielectric substrate, which supports the conductive structure. The photocathode formed in this way enables efficient field emission by using the electric field carried by photons in the terahertz and infrared frequency range. By constructing a conductive structure, also referred to as "antenna" below, to provide a sufficiently high field enhancement β, electron emission can be achieved by concentrating the energy tightly at the antenna / vacuum interface. According to the inventors, this confinement eliminates the difference in electronic potential energy between the antenna material and the vacuum for the electrons and allows the latter to quantum tunnel (emit) from the antenna to the vacuum. By causing coherent electromagnetic radiation to impinge on the antenna, this electron emission process is enhanced in a nonlinear manner. Therefore, the photocathode of the present invention is very useful for the detection of coherent signals, such as laser signals.

[0014] The field enhancement of the present invention can be achieved using a variety of different antenna structures / conductive structures, as described in further detail below, and can also be selected based on the desired properties of the photocathode. For example, some structures can provide electron emission over a wide range of wavelengths of incident electromagnetic radiation. Other structures can be designed to provide resonance for narrow bandwidth applications and thus have high sensitivity within that narrow bandwidth.

[0015] In one embodiment of the photocathode of the present invention, the tip portion is configured to provide a field enhancement β by concentrating the electric field in a volume represented by the confinement volume V.

[0016]

[0017] The confinement volume is highly subwavelength. The photon energy h·f and the free-space electric field E of the N received photons are THz The correlation between them can also be expressed as follows based on the constraint volume V:

[0018]

[0019] Here, the magnetic and electric contributions are assumed to be equal, as in the case of light in a vacuum. By confining the photon energy within a small volume, the electric field strength increases. Therefore, confining the electric field is equivalent to confining the photon energy within its waveform, which in turn affects the electric potential landscape within the confined volume.

[0020] In the present invention, it is the tunnel barrier width that controls the emission onset current threshold. Even for a constant photon energy, higher electric field confinement results in a smaller barrier width. Therefore, the field enhancement β should preferably be maximized to minimize the tunnel barrier width. This can be achieved by minimizing the effective confinement volume V.

[0021] The portion of the electric field that is not correctly located at the metal / vacuum interface is irrelevant.

[0022] Here, the emitter tip is considered as the emission area A em The electric field near the surface bends the potential downward, creating a layer with a thickness of w tu The tunneling volume is then defined as V tun =A em *w tu Any photon energy outside this volume will not result in tunneling, but will instead increase the mass-motive force of the emitted electron.

[0023] Here, it is physically impossible to concentrate all incoming energy into the tunnel volume V tun , while ensuring that the emitted electrons enter the vacuum, and at the same time ensuring a nanometer-scale tunnel barrier thickness. However, the best performance can be achieved by confining the photon field as much as possible in a way that brings as much photon energy as possible into the tunnel volume. The volume that can be achieved in practice is called the confinement volume V. Defined by the formula given in the text, V is the volume where the photon electric field is constant everywhere and the integrated electromagnetic energy in the volume is equal to h*f. This is not equal to the gap volume of the antenna, but the more optimized the antenna, the closer it is. The inventors have shown that for a sufficiently small gap, the first-order derivative dE / dV behaves according to the formula in the text. This means that a large part of the photon energy (but not 100%) is located within the gap volume.

[0024] In one embodiment of the photocathode of the present invention, the tip portion comprises two electrodes separated by a gap having a gap width.

[0025] Having a sufficiently narrow gap helps confine the field to the tip, minimizing V. On the other hand, for large gap values, the field becomes difficult to confine within the gap and develops primarily as a fringe field. For gap widths less than approximately four times the square root of the cross-sectional area of ​​the tip in a plane perpendicular to the substrate, field confinement begins to follow analytical predictions. Therefore, the gap width should preferably be selected to be approximately equal to or less than this value.

[0026] In one embodiment of the photocathode of the present invention, the gap width is in the range of about 1 nm to 1000 nm, such as about 10 nm to 500 nm, or further about 20 nm to 100 nm. It has been found that the gap width in this range can provide good field confinement.

[0027] In one embodiment of the photocathode of the present invention, two electrodes are included as a first electrode and a second electrode, and the geometry of the first electrode is selected to provide a first field confinement, and the geometry of the second electrode is selected to provide a second field confinement, the first field confinement being different from the second field confinement. In this way, the structure can be made sensitive to both the polarization and the absolute field polarity of the received electromagnetic radiation.

