Photocathode device for sub-nanosecond gating and method for the manufacturing thereof, and visible light detection device

Abstract

Photocathode device for sub-nanosecond gating comprising an active photoemissions region (3) surrounded by an electrode region (5), the device comprising: - a substrate (7); - a photocathode layer (9) deposited on the substrate; wherein the device also comprises: - a conductive underlayer (11) deposited between the substrate and the photocathode layer in the active photo-emission region and preferably in the electrode region; - a layer of aluminium oxide (15) deposited between the conductive underlayer and the photocathode layer. Method of manufacturing of the photocathode device, and visible light detection device comprising the photocathode device.

Description

TECHNICAL FIELD

[0001] The present invention generally relates to the field of optoelectronic devices, and more particularly to the field of photocathode devices for sub-nanosecond gating.BACKGROUND

[0002] The current state of the art in the field of sub-nanosecond gating image intensifier tubes involves the use of photocathodes comprising multi-alkali antimonide. These photocathodes are deposited on substrates to achieve high quantum efficiency in the deep ultraviolet spectrum between 200 nm to 400 nm range and visible spectrum between 400 nm and 900 nm range. These devices typically employ conductive grid layers made from materials like chromium, copper, or silver, which are deposited using photolithography techniques. The relatively transparent grid layers are crucial for providing the necessary electrical conductivity to the otherwise highly resistive photocathode materials, which can exhibit square resistance values in the kilo Ohm (kOhm) to Giga Ohm (GOhm) range.

[0003] However, the use of conductive grids for multi-alkali photocathodes presents several significant disadvantages. The fabrication of these grids using photolithography is both complex and costly, significantly increasing the production costs of these devices. Another disadvantage is that the metal of the conductive grid layer may be susceptible to oxidation. Additionally, the grid structures can block a portion of the incoming light, reducing the overall quantum efficiency of the photocathode.

[0004] Document Y. Yang et.al, Nuclear Inst. and Methods in physics research A, 1056, 2023, 168621, discloses a device comprising a layer of alkali antimonide directly deposited onto a gold layer. However, such a device exhibits a very low quantum efficiency.

[0005] It is therefore an object of the present disclosure to provide a device and a method that at least partly overcome one or more of the above-mentioned drawbacks of the prior art.SUMMARY

[0006] In a first aspect, the present disclosure relates to a photocathode device for sub-nanosecond gating comprising an active photo-emissions region surrounded by an electrode region, the device comprising: a substrate; a photocathode layer deposited on the substrate; a conductive underlayer deposited between the substrate and the photocathode layer in the active photo-emission region and preferably in the electrode region; a layer of aluminium oxide deposited between the conductive underlayer and the photocathode layer.

[0007] A multi alkali photocathode device may be understood as an optoelectronic device that is designed to detect visible and / or ultraviolet light.

[0008] The photocathode device may be adapted to detect visible light.

[0009] The device comprises a substrate. A substrate may be understood as the underlying material or layer upon which other materials are deposited or constructed. In this context, the substrate provides the foundational support for the subsequent layers of the device.

[0010] The device comprises a conductive underlayer disposed between the substrate and the photocathode layer. With "conductive" underlayer as used in the present disclosure is meant that the underlayer is electrically conductive. This arrangement includes a layer of indium tin oxide (ITO) deposited directly on the substrate. The layer of ITO is considered a very suitable material due to its high transparency (96%) in the visual range and high conductance.

[0011] The conductive underlayer provides electrical conductance for the photocathode layer to enable a gating duration below 1 ns, preferably below 0.2 ns. It may thus be provided that the conductive underlayer supports gating duration below 1 nanoseconds (ns), preferably below 0.2 ns. This enables the device to operate at high speeds, which is critical for applications requiring rapid response times and high temporal resolution.

[0012] The layer of aluminium oxide layer forms a beneficial interface in terms of growth and quantum efficiency between the conductive underlayer and photocathode layer. A good growth and high quantum efficiency of the photocathode is obtained. The presence of the aluminium oxide layer is important as it functions as a chemical compatible sublayer and it forms an alkali diffusion / reaction barrier.

[0013] Another advantage of this combination of the conductive underlayer and the aluminium oxide layer is that it reduces the overall production costs and complexity compared to traditional grid-based conductive layers, while still delivering high performance.

