Abstract-Flat liquid jet as a highly efficient source of terahertz radiation

Anton N. Tcypkin, Evgenia A. Ponomareva, Sergey E. Putilin, Semen V. Smirnov, Sviatoslav A. Shtumpf, Maksim V. Melnik, Yiwen E, Sergei A. Kozlov, and Xi-Cheng Zhang

Fig. 1 Experimental setup of terahertz generation in flat liquid jets. (a) Experimental layout for energy and spectral terahertz measurements (the inset shows an illustration of optical incident angle ϕ). Laser radiation is splat on pump and probe beams with beam-splitter (BS) with ratio of energy in the channels 1:49, for probe and pump, respectively. Parabolic mirror (PM1 with focal length equal 5 cm) focus the pump radiation on a liquid jet which leads to the generation of terahertz radiation asa result of filamentation inside ionizing liquid jet. The terahertz radiation is collected and collimated by TPX lens (TL) filtered by a teflon filter (F). For spectrum measurements we use conventional electro-optical system (EOS). Parabolic mirror (PM2 with focal length equal 12 cm) focus the terahertz radiation on the ZnTe crystal (EOC) with 1 mm thickness. (b) Photo of laser excitation of the liquid jet. Water moisture plum scatter the laser beam. Temporal terahertz signals (c) and spectrum (d) emitted from the jets of water and ethanol with a thickness of 150 μm at laser pulse duration of 400 fs and optical excitation energy of 600 μJ.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-11-15485

Polar liquids are strong absorbers of electromagnetic waves in the terahertz range, therefore, historically such liquids have not been considered as good candidates for terahertz sources. However, flowing liquid medium has explicit advantages, such as a higher damage threshold compared to solid-state sources and more efficient ionization process compared to gases. Here we report systematic study of efficient generation of terahertz radiation in flat liquid jets under sub-picosecond single-color optical excitation. We demonstrate how medium parameters such as molecular density, ionization energy and linear absorption contribute to the terahertz emission from the flat liquid jets. Our simulation and experimental measurements reveal that the terahertz energy has quasi-quadratic dependence on the optical excitation pulse energy. Moreover, the optimal pump pulse duration, which depends on the thickness of the jet is theoretically predicted and experimentally confirmed. The obtained optical-to-terahertz energy conversion efficiency is more than 0.05%. It is comparable to the commonly used optical rectification in most of electro-optical crystals and two-color air filamentation. These results, significantly advancing prior research, can be successfully applied to create a new alternative source of terahertz radiation.

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