[0028] In one embodiment of the present invention, the first electrode may have a straight pointed geometry, while the second electrode may also have a T-shaped geometry.

[0029] In one embodiment of the photocathode of the present invention, the photocathode is configured to receive electromagnetic radiation at a designed wavelength, the designed wavelength being in the terahertz range or the infrared range. In this way, the performance of the photocathode can be optimized for a specific wavelength.

[0030] In one embodiment of the photocathode of the present invention, the photocathode is configured to receive electromagnetic radiation within a broadband design wavelength range, the broadband design wavelength range being in the terahertz range or the infrared range. In this way, the photocathode can be optimized for broadband use.

[0031] In one embodiment of the photocathode of the invention, the conductive structure has a dipole antenna geometry. This type of geometry is particularly suitable for receiving electromagnetic radiation at the antenna's resonant wavelength.

[0032] In one embodiment of the photocathode of the invention, the conductive structure has a split ring geometry.

[0033] In one embodiment of the photocathode of the present invention, the split ring geometry is a double split ring geometry, which comprises two interconnected rings having a common tip portion and a common gap. This type of structure enables a wider wavelength range and is therefore well suited for receiving electromagnetic radiation within a wide band.

[0034] In one embodiment of the photocathode of the present invention, the conductive structure comprises a conductive material having a high conductivity at infrared wavelengths, for example, greater than 10 5 S / m conductivity, for example, more than 5·10 5 S / m conductivity, or further exceeding 10 6 The conductive structure of the present invention can be made of many different materials that have a sufficiently large conductivity in the relevant wavelength range.

[0035] In one embodiment of the present invention, the conductive material comprises a conductive ceramic.

[0036] In a particular embodiment of the present invention, the conductive ceramic is titanium nitride.

[0037] In another embodiment of the present invention, the conductive material comprises an allotrope of carbon, such as graphene.

[0038] In one embodiment of the present photocathode, the conductive material comprises a metal. Compared to conventional photocathodes, metals are suitable for fabricating the conductive structure of the present invention. Metals tend to have relatively constant material parameters across the entire infrared and terahertz spectral range, which simplifies geometry optimization for different wavelengths.

[0039] In one embodiment of the photocathode of the invention, the metal is from the group of copper, gold, silver, titanium, aluminum and tungsten. These particular metals have been found to be particularly suitable for producing photocathodes.

[0040] In one embodiment of the photocathode of the present invention, the substrate is selected to have a transmittance of incident electromagnetic radiation of greater than 10%, for example greater than 30%, or further greater than 40%. This allows the conductive structures of the photocathode to be back-illuminated, for example, by causing the incident electromagnetic radiation to pass through the substrate before interacting with the conductive structures. In this way, the substrate does not interfere with electron emission from the structures.

[0041] In one embodiment of the photocathode of the present invention, the plurality of conductive structures are arranged in an array. In this manner, the cross-sectional area of ​​the photocathode can be increased while maintaining the size of the individual conductive structures. Thus, the sensitivity of the photocathode can be increased.

[0042] In an embodiment of the photocathode of the present invention, the photocathode includes a metamaterial, the metamaterial includes an array of conductive structures, and the plurality of conductive structures are arranged on a common substrate.

[0043] In an embodiment of the photocathode of the present invention, the vacuum system includes a photomultiplier tube (PMT).

[0044] In one embodiment of the photocathode of the present invention, the vacuum system comprises a multichannel plate.

[0045] According to a second aspect of the invention, an imaging system is disclosed comprising a multi-channel plate having a plurality of conductive structures and a spatially resolved detector system, emission from the conductive structures being spatially mapped onto the spatially resolved detector for generating an image. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Hereinafter, the photocathode of the present invention will be described in more detail with reference to the accompanying drawings, which illustrate one way of implementing the invention and should not be interpreted as limiting other possible embodiments within the scope of the claims.

[0047] Figure 1 Various aspects of embodiments of the photocathode of the present invention are shown, along with simulation results.

[0048] Figure 2 Various aspects of embodiments of the photocathode of the present invention are shown, along with simulation results.

[0049] Figure 3 An embodiment of a substrate for a photocathode according to the present invention and simulation results are shown.

[0050] Figure 4 The working principle of the photocathode of the present invention is shown.

[0051] Figure 5 Another embodiment of the photocathode of the present invention is shown.

