A method for improving specific band quantum efficiency of a streak image converter tube and a streak image converter tube
By integrating a dielectric-metal composite metasurface structure into the incident window of the photocathode in the striped image converter tube, the problem of insufficient quantum efficiency of the S20 photocathode in the 480nm band was solved, and the quantum efficiency and integral sensitivity were significantly improved, meeting the diagnostic requirements of inertial confinement fusion experiments.
Patent Information
- Application Number
- CN202610712398.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-05-22
- Publication Date
- 2026-08-25
AI Technical Summary
The S20 photocathode of the striped image converter tube has insufficient quantum efficiency in the target wavelength band of 480nm, which makes it difficult to meet the diagnostic requirements of inertial confinement fusion experiments.
A dielectric-metal composite metasurface structure was integrated on the photocathode incident window of a striped image converter tube. The subwavelength periodic circular aperture array was optimized by rigorous coupled-wave analysis. The metasurface structure was prepared by combining nanoimprint lithography and reactive ion etching techniques, and an alumina thin film and an S20 photocathode layer were deposited.
The quantum efficiency and integral sensitivity in the 480nm band were significantly improved. The quantum efficiency increased from 14.34% to 19.09%, a relative increase of about 33%, and the integral sensitivity increased from 170μA/lm to 289μA/lm, enhancing the detection sensitivity.
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Figure CN122638403A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of fusion diagnostics and high-energy physics ultrafast diagnostic devices, specifically to a method for improving the quantum efficiency of a stripe image converter tube and a stripe image converter tube, and more particularly to a method for improving the quantum efficiency of a stripe image converter tube in a specific band based on a cathode metasurface structure and a stripe image converter tube. Background Technology
[0002] In diagnostics of extreme physics experiments such as inertial confinement fusion (ICF), streak cameras are core instruments for achieving picosecond to femtosecond time-resolved measurements. ICF experimental environments are accompanied by strong background noise from X-rays and plasma self-luminescence, requiring diagnostic systems to possess a high signal-to-noise ratio to extract weak characteristic signals. The detection sensitivity of streak cameras is limited by the quantum efficiency and spectral response characteristics of their photocathodes. In ICF diagnostics, the blue-green band around 480 nm is typically selected as a critical diagnostic window to balance the resolution of specific physical signals with the avoidance of continuous background noise. Therefore, improving the quantum efficiency of the photocathode in this target band is a direct way to overcome the current limitations in detection capabilities.
[0003] Traditional streak image converter tubes (ABS) typically employ S20 multi-alkali photocathodes, which have a typical quantum efficiency of approximately 14% at 480 nm and an integral sensitivity of about 170 μA / lm, making it difficult to meet the growing diagnostic demands. Although performance can be improved by optimizing the cathode material ratio and activation process, the improvement is limited. In recent years, the guided-mode resonance effect based on subwavelength periodic structures has provided a new technical path for enhancing photocathode absorption. However, there is still a lack of mature design methods and fabrication schemes for applying this type of structure to the photocathode of streak image converter tubes to achieve a significant improvement in quantum efficiency at specific wavelengths. Summary of the Invention
[0004] The technical problem to be solved by this invention is: addressing the current situation of insufficient quantum efficiency of the S20 photocathode in the target wavelength band of the striped image converter tube, and providing a method to improve the quantum efficiency in a specific wavelength band by introducing a cathode metasurface structure and utilizing the guided mode resonance effect, as well as the striped image converter tube prepared by the method of this invention.
[0005] The present invention aims to provide a method for improving the quantum efficiency of a striped image converter tube (SEM) in a specific wavelength band based on a cathode metasurface structure, and the SEM itself. The method includes: integrating a dielectric-metal composite metasurface structure onto the photocathode incident window of the SEM; the metasurface structure consists of a subwavelength periodic circular aperture array fabricated on the surface of the silica window and dielectric pillars filling each aperture; the aperture array is distributed in a hexagonal grid pattern. The geometric parameters of the metasurface structure are optimized according to the target enhanced wavelength band, and its fabrication steps include:
[0006] a) Metasurface structure design and optimization: The electromagnetic simulation of the metasurface structure is carried out using the rigorous coupled-wave analysis method (RCWA). The geometric parameters of the metasurface structure are determined with the light absorption rate in the target band as the optimization target.
[0007] b) Metasurface structure preparation: A master template is prepared by electron beam lithography, and a subwavelength circular hole array structure is fabricated on a silicon dioxide window substrate by nanoimprint lithography combined with reactive ion etching.
[0008] c) Deposition of dielectric and cathode layers: An alumina film is deposited on the structure obtained in step b), followed by the deposition of an S20 photocathode layer under vacuum conditions;
[0009] d) Device integration: The photocathode assembly prepared in step c) is vacuum integrated and assembled with the deflection plate and fluorescent screen of the stripe image converter tube.
