Few-cycle, gapless, and intense terahertz source for nonlinear terahertz spectroscopy (Invited Paper)

Ultrashort energetic terahertz (THz) pulses have opened up exciting new avenues of research in the field of light-matter interactions. For material studies in small laboratories, researchers often require widely tunable femtosecond THz pulses with a peak field strength close to MV/cm. Currently, these pulses can be generated through optical rectification and difference frequency generation in crystals without inversion symmetry. We present in this talk a novel approach for generating THz pulses with no frequency tuning gap. Our method is based on Raman-resonance-enhanced four-wave mixing in centrosymmetric media, specifically diamond. We demonstrate that this technique enables the generation of highly stable, few-cycle pulses with near-Gaussian spatial and temporal profiles. Using a 0.5-mm-thick diamond, we were able to generate THz pulses with a stable and controllable carrier-envelope phase. These pulses carried approximately 15 nJ of energy per pulse at 10 THz, with a peak field strength of about 1 MV/cm at the focus. Experimental measurements of the THz pulse characteristics were in good agreement with theoretical predictions. We also discuss the way to improve output energy.