Generation Of Intense Ultra-short Mid-infrared Pulses By Laser-plasma Interaction

Intense ultra-short mid-infrared(MIR)lasers have irreplaceable value in ultrafast chemistry,strong-field physics,and attosecond science,attracting more and more attention in recent years.However,due to the limitation of laser crystals and nonlinear crystals,it has always been one of the challenge of ultrafast laser technology to generate intense ultra-short MIR pulses,especially long-wavelength infrared(LWIR)pulses.To tackle this issue,we try a new way from the perspective of laser-plasma interaction.We propose and demonstrate experimentally a new scheme of generating tunable relativistic single-cycle LWIR pulses based on "photon deceleration" in a tailored plasma structure for the first time.This new scheme will fill in the blank of intense ultra-short infrared sources in this spectral range and open up new opportunities for many cutting-edge fields,such as strong-field physics and attosecond science.The outline of this dissertation is shown below.In this dissertation,the basic principle of photon frequency downshifting(photon deceleration)in laser-plasma interaction is presented.Based on 1D nonlinear wakefield theory,we get the rate of photon deceleration of the drive laser and accordingly the optimal pulse duration of the drive laser and the optimal plasma length,etc.On this basis,we propose a new scheme to generate relativistic single-cycle tunable infrared pulses efficiently based on a tailored plasma structure.The generation mechanism is described by photon deceleration of an off-the-shelf ultra-short laser in a tailored plasma structure.The plasma structure consists of three modules: the pulse compressor,the infrared converter,and the output coupler.This new scheme is verified by 3D PIC simulations.The new MIR sources based on this scheme have the characterisitics of carrier-envelope phase locking,wavelength tunability,and high feasibility.To experimentally demonstrate this new scheme,the XFROG method based on fourwave mixing in gases is developed to measure ultra-short ultra-broadband MIR pulses.We first generate low-energy ultra-short MIR pulses by two-color filamentation in air.Then we measure and retrieve the MIR pulses by the XFROG method.Through comparison with the measuring results of the prism spectrometer,the validity and feasibility of the XFROG method is verified.Finally,we demonstrate experimentally for the first time the new scheme of generating relativistic single-cycle LWIR pulses in a tailored plasma structure using the XFROG method.In this experiment,we use a blade covering a part of the gas jet to form a plasma density profile similar with the expected plasma structure in our proposal.Then we use this plasma structure to interact with the off-the-shelf Ti:sapphire laser pulse and generate a single-cycle LWIR pulse with the central wavelength of 9.4 ?m and pulse duratin(FWHM)of 33 fs.The generated LWIR pulse reaches relativistic intensity.We also demonstrate that the output IR wavelength can be tuned in the range of 4-15 ?m by changing the laser and plasma parameters.