[0010] Preferably, the array of circular holes in the metasurface structure is distributed in a hexagonal grid pattern.
[0011] Preferably, the geometric parameters of the metasurface structure are: period P: 500-650nm, diameter of the circular hole D: 200-350nm, thickness of the metasurface layer H3: 450-700nm, thickness of the alumina dielectric layer H2: 100-150nm, thickness of the S20 photocathode layer H1: 60-90nm, and thickness of the silicon dioxide window H4: 450-600μm.
[0012] Preferably, in step a), the thickness H1 of the S20 photocathode layer and the thickness H4 of the silicon dioxide window are kept constant, and the metasurface period P, the diameter D of the circular hole, the thickness H3 of the metasurface layer and the thickness H2 of the alumina dielectric layer are used as optimization variables, with the goal of maximizing the average absorption rate in the 400nm-600nm wavelength range.
[0013] Preferably, in step a), a gradient search or parameter scanning method is used to find the combination of geometric parameters that maximizes the absorption rate at a wavelength of 480nm within a preset parameter range.
[0014] Preferably, in step b), the master pattern is complementary to the target metasurface pattern, and the master is made of hard silicon material and is formed by electron beam lithography.
[0015] Preferably, in step b), the reactive ion etching process controls the etching rate and time to ensure that the sidewall verticality of the resulting circular hole array is greater than 80° and the hole depth and periodic processing deviation are controlled within 10 nm.
[0016] Preferably, in step c), the alumina thin film is deposited by magnetron sputtering, and the film thickness deposition accuracy is controlled within 3 nm.
[0017] Preferably, in step c), the S20 photocathode layer is deposited by thermal evaporation, and the vacuum level of the deposition environment is better than 1×10⁻⁶. -6 Pa.
[0018] Preferably, the method is applicable to S20 multi-alkali photocathodes, and by adjusting the metasurface geometry parameters, the quantum efficiency for different target wavelengths can be improved.
[0019] The beneficial effects of this invention are:
[0020] (1) By integrating a subwavelength periodic metasurface structure on the cathode incident window, the present invention effectively couples the incident light of the 480nm target band to the photoelectric emission layer by utilizing the guided mode resonance effect. The measured quantum efficiency is increased from 14.34% to 19.09%, which is a relative increase of about 33%; the integral sensitivity is increased from 170μA / lm to 289μA / lm, which is a relative increase of about 70%, which significantly enhances the detection sensitivity of the stripe tube in the ICF diagnostic window.
[0021] (2) The metasurface structure parameters used in this invention are determined by rigorous electromagnetic simulation optimization. The fabrication process adopts mature nanoimprint lithography and thin film deposition technology, which has high process controllability, good structural size accuracy, and the deviation between the measured geometric parameters and the design values is within 10nm, ensuring the matching of the guided mode resonance effect at the design wavelength.
[0022] (3) The method described in this invention is applicable to S20 and other multi-alkali photocathode systems. It can improve the quantum efficiency of different target wavelengths by adjusting the metasurface geometric parameters, and has strong design flexibility and application scalability. Attached Figure Description
[0023] Figure 1 The simulated comparison curves of the light absorption rate of planar cathode and metasurface cathode in the 400nm-600nm wavelength range are shown.
[0024] Figure 2 Simulation diagrams of electric field intensity distribution: (a) Schematic diagram of planar cathode structure; (b)-(d) Electric field distribution of planar cathode at 400nm, 480nm, and 600nm; (e) Schematic diagram of metasurface cathode structure; (f)-(h) Electric field distribution of metasurface cathode at 400nm, 480nm, and 600nm.
[0025] Figure 3 Transmittance test and simulation comparison curves of planar structure and metasurface structure before deposition of photocathode: (a) comparison of total transmittance; (b) comparison of zero-order transmittance of metasurface structure.
[0026] Figure 4 The optical performance test results for planar cathodes and metasurface cathodes are as follows: (a) spectral response curves; (b) quantum efficiency curves. Detailed Implementation
[0027] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited to the contents described herein.
[0028] This embodiment provides a method for improving the quantum efficiency of a striped image converter tube in a specific band based on a cathode metasurface structure. The specific steps are as follows:
[0029] Step 1: Metasurface Structure Design and Optimization
[0030] Electromagnetic simulation calculations were performed on the metasurface cathode structure using rigorous coupled-wave analysis. The designed cathode structure consists of four parts, from top to bottom: a silicon dioxide optical window, a metasurface layer, an alumina thin film, and an S20 photocathode layer. The metasurface layer is composed of a hexagonal grid-like array of circular holes etched into the silicon dioxide window, with alumina dielectric pillars filling the holes; where H4 is the thickness of the silicon dioxide window, H3 is the thickness of the metasurface layer, H2 is the thickness of the alumina layer, H1 is the thickness of the S20 photocathode layer, P is the metasurface period, and D is the diameter of the circular holes.