[0014] The conductive underlayer preferably comprises a layer of indium tin oxide (ITO), deposited on the substrate. The use of an ITO layer provides sufficient electrical conductance to prevent voltage drops, or in other words to maintain the same electric potential over the entire surface of the photocathode, and dark zones in the image, thereby enhancing the device's operational reliability and performance. This approach also facilitates a more straightforward manufacturing process, leading to improved production yields and reduced costs.

[0015] A further advantage is the low cost price of the ITO deposition method. Also, the refractive index well above 1,5 works as an anti-reflective coating between the high refractive index photocathode (N>3) and low refractive index of many glass like substrates (N=1,5).

[0016] Electrical conductivity (σ) is a measure of a material's ability to conduct electric current. It is the reciprocal of electrical resistivity (ρ) and is expressed in siemens per meter (S / m). Materials with high electrical conductivity allow electric charges to flow easily, while materials with low conductivity resist the flow of current. The relationship between conductivity and resistivity is: σ = 1 / ρ where: σ = electrical conductivity (S / m), and ρ = resistivity (Ω·m).

[0017] The relationship between the square resistance R □ (Ω), electrical resistivity ρ, electrical conductivity σ and thickness d (m) of a conductive thin layer is defined by the following formula: R □ = ρ / d = 1 / σ × d

[0018] The conductance G (in S) of an object in a material is calculated from the conductivity σ and the geometry of the material as defined by the following formula: G = σ × A / L wherein A (in m 2< ) is the cross-sectional area of the object and L (in m) is the length of the object.

[0019] A conductivity of 1 x 10 -8< S / m (at 20 °C) can generally be considered a minimal value to label a material as "conductive," depending on context. In the present invention, "conductive" is defined as having a conductivity over 10 5< S / m, preferably over 10 6< S / m. The high conductivity can be associated with a layer of at least 1 nm.

[0020] The conductive underlayer may be arranged to provide electrical conductance for the photocathode while maintaining a quantum efficiency of the photocathode device over 15%, preferably over 20%, in a wavelength range of 400 nm to 900 nm.

[0021] In an embodiment, the photocathode device comprises a ring electrode in the electrode region to provide electrical conductance for the photocathode. Preferably, the ring electrode comprises a layer of a material selected from the group consisting of chromium, nickel, titanium, silver, gold, and copper.

[0022] Preferably, the ring electrode has a circumference that is larger than a circumference of the photocathode layer defining a flange arranged for sealing the photocathode device in a photocathode vacuum envelope.

[0023] The photocathode device may have in the active photo-emission region a relative quantum efficiency loss of at most 25%, more preferably at most 20%, in the wavelength range of 400 nm to 900 nm, as compared to a photocathode device not comprising a conductive underlayer.

[0024] It is preferred that the photocathode device, more specifically the conductive underlayer, has a square resistance of at most 200 Ohm, more preferably at most 100 Ohm. It may be provided that the conductive underlayer has this specific range of square resistance, ensuring adequate conductance without excessive current leakage. This feature provides optimal electrical performance, preventing voltage drops and ensuring consistent image quality.

[0025] In an embodiment, the photocathode layer is a multi-alkali antimonide selected from the group consisting of sodium, potassium, rubidium, cesium, and combinations thereof, preferably wherein the multi-alkali antimonide is Na x K y Cs z Sb, wherein x=2, y=1, z<0,1. An example of the multi-alkali antimonide is Na 2 KSb. These materials are known for their high quantum efficiency in the visible and ultraviolet spectrum, enhancing the device's sensitivity and performance in detecting visible and ultraviolet light.

[0026] In another embodiment, the photocathode layer has a thickness of 10 nm to 150 nm, preferably 15 nm to 140 nm.

[0027] In a preferred embodiment, the layer of indium tin oxide has a thickness of 5 nm to 40 nm, preferably 10 nm to 30 nm or 15 nm to 20 nm. The indium tin oxide layer having a thickness in such a range is beneficial as it improves the internal transmittance of light, at least in the wavelength range of 400 nm to 900 nm.

[0028] In another embodiment, the layer of aluminium oxide has a thickness of 40 nm to 90 nm, preferably 40 nm to 80 nm. A layer aluminium oxide of a thickness in the defined range avoids a chemical interface reaction between the ITO layer and the photocathode layer. Also, the defined thickness range results in optimization of both conductance and performance of the photocathode, making this approach particularly suitable for high-precision applications.

[0029] Preferably, the layer of aluminium oxide has a density of over 3.2 g / cm 3< . Atomic layer deposition is used as the preferable technique to obtain such a high density.

