Analysis of THz generation using the tilted-pulse-front geometry in the limit of small pulse energies and beam sizes.

Optical rectification in lithium niobate using the tilted-pulse-front geometry is one of the most commonly used techniques for efficient generation of energetic single-cycle THz pulses and the details of this generation scheme are well understood for high pulse energy driving lasers, such as mJ-class, kHz-repetition rate Ti:Sa amplifier systems. However, as modern Yb-based laser systems with ever increasing repetition rate become available, other excitation regimes become relevant. In particular, the use of more moderate pulse energies (in the few µJ to multi-10 µJ regime), available nowadays by laser systems with MHz repetition rates, have never been thoroughly explored. As increasing the repetition rate of THz sources for spectroscopy becomes more relevant in the community, we present a thorough numerical analysis of this regime using a 2+1-D numerical model. Our work allows us to confirm experimental trends observed in this unusual excitation regime and shows that the conversion efficiency is naturally limited by the small pump beam sizes as a consequence of spatial walk-off between the pump and THz beams. Based on our findings, we discuss strategies to overcome the current limitations, which will pave the way for powerful THz sources approaching the watt level with multi-MHz repetition rates.