Cs3Sb Photocathode Research Articles

HERACLES: A high-voltage DC test beamline for high average current photocathodes

We present commissioning results of the High ElectRon Average Current for Lifetime ExperimentS (HERACLES) beamline at Cornell University. 10 mA average beam current at 200 keV was achieved with a Cs3Sb photocathode operated with an Argon laser at 488 nm. HERACLES is intended to be a test facility to improve the robustness of photocathodes developed for high average current beam operation in conjunction with the Cornell photocathode laboratory.

Effects of nanomaterials on Cs3Sb photocathode emission performance

A new approach to preparing the cesium antimonide photocathode has been developed. A tungsten sponge diffusion barrier layer, instead of the conventional nickel pipe, was used as the evaporation source so that a slow and uniform evaporation of the photoemissive materials, whose direction and amount could be well controlled, was realized. Then, the cathode substrate surface was modified with a nanoparticle layer, resulting in better photoemissive material adsorption and higher light absorbance. A new technique of treating the tungsten sponge, which worked as the barrier layer to the diffusion of the photoemissive materials, was studied. The tungsten–copper alloy was subjected to direct high-frequency heating in a vacuum chamber so that the copper in the alloy evaporated rapidly, resulting in smooth and shiny tungsten sponge. In comparison with the conventional method for fabricating the tungsten sponge, in which the copper in the tungsten–copper alloy was removed chemically, the new method removed the copper more completely, took less time, and caused less pollution. By means of direct-current magnetic sputtering, a layer of osmium nanoparticles of 50 nm diameter was deposited on the photocathode substrate surface. Then, a diode tube for the photocathode emission test equipped with a small titanium ion pump, whose pressure was as low as 5 × 10−8 Pa, was developed. A comparative emission performance test was carried out on the photocathode with the substrate surface covered with an osmium film vs that covered with osmium nanoparticles. The highest quantum efficiencies of the stable photoemission from the two photocathodes were 1.80 × 10−3 and 5.13 × 10−3, respectively. Moreover, the light absorptivity of the osmium nanoparticle-coated photocathode was 2.16 times that of the osmium film-coated photocathode, and the quantum efficiency of the former was 2.85 times that of the latter. Therefore, the improvement in the quantum efficiency of the osmium nanoparticle-coated photocathode is mainly attributed to the increase in the light absorptivity.

Prediction model of K<sub>2</sub>CsSb photocathode reflectivity based on LSTM

针对目前K₂CsSb光阴极制备过程中无法预判光阴极生长状态的问题，提出一种基于长短期记忆（LSTM）循环神经网络的K₂CsSb光阴极反射率预测模型。一维原始反射率数据集经过清洗、筛选、序列化等预处理手段后重构为二维数据输入模型。为充分利用反射率数据在时序上高度相关的特性，采用双层LSTM网络提取特征，预测结果通过全连接层输出，以均方误差（MSE）作为模型预测效果的评判标准。实验结果表明，该模型的网络结构合理且在不同数据集下的表现良好，预测准确率可达99.21%。该模型可运用在K₂CsSb光阴极的制作过程中，通过反射率预测值反馈调节工艺参数以趋近目标走势，对提高光阴极性能具有促进作用。

Reduction of surface roughness emittance of Cs3Sb photocathodes grown via codeposition on single crystal substrates

Alkali antimonide photocathodes are capable of generating high brightness electron beams given their high quantum efficiency and low mean transverse energy (MTE). Increasing the brightness of the photoemitted electron beams beyond the current state of the art requires surface control of the photocathode at the atomic scale, since the beam brightness at the cathode is degraded by a rough, inhomogeneous surface. In this work, we grow cesium antimonide photocathodes on single crystal substrates (Al2O3, TiO2, 3C–SiC, and a control sample on Si) and study the resulting surface morphology with scanning tunneling microscopy (STM). We observe dramatic changes in surface morphology across substrates. In particular, we demonstrate 10 times larger island size and significantly reduced roughness on two samples grown on 3C–SiC(100) substrates as compared to samples on other substrates. By computing the local electric fields which these surfaces would generate in an electron accelerator source, we estimate the roughness-induced contribution to MTE. Across samples, the calculated contribution to MTE varies by a factor of 17, and the smallest value computed is 12 meV at an extraction field of 50 MV/m, which is smaller than typical values for alkali antimonides. Additionally, we show that oxidation, commonly encountered in vacuum transfer and in accelerator operation, does not affect the measured surface morphology. Our findings suggest that even in high field environments, the brightness of the photocathodes grown on 3C–SiC will be primarily determined by the material's electronic properties rather than by spurious fields generated by photocathode roughness.

