Two-level tunnelling systems and backward-wave phonon echoes in neutron-irradiated quartz

Abstract

Backward-wave phonon echoes (BWE) in neutron-irradiated quartz crystals are studied experimentally before and after annealing. It is argued that the annealing behaviour of the echo is consistent with a BWE generation mechanism in two-level tunnelling systems (for low input powers). The authors derive a theoretical expression, based on the theory of the mixing of two laser fields, which gives a good agreement with their experimental results.