Intense Tunable Narrowband THz Sources

The THz frequency range plays a critical role in species-resolved standoff sensing and materials imaging and represents a unique testing ground for studying fundamental phenomena related to driving elementary particles, atoms, molecules and solids by strong oscillating electric fields. In the context of the widely known problems surrounding the development of efficient sources in the 0.5-5 THz range, the highly sought-after pulsed tabletop sources with bandwidths below 0.1 THz and energies reaching into tens of microjoules are especially challenging. Currently, energetic tunable narrowband picosecond THz pulses can only be obtained in free-electron facilities, whereas the spectral brightness of THz transients generated on a tabletop via frequency down-conversion of amplified laser pulses remain intrinsically low. Aggravated by the lack of direct efficient spectrally-resolved detection, even the characterization of linear phenomena, such as THz absorption, requires the use of multi-shot cross-correlation techniques based on electro-optic sampling and a Fourier-based time-to-frequency conversion. The primary objective of this bilateral project is to develop generation schemes, based on a femtosecond near- infrared laser amplifier and nonlinear-optical frequency rectification, delivering tunable narrowband THz with the energy sufficient for bolometric detection. These THz sources will be applied in proof-of-concept experiments on simplification of linear THz absorption spectroscopy and in multiphoton applications such as molecular orientation, intensity-dependent Rabi-frequency tuning of samples in Fabry-Perot cavities, and stimulated Raman- scattering. The Austrian and Hungarian partner teams will explore several strategies for inter- and intra-pulse optical rectification of near-infrared laser pulses based on programmable ultrafast regenerative amplifiers capable of delivering a narrowband ps intense THz pulse, enabling bolometric frequency-resolved detection. One approach involves the recently jointly demonstrated method of THz-repetition-rate burst amplification in a Vernier regenerative amplifier. Another approach pursues self-referenced tunable THz generation with two asynchronous amplifiers with application of spectral focusing. 1