| Abstract [eng] |
The propagation of high intensity ultrashort laser pulses in transparent media results in a distinct transformation of the initial pulse due to the interplay of various nonlinear processes. Strong spectral broadening manifests itself as a white light continuum, also known as supercontinuum, generation. The most important feature that employs supercontinuum in various fields of application is its high spatial and temporal coherence and the possibility to modify its spectral characteristics. One of the main applications of supercontinuum is directed to the development of optical parametric amplifiers, where it serves as a seed signal. However, the focusing dynamics of femtosecond pulses in transparent dielectric media in anomalous dispersion region, is still poorly investigated topic. The aim of this study was to investigate femtosecond supercontinuum generation in CaF_2 crystal and determine exact conditions allowing to achieve spectral broadening in the ultaviolet – mid-infrared spectral region without the optical damage of the nonlinear medium. It was shown that a broadband supercontinuum, spanning more than 4 octaves, can be generated in CaF_2 crystal using 170 fs, 2,4 µm laser pulses with no optical damange observed. It was also demonstrated, that with low energy pulses the output spectrum only consists of 3rd, 5th and 7th odd harmonics. However, using more energetic pulses the transition from Kerr-driven to plasma-driven filamentation regime is observed. Plasma-driven filamentation leads to temporal compression of the driving pulse which induces spectral superbroadening around the carrier wavelength and large scale spectral broadening of the individual harmonics spectra. It was determined that exceeding 15 µJ pump pulse energy results in the optical damage of the output face of the crystal after 30-40 laser shots; while at higher input energy the optical damage develops in the volume. |
