Study Of Spatiotemporal Characterization Of Femtosecond-laser-induced-plasmas By Optical Emission Speetroscopy

In the last20years, the study of the characteristics of the laser-induced plasma has been always carried out intensively by the femtosecond laser-induced breakdown spectroscopy (LIBS) technique. Due to the time characteristics of femtosecond laser pulse, the femtosecond laser-induced plasma emission spectroscopy has special spatiotemporal characteristics. That is the initial motivation of this paper.First, For the first time, the angular distribution of femtosecond laser-induced plasma emission spectroscopy involving pulse filamentation in air was experimentally and theoretically investigated. The391.5nm spectral line of N2+and744.2nm spectral line of N2have similar angular distribution behaviors. By using the angular distribution of dipole radiation and laser-induced plasma fluorescence distribution, we build up an analysis model to calculate the the angular distribution of femtosecond laser-induced plasma emission spectroscopy, which is in good agreement with the experimental results. Then, based on the comparison of experimental data and the calculated one, the polarization of the nitrogen molecules and atoms as a function of the position of plasma channel was derived. In the plasma generated during pulse filamentation, the polarization of nitrogen molecules is larger than that of nitrogen atoms. There are similar polarization trend between nitrogen molecules and nitrogen atoms but with obvious difference, because the nitrogen atoms are all from the dissociation of nitrogen molecules and they initial momentum has certain angular distribution.Second:Study of temporal characterization of femtosecond pulse laser induced spectroscopy in five kinds of mixed gas with different pressure (Less than a standard atmospheric pressure). The time-domain evolution of plasma spectroscopy was experimentally investigated in different ratio of nitrogen with argon in different gas pressures. The results show that the decay time of spectral lines of nitrogen molecules and nitrogen ions are approximate20ns. The spectral lines of N+only appear under6kPa and the spectral lines of N2are absent when the pressures are lower than10kPa. The intensity of spectral lines of N2is increased with rising of gas pressure. However, the spectral lines of N2+exist during the whole pressure region. Then we calculated the plasma density and temperature as a function of time by using the spectra of NII. The plasma temperature rapidly goes up during10ns and then remains a constant of about7000K, which does not change with the ratio of argon. The plasma density increases when the pressure changes from1kPa to6kPa. In the time domain under certain pressure, the plasma density reaches the maximum and then gradually decays. With the increasing ratio of Ar, the plasma density decreases under pressure of1kPa and2kPa, but reverse under pressure of4kPa and6kPa. When the pressure is larger than lOkPa, the intensity of spectral lines of N2is increased with the increasing ratio of argon.Third, The spectra of Ni plasma induced by femtosecond laser pulses at sub-atmospheric pressure environment has been studied experimentally in air and argon gas, respectively. The results show that the spectra of laser-induced plasmas are composed of continuous spectra and atomic line spectra in these two surrounding gases. With the reduction of surrounding gas pressure, the spectra at lower surrounding pressure show higher resolution due to the decrease of electron density, compared to that at higher pressure. Besides, the intensity of line spectra undergoes the transition from slow increase to rapid decrease. Additionally, the effect of the composition of the surrounding gas on the intensity is also briefly discussed in this paper.The research results presented in this dissertation are useful for further understanding of the characteristics of femtosecond laser-induced plasma emission spectroscopy, and give support for application of femtosecond laser.