Relativistic Tennis Using Flying Mirror

Upon reflection from a relativistic mirror, the electromagnetic pulse frequency is upshifted and the duration is shortened by the factor proportional to the relativistic gamma-factor squared due to the double Doppler effect. We present the results of the proof-of-principle experiment for frequency upshifting of the laser pulse reflected from the relativistic "flying mirror", which is a wake wave near the breaking threshold created by a strong driver pulse propagating in underdense plasma. Experimentally, the wake wave is created by a 2 TW, 76 fs Ti:S laser pulse from the JLITE-X laser system in helium plasma with the electron density of approximate to 4 -6x10(19) cm(-3). The reflected signal is observed with a grazing-incidence spectrograph in 24 shots. The wavelength of the reflected radiation ranges from 7 to 14 nm, the corresponding frequency upshifting factors are similar to 55-115, and the gamma-factors are gamma = 4-6. The reflected,. X 107 signal contains at least 3 photons/sr. This effect can be used to generate coherent high-frequency ultrashort pulses that inherit temporal shape and polarization from the original (low-frequency) ones. Apart from this, the reflected radiation contains important information about the wake wave itself, e. g. location, size, phase velocity, etc.