[0052] Figure 6 Simulation results for an example of the photocathode of the present invention are shown.

[0053] Figure 7 Simulation results of another embodiment of the photocathode of the present invention are shown.

[0054] Figure 8 The simulation results of different choices of conductive materials for the conductive structure of the photocathode are shown.

[0055] Figure 9 Simulation results of an imaging system corresponding to the second aspect of the present invention are shown.

[0056] Figure 10 Simulation results related to the operation of embodiments of the present invention are shown.

[0057] Figure 11 An embodiment of the present invention is shown.

[0058] Figure 12 Simulation results related to the spectral response of an embodiment of the photocathode of the present invention are shown.

[0059] Figure 13 A photomultiplier tube useful in connection with the present invention is shown.

[0060] Figure 14 A multichannel plate useful in connection with the present invention is shown.

[0061] Figure 15 An imaging system of the present invention is shown.

[0062] Figure 16 An imaging system of the present invention is shown. DETAILED DESCRIPTION

[0063] Figure 1(a) shows a close-up of an embodiment of a photocathode 100 of the present invention. Photocathode 100 comprises a conductive structure 101 and a substrate 102. Conductive structure 101 (hereinafter also referred to as an "antenna") is configured to provide two electrodes 106 separated by a gap 103. The interaction between the electric field 104 of an incident photon and antenna 101 is illustrated, shown as a close-up of the tip portion 105 of antenna 103, which includes the antenna gap 103. These figures illustrate examples of materials that can be used to implement the structure, which have been used in the calculations shown here. Other choices of materials are also contemplated, as discussed elsewhere in this specification. An electric field time trace encompassing a frequency band within the terahertz frequency range is shown to indicate the received electric field 104 propagating through transparent substrate 102. It should be noted that the photocathode of the present invention is not limited to operation in the terahertz frequency range but can also be designed to operate at optical wavelengths. The electric field is concentrated in the volume surrounding gap 103, thereby establishing a strong electric field through the tip portion 105 of conductive structure 101, shown here as electrode 106. The strong electric field enables the electrons to tunnel from the conductive material of the antenna to the surrounding environment. As a final step, the electrons are accelerated in the electric field (4). Figure 1 (b) shows a scanning electron micrograph (SEM) of the conductive structure 101 in the form of a dual split ring resonator (dSRR) antenna. The dotted circle indicates the tip portion 105 including the electrode 106 and the gap 103. Figure 1 (c) shows a finite element simulation of the electric field in the plane of the antenna (101) when the electric field in the tip portion 105 is the strongest.

[0064] Figure 2 (c) shows the magnified images from the Figure 1 Fields of (b) and (c). Figure 2 (d) The electric field in the tunnel volume is shown over time as indicated by the arrows. The field enhancement is close to 400. Figure 2 (e) explains Figure 2 Frequency content of the temporal signal in (d). Obviously, the dSRR antenna geometry enables a wider frequency range to obtain a large temporal field enhancement for a broadband input E-field signal.

[0065] Figure 3The incident electric field Ei is shown, which impinges on the substrate 102. What is shown is the main work of the substrate. The main loss of the electric field occurs at the first interface 201 and is due to Fresnel reflection. The purpose of the substrate 102 is to provide a physical platform for building the photocathode. The substrate 102 itself has no function for the field emission process. One choice of substrate is HR-Si (high resistivity silicon), which does not absorb terahertz and infrared radiation, but has a refractive index of 3.42. Therefore, the Fresnel reflection of the electric field at the first interface 201 is 30%. At the second interface 202, the reflection is considered to be unimportant because the light interacts with the antenna, which is much thinner than the wavelength of the light. However, since the light at the second interface effectively exists partly in the substrate and partly in the vacuum, it is accepted that the effective refractive index should be included when calculating the resonant characteristics of the antenna.

[0066] Figure 4 The tunneling process from the antenna tip is shown, with relevant numerical values ​​inserted for illustration. A spatially invariant electric field is directed toward a conductive surface, represented here by gold 301. The energy difference between the Fermi level of gold and the vacuum level is 5 eV. By integrating the electric field in the direction perpendicular to the gold surface, the distance at which the potential due to the applied electric field is equal to the potential of the Fermi level of gold can be found. This distance is a distance representing the width of the tunnel barrier. Given that the electric field only exists for the duration of the incident signal (typically on the order of 1 ps for broadband transients in the terahertz frequency range), typical values ​​for measurable emission levels in vacuum electron systems are 5 nm or less.

