Plasma-based Novel Radiation Sources Driven By Ultra-intense Laser Pulses

The research of X-rays and terahertz radiation in electromagnetic spectrum has been always paid great attention to over the past two decades.The conventional X-rays source such as synchrotron and free electron laser,with gigantic volume and large quantities of sophisticated devices,is almost impossible to be miniaturization on the table top.The traditional terahertz source such as photoconductive antenna and electro-optical crystal would be completely destroyed under the illumination of super-intense laser,which severely constrain the strength of terahertz emission.By contrast,plasma material,standing for high intensity laser force,is promising for achieving low-cost,table-top,high-intensity and easily-adjust X-rays and T-rays sources.Based on the high-frequency oscillating component and low-frequency envelop component of laser pulse,we proposed novel schemes for X-rays and T-rays generation,respectively.Firstly,harmonic generation from linearly polarized high-intensity short-pulse laser normally impacting a solid plasma grating is investigated using analytical modeling and particle-in-cell simulation.It is found that when the radiation excited by the relativistic electron quiver motion in the laser fields suitably matches a harmonic of the grating periodicity,it will be significantly enhanced and peak with narrow angular spread in specific directions.The corresponding theory shows that the phenomenon can be attributed to an interference effect of the periodic grating on the excitation.Secondly,a simple route toward high-quality THz generation with fine waveform,high intensity（peak field≥100 MV/cm）and easily-tuned frequency is proposed by femtosecond relativistic laser pulses interacting with plasma slabs with density slightly higher than the critical density9)_{（8）.The oscillation of the surface electron layer,resulting from the imbalance of ponderomotive force and electrostatic force,is responsible for THz radiation.It is found that when plasma density is in the range from¹9（8） to⁴9(}（8）,and simultaneously,the normalized laser intensity satisfies(6₀²<（9）₀)/9)_（8）²-1,the slab is opaque for laser pulse and the radiation could be produced in the front surface of plasma.A theoretical model is presented and the scaling law for THz intensity as a function of laser intensity（6₀ and electrons initial density9）₀ is obtained.This scheme can be realized under present experimental conditions and the achieved radiation has many potential applications in intense THz non-linear optics.