## 5 Cooling  
Multi-photon absorption limits the conversion efficiency in the optical rectification process, resulting in the generation of free carriers and heat [21; 22; 37]. The latter can result in thermal damage of the crystal or crystal coating and thermal lensing, while the former can directly reduce the power of the emerging THz radiation. In GaP pumped with 1030 nm, three-photon absorption dominates as two-photon absorption (2 PA) is only possible via indirect bandgap absorption [37]. However, 2 PA is still observed at high pump intensities [22; 38]. As phonon occupation probability is temperature-dependent and indirect bandgap absorption requires assistance from the simultaneous absorption or emission of a phonon, 2 PA is also strongly temperature-dependent [39]. Hence, cooling the crystal leads to a reduction in 2 PA and thus improvements in THz emission. It has been shown that cryogenically cooling GaP can indeed improve conversion efficiency and bandwidth of the generated THz radiation [22; 40] and greatly reduce thermal lensing in GaP under high pump power. While this is of course a desirable improvement, it does require the use of cryogenic equipment, increasing the complexity of the system and requiring constant maintenance. For this reason, we sought to investigate whether improvements in THz generation can be gained by water-cooling GaP--akin to what is reported in [41]. Through initial measurements, we found that GaP absorbs substantial amounts of pump energy, leading to rapid heating of the crystal. We designed a water-cooled mount for the GaP crystal machined from copper. The mount features channels to allow for the ingress and egress of cooled water, enabling the copper mount to dump heat into the cooling loop. Our laser, like most high-power laser sources, requires water cooling to operate. As such, the cooling loop required to cool the GaP was already present in our experiment and is likely present in any experiments working under similar conditions. Our measurements, shown in Figure 5, show a significant increase in the THz generation efficiency when cooling the GaP crystal. Measurements taken of the crystal surface with a thermal camera (Optris Xi 400, Optris, Berlin, Germany) show that water-cooling substantially reduces heat accumulation in the crystal. (Figure 6a,b). These images clearly show the rapid heating of the entire crystal surface without cooling and the development of a hot spot where the pump beam is incident on the surface of the crystal. The peak temperature of the crystal reaches 47.3\({}^{\circ}\)C after 7 min of exposure to 25 W average power. On the other hand, after the same exposure time to the same pump power the cooled crystal shows negligible heating, maintaining an almost even temperature across the crystal over the same time period. Even at the point of pump beam incidence, we observed a temperature increase of only 1.7\({}^{\circ}\)C above ambient temperature. Far field images of the beam profile after transmission through the crystal were also taken (via the Genetec Beamage 4M: Figure 6c,d). These were taken at an average pump power of 23.5 W and show considerable thermal lensing in the uncooled case. This leads to an increase in beam size as well as beam aberrations with much of the beam's intensity being split into two lobes, similar to what was also observed in [22; 41], resulting in the aforementioned increased clipping of the beam during pump rejection, subsequently requiring more filtering following pump rejection. On the other hand, the beam-following transmission through the water-cooled crystal shows very little degradation with the majority of the beam intensity remaining in the central portion, allowing for effective pump rejection via geometric filtering. THz measurements were recorded with and without crystal cooling to determine how the latter affects the signal-to-noise ratio (SNR), measured across the main peak of the THz pulse as described in [42].