Coherent Generation and Field-Resolved Detection of Nonlinear x³ Spectral Broadening in the Multi-THz Regime

Abstract

The availability of ultrabroadband and high-intensity laser sources in the mid-infrared spectral range is of crucial importance for the research field targeting spectroscopy and ultrafast phenomena in condensed matter but remains challenging due to the lack of conventional gain media and sensitive detectors. Although the energetic conversion from the near to the mid infrared via difference-frequency generation (DFG) in nonlinear crystals is widely exploited, general considerations of phase matching reveal a conflict: in the first approximation, a trade-off between the efficiency and the acceptance bandwidth of the DFG process is unavoidable. While the former increases exponentially with the length of the nonlinear medium, the bandwidth decreases rapidly with the inverse square root owing to the finite mismatch of group velocities [1]. In this work, we deal with this conflict by efficiently generating narrowband multi-THz transients with peak electric field strengths exceeding 100 MV/cm in a first step and subsequently compressing these pulses by supercontinuum generation in solids such as alkali halides (NaCl, KBr, CsI) and semiconductors (ZnSe, KRS-5).