Nanocolumn array Na2KSb photoelectric cathode for enhancing light absorption

Abstract

The invention discloses a nano-pillar array NaKSb photoelectric cathode for enhancing light absorption, which is characterized in that through the design of a double-layer TiOtwo-dimensional nano grating and a nano-pillar array structured NaKSb photoelectric cathode layer, the optical path of light with the wave band of 450-900 nm in the photoelectric cathode is increased under the condition that the thickness of the NaKSb photoelectric cathode is not increased, the reflectivity of the wave band of 400-600 nm is reduced, and the light absorption of the NaKSb photoelectric cathode is enhanced. The absorption rate at 400-1000 nm is improved by 32.28% compared with that of a traditional planar film type photoelectric cathode, the absorption rate at 800-1000 nm near-infrared band is improved by 40.27% compared with that of the traditional planar film type photoelectric cathode, the problem that the absorption of the traditional planar film type photoelectric cathode near-infrared band is insufficient is solved, and the photoelectric cathode is suitable for high-performance low-light night vision imaging devices.

Description

Technical Field

[0001] The invention relates to low-light-level night vision technology, and more particularly to a nano-column array Na2KSb photocathode with enhanced light absorption. Background Art

[0002] Low-light-level night vision technology utilizes highly sensitive optoelectronic imaging devices to enhance nighttime observation capabilities and acquire information about the nighttime environment. Low-light-level image intensifiers are vacuum-type imaging devices commonly used for low-light-level night vision. Compared to solid-state low-light-level imaging devices, they offer an electron multiplication gain of over 10,000 times and a nanosecond electronic shutter, making them an indispensable core component for nighttime military operations.

[0003] The low-light-level image intensifier is composed of an input window, a photocathode, an electron multiplication system, an electron focusing system, a fluorescent screen, an output window, a high-voltage power supply and a shell. Together with the objective lens and the eyepiece or a solid-state imaging device, it constitutes a low-light-level imaging system. Figure 1 shown.

[0004] The working principle of low-light image intensifier is as follows Figure 2 As shown in the figure: weak light incident photons enter the photocathode through the input window; the photocathode absorbs light energy and is excited to generate photoelectrons, which are transported to the surface of the photocathode and overcome the work function and are emitted into the vacuum; in the vacuum environment, the electrons are accelerated by the high voltage electric field and enter the electron multiplication system (such as the microchannel plate MCP) for 10 5 The electrons are multiplied by times or more and then bombard the fluorescent screen; the fluorescent screen receives the electron energy and emits light, thereby obtaining an enhanced optical image.

[0005] Current low-light-level image intensifiers usually adopt a close-fitting structure, such as Figure 3 As shown in the figure, the photocathode and the fluorescent screen are naturally focused due to their close proximity, and no additional electron focusing system is required. In addition, a microchannel plate (MCP) with a thickness of only microns is usually used as the electron multiplication system. The distance between the photocathode and the MCP, and the distance between the MCP and the fluorescent screen are also in the micron range, with a simple structure and a small size.

[0006] Na2KSb photocathode is a key component of low-light-level image intensifier, which significantly affects the quantum efficiency and working band of low-light-level image intensifier. The traditional Na2KSb photocathode is a planar thin film type, such as Figure 4 shown. Summary of the Invention

[0007] In response to the above-mentioned defects in the prior art, the present invention proposes a nanopillar array Na2KSb photocathode with enhanced light absorption, which is achieved through the following technical solutions:

[0008] A Na2KSb photocathode with a nanopillar array for enhanced light absorption comprises: a SiO2 window layer, a double-layer TiO2 two-dimensional nanograting layer, and a Na2KSb photocathode layer, wherein the double-layer TiO2 two-dimensional nanograting layer comprises a TiO2 nanopillar array located on one side of the SiO2 window layer and a TiO2 nanohole array located on the other side, the TiO2 nanopillar array being covered by the SiO2 window and coaxially and conformally connected to the Na2KSb photocathode layer; the Na2KSb photocathode layer is supported by the TiO2 nanohole array to form a nanopillar array structure; wherein the TiO2 nanopillar array has a period of 0.8 μm, a diameter of 0.32 μm, and a height of 0.8 μm; the Na2KSb photocathode layer has a thickness of 0.15 μm, and the total thickness of the double-layer TiO2 two-dimensional nanograting layer is 0.15 μm.

