Studies On The Relativistic Ionization Front Produced By Ultraviolet Femtosecond Laser

With the rapid development of ultrashort pulse laser technology, currently unprecedented high intensity can be achieved with even shorter pulse duration. Neutral gases in the focal region can be fully ionized into high-density plasma within the pulse duration, giving rise to a plasma-neutral gas interface called relativistic ionization front which propagates approximately at the speed of light. In the thesis, studies concerning the interactions of electromagnetic waves with the ionization front are performed theoretically and experimentally.Based on the ray-tracing equations, two-dimensional frequency conversion and angel variation of the probe electromagnetic wave induced by the interaction with the front are investigated. It is shown that besides frequency upshifting, frequency downshifting would take place at certain conditions. Using the Maxwell equations and the moving boundary conditions, the coefficients of the reflected wave, the transmitted wave and the static magnetic mode are deduced for the interactions of obliquely oncoming electromagnetic wave with the ionization front. Results show that reflected wave would disappear completely at certain incident angle with low plasma density in the ionization front. To get larger frequency upshifting of the transmitted wave, one should provide ionization front with higher plasma density, and select an appropriate incident angle for the electromagnetic wave.An experimental system has been built for the investigation of fs-ns laser pulses interacting with high density supersonic gas jet. The process of gas ionization by ultraviolet femtosecond lasers is explored for understanding properties of the relativistic ionization fronts. In the experiment, helium and argon jets with various densities are ionized respectively by the ultraviolet femtosecond laser pulses and the spectral blueshifting induced by ultrafast ionization is measured. Numerical simulations of the ionizing process byultraviolet femtosecond lasers are also performed to investigate the evolvement and distribution of plasma density in the ionization front. Two parameters namely, ionization rise time and density rise distance, are proposed to characterize the relativistic ionization front produced by the ionization of gases with high intensity femtosecond lasers. It shows that sharper ionization fronts can only be produced with shorter pulse duration, and higher laser intensity alone does not effectively lead to steeper plasma distribution in the front.Strong atomic emission enhancements have been observed when using fs-ns dual-laser pulse to interact with high density supersonic gas jets. Such a novel effect may be attributed to the different ionizing mechanisms of gases by femtosecond and nanosecond laser. The free electrons produced by tunneling ionization of gases with the femtosecond laser pulse would probably initiate the cascade ionization process by the subsequent nanosecond laser pulse. Spectral measurements of the enhanced emission are performed, and the influences of delays between the two laser pulses for the enhancement effect are also observed.