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In this paper, COMSOL multi-physics field commercial software was used to design the simulation model of GaAs nanostructures array (vertical nanoholes, vertical nanowires, inclined nanoholes and inclined nanowires), and the changes of light absorption of these structures in the wavelength range of 200–840 nm were studied. The electric field distribution, carrier distribution and quantum efficiency of nanostructures are calculated and analyzed under certain structural parameters. The results show that the light absorption performance of nanowire structure in the short-wave region is better than that of the nanoholes structure. With a certain inclined angle, the inclined nanowire structure has a stronger light capture ability than the vertical nanowire structure, and the light absorption of nanowire structure has a minimum value at the 550 nm wavelength.

The built-in electric field of exponential doping can promote the concentration of the photogenerated carrier center to the top surface of the nanowire. The external electric field also can bend the motion trajectory of the emitted electrons toward the collecting side. These field-assisted methods promote the quantum efficiency. In this paper, the emission theory of a single GaN nanowire photocathode is studied for the first time. The effects of height and width of the nanowire, wavelength, intensity of electric field on quantum efficiency of uniformly doped or exponentially doped GaN nanowire photocathodes were explored. It shows that the top of the exponentially doped cathode has a higher quantum efficiency than uniformly doped cathode. With the absence of the field, quantum efficiency of a uniformly doped cathode reaches a maximum value of 55.29% when the width is 150 nm and the wavelength is 220 nm. The form of exponentially doped cathode can generate an internal field. With the internal field, a maximum value rises 56.73% when the height is 900 nm and the wavelength is 230 nm. The theoretical results can direct the preparation of this kind of photocathode.

To enhance the near-infrared response of photocathodes in the application of image detection, two kinds of multilayer complex structures of GaAs-based photocathodes are developed by changing the structure of the buffer layer underneath the emission layer, wherein one is the graded bandgap structure, and the other is the distributed Bragg reflector (DBR) structure. The theoretical quantum efficiency suitable for the reflection-mode (r-mode) GaAs-based photocathode with these two kinds of buffer structures are deduced based on one-dimensional continuity equations, among which the reflectivity changing with the wavelength of incident light is considered in particular. By comparison of spectral characteristics of photocathodes with the two different structures, and analysis of cathode structure parameters, it is found that the photoelectric performance of the photocathodes with the two structures are quite different, and the structure parameters especially the thickness of GaAs emission layer have a great impact on the spectral characteristic. The quantum efficiency of photocathode with graded bandgap structure is improved due to the introduction of the built-in electric field and the decrement of interface recombination, while for cathode with DBR structure, the quantum efficiency is improved by the multiple reflection of light between two parallel mirrors in the stack of alternating quarter-wave layers of high and low refractive. The theoretical quantum efficiency calculation and analysis would provide theoretical guidance for the better design of GaAs-based photocathodes.

In the design of photocathode, the internal electric field could be formed due to the graded Al compositional ${\mathrm{\text{Al}}}_x{\mathrm{\text{Ga}}}_{1-x}\mathrm{\text{As/GaAs}}$ nanostructure, which can improve the top surface emission probability of carriers. In this paper, ${\mathrm{\text{Al}}}_x{\mathrm{\text{Ga}}}_{1-x}\mathrm{\text{As/GaAs}}$ nanostructure array photocathode composed of two sub-layers is presented. Based on the finite element method, the influence of graded geometrical parameters on their optoelectronic characteristics is investigated. The results show that when the thickness of the sublayer is equal, the difference of the Al composition between the two sublayers of nanostructure is larger, the sub-layers are less, and the quantum efficiency is higher. The light capture ability of the photocathode can be enhanced by increasing the thickness and the array spacing of the first sublayer. Compared with the hexagonal cross-section structure, the light trapping effect and spectral response of the circular cross-section structure are better.

In recent years, with the development of wide-spectrum response photodetectors, InGaN as a semiconducting material has been widely studied. The nanowire array structure has excellent trapping ability, but different structures and shapes have different absorption abilities. It is necessary to optimize the nanowire array continuously in order to obtain the highest absorption efficiency possible. Based on this background, we study the effects of the geometry and structural parameters of InGaN nanowires on the optical response properties. We define the cone ratio and fill factor, respectively, and compare the optical absorption characteristics of InGaN nanowires by using the finite difference time domain (FDTD) method. The calculation results show that the truncated nanocone arrays can enhance the light capture ability and obtain the high sensitivity of the cut-off wavelength. Its optical absorption is at least 15% higher than that of nanowires. Therefore, the research of this paper can provide a certain theoretical reference for the experiment and preparation of InGaN photocathode.

