Abstract
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.
Highlights
Over the last decades, ultrafast laser-driven terahertz time domain spectroscopy (THz-TDS) has developed into a mature technique [1]
The overlap between THz and pump persists over a long propagation distance leading to strong depletion, which eventually destroys the temporal structure of the pulse and causes a drop in intensity
It becomes clear from these observations that the optimal pulse duration for highest THz generation efficiency depends on a complex interplay between several parameters and cannot be extrapolated from other excitation geometries; i.e., for example those observed at higher energies and larger spot sizes, further justifying the need for detailed numerical investigations
Summary
Ultrafast laser-driven terahertz time domain spectroscopy (THz-TDS) has developed into a mature technique [1]. With respect to average power scaling of OR in LN using TPFs, Kramer et al [9] recently reported on the use of an Yb:YAG innoslab amplifier with a repetition rate of 100 kHz and 3.4 mJ pulse energy (i.e. 340 W of average power) to generate 144 mW at 0.6 THz, which is currently the highest average power of a single-cycle laser-driven THz source Whereas this is an impressive realization, proving the potential of OR in LN using TPFs for average power scaling, the nonlinear generation mechanism is unaltered compared to the usual mJ-class excitation except for potential thermal effects due to the high average power, which have not been explored in detail. The excitation regime using multi-mJ pulse energies and above is well explored experimentally as illustrated by the above-mentioned state-of-the-art and in theory [20,21]
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