[0030] The photocathode device, in the active photo-emission region, may have an internal transmittance of at least 96%, preferably at least 98%, more preferably at least 99%, in a wavelength range of 400 nm to 900 nm. With "internal transmittance" as used in the present disclosure is meant the remaining energy not lost by absorption of incident electromagnetic power, determined by the sum of transmittance and reflectance of the incident electromagnetic power.

[0031] The active photo-emission region surrounded by the electrode region may have a surface area of at least 0.25 cm 2< , preferably at least 0.5 cm 2< .

[0032] In the active photo-emission region, the substrate and the aluminium oxide layer may be devoid of contact. Indeed, the conductive layer allows optical transmission that avoids the use of a conductive grid under the photocathode layer. The conductive layer can be uniformly deposited under an entire surface where the photocathode layer is deposited.

[0033] In an embodiment, the ring electrode, if any, has a thickness of 50 to 280 nm, preferably 100 nm to 200 nm.

[0034] In an embodiment, the substrate is a transparent substrate and / or a diffractive substrate and / or a fibre optic substrate in a wavelength range of 400 nm to 900 nm.

[0035] In another embodiment, the substrate has a thickness of 1 mm to 100 mm, preferably 2 mm to 10 mm.

[0036] In yet another embodiment, the substrate is made of a material selected from the group consisting of glass, quartz, sapphire, fused silica, magnesium fluoride, calcium fluoride and lithium fluoride. This feature ensures minimal light absorption and high structural integrity, enhancing the overall performance of the photocathode device.

[0037] In a second aspect, the present disclosure relates to a method of manufacturing a photocathode device for sub-nanosecond gating comprising an active photo-emission region surrounded by an electrode region, the method comprising the steps of: 1) providing a substrate, preferably a transparent substrate and / or a diffractive substrate and / or a fibre optic substrate; 2) depositing a conductive underlayer on the substrate in the active photo-emission region; 3) depositing or forming a layer of aluminium oxide on the conductive underlayer; 4) depositing a photocathode layer on the layer of aluminium oxide.

[0038] The step of deposition a conductive underlayer is preferably followed by a step of heat treatment of the conductive underlayer so that the electrical conductance of the conductive underlayer is set to a predetermined value.

[0039] The conductive underlayer comprises a layer of indium tin oxide, deposited on the substrate.

[0040] The step of depositing or forming the layer of aluminium oxide may comprise the sub-step(s) of: i) depositing aluminium oxide on the conductive layer in the active photo-emission region, and / or ii) depositing aluminium on the conductive underlayer in the active photo-emission region and subsequently exposing the deposited aluminium to an oxidizing gas, such as air, thereby forming the layer of aluminium oxide on the conductive underlayer.

[0041] The conductive underlayer may be arranged to provide electrical conductance for the photocathode while maintaining a quantum efficiency of the photocathode device over 20%, preferably over 25%, in a wavelength range of 400 nm to 900 nm.

[0042] The conductive layer is at least deposited in the active photo-emission region and may also be deposited in the electrode region. This is beneficial as it provides a good electrical contact between the ring electrode and the ITO film (i.e., the layer of indium tin oxide).

[0043] Prior to step 1), the substrate is preferably polished such that a polished substrate is provided in step 1). Preferably, the substrate to be provided is also cleaned and / or treated with ozone in an air atmosphere and / or provided with a glow discharge treatment in an evaporator.

[0044] The method may further comprise a step of depositing a ring electrode in the electrode region in between steps 3) and 4) to provide electrical conductance for the photocathode layer. Preferably, the step of depositing the ring electrode in the electrode region comprises depositing a layer of a material selected from the group consisting of chromium, nickel, titanium, silver, gold, and copper.

[0045] The depositing of the ring electrode is preferably done in a manner such that the ring electrode has a circumference that is larger than a circumference of the photocathode layer defining a flange arranged for sealing the photocathode device in a photocathode vacuum envelope.

[0046] In an embodiment, the step of depositing the layer of chromium of the ring electrode comprises depositing a layer with a thickness of 50 to 280 nm, preferably 100 nm to 200 nm.

[0047] In another embodiment, the depositing the layer of the indium tin oxide comprises depositing a layer with a thickness of 5 nm to 40 nm, preferably 10 nm to 30 nm.

