High Power THz Generation Using Tilted Pulse Fronts with Low Pump Pulse Energies

Optical rectification using tilted pulse fronts in lithium niobate (LN) is currently the method of choice for the generation of strong-field THz pulses for THz-TDS. The success of this technique owes mostly to the high conversion efficiencies achievable, reaching the percent level at room temperature [1] . So far, this method has been mostly used with amplified laser systems, with repetition rates up to the kHz range and corresponding pulse energies of more than 1 mJ. To increase the DR and SNR of today’s THz sources, there is an increasing demand for high average power, high repetition rate THz sources. Recently, we have demonstrated up to 66 mW THz average power, driven by a >100-W average power mode-locked thin-disk oscillator with 13.3 MHz repetition rate and pulse energies on the 10 µJ level [2] , enabling us to achieve the highest average power of a laser-driven THz source at MHz repetition rates. Despite this promising first achievement, the conversion efficiency of 6∙10 -4 was significantly lower than the record conversion efficiencies obtained with lower repetition rates and higher pulse energies. Furthermore, the scaling laws in this unusual excitation regime remain so far unexplored. Here we present an in-depth investigation of this excitation regime using a 2+1D model including pump beam depletion. It is shown that a combination of spatial walk-off and pump beam break-up is responsible for a reduction in efficiency at small beam sizes. Furthermore, we discuss possibilities to overcome the current limitations and predict that watt-level THz sources at MHz repetition rates will become available in the very near future.