| Abstract [eng] |
The aim of this work is to use high-power focused femtosecond laser pulses of the first and second harmonics to investigate the interactions between them and the air, with the two beams being brought together at small angles (up to 10 mrad). When the pulses are aligned in time and space through fifth-order nonlinearity, six-wave frequency mixing can occur, whereby the interaction of four first-harmonic photons and one second-harmonic photon generates an additional second-harmonic photon, thus enhancing the second harmonic. Normally, the second harmonic is generated using crystals, but the use of high-energy pulses increases the likelihood of damaging the crystal and rendering it incapable of generating the second harmonic, which encourages the search for other ways to perform the generation of the second harmonic. Combining the first and second harmonic pulses in space and time, and bringing them together at zero angle, initially resulted in an increase in the second harmonic beam divergence due to the phenomenon of six-wave frequency mixing. However, by increasing the delay of the second-harmonic pulse, changes in the second-harmonic beam divergence were observed, due to the diffraction of the second harmonic beam by the plasma filament produced by the first harmonic pulse. By combining the pulses at small angles and delaying the second harmonic pulse, the generation of the second harmonic cone beam was observed due to the reflection of the second harmonic from the plasma filament created by the first harmonic pulse. A titanium-sapphire laser generating 800 nm wavelength radiation with a pulse repetition rate of 1 kHz and a pulse duration of 35 fs was used for the measurements. |