[0009] Preferably, the TiO2 nanocolumn array and the Na2KSb photocathode layer are arranged in a hexagonal periodic arrangement, and the diameters D1, D2, and D3 of the TiO2 nanocolumn array are all 0.32 μm, and the heights H1, H2, and H3 are 0.8 μm, 0.075 μm, and 0.075 μm.

[0010] Preferably, the period of the TiO2 nanopore array is 1 / 4 of the period of the TiO2 nanocolumn array. times, and the diameter of the TiO2 nanopore matches the diameter of the TiO2 nanocolumn to achieve the formation of a refractive index gradient layer.

[0011] The present invention also provides a low-light-level image intensifier, comprising any one of the nanorod array Na2KSb photocathode described above, wherein the photocathode is coupled to a fluorescent screen via a vacuum electron multiplication system to form an enhanced photoelectric device.

[0012] The present invention also provides a method for preparing the nanopillar array Na2KSb photocathode according to claim 1, comprising the following steps:

[0013] a) forming a TiO2 nanocolumn array on the SiO2 window layer by nanoimprinting or electron beam etching technology;

[0014] b) forming a TiO2 nanohole array on the surface of the TiO2 nanocolumn array by anodic oxidation or chemical vapor deposition to form a double-layer TiO2 grating layer;

[0015] c) depositing Na2KSb material on the TiO2 nanopore array by molecular beam epitaxy or magnetron sputtering to form a nanocolumn array Na2KSb photocathode layer;

[0016] d) performing surface passivation treatment on the photocathode layer to suppress photoelectron recombination.

[0017] Preferably, the period, diameter and height of the TiO2 nanocolumn array are precisely controlled by adjusting the nanoimprint template or electron beam exposure parameters, and the thickness of the Na2KSb layer is regulated by deposition time or sputtering power.

[0018] The present invention also provides a low-light-level night vision imaging system, comprising the above-mentioned low-light-level image intensifier. The system also includes an objective lens, an eyepiece, a high-voltage power supply and a housing. The average absorption rate of the absorption spectrum of the photocathode in the 400-1000nm band is ≥77.85%.

[0019] This invention utilizes a TiO2 / SiO2 interface grating design to reduce reflectivity and a nanopillar structure to increase optical path length, significantly improving light absorption across a wide band of 400-1000nm. Experimental data shows that this structure achieves a 40.27% increase in absorption in the near-infrared band of 800-1000nm compared to conventional planar thin-film devices, and increases quantum efficiency by approximately 32.28%. This proposed three-dimensional nanostructure addresses the insufficient near-infrared absorption of conventional photocathodes without increasing material thickness, making it suitable for high-performance low-light-level night vision devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The relationship between the low-light-level image intensifier and the low-light-level imaging system;

[0021] Figure 2 This is the working principle diagram of the low-light-level image intensifier;

[0022] Figure 3 Schematic diagram of the structure of a close-fitting image intensifier;

[0023] Figure 4 It is the optical effect of incident light on the planar SiO2-photocathode interface;

[0024] Figure 5 (a) Optical properties of a Na2KSb photocathode with a thickness of t = 150 nm and (b) its inverted refractive index;

[0025] Figure 6 Figure 3. Structure and parameters of the nanopillar array Na2KSb photocathode: (a) Top view of the nanopillar array photocathode, the inset on the lower right shows the back view; (b) xz cross-section of the nanopillar array photocathode; (c) TiO2 nanopillar array in the SiO2 window; (d) Na2KSb nanopillar array in the TiO2 layer; (e) xy cross-section of the vacuum nanohole in the Na2KSb layer;

[0026] Figure 7are the real (n) and imaginary (k) parts of the refractive index of TiO2 material;

[0027] Figure 8 This is a comparison of the reflectivity of the grating-free photocathode and the TiO2 / SiO2 interface grating photocathode;

[0028] Figure 9 Comparison of absorption spectra of planar thin film and nanopillar array Na2KSb photocathodes;

[0029] Figure 10 The electric field distribution diagram of planar thin film and nanocolumn array Na2KSb photocathodes. DETAILED DESCRIPTION

[0030] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments. Obviously, the embodiments described are only some examples of the present invention, not all examples. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without expending creative work are within the scope of protection of the present invention.

