| Abstract [eng] |
The aim of this work was to investigate the dependence of intensity of the fourth harmonic generated in air by a femtosecond Ti:Sapphire laser on the excitation radiation parameters, using a two-beam fourth harmonic generation scheme in which the fundamental and second harmonic beams are crossed in air at a small angle. First, the beam crossing angles were calculated for three possible six-wave mixing processes responsible for fourth harmonic generation in air. Depending on the process, the obtained values varied between 10 and approximately 17 milliradians. Next, a two-beam optical setup was assembled, a beam crossing angle of 10 mrad was selected, and after temporal overlap of the fundamental and second harmonic pulses, a fourth harmonic signal was obtained and measured using a spectrometer. Measurements of the fourth harmonic intensity as a function of the fundamental and second harmonic pulse energies and their ratio were performed. It was observed that increasing the laser energy resulted in a narrower range of energy ratios for which efficient fourth harmonic generation was achieved. During the measurements it was also noticed that the laser operated unstably, i.e. the beam direction and/or pulse energy changed over time, which affected the measurement uncertainties. The dependence of the fourth harmonic generation efficiency on the angle between the polarization directions of the excitation beams was also measured. A maximum signal was obtained for parallel polarizations, while a minimum was observed for perpendicular polarizations. It was noticed that every second maximum was larger; this was caused by the low-order wave plate used in the experiment, which introduced an additional pulse delay in one of the orientations. The dependence of the fourth harmonic intensity on the delay between the excitation pulses was also measured, corresponding essentially to a higher-order correlation function. It was found that at lower laser energies the obtained function had a Gaussian shape, whereas at the maximum energy (4.8 mJ) a second maximum appeared, indicating that the excitation pulse split due to filamentation. Finally, the dependence of the fourth harmonic signal on the distance between the second harmonic crystal and the filament was investigated. The purpose was to determine whether sinusoidal oscillations, similar to those observed in previous work due to phase differences and interference between several different fourth harmonic generation sources, could be measured. No oscillations were observed, indicating that only a single fourth harmonic generation process was present. |
