Research On Terahertz Generation And Detection In Femtosecond Laser-Gas Interactions

THz generation and detection employing femtosecond laser-gas interaction are important research directions in THz gas photonics, as well as recent research highlights in THz science. Aiming to lead a deep understanding of THz generation in two-color field and THz air-biased-coherent-detection(ABCD), the thesis tries to clarify the controversial terahertz mechanism, solve the problems in THz-ABCD and propose a polarizationresolved THz detection scheme. The main content consists of three parts:First part is the study on the mechanism of THz generation in linear two-color field.A plasma reflection model is proposed to detect the gas ionization rate in situ measurement method. Utilizing the plasma density gradient and the nonlinearity of the Fresnel reflection in the case of grazing incidence, the relationship of dependence of THz radiation and gas ionization on the relative phase has been researched. Systematically study of the third-order harmonic has been carried out, which indicates the optimized phase for THz wave keeps stable, while a phase hopping for third-order harmonic has been observed as the laser intensity increases. The optimized phase for THz wave has also been calibrated with attosecond-resolved birth time of high harmonic generation. Analysis shows the rescattering of the ionized electron plays an important role. The generation of terahertz radiation and high harmonic is explained under a unified image.The second party is the study of THz-ABCD. The mechanism has been clarified,which shows THz-ABCD works as phase sampling. The expression for TFISH generation under the influence of Gouy phase shift has been provided. A plasma transient polarization model has been presented to explain the asymmetry response reported recently.Experiment shows plasma transient polarization is the main factor to destroy the measuring accuracy. Strong ionized case has also analyzed. A polarization-resolved THz ABCD scheme has been proposed. In the scheme THz polarization information is modulated on the final output signal. Employing a phase-locked amplifier, arbitrary polarized THz can be measured with a single scanning. This method can eliminate the optical repeatability error and laser energy jitter to achieve high polarization resolution.The last part is the study of THz generation in elliptical two-color field. Employing the polarization-resolved detection, the characteristic of THz generation from gas cell and gas jet has been obtained. THz generation in linear and elliptical two-color field has been connected. Experimental results show that the Coulumb potential can be ignored in elliptical two-color field. The 2D photocurrent model fits the fact very well. Based on this conclusion, a preliminary study of tunneling ionization delay measurement has been carried out.