[0067] Example: For -10 directed to a gold surface 9 For a constant electric field of V / m, the potential energy is given by:

[0068]

[0069] Therefore, the tunnel barrier width The result shown at point (1) is a well-known result of the type known as Fowler-Nordheim field emission. It is assumed that the applied electric field is independent of time.

[0070] In the ultrafast field emission regime, as in the photon-driven emission regime, time independence no longer holds. This can be corrected by assuming a quadratic potential barrier. Here, Φ is the system work function assuming a triangular potential barrier close to the emitting surface.

[0071] Figure 5 Different possible configurations of the conductive structure (101) are shown, for example, (a) a single resonant dipole and (b) two dipoles separated by a gap 103. Each pole of the dipole can be considered as an electrode 106, forming part of the tip portion 105.

[0072] Record the number of electrons emitted as a function of the incident field strength:

[0073] Due to the Fowler-Nordheim emission physics, the electron emission current is highly nonlinear with respect to the incident field strength. In addition, the sharpness of the tip from which the electrons are emitted affects the emission. Figure 6 These two points are shown. The left figure shows the emission current versus incident field strength for a circular tip (radius of curvature 1.5 μm), and the right figure shows the emission from a similar structure where the emission tip is pointed, resulting in a higher field enhancement. In the latter case, the high emission range extends to lower incident field strengths. A Fowler-Nordheim fit (the “FN Fit” curve in each figure) allows the absolute incident field strength to be determined (x-axis of the curve), which is in excellent agreement with the field strength value determined from calibrated free-space electro-optical sampling of the terahertz waveform.

[0074] Emission records for different geometries:

[0075] The antenna geometry determines the field enhancement factor and field confinement. Therefore, electron emission is not limited to a specific geometry, such as Figure 6 and Figure 8 The comparison is shown in Figure 6 In the antenna, the I structure is adopted ( Figure 5 (a)), in Figure 8 In the antenna, the form of dSRR is adopted ( Figure 1 (b)). Thus, the inventors have recognized that many antenna geometries can result in a given field enhancement and field localization. A particular geometry can be chosen to emphasize field localization at a small point and minimize electromagnetic coupling between adjacent elements (as embodied in a dSRR), or to allow coupling between adjacent antennas (I-structure). Of course, many other geometries can exhibit similar characteristics.

[0076] Absolute polarity measurement record:

[0077] Electron emission is only efficient when a tunneling channel is opened from the metal to the surrounding medium. This is ensured by the polarity of the driving electric field towards the tip. Therefore, reversing the field direction at a constant field strength significantly reduces the electron emission efficiency. This means that the asymmetric dSRR design is able to detect the absolute polarity of the driving terahertz field. Figure 7 As shown in the figure, this capability allows the absolute polarity of asymmetric drive fields to be detected. As shown, the incident terahertz field can be asymmetric in a single-period form (upper left), where the degree of asymmetry can be defined as 1-|E2 / E1|. If the polarity of this input field is rotated relative to the orientation of the dSRR antenna (lower right panel), the emission current varies with the angle of incidence, as shown in the left portion of the figure. A very pronounced anisotropy is observed in the emission current, and the absolute polarity of the field can be determined.

[0078] Figure 8 shows the output from a simple dipole antenna (e.g. Figure 5 Electron emission from the (structure I) shown in (a) as a function of the incident electric field strength. Emission is shown for antenna 701 made of gold and antenna 702 made of graphene. The Fowler-Nordheim plot (below) determines the effective work function of each material. As observed in many previous studies, for gold, the work function was found to be lower than that of bulk gold. For graphene, the extracted work function closely matches that of the armchair configuration.

[0079] Array arrangement and its use in terahertz beam imaging:

[0080] If individual antennas are placed in an array and the metamaterial is read from each individual antenna (e.g., a collection of antennas), imaging can be achieved. Figure 9 In this example, a metasurface with an array of antennas is placed in argon gas, where the emitted electrons can collide with argon atoms. The collisions transfer energy to the argon atoms, which are excited to higher electronic states. Subsequent relaxation to the ground state results in the emission of visible light similar to a glow discharge. To form such an image, a closed antenna design is preferably used to avoid field coupling between antennas, which makes it difficult to interpret the emission of a single antenna as a representation of the peak field at that precise point only. The dSRR is a design that solves the field encapsulation problem very well ( Figure 9 (b)). Image formation can be performed in a vacuum, for example, using a multichannel plate rather than a gas. This lowers the detection threshold and thus increases imaging sensitivity. Generally, all antenna types can be imaged.

