Numerical and theoretical study of the generation of extreme ultraviolet radiation by relativistic laser interaction with a grating

Abstract

The generation of harmonics by the interaction of a femtosecond, relativistic intensity laser pulse with a grating of subwavelength periodicity was studied numerically and theoretically. For normal incidence, strong, coherent emission at the wavelength of the grating period and its harmonics is obtained, nearly parallel to the target surface, due to relativistic electron bunches emanating from each protuberance. For oblique incidence (30°), only even harmonics of the grating periodicity are seen, but with an even higher intensity. This is due to constructive interference of the emission from the grating protuberances. The emission along the grating surface is composed of trains of attosecond pulses; therefore there is no need to use a filter. An efficiency greater than 10−4 is obtained for the 24th harmonic. The conversion efficiency is fairly constant when the similarity parameter S=ne/(a0nc)(∝neλL/IL1/2) is held fixed, and is optimum when S≃4. Here, ne and nc are the electron density and the critical density; a0=eEL/(meωLc) is the quiver momentum in the laser field EL normalized to mec.