Modulation And Generation Of UV Harmonic In Filaments Induced By Dual Pulses

Due to its high peak power and high time/frequency resolution, femtosecond laser has become an important tool in physics, chemistry, and biology. The ultrafast laser technology is expanding to new waveband as well as to a less during time. When high intensity laser pulses propagate in many gases, due to the interplay between the Kerr self-focusing and plasma defocusing effect, long plasma filament would be formed. The temporal-space property of the laser pulses would be strongly modulated in the filament, thus leads to the generation of super continuum, third harmonic, THz radiation and pulse self-compression.This thesis is based on the modulation effect between the plasma filament and the laser pulse, and is composed by two aspects, the enhancement of TH and spectra broadening. The enhancement of TH in air, argon and nitrogen was achieved when focusing dual femtosecond laser beams of the same frequency into the gas ceil, and the two pulses are overlapped both in time and space. The enhancement in molecule gases was also effected by the cross-phase-modulation of the two pulses and the delayed Raman effect in the gas. This method provides an effective way to achieve ultrashort UV laser pulse. Spectra width of the UV laser was slightly broadened in argon gas through filamentaion, and then a two-color filamentaion method was applied to broaden its spetrca further, providing a strong support the generation of high power ultrashort UV pulses.The main work in the thesis includes:1. The research on the enhancement of third harmonic in filamentWhen the power of the pump beam is very high in gases, the induced TH signal evolves into two parts, an on-axis component and a dispersive ring structure, and its total efficiency is merely 10"4. The author introduced another laser beam, and generated dual filaments by non-collinearlly focusing the two beams in the gas cell. Great enhancement was achieved when the two beams overlapped both in time and space. Background-free TH was observed in argon gas, while enhancement of TH on the axis was found in air and nitrogen. Spectra width of TH in nitrogen wassignificantly broadened from 66 THz to 100 THz, which showed a latency value inthe generation of ultrafast UV laser pulse.2. Spectra broadening in filamentThe generalized broadening mechanisms in filament were analyzed, then single-beam induced spetra broaden by filamentation was performed both in IR and UV waveband. Supercontinuum with bottom width from 200nm to 1100nm was achieved in ir waveband; while a broadening from 6.3 THz to 13.8 THz was achieved in UV region. A two-color co-filamentation setup in argon gas was used to further broaden the UV pulses. By adjusting the time delay, the background pressure and the overlapping of two filaments, the spetca of the UV laser was further broadened (from 6.3 THz to 26.7 THz)...