[0031] During the simulation, the thickness of the S20 photocathode layer H1 (60–90 nm) and the thickness of the silicon dioxide window H4 (450–600 μm) were kept constant. The metasurface period P, the diameter of the circular hole D, the thickness of the metasurface layer H3, and the thickness of the alumina layer H2 were used as optimization variables. The optimization was carried out in the 400 nm–600 nm wavelength range with the goal of maximizing the average absorption rate.
[0032] The optimized geometric parameters are as follows: metasurface period P is 550nm, circular hole diameter D is 275nm, metasurface layer thickness H3 is 600nm, alumina layer thickness H2 is 120nm, S20 photocathode layer thickness H1 is 75nm, and silicon dioxide window thickness H4 is 500μm.
[0033] Figure 1 A comparative absorption simulation of planar cathodes and metasurface cathodes in the 400nm-600nm wavelength range is presented. Within the 400nm-520nm range, the average absorptivity of the metasurface structure is 69.92%, an increase of approximately 9.53 percentage points compared to the 60.39% of the planar structure. At the target wavelength of 480nm, the absorptivity increases from 56.42% to 71.65%, a relative increase of approximately 15.23%.
[0034] The absorptivity A is calculated from the reflectivity R and the transmittance T1 and T2 according to the formula A=1-R-T1-T2, where T1 and T2 are the transmittance components of each interface.
[0035] Figure 2The simulation results show the electric field distribution of the two structures at different wavelengths. Figure 2 (a) is a schematic diagram of a planar cathode structure. Figure 2 (b)-(d) show its electric field distribution at 400 nm, 480 nm, and 600 nm, respectively; Figure 2 (e) is a schematic diagram of a metasurface cathode structure. Figure 2 (f)-(h) represent the electric field distribution at the corresponding wavelengths, respectively. Figure 2 As can be seen from (f) to (h), the metasurface structure forms a significant local electric field enhancement within the cathode active layer, proving that the guided mode resonance effect effectively couples the incident light into the photoelectric emission layer, thereby enhancing light absorption.
[0036] Step 2: Preparation of metasurface structures
[0037] First, a hard silicon master pattern is prepared using electron beam lithography, with the pattern on the master pattern being complementary to the pattern on the target metasurface. Then, nanoimprint lithography is used to transfer the master pattern to a photoresist layer on a silicon dioxide window substrate, and finally, reactive ion etching is used to transfer the pattern into the silicon dioxide substrate, forming a hexagonal grid-like array of circular holes.
[0038] During reactive ion etching, by controlling the etching gas ratio, radio frequency power and etching time, the sidewall perpendicularity of the resulting circular hole is greater than 80°, and the processing deviation of hole depth and period is controlled within 10nm.
[0039] The prepared 18mm silica metasurface structure has a measured period of 550nm, a pore size of 280nm, a depth of 610nm, and a sidewall perpendicularity greater than 80°. The deviation of each dimension from the design target is within 10nm.
[0040] Step 3: Deposition of dielectric and cathode layers
[0041] On the aforementioned metasurface structure, an alumina thin film was first deposited by magnetron sputtering with a thickness of 120 nm. The film thickness accuracy was ensured to be within 3 nm by controlling parameters such as sputtering power, time, and target-substrate distance.
[0042] Subsequently, an S20 photocathode layer was deposited under vacuum conditions using a thermal evaporation method, with a deposition thickness of 75 nm. The deposition process was conducted at a vacuum level better than 1 × 10⁻⁶. -6 The process is carried out in a Pa environment to avoid oxidation and contamination of the cathode material. After the cathode layer deposition is completed, in-situ activation treatment is performed in a vacuum environment.
[0043] Step 4: Device Integration and Performance Testing and Verification
[0044] Before completing the photocathode deposition, to verify the consistency between the fabricated structure and the simulation model, the transmittance of the planar structure and the metasurface structure without a cathode layer was measured, and the results are as follows: Figure 3 As shown in (a), in the range of 400nm-600nm, the measured average transmittance of the planar structure is 92.03%, which is 4.48 percentage points lower than the simulated value of 96.51%. The difference is mainly due to non-ideal factors such as actual surface roughness.
