Picosecond VIS, UV and Deep UV Beams Generated at 100 kHz Diode-Pumped Yb:YAG Thin Disk Laser System

Generation of harmonic frequencies effectively broadens the spectral region of a laser system and thus extends its application potential. In our case the fundamental beam of 1030 nm wavelength is produced by PERLA C system at the HiLASE laser centre in Dolni Brezany, Czechia. The system is Yb:YAG thin-disk based operating at a repetition frequency of 50-100 kHz and delivers pulses of 1-2 ps duration [1] , [2] . The second harmonic (2H, 515 nm) and third harmonic (3H, 343 nm) are generated in an LBO(2H) crystal and an LBO(3H) crystal, respectively. Both crystals were cascaded with a dual waveplate inbetween them, so that the phase matching in the LBO(3H) could be of Type I providing a higher effective nonlinear coefficient than of Type II (0.69 pm/V compared to -0.45 pm/V). The fourth harmonic (4H, 257.5 nm) is generated as a second harmonic of 515 nm in a CLBO(4H) crystal. The fifth harmonic (5H, 206 nm) is produced in SFG process of 4H+1H in a CLBO(5H) crystal, where the 1H beam is the residual beam after the second harmonic [3] . A good conversion in all the nonlinear processes is conditioned by high quality crystals, excellent fundamental beam and proper pulse temporal overlap in sum frequency processes. The output powers achieved in all the harmonics confirm high standard of our harmonics generation system as well as the top quality of our fundamental beam. At present we have at a disposal a 200 W fundamental beam with 1 ps pulses, 90 W in the second harmonic, and 38 W in the third one. Our best result in the FHG was 20 W at 257.5 nm, however, up to now our users prefer lower DUV outputs, see the figure below. It presents the output power time progression of the harmonics generated from a fundamental beam of about 60 W. The second harmonic was used e.g. for green optical transmission tests of optical fibres based on phosphate glasses [4] . A unique application of the fourth harmonic was a crystallization of amorphous TiO2 nanotube layers via laser annealing [5] . The fifth harmonic is used at present e.g. for irradiation of different chemical compounds with the aim of their photolysis and subsequent analysis of the constituents. All harmonics are offered for user experiments related to laser matter interaction.