Demonstration of light reflection from the relativistic mirror

Electromagnetic wave frequency upshifting upon reflection from a relativistic mirror (the double Doppler effect) can be used for the generation of coherent high-frequency radiation. The reflected high-frequency pulse inherits the coherence, polarization, and temporal shape from the original laser pulse. A partly reflecting relativistic mirror (flying mirror) can be formed by a breaking wake wave created by a strong laser pulse propagating in underdense plasma [Bulanov S V et al. 2003 Phys. Rev. Lett. 91, 085001]. We present the results of the proof-of-principle experiment for frequency upshifting of the laser pulse reflected from the flying mirror. In the experiment, the breaking wake wave is created by a Ti:S laser pulse (2 TW, 76 fs) in helium plasma with the electron density of ~5x10 cm. The incidence angle of the second laser pulse on the flying mirror is 45°. The reflected signal is observed in 24 shots, with the wavelength from 7 to 14 nm, which corresponds to the frequency upshifting factors from 55 to 114 and the relativistic gamma-factors from 4 to 6. The reflected signal contains at least 3x10 photons/sr. The new source promises the generation of coherent ultrashort XUV and x-ray pulses with tunable wavelength and duration, with the possibility of focusing to record intensities.