[0048] In yet another embodiment, the depositing the layer of aluminium oxide or depositing the layer of aluminium comprises depositing of a layer of aluminium oxide with a thickness of 40 nm to 90 nm, preferably 40 nm to 80 nm or depositing a layer of aluminium with a thickness of 40 nm to 90 nm, preferably 40 nm to 80 nm.

[0049] The depositing of each of the layer of indium tin oxide, the conductive underlayer, the aluminium oxide layer, the aluminium layer, and the ring electrode, if any, may be done by an e-beam evaporation process, a sputtering process, or an atomic layer deposition process.

[0050] The depositing of the photocathode layer may be done by chemical vapour deposition (CVD). The CVD process allows precise control over the thickness and uniformity of the layer, which ensures high-quality deposition, enhancing the performance and reliability of the photocathode device.

[0051] Furthermore, the depositing of the layers may be done by evaporation, preferably at a high temperature (200 °C to 300 °C). This is beneficial to obtain the desired properties for conductance, transparency, and diffusion, which minimalizes the formation of alkali oxides. The aluminium oxide is preferably deposited with the atomic layer deposition technique.

[0052] The deposited layer of indium tin oxide may be subjected to a heat treatment done by heating the deposited layer of ITO in an air and / or oxygen atmosphere at a temperature of 250 °C to 350 °C for 0.5 hours to 3 hours.

[0053] Embodiments of the photocathode device according to the first aspect of the present disclosure as presented herein are also applicable to the method according to the second aspect of the present disclosure, and vice versa.

[0054] Effects of the photocathode device according to the first aspect of the present disclosure as presented herein correspond to or are similar to effects of the method according to the second aspect of the present disclosure.

[0055] In a third aspect, the present disclosure relates to a visible light detection device, comprising: a photocathode device for sub-nanosecond gating comprising an active photo-emission region surrounded by an electrode region, the photocathode device comprising: a substrate; a photocathode layer disposed on the substrate; a conductive underlayer, preferably comprising a layer of indium tin oxide, disposed between the substrate and the photocathode layer in the active photo-emission region and preferably in the electrode region; a layer of aluminium oxide deposited between the conductive underlayer and the photocathode layer, wherein the photocathode layer is a multi-alkali antimonide, wherein the conductive underlayer is preferably arranged to provide electrical conductance for the photocathode while maintaining quantum efficiency of the photocathode device over 20%, preferably over 25%, in a wavelength range of 400 nm to 900 nm, wherein the visible light detection device is configured for use in applications requiring sensitivity to visible radiation.

[0056] Optionally, the visible light detection device further comprises a ring electrode in the electrode region being in electrical contact with the conductive underlayer and configured to provide electrical conductance for the photocathode device.

[0057] The visible light detection device may be implemented in one or more of the following applications: (UV) image intensifiers; photon counting detectors and modules; photomultiplier tubes; night vision and low-light imaging devices; ultraviolet spectroscopy instruments; radiation monitoring equipment; scientific and medical imaging cameras; aerospace and satellite-based UV detectors; UV detection in fire and explosion sensors; fast-gated photocathode detectors for laser-induced fluorescence; range gated imaging devices; and UV astronomy instruments.

[0058] The visible light detection device according to the third aspect of the present disclosure may comprise a photocathode device according to the first aspect of the present disclosure or a photocathode device manufactured according to the method according to the second aspect of the present disclosure.

[0059] Embodiments of the photocathode device according to the first aspect of the present disclosure as presented herein are also applicable to the visible light detection device according to the third aspect of the present disclosure, and vice versa.

[0060] Effects of the photocathode device according to the first aspect of the present disclosure as presented herein correspond to or are similar to effects of the visible light detection device according to the third aspect of the present disclosure.BRIEF DESCRIPTION OF THE DRAWINGS

[0061] The present disclosure will be explained in more detail below by means of examples of a device and method according to the present disclosure shown in the drawings, in which: Fig. 1 shows a photocathode device for sub-nanosecond gating according to a first aspect of the present disclosure; Fig. 2 shows a method step of manufacturing a photocathode device for sub-nanosecond gating according to a second aspect of the present disclosure. DETAILED DESCRIPTION

[0062] The present disclosure will now be described in detail with reference to the accompanying drawings, which illustrate exemplary embodiments of the photocathode device 1 for sub-nanosecond gating and the method 101 of manufacturing the same. It is to be understood that these embodiments are provided by way of example only and are not intended to limit the scope of the present disclosure. The following detailed description should be read in conjunction with the claims and the figures.