Robust method for broadband efficiency enhancement of electron photocathodes using optical interferences

We demonstrate the key features of an interference cathode using both simulations and experiments. We deposit Cs3Sb photocathodes on Ag to produce an interference enhanced photocathode with 2–5× quantum efficiency (QE) enhancement using a robust procedure that requires only a smooth metal substrate and QE monitoring during growth. We grow both an interference cathode (Ag substrate) and a typical photocathode (Si reference substrate) simultaneously to confirm that the effects are due to optical interactions with the substrate rather than photocathode composition or surface electron affinity differences. Growing the cathodes until the QE converges shows both the characteristic interference peaks during growth and the identical limiting case where the cathode is “infinitely thick,” in agreement with simulations. We also grow a cathode until the QE on Ag peaks and then stop the growth, demonstrating broadband QE enhancement.

The effects of oxygen-induced phase segregation on the interfacial electronic structure and quantum efficiency of Cs3Sb photocathodes.

High-performance photocathodes for many prominent particle accelerator applications, such as x-ray free-electron lasers, cannot be grown in situ. These highly reactive materials must be grown and then transported to the electron gun in an ultrahigh-vacuum (UHV) suitcase, during which time monolayer-level oxidation is unavoidable. Thin film Cs3Sb photocathodes were grown on a variety of substrates. Their performance and chemical state were measured by x-ray photoelectron spectroscopy after transport in a UHV suitcase as well as after O2-induced oxidation. The unusual chemistry of cesium oxides enabled trace amounts of oxygen to drive structural reorganization at the photocathode surface. This reorganization pulled cesium from the bulk photocathode, leading to the development of a structurally complex and O2-exposure-dependent cesium oxide layer. This oxidation-induced phase segregation led to downward band bending of at least 0.36 eV as measured from shifts in the Cs 3d5/2 binding energy. At low O2 exposures, the surface developed a low work function cesium suboxide overlayer that had little effect on quantum efficiency (QE). At somewhat higher O2 exposures, the overlayer transformed to Cs2O; no antimony or antimony oxides were observed in the near-surface region. The development of this overlayer was accompanied by a 1000-fold decrease in QE, which effectively destroyed the photocathode via the formation of a tunnel barrier. The O2 exposures necessary for degradation were quantified. As little as 100 L of O2 irreversibly damaged the photocathode. These observations are discussed in the context of the rich chemistry of alkali oxides, along with potential material strategies for photocathode improvement.

Fabrication of Large-area, Planar, UV Lighting Devices Based on flat Cs3Sb Photocathode Emitters

Planar lighting devices are ideal for many applications. Requirements for planar lighting devices are high luminance and good luminance uniformity, together with low power consumption. In this article, a large-area, planar lighting device with high luminance, excellent luminance uniformity, and controllable luminance characteristic that was successfully demonstrated for visible light emission is discussed. The same device with sufficient UV intensity and tunable intensity for UV light emission is also discussed. The design concept involves a diode panel structure that utilizes photocathodes operated in a reflection mode with an external excitation light source. The excitation light source is in the blue-visible light range. Commercially available phosphors are used as light sources for both visible and UV emissions. The light emission characteristic of the devices is investigated by measuring the optical and the electrical parameters as functions of the applied anode voltage. The results show that the luminance is high for visible light emission and that the intensity is sufficient for UV light emission. In particular, UV emission exhibits a characteristic of cold-UV light with a near-monochromatic spectrum. The optical and the electrical properties of the large-area planar device are good enough for many industrial applications.

Preliminary Systematic Study of the Temperature Effect on the K–Cs–Sb Photocathode Performance Based on the K and Cs Co-Evaporation**Supported by the National Natural Science Foundation of China under Grant No. 11405251, and the Shanghai Municipal Science and Technology Major Project.

It is very important to increase the quantum efficiency (QE) and prolong the lifetime of the photocathode in a variety of applications. We have succeeded in preparing a high QE cesium potassium antimonide (K–Cs–Sb) photocathode by K and Cs co-evaporation in the photocathode preparation facility. In order to better understand the effect of the substrate (photocathode) temperature on the photocathode performance, the photocathode preparation, photocathode performance degradation, photocathode performance recovery and photocathode removal are studied in detail.

Development of a Multialkali Photocathode Dc Gun for a Smith-Purcell Terahertz Free-Electron Laser

We have developed a photocathode dc gun for a compact Smith-Purcell free-electron laser in the terahertz wavelength region. The gun system consists of an alkali antimonide photocathode preparation chamber, a dc gun with a 250 kV-50 mA Cockcroft-Walton high-voltage power supply, and a downstream beamline with a water-cooled beam dump to accommodate a beam power of 5 kW. We fabricated a Cs3Sb photocathode with quantum efficiency of 5.8% at a wavelength of 532 nm and generated a 150 keV beam with current of up to 4.3 mA with a 500 mW laser. A vacuum chamber for the Smith-Purcell free-electron laser has been installed in the downstream beamline. We describe the present status of our work.

