To achieve negative-electron-affinity state, the atomically clean surface of GaAs-based photocathode is usually activated by cesium and oxygen in the ultrahigh vacuum environment. In view of the required computer-control of evaporation flow rates, the solid oxygen dispenser instead of gaseous oxygen is urgently needed just as the regular cesium dispenser. Accordingly, the solid cesium and oxygen dispensers were applied to activate epitaxial GaAs cathode samples. Two types of solid oxygen dispensers composed of barium peroxide powder and silver oxide powder respectively are employed to improve cathode photoemission performance. The experimental results show that the barium peroxidebased oxygen dispenser can release more oxygen and bring in higher activation photocurrent and spectral response than the silver oxide-based one. The unsatisfactory feature is that the silver oxide-based oxygen dispenser released effectual oxygen gas more slowly than the barium peroxide-based oxygen dispenser. Therefore, an effective activation technique was proposed to ameliorate this unfavorable phenomenon for the silver oxide-based dispenser, which can bring out the desired symmetry of photocurrent curve shape during the Cs/O alternate activation process. The improved activation technique would provide guidance for the optimization of activation craft.
In view that enhancing near-infrared response of photocathodes is critical to the detection performance, we propose two technical approaches by changing the structure of buffer-layer underneath the active-layer, wherein one is to produce a graded band gap using the graded-composition structure, and the other is to produce a distributed Bragg reflector using the AlAs/GaAs supperlattice structure. Three types of reflection-mode GaAs photocathode samples grown by molecular beam epitaxy were prepared under the same condition. By comparison of activation photocurrent and spectral response among the three different samples, it is found that compared with the conventional sample, the samples with graded-composition and distributed Bragg reflector can obtain higher photocurrent and better response. The measured results of spectral response indicate that the samples without a distributed Bragg reflector exhibit a typical smooth spectral behavior, while the spectral response of the sample with a distributed Bragg reflector structure has a different resonance feature. The sample with the distributed Bragg reflector structure can obtain higher response than those without distributed Bragg reflector at some near-infrared wavelength positions. The peak positions of spectral response curve agree quite well with the dip positions in the reflectivity spectrum. This agreement demonstrates that the response enhancements are ascribed to the resonant absorption effect.
In order to measure spectral transmittance of solar-blind filter ranging from ultraviolet to visible light accurately, a high-precision filter transmittance measuring system based on the ultraviolet photomultiplier is developed. The calibration method is mainly used to measure transmittance in this system, which mainly consists of an ultraviolet photomultiplier as core of the system and a lock-in amplifier combined with an optical modulator as the aided measurement for the system. The ultraviolet photomultiplier can amplify the current signal through the filter and have the characteristics of low dark current and high luminance gain. The optical modulator and the lock-in amplifier can obtain the signal from the photomultiplier and inhibit dark noise and spurious signal effectively. Through these two parts, the low light passing through the filters can be detected and we can calculate the transmittance by the optical power detected. Based on the proposed system, the limit detection of the transmittance can reach 10-12, while the result of the conventional approach is merely 10-6. Therefore, the system can make an effective assessment of solar blind ultraviolet filters.
Microchannel Plates(MCP) have been widely used in X-ray detection, night vision and other fields. X-ray detection used
in the field of space usually requires a lot of large area of MCPs. A set of multi-station electron scrubbing and
performance testing device for large area MCP is developed in this paper. Four sets of large area electron source are
designed for electron scrubbing. Aiming at single MCP and dual-MCP structure, the high voltage power system, signal
processing module and mechanical control structure are designed to achieve scrubbing and testing of 4 groups of large
area MCP at the same time. By using this device, the scrubbing and testing of large area MCPs of 106mm in diameter are
achieved. The test results are given and analyzed.
The UV image intensifier is one kind of electric vacuum imaging device based on principle of photoelectronic imaging.
To achieve solar-blind detection, its spectral response characteristic is extremely desirable. A broad spectrum response
measurement system is developed. This instrument uses EQ-99 laser-driven light source to get broad spectrum in the
range of 200 nm to 1700 nm. A special preamplifier as well as a test software is work out. The spectral response of the
image intensifier can be tested in the range of 200~1700 nm. Using this spectrum response measuring instrument, the
UV image intensifiers are tested. The spectral response at the spectral range of 200 nm to 600 nm are obtained. Because
of the quantum efficiency of Te-Cs photocathode used in image intens ifier above 280nm wavelength still exists,
especially at 280 nm to 320nm.Therefore, high-performance UV filters is required for solar blind UV detection. Based on
two sets of UV filters, the influence of solar radiation on solar blind detection is calculated and analyzed.
There exist limitations of conventional quantum efficiency models for both reflection-mode (r-mode) and
transmission-mode (t-mode) exponential-doped GaAs photocathodes in some cases. The revised quantum efficiency
models of the r-mode and t-mode photocathodes are solved from the one-dimensional continuity equations, wherein the
built-in electric field in the GaAs layer and the electrons generated from the AlGaAs layer are considered. According to
the revised models, the effects of some relational performance parameters are analyzed, such as the thicknesses of GaAs
layer and AlGaAs layer, and the interface recombination velocity on the quantum efficiency for t-mode and r-mode
photocathodes in combination with the conventional models. The results show that the main contribution of
photoelectrons generated from AlGaAs layer to quantum efficiency in the shortwave (i.e. high incident photon energy)
region, depends on the factors including cathode thickness and interface recombination velocity.