Controlling Cherenkov angles with resonance transition radiation

Cherenkov radiation provides a valuable way to identify high-energy particles in a wide momentum range, through the relation between the particle velocity and the Cherenkov angle. However, since the Cherenkov angle depends only on the material's permittivity, the material unavoidably sets a fundamental limit to the momentum coverage and sensitivity of Cherenkov detectors. For example, ring-imaging Cherenkov detectors must employ materials transparent to the frequency of interest as well as possessing permittivities close to unity to identify particles in the multi-gigaelectronvolt range, and thus are often limited to large gas chambers. It would be extremely important, albeit challenging, to lift this fundamental limit and control Cherenkov angles at will. Here we propose a new mechanism that uses the constructive interference of resonance transition radiation from photonic crystals to generate both forward and backward effective Cherenkov radiation. This mechanism can control the radiation angles in a flexible way with high sensitivity to any desired range of velocities. Photonic crystals thus overcome the material limit for Cherenkov detectors, enabling the use of transparent materials with arbitrary values of permittivity, and provide a promising versatile platform well suited for identification of particles at high energy with enhanced sensitivity.