# Advances in High-Power, Ultrashort Pulse DPSSL Technologies at HiLASE
## 3 Harmonics Generation  
Various applications benefit from higher photon energy in visible, UV, and DUV (Deep Ultraviolet) wavelength ranges, which can better match the absorption of materials such as copper, carbon fiber-reinforced plastics, glasses, etc. When the picosecond pulses are considered, the laser-matter interaction is specified by a very small heat-affected zone. Therefore, the pulses of 100 W, 100 kHz, 1030 nm first Perla C amplifier were converted into harmonic frequencies in order to fulfill the requirements of user experiments. For the harmonics generation, we use borate crystals LBO (lithium triborate), BBO (beta-barium borate), and CLBO (cesium lithium borate) due to their suitable properties, such as relatively high nonlinear coefficient, high transparency, and commercial availability [44]. The second harmonic (515 nm) of the fundamental beam is generated in the LBO crystal (Type I phase-matching (PM), 8 \(\times\) 8 mm aperture, 10 mm long, antireflective (AR) coated on both sides for 1030 nm and 515 nm, routine conversion efficiency of 56%) at the output powers up to 40 W (for 70 W input), and it is further is used for the generation of the third and fourth harmonics in cascade conversions (Figure 5). The power of the third harmonic--achieved by 2\(\omega\) + 1\(\omega\) sum frequency generation in an LBO crystal (Type II PM, AR/AR@1030 + 515 + 343 nm, 8 \(\times\) 8 mm, 5 mm long, conversion efficiency of 35% related to the second harmonic), placed just behind the first LBO, without any optimization--is 6.7 W, and will soon be a subject of the next system upgrades. The fourth harmonic (257.5 nm) in the DUV range is produced either in the BBO (8 \(\times\) 8 mm, 3 mm long, with AR/AR@515 + 257.5 nm, conversion efficiency of 14% related to the second harmonic) or CLBO (6 \(\times\) 6 mm, 6 mm long, uncoated) crystals (both Type I PM) by the second harmonic generation of a 515-nm beam. The output power was up to 6 W at the conversion efficiency of about 18% (related to the second harmonic, neglecting the Fresnel reflections), while the CLBO crystal resulted as a better convertor showing higher output power without a trend for saturation in efficiency [9]. The fifth harmonic (206 nm) is realized by 4\(\omega\) + 1\(\omega\) sum frequency generation in a CLBO crystal (Type I PM, 12 \(\times\) 12 mm, 4 mm long, uncoated, conversion efficiency of 20% related to the fourth harmonic) at the output power of 0.8 W [45]. The ovens with the crystals for the DUV radiation are placed in a metal box filled with argon so as to protect the crystal surfaces against moisture and DUV light-generated ozone. The CLBO crystals are always kept at a recommended temperature of 150 \({}^{\circ}\)C to prolong their lifetime [46]. The present values of the harmonics outputs derived from the Perla C beam (first regenerative amplifier only) are the values after a long-term operation of the system and do not represent the fully optimized fundamental beam (e.g., higher orders of pulse dispersion were not fully compensated).  
When the 1-kW, 1-MHz upgrade of the thin-disk amplifiers is accomplished, it will be used for harmonic generation as well. The second and third harmonic aims to reach 500 W and 400 W, respectively. Up to 100 W average power should be generated in the DUV. The increased thermal issues, which normally lead to phase mismatch and conversion efficiency reduction, will be decreased by the sandwich structure of a transparent heat spreader and nonlinear crystal or actively cooled multi-plate design of the harmonic generation stage.