[0063] As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items. Expressions such as "at least one of," when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list.

[0064] As the inventive concept allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in more detail in the written description. Effects, features, and a method of achieving the inventive concept will be obvious by referring to exemplary embodiments of the inventive concept with reference to the accompanying drawings. The inventive concept may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein.

[0065] In the embodiments described in the present specification, an expression utilized in the singular encompasses the expression of the plural, unless it has a clearly different meaning in the context. Also, it is to be understood that the terms such as "including," "having," and / or "comprising" are intended to indicate the presence of the stated features or components, and are not intended to preclude the presence or addition of one or more other features or components.

[0066] It will be understood that when a layer, region, or component is referred to as being "on" or "onto" another layer, region, or component, it may be directly or indirectly formed on the other layer, region, or component. That is, for example, intervening layer(s), region(s), or component(s) may be present.

[0067] Sizes of components in the drawings may be exaggerated for convenience of explanation. In other words, since sizes and thicknesses of components in the drawings are arbitrarily illustrated for convenience of explanation, the following embodiments of the present disclosure are not limited thereto.

[0068] Fig. 1 shows schematically an example of a photocathode device 1 for sub-nanosecond gating according to the first aspect of the present disclosure. The photocathode device 1 comprises two regions: an active photo-emission region 3 that is surrounded by an electrode region 5. The device 1 further comprises a substrate 7, a conductive underlayer 11 (comprising a layer of indium tin oxide (ITO) 13), a layer of aluminium oxide 15, and a photocathode layer 9 (being a multi-alkali antimonide). As can be seen, the layer of aluminium oxide 15 is deposited between the ITO layer 13 and the photocathode layer 9. As can further be seen, the conductive underlayer 11 is deposited between the substrate 7 and the photocathode layer 9, preferably between the substrate 7 and the aluminium oxide layer 9 in the active photo-emission region 3 and in the electrode region 5. The conductive underlayer 11 is arranged to provide electrical conductance for the photocathode while maintaining a quantum efficiency over 20%, preferably over 25%, in a wavelength range of 400 to 900 nm.

[0069] The device 1 shown in Fig. 1 further comprises a ring electrode 19 in the electrode region 5 to provide conductance for the photocathode device 1. The ring electrode 19 comprises a layer of a material selected from the group consisting of chromium, nickel, titanium, silver, gold, and copper.

[0070] The example of the device 1 shown in Fig. 1 has in the active photo-emission region 3 a relative quantum efficiency loss of at most 25%, preferably at most 20%, in the wavelength range of 400 to 900 nm as compared to a photocathode device not having a conductive underlayer, and a square resistance of at most 200 Ohm, preferably at most 100 Ohm.

[0071] Furthermore, in the example shown in Fig. 1, the multi-alkali of the multi-alkali antimonide of the photocathode layer 9 is selected from the group consisting of sodium, potassium, rubidium, cesium, and combinations thereof, preferably wherein the multi-alkali antimonide is Na x K y Cs z Sb, wherein x=2, y=1, z<0,1. As an example, the multi-alkali material of the multi-alkali antimonide is Na 2 KSb.

[0072] In the photocathode device 1 shown in Fig. 1, the layer of indiumtinoxide 13 has a thickness of 5 to 40 nm, but it may also have a thickness between 10 to 30 nm or 15 to 20 nm. The layer of aluminium oxide 15 has a thickness of 40 to 90 nm, preferably 40 to 80 nm. The layer of aluminium oxide 15 has a density of over 3.2 g / cm 3< .

[0073] The device 1 of Fig. 1, in the active photo-emission region 3, has an internal transmittance of at least 96%, preferably at least 98%, more preferably at least 99%, in a wavelength range of 400 to 900 nm. The conductive underlayer 11 provides electrical conductance for the photocathode layer 9 to enable a gating duration below 1 ns, preferably below 0.2 ns. Furthermore, the active photo-emission region 3 surrounded by the electrode region 5 has a surface area of at least 0.25 cm 2< , preferably at least 0.5 cm 2< .

[0074] The substrate 7 is a transparent substrate and a diffractive substrate in a wavelength range of 400 to 900 nm. The substrate 7 can be a fibre optic substrate. However, in other examples, the substrate 7 may be a transparent substrate and / or a diffractive substrate and / or a fibre optic substrate in a wavelength range of 400 to 900 nm. The substrate is made of a material selected from the group consisting of glass, quartz, sapphire, fused silica, magnesiumfluoride, calciumfluoride and lithiumfluoride.

