Abstract
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.
Highlights
The process of Cherenkov Radiation (ChR) emission [1, 2, 3] is widely used in different fields in physics as e.g. in elementary particle physics where ChR is applied in design and construction of detectors [4, 5], in beam diagnostics of modern particle accelerators [6], and as intense source of electromagnetic radiation [7]
This paper reports about the first observation of quasi– monochromatic optical Cherenkov radiation generated in a quartz crystal and observed in air, and it presents a theoretical foundation in order to explain the measurements
Based on the theoretical model presented in the first part of this work it was shown that Cherenkov light from a dispersive medium becomes monochromatic if beam divergence, multiple scattering angle inside the target, and angular acceptance of the spectrometer are much smaller than the Cherenkov cone “intrinsic” spread
Summary
The process of Cherenkov Radiation (ChR) emission [1, 2, 3] is widely used in different fields in physics as e.g. in elementary particle physics where ChR is applied in design and construction of detectors [4, 5], in beam diagnostics of modern particle accelerators [6], and as intense source of electromagnetic radiation [7]. The progress in the design of artificial structures allowed to fabricate materials with exotic electromagnetic properties (metamaterials, photonic crystals, metastructures etc.) and achieved the nanoscale level. Such materials are used as ChR targets and lead to a significant modification of the emission characteristics. [8, 9] considered ChR generation in photonic crystals and anisotropic metamaterials respectively. It was demonstrated that the ChR threshold can be decreased significantly if the radiation is generated in metamaterials or photonic crystals [12, 13].
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