[0081] Due to the engineered periodicity of the antenna array, any pixelated image can be efficiently reconstructed using 2D FFT filtering, such as Figure 9 (b) shown.

[0082] Record of lightning rod effect (mid-infrared driven emission):

[0083] In addition to the resonant field enhancement, another physical effect, the so-called lightning rod effect, affects the launch process. This effect enhances the field confinement at the tip to a 2 , where a is the tip radius.

[0084] For example, consider two tips with radii of 1500nm and 150nm, respectively, illuminated with 3.2um light. The illuminated antenna has no resonant enhancement for this frequency, so the enhancement depends solely on the lightning rod effect. For a 10-fold decrease in a (resulting in a 10-fold decrease in tip radius), the theoretical lightning rod enhancement should be a 2 =100.

[0085] The generated field-dependent emission current conforms to the Fowler-Nordheim emission model with a quadratic potential and a relative field enhancement fraction of 100. Figure 10 As shown, the consistency is very good.

[0086] Therefore, for all applications, a core feature of the antenna is a geometry with a tip portion that is as sharp or pointed as possible to maximize field confinement. This will enhance both the resonance-driven electron emission and the emission due to the lightning rod effect. Here, a sharp electrode is an electrode that includes a taper (see Figure 11 ), which starts at the antenna line width 1001 and ends in a plane parallel to the substrate surface with a significantly smaller dimension 1002, which is preferably as small as possible. It should be noted that the cone does not have to be continuous, as the tip can include a non-tapered portion 1003 at the narrow end of the cone, for example towards the gap. In practice, the minimum achievable dimension is limited by the manufacturing technology. Typical antenna line widths are hundreds of nanometers, further up to several micrometers. Typical tip dimensions are 20nm or further 10nm. In the direction perpendicular to the substrate surface, for example, the "antenna thickness", the optimal thickness can be found as a trade-off between a large thickness, which supports low resistance in the antenna and therefore optimal use of conductivity, and a small thickness, which supports the smallest possible tip cross-sectional area.

[0087] Practical applications of the lightning rod effect include Figure 12 Figure 3 is shown, which illustrates the emission driven by mid-infrared wavelengths (2.5-5.5μm wavelength). The average power of the femtosecond mid-infrared light source is shown as a substantially constant curve at the top of the figure. The curves labeled "0.5THz PMT" and "5.0THz PMT" are the electron emission signals detected with antenna structures of a = 1500nm and a = 150nm, respectively. The mid-infrared light source passes through a fused silica window before reaching the antenna. It is well known that fused silica has a strong absorption band at wavelengths exceeding 3.9μm. Therefore, for a 150nm tip radius (one order of magnitude smaller), the electron emission observed at an excitation wavelength of 4.0μm increases by two orders of magnitude, compared to the value of a 2 Consistent with the increased field confinement of the scale, there is evidence that a lightning rod effect favors efficient electron emission driven by a mid-infrared light source.

[0088] Figure 13 : is a cross-sectional view showing an example of an electron tube. The electron tube 1 is a photomultiplier tube that outputs an electric signal in response to the incidence of electromagnetic waves. When electromagnetic waves are incident, the electron tube 1 emits electrons inside and multiplies the emitted electrons. In this specification, the "electromagnetic waves" incident on the electron tube are electromagnetic waves included in the frequency band from so-called millimeter waves to infrared light. Figure 13As shown, the electron tube 1 includes a housing 10 , an electron emission unit (photocathode) 20 , an electron multiplying unit 30 and an electron collecting unit 40 .

[0089] The housing 10 includes a valve 11 and a valve stem 12. The interior of the housing 10 is hermetically sealed by the valve 11 and the valve stem 12 and is maintained in a vacuum. Vacuum includes not only an absolute vacuum but also a state where the housing is filled with a gas having a pressure lower than atmospheric pressure. For example, the interior of the housing 10 is maintained at 1×10 -4 to 1×10 -7 Pa. Valve 11 includes a window 11a that transmits electromagnetic waves. For example, housing 10 has a cylindrical shape. In this embodiment, housing 10 has a cylindrical shape. Valve stem 12 forms the bottom surface of housing 10. Valve 11 forms the side surface of housing 10 and the bottom surface facing valve stem 12.