An external electric field-assisted method is proposed to improve the electron collection capability of InGaN photocathode. We use the diffusion drift equation to calculate the photoemission performance of the nanowire arrays. The results show that nanowire array has better electron collection ability with the aid of external electric field. When the incidence angle is 88^∘ and 70^∘, the quantum efficiency and collection efficiency reach the maximum respectively. When the external electric field is 0.6V/$\mu$m, the maximum collection efficiency is 34.2%. Compared with no electric field, the electron collection ratio increased by 69.4%. The external electric field- assisted InGaN photocathode can significantly improve the photoelectric emission performance, which is expected to be realized in future experiments.

In this paper, three structures (cylinder, square column, and hexagonal prism) of InGaAsP nanowire arrays are designed based on the excellent light trapping effect of nanostructures. The effects of nanowire aperture, array period, and nanowire height on the light absorption properties are simulated and analyzed using the finite-domain time-difference (FDTD) method. The photoelectron emission capacity of the nanowire arrays was also calculated using MATLAB. The results show that the cylindrical nanowire array has phenomenon of resonance enhancement (absorption peak) in the near-infrared band of 820–1000nm, and the shift of absorption peaks can be achieved by adjusting the geometric parameters. Meanwhile, the quantum efficiency is taken to 9.98%. These simulation results provide some reference for the photocathode design of InGaAsP in the near-infrared band.

The latest generation of light sources, the Free Electron Lasers that are capable of delivering high-intensity photon beams of unprecedented brilliance and quality, provide a substantially novel way to probe matter with a very high, largely unexplored potential for science and innovation. The CompactLight Collaboration intends to design a hard X-ray FEL facility beyond today’s state of the art using the latest concepts for bright electron photoinjectors, very high-gradient X-band structures at 12 GHz, and innovative compact short-period undulators. When compared to existing facilities, the proposed facility will benefit from a lower electron beam energy, due to the enhanced undulator performance, will be significantly more compact with a smaller footprint, as a consequence of both the lower energy and the high-gradient X-band structures, and will have a much lower electrical power demand. This paper gives an overview of different candidate photocathode materials for use in conjunction with a laser and presents the proposed design of the photocathode and the related laser. Also presented are the e-gun with solenoid selection and design, together with relevant simulation results.

The state of the art DC guns with GaAs, type photocathodes have been successfully operated as polarized electron sources for accelerators, the beam emittance is regretfully very high because of the bunch compressing after the gun. Though not all of the high energy physics experiments using polarized beams are sensitive to the source emittance, but a new source with lower emittance can simplify the injector system and lower radiation load during the beam transport. A normal conducting RF gun can produce beams with low emittance, but the limit on vacuum is still an open question for the currently designed RF guns. In this paper a new type of polarized source is reported: an FZD polarized SRF gun, i.e. an FZD Superconducting RF gun (SRF gun) with a GaAs-type photocathode. The SRF gun is able to produce a 1 mA CW beam with 9.5 MeV energy and 1 mm mrad emittance. It has higher a accelerating field and thus lower emittance than DC guns, at the same time vacuum condition much better than RF guns. Based on the successful running experience in last two years, an SRF gun applied with GaAs-type cathode is considered as a promising alternative for current polarized guns. Some interesting questions will be discussed here, including the back bombardment on cathode, cathode dielectric loss in strong RF field and the cathode time-response.

A high-flux mono-energetic γ-ray beam can be generated via Compton scattering of high-power laser by high-brightness electron beam. We have developed a high-brightness and high-current electron gun for generation of the high-flux γ-ray beam. Recently we demonstrated 500 keV electron beam generation, which meets the high-brightness requirement, from our DC photocathode gun at Japan Atomic Energy Agency. The gun was transported to High Energy Accelerator Research Organization (KEK) and connected to the following accelerator system. The gun operational status at KEK and our plan to develop a multialkali photocathode with a long lifetime are presented.