[0031] This embodiment provides a light response process of a low-light-level image intensifier, which involves the following seven main steps:

[0032] Step 1: Light input and initial interaction: the number is N ph The photons are incident from the optical window made of SiO2, and part of the incident light is emitted with a certain probability ( Figure 4 The reflectivity Ref in the photocathode is reflected at the SiO2-Na2KSb photocathode interface, and a part of the incident light is reflected with a certain probability ( Figure 4 Absorption rate Abs) enters the photocathode layer with thickness t and is absorbed. The remaining incident light is absorbed with a certain probability ( Figure 4 The transmittance Trans) in is transmitted.

[0033] Step 2: Photoelectron generation: The valence band electrons of the photocathode are excited to the conduction band after absorbing incident photons with energy greater than the band gap of the photocathode material. They are excited to the conduction band with a certain probability (transition probability) to form photoelectrons. The generation efficiency of electron-hole pairs QE is used. gen describe.

[0034] Step 3, photoelectron transport: Photoelectrons are transported to the emission surface of the photocathode through diffusion or drift effect. In this process, photoelectrons lose some energy due to lattice defects and collisions with atoms. The transport efficiency QE is used to express the energy loss. trans describe.

[0035] Step 4: Photoelectron emission: When the photoelectron reaches the emission surface, it escapes into the vacuum through the surface potential barrier. The electron escape efficiency QEel describe.

[0036] Steps ②, ③, and ④ are called the "three-step model" of the photocathode, and each step has its own quantum efficiency, or QE gen , QE trans and QE el The product of the quantum efficiencies of the three steps multiplied by the absorptivity Abs of the photocathode is the external quantum efficiency (EQE) of the photocathode:

[0037]

[0038] Where N ph is the number of incident photons, N el is the number of photoelectrons escaping from the photocathode into the vacuum.

[0039] Step 5, electron multiplication: The photoelectrons that escape into the vacuum move toward the MCP input end under the action of the cathode electric field (the electric field between the photocathode and the MCP input end) and enter the channel of the microchannel plate. Under the action of the electric field inside the channel (the electric field between the MCP input end and the output end), the photoelectrons move from the input end to the output end. In this process, electrons continuously collide with the inner wall of the channel and generate secondary electrons. Since the secondary electron emission coefficient of the inner wall of the channel is greater than 1, when electrons are output from the MCP output end, the number of output electrons far exceeds the number of input electrons, thus achieving about 10 5 times the electron multiplication.

[0040] Step 6: Photoelectron display and image formation: The multiplied photoelectrons are incident on the fluorescent screen located at the anode, and the photoelectrons are displayed as optical images through "electro-optical" conversion.

[0041] Step 7, optical coupling: The optical image is projected into the eye through the eyepiece of the night vision device for viewing, or coupled with the CCD / CMOS imaging device to become an enhanced CCD / CMOS, referred to as ICCD / ICMOS.

[0042] As can be seen from the above process, increasing the photocathode's absorptivity (Abs) is an important way to improve the photoresponse sensitivity of a low-light-level image intensifier by minimizing light energy loss. There are two main methods for increasing the photocathode's absorptivity: 1. Using a multilayer thin-film structure to reduce the reflectivity of the SiO₂-Na₂KSb interface; 2. Increasing the thickness of the Na₂KSb layer to increase the light transmission path (i.e., increasing the optical path). Therefore, by analyzing the optical properties of the Na₂KSb photocathode in a low-light-level image intensifier, the effectiveness of these two methods for increasing absorptivity and their application limitations are discussed.