[0075] Fig. 2 shows an example of a method 101 according to the second aspect of the present disclosure of manufacturing of a photocathode device 1 for sub-nanosecond gating comprising an active photo-emission region 3 surrounded by an electrode region 5, such as the device 1 according to the first aspect of the present disclosure. The method 101 comprises the steps of 1) providing 103 a substrate 7, preferably a transparent substrate and / or a diffractive substrate and / or a fibre optic substrate; 2) depositing 105 a conductive underlayer 11, comprising a layer of ITO 13, on the substrate 7 in the active photo-emissions region 3; 3) depositing or forming 107 a layer of aluminium oxide 15 on the conductive underlayer 11; and 4) depositing 109 a photocathode layer 9 on the layer of aluminium oxide 15.

[0076] The step of depositing 105 the conductive underlayer 11 is followed by a step 111 of heat treatment of the conductive underlayer 11 so that the electrical conductance of the conductive underlayer 11 is set to a predetermined value. Preferably, the heat treatment is performed in air and / or in oxygen.

[0077] The step of depositing or forming 107 the layer of aluminium oxide comprises the sub-step(s) of i) depositing 107-l aluminium oxide on the conductive underlayer 11 or ii) depositing 107-iia aluminium on the conductive underlayer 11 and subsequently exposing 107-iib the deposited aluminium to an oxidizing gas, such as air, thereby forming the layer of aluminium oxide 15 on the conductive underlayer 11. It is also possible to first perform step i) and then step ii) or vice versa.

[0078] The method 101 further comprises the step of depositing 113 a ring electrode 19 in the electrode region 5 of the photocathode device 1 in between step 3) of depositing 107 the layer of aluminium oxide 15 on the conductive underlayer 11 and step 4) of depositing 109 a photocathode layer 9 on the layer of aluminium oxide 15. The step of depositing 113 the ring electrode 19 comprises depositing a layer of a material selected from the group consisting of chromium, nickel, titanium, silver, gold, and copper.

[0079] Furthermore, the depositing 105,107-i,107-iia,113 of each of the conductive underlayer 11, the aluminium oxide, the aluminium, and the ring electrode 19 are done by an e-beam evaporation process, a sputtering process, or an atomic layer deposition process. The depositing 109 of the photocathode layer 9 is done by chemical vapour deposition.

[0080] The foregoing description of the embodiments of the disclosure has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the claimed disclosure to the precise form disclosed. Those skilled in the art will readily appreciate that many modifications and variations to the claimed disclosure are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present disclosure and its practical application to thereby enable others skilled in the art to best utilize the present disclosure in various photocathode embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the present disclosure be defined exclusively by the following claims, and equivalents thereof.

[0081] A single unit or component may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope thereof.EXAMPLES

[0082] Several different combinations of material layers and some additional variations were tested to achieve a suitable replacement of the current gating technology with a chromium grid. Samples were cleaned with Iso Propyl Alcohol and dry spinned. Subsequently, an ozone treatment was done. Windows were made and tested using quartz and diffractive substrates.

[0083] With "windows" as used in the present disclosure is meant a substrate having a region or layer that allows incoming light to reach the photocathode's active area.

[0084] Different regimes were tested in terms of indium tin oxide (ITO) thicknesses and oxygen background pressure regimes. The square resistance (Rsquare) of the samples was around 200 Ohm using a minimal thickness of 28 nm and the Internal Transmittance was above 96% (Internal Transmittance = Transmittance + Reflectance), as shown in Table 1. Table 1. Resistivity, transmittance and reflectance data.Thickness (nm)Resistivity (Ω·m·10 -6< )Visible domain (380-750 nm)UV domain (200-380 nm)Avg. trans.%Avg. reflect.%Avg. abs.%Avg. trans.%Avg. reflect. %Avg. abs.%753.192.47.310.2980.4111.5813.641414.291.49.55-0.9571.7717.8615.4218.7589.869.190.9566.0820.1118.44285.788.9910.870.1460.6424.2719.333510.588.169.52.3461.3923.3019.71

[0085] As becomes clear from Table 1, the light absorption is close to zero. Resistivity drops with the thickness. Useful resistance range comes in sight above 14 nm.

