High-Order-Harmonic Generation from a Relativistic Circularly Polarized Laser Interacting with Over-Dense Plasma Grating**Supported by the National Natural Science Foundation of China under Grant Nos 11375265, 11475259 and 11675264, the National Basic Research Program of China under Grant No 2013CBA01504, and the Science Challenge Project under Grant No JCKY2016212A505.

Abstract

The simple surface current model is extended to study the generation of high-order harmonics for a relativistic circularly polarized laser pulse interacting with a plasma grating surface. Both exact relativistic electron dynamics and optical interference of surface periodic structure are considered. It is found that high order harmonics in the specular direction are obviously suppressed whereas the harmonics of the grating periodicity are strongly enhanced and folded into small solid angles with respect to the surface direction. The conversion efficiency of certain harmonics is five orders of magnitude higher than that of the planar target cases. It provides an effective approach to generate a coherent radiation within the so-called ‘water window’ while maintaining high conversion efficiency and narrow angle spread.