[0043] like Figure 5Figure (a) shows the optical properties of a Na2KSb photocathode of a certain type of low-light-level image intensifier with a thickness of t = 150nm. It can be seen that: in the 400-1000nm band, the average absorptivity of the Na2KSb photocathode is only 45.58%, especially in the long-wave band, where it decreases with increasing wavelength; the energy loss of reflected light in the 400-600nm band is dominant, with an average reflectivity of about 27.28%; the energy loss of transmitted light in the 600-1000nm band is dominant, with an average transmittance of about 45.80%; it is obvious that the energy loss of light is large when it is transmitted in the photocathode, especially in the near-infrared band of 750-1000nm, most of which is transmitted.

[0044] From the above optical properties, it can be seen that the anti-reflection method can increase the light absorption rate in the 400-800nm ​​band, with the maximum increase of about 20% on average; however, since the imaginary part of the refractive index of the Na2KSb material decreases with increasing wavelength, Figure 5 As shown in (b), this method can hardly increase the light absorption rate in the 800-1000nm band.

[0045] Increasing the optical path length can increase light absorptivity in the 450-1000nm band, with the maximum increase in average absorption being approximately 30% in the 450-800nm ​​band and approximately 75% in the 800-1000nm band. However, since a 150nm thickness provides a long enough optical path length for the 400-450nm band, resulting in near-zero transmittance, increasing the optical path length will not improve light absorptivity in this band. Furthermore, increasing the thickness of the Na2KSb layer increases the distance photoelectrons travel to the emitting surface, thereby increasing the probability of recombination with holes and reducing the photoelectron transport efficiency. The "three-step model" shows that while absorptivity increases, photoelectron transport efficiency decreases accordingly. Therefore, increasing the optical path length does not generally improve the quantum efficiency of the photocathode.

[0046] Therefore, based on the above analysis, this patent innovatively proposes a nanocolumn array photocathode structure with enhanced light absorption, which increases the optical path of light in the 450-900nm band in the photocathode and reduces the reflectivity in the 400-600nm band without increasing the thickness of the Na2KSb photocathode, ultimately achieving the photocathode's absorption of light in a wide band of 400-1000nm.

[0047] This patent innovatively proposes a nanopillar array Na2KSb photocathode structure, as shown in the schematic diagram. Figure 6As shown, the device consists of three main components: a SiO2 window, a double-layer TiO2 two-dimensional nanograting, and a nanopillar array Na2KSb photocathode layer. The TiO2 two-dimensional nanograting consists of a TiO2 nanopillar array on one side of the SiO2 layer and a TiO2 nanohole array on the opposite side of the TiO2 layer. The TiO2 nanopillar array is covered by the SiO2 window. This configuration facilitates the conformal deposition of Na2KSb to form a nanopillar array Na2KSb photocathode. The nanopillars and nanoholes are coaxially arranged with a hexagonal period (period_x, period_y), pillar diameters (D1, D2, and D3), pillar heights (H1, H2, H3), TiO2 layer thickness (anti_H), and Na2KSb photocathode thickness (cath_H).

[0048] according to Figure 6 The geometric parameters shown in the figure set the thickness of the Na2KSb photocathode to a fixed value of 150 nm. The geometric parameters of the nanopillar array Na2KSb photocathode structure designed in this patent are shown in Table 1.

[0049] Table 1 Geometric parameters of the nanopillar array Na2KSb photocathode structure designed in this patent

[0050]

[0051]

[0052] Depend on Figure 5 (b) It can be seen that the average refractive index of the Na2KSb photocathode in the 400-800nm ​​band is about 3.3, and the average refractive index of SiO2 is 1.45. The large refractive index difference will produce a large reflection at the SiO2-photocathode interface. In order to reduce the reflectivity in the 400-600nm band and increase the photocathode absorptivity, this patent introduces a TiO2 micro-nano grating as a refractive index gradient layer to reduce the refractive index difference at the SiO2-photocathode interface, thereby reducing the reflectivity at the interface. According to the equivalent medium theory, the dielectric grating can be used as an anti-reflection layer to reduce the reflectivity. The equivalent refractive index of the dielectric grating is controlled by changing the material, duty cycle and filling of the grating groove, and finally a gradient layer with gradually increasing refractive index of SiO2 / TiO2 grating layer / photocathode is formed. TiO2 is a common optical material. The average value of the real part of its refractive index in the range of 400-800nm ​​is 2.2, which is just between 1.45 and 3.3, which is conducive to the formation of a refractive index gradient layer. In addition, the imaginary part of the refractive index of TiO2 in the range of 400-600nm is almost 0, and there is almost no absorption of photons in this band, which is conducive to the entry of photons into the photocathode. The real and imaginary parts of the refractive index of TiO2 material are as follows: Figure 7 shown.