[0086] Sub-nanosecond gating was proven with a functionality test. The average was around 0.6 ns (n=69), see Table 2 for the results. Table 2. Functionality test results for sub-nanosecond gating.Sub-nanosecond qatinq. time (ns)Amount (#)0.110.210.3120.460.520.6270.720.820.9101.021.101.231.31

Claims

1. Photocathode device (1) for sub-nanosecond gating comprising an active photo-emissions region (3) surrounded by an electrode region (5), the device comprising: - a substrate (7); - a photocathode layer (9) deposited on the substrate; wherein the device also comprises: - a conductive underlayer (11) deposited between the substrate and the photocathode layer in the active photo-emission region and preferably in the electrode region; - a layer of aluminium oxide (15) deposited between the conductive underlayer and the photocathode layer.

2. Photocathode device according to claim 1, wherein the conductive underlayer comprises a layer of indium tin oxide (13), deposited on the substrate.

3. Photocathode device according to claim 1 or 2, wherein the photocathode layer is a multi-alkali antimonide.

4. Photocathode device according to any of the preceding claims, wherein the conductive underlayer is arranged to provide electrical conductance for the photocathode while maintaining a quantum efficiency of the photocathode device over 15%, preferably over 20%, in a wavelength range of 400 nm to 900 nm.

5. Photocathode device according to any of the preceding claims, wherein the photocathode device comprises a ring electrode (19) in the electrode region to provide electrical conductance for the photocathode, preferably wherein the ring electrode comprises a layer of a material selected from the group consisting of chromium, nickel, titanium, silver, gold, and copper.

6. Photocathode device according to any of the preceding claims, wherein the photocathode device has a square resistance of at most 200 Ohm, preferably at most 100 Ohm.

7. Photocathode device according to claim 3 or any claim dependent thereof, wherein the photocathode layer is a multi-alkali antimonide selected from the group consisting of sodium, potassium, rubidium, cesium, and combinations thereof, preferably wherein the multi-alkali antimonide is NaxKyCszSb, wherein x=2, y=1, z<0,1.

8. Photocathode device according to claim 2 or any claim dependent thereof, wherein the layer of indium tin oxide has a thickness of 5 nm to 40 nm, preferably 10 nm to 30 nm or 15 nm to 20 nm.

9. Photocathode device according to any of the preceding claims, wherein the layer of aluminium oxide has a thickness of 40 nm to 90 nm, preferably 40 nm to 80 nm.

10. Photocathode device according to any of the preceding claims, wherein the layer of aluminium oxide has a density of over 3.2 g / cm3.

11. Photocathode device according to any of the preceding claims, wherein the photocathode device, in the active photo-emission region, has an internal transmittance of at least 96%, preferably at least 98%, more preferably at least 99%, in a wavelength range of 400 nm to 900 nm.

12. Method (101) of manufacturing a photocathode device (1) for sub-nanosecond gating comprising an active photo-emission region (3) surrounded by an electrode region (5), the method comprising the steps of: 1) providing (103) a substrate (7), preferably a transparent substrate and / or a diffractive substrate and / or a fibre optic substrate; 2) depositing (105) a conductive underlayer (11) on the substrate in the active photo-emission region; 3) depositing or forming (107) a layer of aluminium oxide (15) on the conductive underlayer; 4) depositing (109) a photocathode layer (9) on the layer of aluminium oxide.

13. Method according to claim 12, wherein the step of depositing the conductive underlayer is followed by a step (111) of heat treatment of the conductive underlayer so that the electrical conductance of the conductive underlayer is set to a predetermined value.

14. Method according to claim 12 or 13, wherein the conductive underlayer comprises a layer of indium tin oxide (13), deposited on the substrate.

15. Method according to any of the claims 12 to 14, wherein the step of depositing or forming the layer of aluminium oxide comprises the sub-step(s) of: i) depositing (107-i) aluminium oxide on the layer of indium tin oxide in the active photo-emission region, and / or ii) depositing (107-iia) aluminium on the layer of indium tin oxide in the active photo-emission region and subsequently exposing (107-iib) the deposited aluminium to an oxidizing gas, such as air, thereby forming the layer of aluminium oxide on the conductive underlayer.

16. Method according to any of the claims 12 to 15, further comprising a step of depositing (113) a ring electrode (19) in the electrode region in between steps 3) and 4) to provide electrical conductance for the photocathode layer, preferably wherein the step of depositing the ring electrode in the electrode region comprises depositing a layer of chromium.

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