Abstract

The spectral broadening of high energy pulses after filamentation is an interesting physical phenomenon that can be applied for pulse shortening. This phenomenon has been observed also in the UV spectral range in a number of experiments [1-3] and the UV filamentation in the air has been compared theoretically and experimentally with the filamentation in the IR region. Here, we report the generation of sub 20 fs UV pulses with 300 µJ energy at 1 kHz repetition rate by applying spectral broadening of the UV pulses during filamentation in argon. The original UV pulses, applied in the experiment, are generated as the third harmonic (TH) of an amplified Ti:sapphire laser system at 268 nm. The THG stage is pumped by the fundamental frequency (FF) of a regenerative/multipass Ti:sapphire amplifier at 805 nm. The laser provides up to 12 W of average power at 805 nm with 135 fs pulse duration and 1 kHz repetition rate. In this experiment 7 mJ of the FF pulse energy was applied. The generated TH pulse energy exceeds 1 mJ and the duration is ~100 fs. After focusing the UV beam into a gas cell filled with argon, because of the high peak power of the pulses (which is far above the critical power for self-focusing) a bright filament is generated. The length of the UV filament, which can be up to 25 cm, depends on the focusing conditions. The stability of the generated filament and also the spectral bandwidth of the UV pulses after broadening in the filament depend on the gas pressure. Because of the promising spectral bandwidth and also the stability of the UV pulses in the range of 500–800 mbar, we performed the experimental measurements at 650 mbar. In order to compress the UV pulses we applied a prism pair compressor in a double-pass configuration consisting of two Brewster-angled prisms of magnesium fluoride (MgF₂) with a separation of ~50 cm. Figure 1(a) shows the retrieved temporal intensity and phase profiles from a self diffraction frequency resolved optical gating (SD-FROG) measurement for a typical compressed UV pulse at the pressure of 650 mbar (FROG error 0.008). The SD FROG was performed in a 100-µm-thick Suprasil quartz plate. The retrieved pulse duration of 19.8 fs (FWHM) has a time-bandwidth product of 0.65. More investigation on the spectral broadening in the argon gas filament and its application for pulse shortening of the UV pulses with lower energies was performed by spectral and temporal characterization of the pulses at different energy levels. The results of these measurements for different input pulse energies are shown in Fig. 1(b).

© 2011 Optical Society of America