[0045] For metasurface structures, multi-order diffraction occurs when the incident wavelength is shorter than the grating period. However, the transmittance testing instrument only collects the zero-order diffracted light, resulting in measured values lower than the simulated total transmittance. Therefore, the zero-order transmittance of the metasurface structure was specifically simulated, such as... Figure 3 As shown in (b), the deviation between the measured average transmittance of 69.56% across the entire band and the simulated zero-order transmittance of 73.50% is only 3.94 percentage points.
[0046] Figure 4 (a) shows the spectral response test curves for the two cathode structures. Across the entire wavelength range, the spectral response of the metasurface cathode is significantly higher than that of the planar cathode, with the spectral response at 480 nm increasing from 55.52 mA / W to 73.91 mA / W, an absolute increase of 18.39 mA / W. Figure 4 (b) shows the quantum efficiency test curve. At 480 nm, the quantum efficiency increased from 14.34% to 19.09%, an absolute increase of 4.75 percentage points and a relative increase of about 33%. The integral sensitivity test results are: 170 μA / lm for planar cathode and 289 μA / lm for metasurface cathode, with a relative increase of about 70%.
[0047] In another batch of three sets of samples, the peak quantum efficiencies at 480 nm were 16.8%, 18.1%, and 19.1%, respectively, with corresponding integral sensitivities of 289 μA / lm, 304 μA / lm, and 348 μA / lm, further verifying the effectiveness of the method and the reproducibility of the process.
[0048] The test results above confirm that the introduction of a cathode metasurface structure can effectively improve the quantum efficiency and integral sensitivity of the striped image converter tube in a specific wavelength band.
[0049] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.
Claims
1. A method for improving the quantum efficiency of a striped image converter tube in a specific wavelength band, characterized in that, Integrating a dielectric-metal composite metasurface structure on the photocathode incident window of a striped image converter tube, the metasurface structure comprising a subwavelength periodic circular aperture array fabricated on the surface of a silicon dioxide window and dielectric pillars filling each aperture, the method includes the following steps: a) Metasurface structure design and optimization: Electromagnetic simulation of metasurface structure is performed using rigorous coupled-wave analysis. The geometric parameters of metasurface structure are determined with the light absorption rate in the target band as the optimization target. b) Metasurface structure preparation: A master template is prepared by electron beam lithography, and a subwavelength circular hole array structure is fabricated on a silicon dioxide window substrate by nanoimprint lithography combined with reactive ion etching. c) Deposition of dielectric and cathode layers: An alumina film is deposited on the structure obtained in step b), followed by the deposition of an S20 photocathode layer under vacuum conditions; d) Device integration: The photocathode assembly prepared in step c) is vacuum assembled with the deflection plate and fluorescent screen of the stripe image converter tube.
2. The method according to claim 1, characterized in that, The array of circular holes in the metasurface structure is distributed in a hexagonal grid pattern.
3. The method according to claim 1, characterized in that, The geometric parameters of the metasurface structure are as follows: period P: 500 nm to 650 nm, diameter of the circular hole D: 200 nm to 350 nm, metasurface layer thickness H3: 450 nm to 700 nm, alumina thin film layer thickness H2: 100 nm to 150 nm, S20 photocathode layer thickness H1: 60 nm to 90 nm, and silicon dioxide window thickness H4: 450 μm to 600 μm.
4. The method according to claim 1, characterized in that, In step a), the thickness H1 of the S20 photocathode layer and the thickness H4 of the silicon dioxide window are kept constant. The metasurface period P, the diameter of the circular hole D, the thickness H3 of the metasurface layer and the thickness H2 of the alumina dielectric layer are used as optimization variables. The optimization objective is to maximize the average absorption rate in the 400nm-600nm wavelength range.
5. The method according to claim 4, characterized in that, In step a), a gradient search or parameter scanning method is used to find the combination of geometric parameters that maximizes the absorption rate at a wavelength of 480nm within a preset parameter range.
6. The method according to claim 1, characterized in that, In step b), the master pattern is complementary to the target metasurface pattern. The master is made of hard silicon material and is fabricated by electron beam lithography.
7. The method according to claim 1, characterized in that, In step b), the reactive ion etching process controls the etching rate and time to ensure that the sidewall verticality of the resulting circular hole array is greater than 80° and the processing deviation of hole depth and period is controlled within 10nm.
8. The method according to claim 1, characterized in that, In step c), the alumina thin film is deposited using magnetron sputtering, with the film thickness controlled within 3 nm; in step c), the S20 photocathode layer is deposited using thermal evaporation, with a deposition environment vacuum level better than 1×10⁻⁶. -6 Pa.
9. The method according to claim 1, characterized in that, The method is applicable to S20 multi-alkali photocathodes, and by adjusting the metasurface geometry parameters, the quantum efficiency of different target wavelength bands can be improved.
10. A stripe image converter tube, characterized in that, Prepared by the method described in any one of claims 1-9.