Theoretical seminar | 14 June 2023

Nikita Abramovsky
University of Nizhny Novgorod
New optical-to-terahertz conversion regimes with non-collinear propagation of optical and terahertz beams

We propose and experimentally demonstrate significant improvements of two main modern methods of optically generating high-field terahertz pulses in electro-optic crystals, i.e., the tilted-pulse-front excitation technique and Cherenkov scheme. 

Regarding the tilted-pulse-front technique, we propose a new conversion scheme based on using a LiNbO3 plate clamped between two dielectric prisms. The scheme provides high quality of the generated terahertz beam and has the potential of scaling up the generated terahertz energy by using large-aperture terawatt-power pump laser beams and commercially available large-diameter LiNbO3 wafers, instead of expensive large-size LiNbO3 prisms, as in the conventional tilted-pulse-front scheme. 

We also present a new design of the Cherenkov-type optical-to-terahertz converter with a thin (a few tens of micrometers thick) layer of LiNbO3 sandwiched between two silicon prisms. The converter ensures bilateral emission of terahertz beams from the LiNbO3 layer and their further combining into a single beam. Bilateral emission allows to overcome such a drawback of the unilateral emitters as a notch in the generated spectrum. A conversion efficiency as high as 0.35% (close to 100% quantum efficiency) was achieved with μJ-level pump laser pulses.

We also demonstrate scaling up the terahertz energy and field strength by extending the size of a Cherenkov-type converter and increasing the pump pulse energy. In particular, 0.5 MV/cm peak terahertz field was obtained with 200-µJ laser pulses. 

Finally, we predict theoretically and demonstrate experimentally that Cherenkov-type terahertz radiation produced by optical rectification of ultrashort laser pulses in electro-optic crystals can experience strong spectral broadening in the regime of multiphoton optical absorption.