[0053] Figure 8 This is a comparison chart of the reflectivity of a gratingless photocathode and a TiO2 / SiO2 interface nanopillar array photocathode. As can be seen from the figure, within the 400-700nm band, the average reflectivity of the TiO2 / SiO2 interface nanopillar array photocathode decreased from 24.37% to 8.62%, a decrease of 15.75%. Within the 700-1000nm band, the average reflectivity increased from 3.14% to 5.43%, a change of only 2.3%. In summary, within the 400-1000nm range, the TiO2 / SiO2 interface nanopillar array photocathode can significantly reduce the spectral reflectivity, which facilitates the entry of photons into the photocathode for absorption.

[0054] Figure 9 The absorption spectra of the planar thin film type and the nanopillar array Na2KSb photocathode were compared. Compared with the planar thin film type, the nanopillar array Na2KSb photocathode of this patent achieves excellent absorption. Figure 9 It can be seen that at 400-1000nm, the average absorptivity of the nanopillar-type photocathode increased from 45.57% for the planar thin-film type to 77.85%, an average increase of approximately 32.28%, a significant increase in absorptivity. In particular, in the near-infrared band of 800-1000nm, the average absorptivity of the nanopillar-type photocathode increased from 30.08% for the planar thin-film type to 70.35%, an average increase of approximately 40.27%. Therefore, the performance of this nanopillar array Na2KSb photocathode far exceeds that of a planar thin-film photocathode.

[0055] Figure 10 The electric field distribution of planar thin film and nanopillar array Na2KSb photocathodes is shown. Figure 10 (a)-(c) are the electric field distribution diagrams of the planar thin-film photocathode at 400nm, 800nm, and 1000nm, respectively. The photocathode is at 0-0.15μm on the z-axis in the figure, the SiO2 optical window is at 0.15-0.5μm, and the light source is at 2μm. Therefore, the spectral reflection region is greater than 2μm, and the spectral transmission region is less than 0μm. From the electric field energy distribution diagram, in the transmission region, the energy at 400nm is almost 0, and the energy at 800nm ​​and 1000nm is not 0, indicating that some light in the near-infrared part is transmitted out; in the reflection region, the reflection energy at 400nm and 1000nm is high, and the reflection energy at 800nm ​​is low, indicating that the reflectivity is higher near the ultraviolet region and the near-infrared region, which is consistent with the Figure 5 (a) is consistent with the pattern shown.

[0056] Figure 10(d)-(f) are electric field distribution diagrams of the nanopillar photocathode at 400nm, 800nm, and 1000nm, respectively. The photocathode is located at 0-0.3μm on the z-axis, the SiO2 / TiO2 micro-nano grating is located at 0.3-1.22μm, the SiO2 optical window is located at 1.22-1.37μm, and the light source is located at 2μm. Therefore, the spectral reflection range is greater than 2μm, while the spectral transmission range is less than 0μm.

[0057] From the electric field energy distribution diagram, in the transmission area, the energy at 400nm is almost 0, and the energy at 800nm ​​and 1000nm is not 0, indicating that some light in the near-infrared part is transmitted out, but it is very small; in the reflection area, the reflection energy at 400nm, 800nm ​​and 1000nm is low, indicating that the reflectivity is low in the entire band, which shows that the nanocolumn array structure is beneficial to reducing the reflectivity of the photocathode in the entire band and the transmittance of the near-infrared spectrum, thereby improving the absorption rate of the photocathode, which is consistent with the above. Figure 9 The rules shown are consistent.

