First observation of quasi-monochromatic optical Cherenkov radiation in a dispersive medium (quartz)

The present article summarizes the results of a study of optical Cherenkov radiation (ChR) spectral properties both theoretically and experimentally. This type of radiation has a continuous spectral distribution which allows to use it in different fields of physics as for charged particle identification or generation of intense THz radiation. By exploiting the frequency dependency of the target permittivity it is possible to observe quasi-monochromatic radiation. A theoretical model based on a surface current approach is presented which allows to predict angular and spectral properties of ChR. In order to test the model predictions, an experiment was carried out using 855 MeV electrons and a 0.2 mm thick quartz target as radiator which could be rotated with respect to the beam axis. Quasi-monochromatic ChR was observed with a spectrometer placed at a fix observation angle, and tilting the radiator crystal offered the possibility to tune the radiation wavelength. The monochromatization effect is attributed to the frequency dependency of the quartz permittivity, and taking into account the refraction law for emitted ChR crossing the boundary between radiator target and vacuum it is possible to deduce a dispersion relation which connects ChR wavelength and outgoing photon angle - or in an alternative way ChR wavelength and target tilt angle for fixed observation angle. The dispersion relation is clearly confirmed in the experiment, and the model predictions show a satisfactory agreement with the measurements. Exploiting the ChR monochromatization mechanism might offer versatile tools which can find applications for example in beam diagnostics at modern particle accelerators. (C) 2021 The Authors. Published by Elsevier B.V.

Automated summary

The study summarizes the spectral properties of optical Cherenkov radiation, demonstrating the observation of quasi-monochromatic radiation through exploiting the frequency dependency. Experimental results confirm the theoretical model predictions, showing satisfactory agreement. The monochromatization mechanism could potentially be applied in beam diagnostics at modern particle accelerators.