[0090] The window 11a constitutes the bottom surface facing the valve stem 12. For example, the window 11a has a circular shape in a plan view. 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, and calcium carbonate. In the present embodiment, the window 11a is made of quartz. The frequency characteristics of the transmittance of electromagnetic waves vary depending on the material. Therefore, the material of the window 11a can be selected according to the frequency band of the electromagnetic wave passing through the window 11a. For example, quartz can be selected as the material of the component that transmits electromagnetic waves with a frequency band of 0.1 to 5 THz, silicon can be selected as the material of the component that transmits electromagnetic waves with a frequency band of 0.04 to 11 THz and 46 THz or above, magnesium fluoride can be selected as the material of the component that transmits electromagnetic waves with a frequency band of 40 THz or above, germanium can be selected as the material of the component that transmits electromagnetic waves with a frequency band of 13 THz or above, and zinc sulfide can be selected as the material of the component that transmits electromagnetic waves with a frequency band of 14 THz or above.

[0091] The electron tube 1 includes a plurality of wires 13 for electrically connecting the exterior and interior of the housing 10. The wires 13 are, for example, leads or pins. In this embodiment, the wires 13 are pins that penetrate the valve stem 12 and extend from the interior to the exterior of the housing 10. At least one of the wires 13 is connected to various components disposed within the housing 10.

[0092] The electron emission unit 20 is disposed in the housing 10 and emits electrons in response to electromagnetic waves incident on the housing 10. The electron emission unit 20 includes a metasurface 50 and a substrate 21 provided with the metasurface 50. The substrate 21 is transparent to electromagnetic waves passing through the window 11a. In this specification, "transparency" refers to the property of transmitting at least a portion of the frequency band of the incident electromagnetic wave. That is, the substrate 21 transmits at least a portion of the frequency band of the electromagnetic wave passing through the window 11a. The substrate 21 is made of, for example, silicon. It has a rectangular shape in plan view. The substrate 21 is separate from the window 11a and the electron multiplying unit 30.

[0093] Figure 14 1 is a perspective cross-sectional view of an example of a microchannel plate (multichannel plate). In this modified example, as shown in the figure, the microchannel plate 70 includes a substrate 73, a plurality of channels 74, a partition wall portion 75, and a frame member 76. The substrate 73 includes an input surface 73a and an output surface 73b opposite the input surface 73a. The substrate 73 is formed into a disc shape. The input surface 73a faces the substrate 21. The output surface 73b faces the anode 41. The input surface 73a and the output surface 73b are arranged parallel to the window 11a, the substrate 21, and the metasurface 50. The anode 41 has a flat plate shape and is arranged parallel to the output surface 73b of the microchannel plate 70.

[0094] A plurality of channels 74 are formed in the substrate 73 from the input surface 73a to the output surface 73b. Specifically, each channel 74 extends from the input surface 73a to the output surface 73b in a direction perpendicular to the input surface 73a and the output surface 73b. The plurality of channels 74 are arranged in a matrix shape in a plan view. Each channel 74 has a circular cross-sectional shape. Partition wall portions 75 are provided between the plurality of channels 74. In order to function as an electron multiplier, the microchannel plate 70 has a resistance layer and an electron emission layer (not shown) on the surface of the partition wall portions 75 in the channels 74. A frame member 76 is provided on the outer peripheral edge portions of the input surface 73a and the output surface 73b of the substrate 73.

[0095] In the electron tube 1E, one of the plurality of wires 13 is connected to each of the attachment members 71 and 72. In the microchannel plate 70, a voltage is applied between the input surface 73a and the output surface 73b via the wires 13 and the attachment members 71 and 72. When electrons emitted from the metasurface 50 are incident on the input surface 73a, the electrons are multiplied by the channels 74 and emitted from the output surface 73b. The electrons multiplied by the microchannel plate 70 are collected by the anode 41 and output from the anode 41 via the wires 13 as an output signal.

[0096] Next, we will refer to Figure 15 and Figure 16 An electron tube according to a modified example of this embodiment will be described. Figure 15 This is a partial cross-sectional view of an example electron tube. Figure 16 It is shown Figure 15 Cross-section of the electron tube. Figure 15 and 16 The modified example is substantially similar or identical to the above-described embodiment. However, the modified example differs from the embodiment in that the electron tube in the embodiment is a so-called image intensifier. The following mainly describes the differences between the embodiment and the modified example.

