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 ≈4–6×1019 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 ∼55–115, and the gamma‐factors are y = 4–6. The reflected signal contains at least 3×107 photons/sr. This effect can be used to generate coherent high‐frequen...