A method of coherent high-frequency electromagnetic radiation generation, proposed by
Bulanov, Esirkepov, and Tajima [Phys. Rev. Lett. 91, 085001 (2003)], is experimentally
demonstrated. This method is based on the radiation frequency multiplication during
reflection at a mirror flying with relativistic velocity. The relativistic mirror is formed by the
electron density modulations in a strongly nonlinear wake wave, excited in an underdense
plasma in the wake behind an ultrashort laser pulse. In our experiments, the reflection of a …