| Due to the characteristics such as the unique transmission and reflection properties,biological safety of the low-energy photons,as well as the matching with molecular vibration or rotational energy levels,terahertz radiation has many promising applications in the fields including security inspection,imaging,communication,spectroscopic detection,as well as study or regulation of matter property.This dissertation has been organized in two parts to summarize my research.The first part is on the generation and detection of high-field terahertz radiation.One of the most important aspects of terahertz science is intense terahertz source.Among high-field terahertz sources,sources based on a pulse-front-titled femtosecond laser pulse pumping lithium niobate crystals are fairly common,whose maximum focused field strength can reach several MV/cm.However,considering the damage threshold and the saturation of the crystal,it is difficult to further increase the terahertz intensity.Alternatively,plasmas,free of optical damage,could generate higher-power terahertz radiation.The highest terahertz energy up to 200-m J has been reported in laboratory from ultraintense laser-solid interaction.This dissertation experimentally studies the terahertz radiation emitted from intense laser-irradiated wire targets,to explore how the distance from the laser interaction point to the wire,wire radius and plastic coating influence the terahertz radiation: It has been observed that,the terahertz energy increases with the distance from the laser interaction point to the wire end,and then becomes saturated;the terahertz radiation is radially polarized,and the energy is optimized when the wire radius is in the range of 100-250 μm;plastic coating also contributes to the propagation of terahertz waves along the wire target.In addition,two single-shot terahertz spectral measurement methods,based respectively on integrated terahertz filters and Michelson interferometer,are introduced.The second part of this dissertation is on the terahertz pulse-electron interactions,involving two cases,where electrons are generated from laser-solid interactions and ultrafast electron diffraction,respectively.With the increasing terahertz field strength,increasing attention has been paid to the terahertz-driven electron acceleration or streak cameras recently.Ultraintense laser-solid interactions can produce not only intense terahertz radiation,but also hot electrons as a natural electron source,providing a convenient platform for the study of terahertz field-electron interactions.This dissertation has discussed two basic layouts where the terahertz pulse and electron beam propagate co-directionally or perpendicularly.By simulations,the deflection and acceleration of electrons by the terahertz pulse in the case of laser-solid interactions are studied,and the effects of parameters such as the terahertz field strength,pulse duration,and waveform are discussed.A preliminary experimental demonstration is carried out to confirm the deflection of electrons by the terahertz pulse.Strong-field terahertz radiation can directly stimulate atomic-scale processes like lattice phase transitions,and ultrafast electron diffraction is an important tool for studying lattice dynamics.The effect of the terahertz pump on the electron probe must be considered in order to realize terahertz pumped ultrafast electron diffraction.This dissertation establishes two basic cases,including the obverse set-up and the reversal set-up,based on actual experimental situations.The deflection angle and distance of electrons arriving at the sample under different electron-terahertz pulse delays are calculated and analyzed via simulations.The effects of parameters such as sample transmittance,electron velocity,terahertz frequency,terahertz waveform,and off-axis parabolic mirror focal length on the deflection angle are also discussed.Furthermore,three types of improvement schemes are proposed to reduce the deflection effects of the terahertz field on electrons: Choose an appropriate terahertz waveform to counteract the effects of the electron in both positive and negative directions;use a mirror or a noncollinear scheme to reduce the interaction distance;use a metal mesh in the reversal setup to shield the terahertz wave in the electron path before the sample.These schemes are important for realizing a terahertz pump-ultrafast electron diffraction probe experiment. |
