Relativistic flying forcibly oscillating reflective diffraction gratings are formed by an intense laser pulse (driver) in plasma. The mirror surface is an electron density singularity near the joining area of the wake wave cavity and the bow wave; it moves together with the driver laser pulse and undergoes forced oscillations induced by the field. A counterpropagating weak laser pulse (source) is incident at grazing angles, being efficiently reflected and enriched by harmonics. The reflected spectrum consists of the source pulse base frequency and its harmonics, multiplied by a large factor due to the double Doppler effect.

ELI Beamlines Project Institute of Physics of the ASCR Na Slovance 2 18221 Prague Czech Republic

Faculty of Nuclear Sciences and Physical Engineering Czech Technical University Prague Brehova 7 11519 Prague Czech Republic

Kansai Photon Science Institute National Institutes for Quantum and Radiological Science and Technology 8 1 7 Umemidai Kizugawa Kyoto 619 0215 Japan

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