Fabrication of a planar UV lighting device utilizing Cs3Sb photocathode emitters

Planar ultraviolet (UV) light sources with uniform intensity are ideal for UV treatments. A newly conceived planar UV lighting device utilizing Cs3Sb mono-alkali antimonide photocathodes as flat electron emitters is presented and analyzed here. The design concept involves a panel device that utilizes photocathodes operated in a reflection mode with an external excitation light source. The excitation light source is in the blue visible light range. Commercially available UV phosphors are used as a light source of desirable UV wavelengths. The light emission characteristics of the device are investigated by measuring the optical parameters as function of applied anode voltage. The UV light emission shows that cold UV light with the spectral characteristics of near-monochromatic distribution are most effective. It also demonstrates that the UV intensity is sufficient for the UV treatment. More importantly, the planar UV lighting device is mercury free. These make them suitable for all UV applications.

Electrical properties of Cs3Sb photocathode emitters in panel device applications

Nonvacuum process technology was used to produce Cs3Sb photocathodes on substrates, and in situ panel devices were fabricated. The performance of the devices was characterized by measuring anode current as functions of the devices' operation times. An excitation light source with 475 nm wavelength was used for the photocathodes. The device has a simple diode structure, providing unique characteristics such as a large gap, vertical electron beam directionality, and resistance to surface contamination from ion bombardment and poisoning by outgassing species. Accordingly, Cs3Sb photocathodes function as flat emitters, and the emission properties of the photocathode emitters depend on the vacuum level of the devices. Improvement of current stability has been observed after conducting the electrical conditioning process to remove possible adsorbates on Cs3Sb flat emitters.

Fabrication and electrical characterization of planar lighting devices with Cs3Sb photocathode emitters

Non-vacuum process technology was used to produce Cs3Sb photocathodes on substrates, and in-situ panel devices were fabricated. The performance of the devices was characterized by measuring the anode current as functions of the devices’ operation times. An excitation light source with a 475-nm wavelength was used for the photocathodes. The device has a simple diode structure, providing unique characteristics such as a large gap, vertical electron beam directionality, and resistance to surface contamination from ion bombardment and poisoning by outgassing species. Accordingly, Cs3Sb photocathodes function as flat emitters, and the emission properties of the photocathode emitters depend on the vacuum level of the devices. An improved current stability has been observed after conducting an electrical conditioning process to remove possible adsorbates on the Cs3Sb flat emitters.

Kinetics of alkali-based photocathode degradation

We report on a kinetic model that describes the degradation of the quantum efficiency (QE) of Cs3Sb and negative electron affinity (NEA) GaAs photocathodes under UHV conditions. In addition to the generally accepted irreversible chemical change of a photocathode’s surface due to reactions with residual gases, such as O2, CO2, and H2O, the model incorporates an intermediate reversible physisorption step, similar to Langmuir adsorption. This intermediate step is needed to satisfactorily describe the strongly non-exponential QE degradation curves for two distinctly different classes of photocathodes –surface-activated and “bulk,” indicating that in both systems the QE degradation results from surface damage. The recovery of the QE upon improvement of vacuum conditions is also accurately predicted by this model with three parameters (rates of gas adsorption, desorption, and irreversible chemical reaction with the surface) comprising metrics to better characterize the lifetime of the cathodes, instead of time-pressure exposure expressed in Langmuir units.

Experiments for improving fabrication, recovery and surface-protection of Cs3Sb photocathode

We examined 1) the photocurrent from Cs3Sb photocathode as a function of anode voltage below 200V, 2) the relationship between the quantum efficiency of photoemission and the conditions for fabrication by the sandwich method, 3) recovery of the photoemission by additional Cs deposition, and 4) the effects of surface protection of Cs3Sb photocathodes by WO3 and Cr2O3 films in the passive state. The photocurrent had a maximum at approximately 68V except when we increased the anode voltage extraordinarily slowly. Cs3Sb photocathodes were fabricated by increasing the temperature of sandwiched layers of Sb, Cs and Sb deposited on the fine tips of eight cathodes at less than −12°C. Cs3Sb photocathodes having higher quantum efficiency were fabricated by smoothly increasing the temperature of the layers quickly after we deposited the second Sb layer. The photocurrent from the Cs3Sb photocathodes was significantly higher when Cs was deposited at temperatures of 50–70°C. Deposition of a one- to three- atomic-layer W or Cr film extended the photoemission lifetime after the layers were oxidized to WO3 or Cr2O3 in the passive state due to oxidation. The WO3 or Cr2O3 in the passive state provided more surface protection as their thickness increased.