[0097] exist Figure 15 In the illustrated electron tube 1F, the electron emission unit 20, the electron multiplying unit 30, and the electron collecting unit 40 are disposed in a housing 80. The electron multiplying unit 30 includes a microchannel plate 70 instead of the focusing electrode 31 and the dynodes 32a to 32j. In the electron tube 1F, the electron collecting unit 40 includes a phosphor 81 instead of the anode 41. In the electron tube 1F, the metasurface 50, the microchannel plate 70, and the phosphor 81 are located close to each other in the housing 80.

[0098] The housing 80 includes a side wall 82, an incident window 83 (window 11a), and an exit window 84. The side wall 82 has a hollow cylindrical shape. The incident window 83 and the exit window 84 each have a disc shape. By hermetically sealing both ends of the side wall 82 with the incident window 83 and the exit window 84, the interior of the housing 80 is maintained in a vacuum. For example, the interior of the housing 80 is maintained at 1×10 -5 to 1×10 -7 Pa.

[0099] For example, the side wall 82 includes a side tube 85, a mold member 86 covering the side of the side tube 85, and a shell member 87 covering the side and bottom of the mold member 86. Each of the side tube 85, the mold member 86, and the shell member 87 has a hollow cylindrical shape. The side tube 85 is made of, for example, ceramic. The mold member 86 is made of, for example, silicone rubber. The shell member 87 is made of, for example, ceramic.

[0100] A through hole is formed at each end of mold member 86. One end of shell member 87 is open. The other end of shell member 87 is provided with a through hole. The through hole of shell member 87 includes an edge positioned so as to coincide with the edge position of one of the through holes of mold member 86. At one end of mold member 86, an incident window 83 is bonded to a surface surrounding the through hole of mold member 86. Similar to window 11a of electron tube 1, incident window 83 transmits electromagnetic waves. Similar to window 11a of electron tube 1, incident window 83 comprises at least one selected from the group consisting of quartz, silicon, germanium, sapphire, zinc selenide, zinc sulfide, magnesium fluoride, lithium fluoride, barium fluoride, calcium fluoride, magnesium oxide, and calcium carbonate.

[0101] In the electron tube 1F, the metasurface 50 is disposed directly on the entrance window 83 in the housing 80. The metasurface 50 faces the microchannel plate 70. The microchannel plate 70 is disposed between the metasurface 50 and the phosphor 81. The microchannel plate 70 is separated from the metasurface 50 and the phosphor 81.

[0102] On the other end side of the mold member 86, the emission window 84 is fitted to the other through-hole of the mold member 86. The emission window 84 is, for example, a fiber disk constituted in a disk shape by gathering a large number of optical fibers. Each optical fiber of the fiber disk is constituted so that the inner side end face 84a of the housing 80 is flush with each optical fiber. The end face 84a is disposed parallel to the metasurface 50.

[0103] The phosphor 81 is disposed on the end face 84a. The phosphor 81 is, for example, formed by applying a fluorescent material to the end face 84a. The fluorescent material is, for example, (ZnCd)S:Ag (zinc cadmium sulfide doped with silver). On the surface of the phosphor 81, a metal back layer and a low electron reflection layer are sequentially laminated. The metal back layer is formed by evaporation of Al, for example, and has a high reflectivity with respect to light passing through the microchannel plate 70 and a high transmissivity with respect to electrons emitted from the microchannel plate 70. The low electron reflection layer is formed by evaporation of carbon (C), beryllium (Be), or the like, for example, and has a relatively low reflectivity with respect to electrons emitted from the microchannel plate 70.

[0104] Similar to the electron tube 1E, in the electron tube 1F, one of the plurality of wires 13 extending to the outside of the housing 80 is connected to each of the attachment members 71 and 72 holding the microchannel plate 70. In the microchannel plate 70, a voltage is applied between the input face 73a side and the output face 73b side by the attachment members 71 and 72.

[0105] When electrons emitted from the metasurface 50 are incident on the input surface 73a, the electrons are multiplied by the channels 74 and emitted from the output surface 73b. In the electron tube 1F, the electrons multiplied by the microchannel plate 70 are collected in the phosphor 81. The phosphor 81 receives the electrons multiplied by the microchannel plate 70 and emits light. The light emitted from the phosphor 81 passes through the fiber plate and is emitted from the emission window 84 to the outside of the housing 80.

