The SHG conversion efficiency (Fig. 2) exhibits the characteristic tanh2 behavior of a frequency-doubling process with depleted pump . Even for an intensity of 70 GW/cm2, the saturation is not reached as the conversion efficiency still rises. Higher intensities would, however, increase the risk of damaging the coating and the crystal. At the maximum investigated pump power (0.5 TW), the 515 nm output reaches 295 mJ, i.e. ~ 0.3 TW peak power and 295 W average power, for a spectral width of 1.6 nm (Fig. 3b). This corresponds to an energy conversion efficiency of 59%. As mentioned in the introduction, simultaneous high peak and high average power in the green spectral region was not reported before. For example, the impressively high yield (80%) and energy (3.2 J) reported in  only correspond to an average power of 16 W. The divergence of the SHG beam was minimal, with a beam diameter growing from 3 to 5 cm over 127 m propagation, i.e., a divergence of 0.16 mrad comparable to that of the NIR pump beam. No significant modification of the divergence due to thermal lensing in the crystals was observed.
Figure 4 displays the output energy and conversion efficiency into the third harmonic. The 1 ps pulses reach a peak conversion efficiency of 26%, while the maximum output energy (117 mJ, i.e., 27% conversion efficiency, 117 W average power and 39 GW peak power) is obtained with pulses of 3 ps. The corresponding spectral width is 0.8 nm (Fig. 3a)
This is most likely due to c(3)-induced phase mismatch effects via cross- and self-phase modulation occurring at higher intensities, and to back conversion . Nonlinear absorption in the crystal and the associated thermal effects may also contribute. Two-photon absorption at 515 nm can be excluded, and two-photon absorption at 343 nm is unlikely since the LBO bandgap lies at 7.78 eV . However, some three photons absorption may still occur, as already observed in LBO . Slight misalignments may also have contributed. On the other hand, multiphoton absorption in the UV often induces photo-damage or long-term loss of conversion efficiency , which was not observed in our case, even after more than 100 hours of operation to date. Moreover, the smooth spectral tails show no sharp cut, excluding issues with the angular acceptance of the crystal.
Reducing the peak power by stretching the pulse is a common and efficient way of reducing the B-integral and therefore shifting the onset of filamentation to longer distances [7, 29, 30]. Indeed, due to nonlinear effects in air, such as Kerr self-focusing, a 96 GW peak power laser pulse at 343 nm would self-collapse as a bundle of filaments after only 33 m of propagation [28, 29]. Chirping the pulse to 3 ps duration shifts the saturation of the conversion efficiency to higher input energies, so that the THG beam reaches 27% of conversion efficiency (118 mJ, 40 GW peak- and 118 W average power). Further chirping the pulse to 5 ps reduces the maximum conversion efficiency to 21%.
In conclusion, we demonstrated SHG and THG conversion efficiencies of up to 59% and 27%, respectively with a high energy, high repetition rate Yb thin disk laser system. These performances are unprecedented for laser systems simultaneously delivering high peak and average powers, in the sub-kW and sub-TW, respectively.