Cold electron beams from cryocooled, alkali antimonide photocathodes

In this letter we report on the generation of cold electron beams using a Cs3Sb photocathode grown by co-deposition of Sb and Cs. By cooling the photocathode to 90 K we demonstrate a significant reduction in the mean transverse energy validating the long standing speculation that the lattice temperature contribution limits the mean transverse energy or thermal emittance near the photoemission threshold, opening new frontiers in generating ultra-bright beams. At 90 K, we achieve a record low thermal emittance of 0.2 $\mu$m (rms) per mm of laser spot diameter from an ultrafast (sub-picosecond) photocathode with quantum efficiency greater than $7\times 10^{-5}$ using a visible laser wavelength of 690 nm.

Fabrication of a planar light source utilizing monoalkali antimonide photocathodes as emitters

This paper presents a new concept for a planar light source utilizing Cs3Sb photocathodes as flat electron emitters. The design concept of the devices with a diode structure was that the photocathodes were to be operated in a reflective mode. Commercially available P-22 phosphors were used as a light source of desirable wavelengths. Light emission from the devices was induced by photoemitted electrons, accelerated by an anode electric field that was shielded from the photoemitter surface. The luminescent characteristics of the devices were investigated by measuring the optical parameters as functions of applied anode voltages. The results showed the possibility to produce a more directable and easily controllable stream of light. These make the devices suitable for a variety of planar lighting applications.

Electron emission properties of Cs3Sb photocathode emitters in a panel device

A Cs3Sb photocathode was deposited on a flat metal surface using a nonvacuum processing technique and in-situ panel devices were fabricated. The technique does not require ultrahigh vacuum conditions as in conventional techniques. The electrical characteristics of the panel devices were investigated by measuring the anode current as functions of devices operation times. Blue light with a 450 nm wavelength was used as an excitation source. The result shows that the Cs3Sb photocathode emitter has the potential of a long lifetime with stable electron emission characteristics in the panel making it a suitable cathode material for panel devices.

Lifetime test of photoemission from Cs3Sb photocathode coated with W or Cr film

Cs3Sb photocathodes were fabricated at 8–16°C with sandwiched layers of Sb, Cs, and Sb deposited onto the fine tips of three cathodes at 15°C. After examining the influence of the cathode tip temperature on the changes in the quantum efficiencies of the Cs3Sb photocathodes during and after additional Cs depositions, we performed lifetime tests of the three Cs3Sb photocathodes using a 405-nm semiconductor laser and 488-nm Ar ion laser. The decrease in the photocurrent with time was more rapid with the 488-nm laser irradiation than with the 405-nm laser irradiation, and continuous laser irradiation caused a much more rapid decrease in photoelectrons with time than intermittent laser irradiation did. We deposited a 0.32–0.64-nm-thick W film or a 0.43-nm-thick Cr film onto the Cs3Sb photocathode during the lifetime test at 0.9–1.0×10−7Pa. We found that the passive WO3 or Cr2O3 film, which was formed at a reduced vacuum level of 1.6×10−7Pa or during the continuous 405-nm laser irradiation, increased the lifetime of the Cs3Sb photocathode by effectively protecting its surface against oxidation and evaporation of Cs. This protection effect was most effective at approximately 90°C.

Thermal emittance and response time of a cesium antimonide photocathode

Measurements of the intrinsic emittance and response time of a Cs3Sb photocathode are presented. The emittance is obtained with a solenoid scan technique using a high voltage dc photoemission gun. Photoemission response time is evaluated using a RF deflecting cavity synchronized to a picosecond laser pulse train. We find that Cs3Sb has both small mean transverse energy, 160 ± 10 meV at 532 nm laser wavelength, and a prompt response time (below the resolution of our measurement) making it a suitable material for high brightness electron photoinjectors.

Ion-induced secondary electron emission from K–Cs–Sb, Na–K–Sb, and Cs–Sb photocathodes and its relevance to the operation of gaseous avalanche photomultipliers

The operation of visible-sensitive gaseous and to some extent, vacuum-photomultipliers is critically affected by secondary electrons originating from ion impact on the photocathode. A simple method for indirect measurement of the effective ion-induced secondary-electron emission (IISEE) coefficient from the photocathode into a gas medium γ+eff was developed. The experimental results with visible-sensitive K–Cs–Sb, Na–K–Sb, and Cs–Sb photocathodes, yielded γ+eff-values between 0.02 and 0.03 in Ar/CH4(95/5) at 700 mbar; these are in good agreement with theoretical calculations. The corresponding vacuum IISEE coefficients γ+ were estimated, based on a theoretical model, to be 0.47, 0.49, and 0.47 for K–Cs–Sb, Na–K–Sb, and Cs–Sb photocathodes, respectively. The ratio of gas γ+eff and vacuum γ+ IISEE coefficients, calculated to be ∼0.06, is the fraction of secondary electrons surmounted the backscattering in the gas media.