[0106] Although the present application has been described in connection with certain embodiments, it is not intended to be limited to the described examples but rather can be practiced with modifications and alterations. The scope of the application is indicated by the claims and not by the supporting text. In the claims, the term "comprising" does not exclude other elements or steps. Furthermore, the term "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The use of reference signs in the claims with following a colon even indicates multiple referents. Furthermore, the use of the term "including" as well as other forms for example "comprising", "having" or "containing" should be understood as meaning "including but not limited to". Furthermore, the term "example" does not mean "preferred" or "exemplary".

Claims

1. A photocathode for a vacuum system, wherein: The photocathode is configured to receive electromagnetic radiation having an incident wavelength and to emit electrons from the two dipoles in response thereto, The photocathode comprises: An electrically conductive structure having a geometric shape comprising a pointed portion having a pointed shape that concentrates the electric field in such a way as to provide a field enhancement β when the electrically conductive structure is irradiated with electromagnetic radiation, β being greater than 10 2 ;as well as a substrate, the substrate being a dielectric substrate or including a dielectric substrate, the substrate supporting the conductive structure, the conductive structure comprising a conductive material selected from the group consisting of copper, gold, silver, titanium, aluminum, and tungsten, having the two dipoles comprising the tip portion and separated by a gap, The conductive materials face each other in the two tip portions included in the two dipoles.

2. The photocathode according to claim 1, wherein The tip portion is configured to provide a field enhancement β by concentrating the electric field in a volume represented by a confining volume V, The confined volume is highly subwavelength.

3. The photocathode according to claim 1, wherein The gap width of the gap is in the range of 1 nm to 1000 nm.

4. The photocathode according to claim 1, wherein The gap width of the gap is in the range of 10 nm to 500 nm.

5. The photocathode according to claim 1, wherein The gap width of the gap is in the range of 20 nm to 100 nm.

6. The photocathode according to claim 1, wherein The two dipoles are included as a first electrode and a second electrode, the geometry of the first electrode being selected to provide a first field confinement and the geometry of the second electrode being selected to provide a second field confinement, the first field confinement being different from the second field confinement.

7. The photocathode according to any one of claims 1 to 6, wherein The photocathode is configured to receive the electromagnetic radiation at a designed wavelength, the designed wavelength being in the terahertz range or the infrared range.

8. The photocathode according to any one of claims 1 to 6, wherein The photocathode is configured to receive the electromagnetic radiation within a broadband design wavelength range, the broadband design wavelength range being in the terahertz range or the infrared range.

9. The photocathode according to claim 1, wherein The conductive structure has a double split ring geometry comprising two interconnected rings having a common tip portion and a common gap.

10. The photocathode according to any one of claims 1 to 6, wherein The substrate is selected to have a transmittance of incident electromagnetic radiation greater than 10%.

11. The photocathode according to any one of claims 1 to 6, wherein The substrate is selected to have a transmittance of incident electromagnetic radiation greater than 30%.

12. The photocathode according to any one of claims 1 to 6, wherein The substrate is selected to have a transmittance of incident electromagnetic radiation greater than 40%.

13. The photocathode according to any one of claims 1 to 6, wherein The plurality of conductive structures are arranged in an array.

14. The photocathode according to claim 13, wherein The photocathode comprises a metamaterial, the metamaterial comprises an array of the conductive structures, and the plurality of conductive structures are arranged on a common substrate.

15. The photocathode according to any one of claims 1 to 6, wherein The vacuum system includes a photomultiplier tube.

16. The photocathode according to any one of claims 1 to 6, wherein The vacuum system includes a multi-channel plate.

17. An imaging system, wherein: comprising a multi-channel plate having a plurality of conductive structures, and a spatially resolved detector system onto which emissions from the conductive structures are spatially mapped to generate an image, The conductive structure has a geometric shape comprising a pointed portion having a pointed shape that concentrates the electric field in a manner that provides a field enhancement β when the conductive structure is irradiated with electromagnetic radiation, β being greater than 10 2 , The conductive structure comprises a conductive material selected from the group consisting of copper, gold, silver, titanium, aluminum, and tungsten, having two dipoles including the tip portion and separated by a gap, The conductive materials face each other in the two tip portions included in the two dipoles.

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