386 / 2019-03-13 16:18:17

Surface deformation induced harmonic generation in non-specular direction from relativistic plasma surfaces

high order harmonics

全体主题 > Fundamental Physics at Extremes

High-order harmonic generation (HHG) from the interactions of intense laser pulses with solid-density plasma surfaces is a promising way for the production of energetic coherent x-ray radiation. Owing to the high temporal coherence, the superposition of harmonics leads to the generation of attosecond pulse trains. There are two well-recognized generation processes attributed to HHG by plasma surfaces, coherent wake emission (CWE) and relativistically oscillating mirror (ROM).
Besides the spectral and temporal characteristics, the spatial beam quality is also crucial for the potential applications of HHG source. In recent experiments on HHG, high contrast and shorter laser pulses are used to keep a steep mirror-like surface so that the harmonics are emitted in the specular direction.
We use the 200 TW laser system (800 nm, 25 fs) at the Laboratory for Laser Plasmas in Shanghai Jiao Tong University to investigate the influence of plasma surface deformation on the beam divergence of ROM harmonics. The laser temporal contrast is improved to be better than ~10-11 at 10 ps prior to the main peak by combing a single plasma mirror (PM) system and a cross-polarized wave (XPW) generation technique. The pulse with peak intensity of 3.5×1019 W/cm2 was focused onto the polished fused silica targets at 40.8° and the HHG spectra are measured by a flat field spectrometer.
Two kinds of pre-pulse are introduced to ionize the target, one is the same focal spot diameter as the main pulse and the other is much larger than the main pulse. The experiment results show that when the pre-pulse focal spot diameter is much larger than the main pulse, the harmonics are generated only in specular direction of 40.8°. However, when the pre-pulse focal spot diameter is the same as the main pulse, the plasma surface deformation induced the harmonics irradiate in non-specular direction. At incidence angle of 20°, 25°, 30° and 35°, the harmonic radiations can be measured in non-specular direction of 61.6°, 56.6°, 51.6° and 46.6°.