Studies On Third-harmonic Emission Of Plasma Channels In Gaseous Media

With the development of Chirped Pulse Amplification (CPA) technology, the laser power increases so much, so the propagation of intense laser pulse in gaseous media forms the plasma channel and causes strong nonlinear effects such as self-focusing, self-phase modulation, four-wave mixing, and stimulated Raman processes. The plasma channel has attracted a lot of interest because of many promising applications such as lightning control, remote sensing et al.This paper focuses on research of third-harmonic emission of plasma channels in gaseous media. The plasma channel in gaseous media is beneficial to get high conversion efficiency of third-harmonic generation, because the conversion efficiency does not only depend on laser intensity, but also on interaction length. The formation of long plasma channel enhances interaction length effectively. The research of third-harmonic generation in gaseous media is also important to high harmonic generation. The main researches have been summarized as follows.1. The mechanism of plasma channel in gaseous media is investigated theoretically, including nonlinear effects and physics phenomena. The plasma channel can satisfy the phase matching condition of third-harmonic generation. There exists an optimum focal length for the conversion efficiency to the third-harmonic emission for a constant input energy.2. In our experiment condition we explore the optimum focal length for the third-harmonic emission by using rebuilt TSA-25 system is 0.5m. The spectrum width of third-harmonic was 15nm (FWHM). With different chirps of the laser pulse, the spectrum of TH red shift or blue shift respectively. When the group velocity dispersion (GVD) of the pulses equal +1.3×10~5fs~2, the peak of TH red shift and the intensity increases two times. The rising wing of pulses can obtain high conversion efficiency, and proper positive dispersion can increase the intensity of third-harmonic spectrum peak and form a sharp spectrum peak. By changing the hole position of Acoustic-Optic Programmable Dispersive Filter (AOPDF) in laser system, the spectrum of third hannonic shift too. AOPDF can control the spectrum peak position of third harmonic emission in a certain range.3. We focused ultrafast and intense laser in Ar with different pressure and spectra after the plasma channel is studies. We discussed the relationship between the supercontinuum and third-harmonic. The experiment shows effective third-harmonic can extend the UV part of supercontinuum.