No abstract

Accelerator scientists have high demands on photocathodes possessing high quantum efficiency (QE) and long operational lifetime. p-GaN, as a new photocathode type, has recently gained more and more interest because of its ability to form a negative electron affinity (NEA) surface. Being activated with a thin layer of cesium, p-GaN:Cs photocathodes promise higher QE and better stability than the known photocathodes. In our study, p-GaN samples grown on sapphire or silicon were wet chemically cleaned and transferred into an ultra-high vacuum (UHV) chamber, where they underwent a subsequent thermal cleaning. The cleaned p-GaN samples were activated with cesium to obtain p-GaN:Cs photocathodes, and their performance was monitored with respect to their quality, especially their QE and storage lifetime. The surface topography and morphology were examined by atomic force microscopy (AFM) and scanning electron microscopy (SEM) in combination with energy dispersive X-ray (EDX) spectroscopy. We have shown that p-GaN could be efficiently reactivated with cesium several times. This paper systematically compares the influence of wet chemical cleaning as well as thermal cleaning at various temperatures on the QE, storage lifetime and surface morphology of p-GaN. As expected, the cleaning strongly influences the cathodes’ quality. We show that high QE and long storage lifetime are achievable at lower cleaning temperatures in our UHV chamber.

The optical limiting effect was numerically simulated and experimentally observed for a 25-layer thin-film Fabry-Perot microresonator by 7 ns laser pulses at 532 nm. The sample, made by vacuum evaporation and consisting of alternating Nb2O5 and SiO2 layers, has an ultranarrow line of transparency at near 532 nm within a wide spectral band of reflection. By adjusting simulated results in accordance with experimental dependencies of transmittance, reflectance, and absorbance on incident light intensity, the coefficient of optical nonlinearity of Nb2O5 was estimated at (6+1i)·10(-12)  cm2/W.

A multilayer GaAs/AlAs heterostructure forming a Fabry-Perot microcavity with a narrow resonance at 1.1 µm was produced by molecular-beam epitaxy. Under nanosecond pulsed laser radiation, a blue shift of the resonant line, associated with a photo-induced negative change in refractive index in GaAs, was experimentally registered by using an optical parametric oscillator. The spectral shift was accompanied by a reduction in peak transmittance, associated with nonlinear intracavity absorption. Such a cavity can be used as an optical limiter at the resonant wavelength when both the spectral shift and the transmittance reduction contribute to the limiting effect. An exceptionally low limiting threshold of about 1 mJ/cm 2 was observed in the experiment.

This paper presents the results of the development of modules for protecting the eyes from laser radiation in the visible and near-IR regions. A description is given of a nonlinear-optical module with a medium based on a suspension of carbon nanoparticles in a silicone liquid, a multispectral interference filter having narrow reflection bands with minimal transmission at wavelengths of 355, 532, and 1064 nm, and a system adequate for observational devices with embedded protective elements. The combined use of the protective modules makes it possible to reduce the energy at the output of the system to a level no greater than 200 pJ when the input energy is 50 mJ and the pulse-repetition rates are as much as 10 Hz, thus ensuring the safety of the observer's eyes.