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Negar Hekmat, Tim Vogel, Yicheng Wang, Samira Mansourzadeh, Farhad Aslani, Alan Omar, Martin Hoffmann, Frank Meyer, Clara J. Saraceno

• 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 (< 50 fs) excitation at even higher average power (>> 100 W) - performance that is readily available from state-of-the-art ultrafast Yb-doped solid-state lasers.