[0058] In addition, within the same band, e.g. Figure 10 (a) and (d), Figure 10 (b) and (e), Figure 10 (c) and (f) show that the nanopillar array structure allows photons to enter the photocathode through the nanopillar array slits. As shown in the energy diagram in the figure, the maximum energy of the nanopillar photocathode is 3.85, while the maximum energy of the planar thin-film photocathode is 1.58. The maximum energy of the nanopillar photocathode is 2.4 times that of the planar thin-film photocathode. Therefore, the nanopillar photocathode can better concentrate the energy of the incident light, which is beneficial to improving the quantum efficiency of the photocathode.

[0059] In summary, the nanopillar array Na2KSb photocathode can, on the one hand, reduce the reflectivity across the entire wavelength range by providing a graded refractive index layer, and on the other hand, increase the optical path without changing the thickness of the photocathode, thereby reducing the transmittance in the near-infrared band. Therefore, compared with traditional thin-film Na2KSb photocathodes, the nanopillar array Na2KSb photocathode can effectively improve the absorptivity across the entire wavelength range, thereby increasing the quantum efficiency of the photocathode.

[0060] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A nanopillar array Na2KSb photocathode with enhanced light absorption, characterized in that: include: A SiO2 window layer, a double-layer TiO2 two-dimensional nanograting layer and a Na2KSb photocathode layer, wherein the double-layer TiO2 two-dimensional nanograting layer is composed of a TiO2 nanocolumn array located on one side of the SiO2 window layer and a TiO2 nanohole array located on the other side, the TiO2 nanocolumn array is covered by the SiO2 window, and the TiO2 nanocolumn array is coaxially conformally connected to the Na2KSb photocathode layer; the Na2KSb photocathode layer is supported by the TiO2 nanohole array to form a nanocolumn array structure; wherein the TiO2 nanocolumn array has a period of 0.8 μm, a diameter of 0.32 μm and a height of 0.8 μm; the thickness of the Na2KSb photocathode layer is 0.15 μm, and the total thickness of the double-layer TiO2 two-dimensional nanograting layer is 0.15 μm.

2. The nanorod array Na2KSb photocathode according to claim 1, characterized in that: The TiO2 nanocolumn array and the Na2KSb photocathode layer are arranged in a hexagonal periodic arrangement, and the diameters D1, D2, and D3 of the TiO2 nanocolumn array are all 0.32 μm, and the heights H1, H2, and H3 are 0.8 μm, 0.075 μm, and 0.075 μm.

3. The nanorod array Na2KSb photocathode according to claim 1, characterized in that: The period of the TiO2 nanopore array is 1 / 4 of the period of the TiO2 nanocolumn array. times, and the diameter of the TiO2 nanopore matches the diameter of the TiO2 nanocolumn to achieve the formation of a refractive index gradient layer.

4. A low-light-level image intensifier, characterized in that: A Na2KSb photocathode comprising the nanocolumn array according to any one of claims 1 to 3, wherein the photocathode is coupled to a fluorescent screen via a vacuum electron multiplication system to form an enhanced photoelectric imaging device.

5. A method for preparing the nanorod array Na2KSb photocathode according to claim 1, characterized in that: The following steps are involved: a) forming a TiO2 nanorod array on the SiO2 window layer by nanoimprinting or electron beam etching technology; b) forming a TiO2 nanohole array on the surface of the TiO2 nanorod array by anodic oxidation or chemical vapor deposition to form a double-layer TiO2 grating layer; c) depositing Na2KSb material on the TiO2 nanopore array by molecular beam epitaxy or magnetron sputtering to form a nanocolumn array Na2KSb photocathode layer; d) performing surface passivation treatment on the photocathode layer to suppress photoelectron recombination.

6. The method according to claim 5, characterized in that The period, diameter and height of the TiO2 nanocolumn array are precisely controlled by adjusting the nanoimprint template or electron beam exposure parameters, and the thickness of the Na2KSb layer is regulated by deposition time or sputtering power.

7. A low-light-level night vision imaging system, characterized in that: The system comprises the low-light-level image intensifier according to claim 4, wherein the system further comprises an objective lens, an eyepiece, a high-voltage power supply and a housing, and the average absorption rate of the absorption spectrum of the photocathode in the 400-1000 nm band is ≥77.85%.

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