We present a detailed exploration of the behavior of gallium phosphide (GaP) crystals used for optical rectification (OR) of high average power (> 100 W), MHz repetition rate ultrafast lasers. We measure thermal load, Terahertz (THz) refractive index and THz yield over a wide temperature range (77 K to 500 K) in this unusual excitation regime. Our thermal load measurements indicate that nonlinear absorption remains the main contribution to crystal heating and thus the main limitation to scaling the conversion efficiency and show that cryogenic cooling can partly relax these limitations. Furthermore, we present first temperature-dependent refractive index measurements of GaP for frequencies up to 4 THz, showing only minor deviation from room temperature values and no significant degradation of coherence length. Last but not least, we present first experiments of OR in GaP at cryogenic temperatures and observe an increase in THz yield (30%) at cryogenic temperatures when using short pulse duration excitation, due to reduced THz absorption at broad THz bandwidth. Our results indicate that OR in cryogenically cooled GaP is a promising approach for achieving broadband, high-average power THz radiation using short-pulse ( > 100 W) - performance that is readily available from state-of-the-art ultrafast Yb-doped solid-state lasers.

中文翻译：

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用于高激发平均功率下的光学整流的低温冷却 GaP

我们详细探索了用于高平均功率 (> 100 W)、MHz 重复率超快激光器的光学整流 (OR) 的磷化镓 (GaP) 晶体的行为。我们在这种不寻常的激发机制中测量了很宽的温度范围（77 K 到 500 K）内的热负荷、太赫兹 (THz) 折射率和太赫兹产量。我们的热负载测量表明非线性吸收仍然是晶体加热的主要贡献，因此是缩放转换效率的主要限制，并表明低温冷却可以部分放松这些限制。此外，我们首次对频率高达 4 THz 的 GaP 进行了温度相关的折射率测量，结果表明与室温值的偏差很小，相干长度没有显着降低。最后但并非最不重要的，我们在低温下展示了 GaP 中 OR 的首次实验，并观察到在使用短脉冲持续时间激发时低温下太赫兹产率增加（30%），这是由于在宽太赫兹带宽下太赫兹吸收减少。我们的结果表明，低温冷却 GaP 中的 OR 是一种使用短脉冲（> 100 W）实现宽带、高平均功率太赫兹辐射的有前途的方法 - 性能可从最先进的超快掺镱中轻松获得固态激光器。