Experimental And Simulation Study On High Harmonic Generation By The Interaction Of Ultra-Short And Ultra-Intense Circularly Polarized Laser With Solid Target

With the rapid development of laser technology,the interaction between laser and matter has expanded to relativistic domain.High order harmonic generation from the highly nonlinear process of ultra-short and intense laser interactions with solid-density plasma provides a new approach to generate coherent extreme ultra-violet and soft X-ray light.It is found that such light source has high brightness and spatial resolution with attosecond(as)temporal duration.According to the experimental and simulation results,high order harmonics with controlled polarization are also possible to be obtained by a circularly polarized relativistic laser.Extreme ultra-violet sources with a controllable polarization state are powerful tools for investigating the magnetic properties of materials.We investigate the properties of high-order harmonics generated from the interaction of circularly polarized relativistic laser with solid target by experiments and simulations.The experiment was carried out using the 200 TW Ti:sapphire laser system at the Laboratory for Laser Plasmas in Shanghai Jiao Tong University.We explored the influence of laser polarization on high order harmonic generation.It is found that high order harmonic radiation can also be efficiently generated by a circularly polarized laser at large incidence angle(40°).The generation efficiency can be comparative with that by a linearly polarized laser.We use a prepulse with different time delays to control plasma density scale length.This helps us to study the high order harmonic generation efficiency with plasma density scale length by a circularly polarized laser.The experimental results show that the high order harmonic generation efficiency decreases gradually with the increase of plasma density scale length.For simulations,high order harmonics generated by the interaction of ultra-short and ultra-intense laser with solid target are studied using the particle-in-cell code OSIRIS.The major contents are as follows: 1.The effect of laser polarization(linear polarization and circular polarization)on the generation of high order harmonic is investigated.The simulation results show that the high-order harmonic intensity generated by circularly polarized laser is lower than the harmonic intensity by linearly polarized laser,but there is no difference in the order of magnitude.This is in good agreement with the experimental results.2.The effect of laser incidence angle on the high order harmonic generation efficiency using circulariy polarized laser is simulated.3.The effect of plasma density scale length on the high order harmonic generation efficiency with linearly and circularly polarized laser is studied,and the results can be explained by the mechanism of relativistically oscillating mirror.4.High order harmonic polarization is investigated.The simulations results show that high order harmonic pulse is elliptically polarized,which indicates that we have obtained elliptically polarized high order harmonics in our experiment.High order harmonics with circular polarization is possible to be obtained by elliptically polarized laser in experiments.