Characteristics of light reflected from a dense ionization wave with a tunable velocity

Zhidkov, A.*; Esirkepov, T. Z.; Fujii, Takashi*; Nemoto, Koshichi*; Koga, J. K.; Bulanov, S. V.

An optically dense ionization wave produced by two femtosecond ( 10/30 fs) laser pulses focused cylindrically and crossing each other may become an efficient coherent X-ray converter in accordance with the Semenova-Lampe theory. The resulting velocity of a quasiplane ionization wave in the vicinity of pulse intersection changes with the angle between the pulses from the group velocity of ionizing pulses to infinity allowing a tuning of the wavelength of X rays and their bunching. The X-ray spectra after scattering of a lower frequency and long coherent light pulse change from the monochromatic to high order harmonic-like with the duration of the ionizing pulses.