Study Of High Order Harmonic Generation In The Laser Grating-target Interaction

Ultra-fast laser pulse is a promise tool in the research of ultra-fast physical process.Traditionally,such a kind of laser pulse is generated by synchrotron radiation or free electron radiation.For the disadvantage of huge volume,intricate structure,expensive cost and difficult in miniaturization,the application of radiation from this kind of facility is heavily restricted.Moreover,the mechanism of these two lights determines that their coherence is not that good.Fortunately,with the development of high intense laser technology,we could attain high order harmonic and ultra-fast pulse in the interaction of relativistic laser and plasma.Through this method,the volume and cost is considerably diminished compared to the traditional ones.Furthermore,purified harmonics can be gained through target design and consequential filter technology.In this paper,high order harmonic generation and sensitive parameter analysis are proposed and investigated.Firstly,harmonic generation by circularly polarized laser impacting on a over-dense plasma grating target is analyzed both theoretically and experimentally.It is found in our surface current model that harmonics is generated and transmitting in the direction along the target surface.Interference phenomenon of the grating enhances the harmonics intensity and conversion efficiency considerably.In the particle in cell simulation,we observed as high as 48 th order harmonics transmitting along the target surface.Further investigation to the harmonics indicates that the polarization is different as the transmittance direction changes.Secondly,target surface roughness is discussed and associated numerical simulation is operated.The result shows that minute surface roughness,even its value is shorter than the tenth of the incident wavelength,considerably affects the spectrum of high order harmonics.The harmonic intensity is greatly weakened and the main transmitting direction heads away from the target surface.Besides,orders do not exist in the spectrum of ideal grating arise.All these features indicate that harmonic generation is sensitive to the target surface roughness.In experiment,researchers should modulate the target geometric structure finely to avoid the defects of surface roughness.