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.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-High Kerr nonlinearity of water in THz spectral range

Anton N. Tcypkin, Maksim V. Melnik, Maria O. Zhukova, Irina O. Vorontsova, Sergey E. Putilin, Sergei A. Kozlov, and Xi-Cheng Zhang

Fig. 1 (a) The experimental setup for measuring the nonlinear refractive index (n2) of a liquid jet in the THz spectral range. Two parabolic mirrors (PM1 and PM2) with a focal length of 12.5 mm form the caustics area where the water jet (jet) is scanned along the z axis. The synchronization is performed using the mechanical modulator (M) located between the lens and the Golay cell (GC). The aperture (A) is moved from open to closed position to change the geometry of Z-scan from open to closed aperture. Insert - Geometrical position of the jet moved along the z axis relative to the THz radiation. The temporal waveform (b) and its spectrum (c) of the THz pulse generated by the TERA-AX system.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-8-10419

The values of the nonlinear refractive index coefficient for various materials in the terahertz frequency range exceed the ones in both visible and NIR ranges by several orders of magnitude. This allows to create nonlinear switches, modulators, systems requiring lower control energies in the terahertz frequency range. We report the direct measurement of the nonlinear refractive index coefficient of liquid water by using the Z-scan method with broadband pulsed THz beam. Our experimental result shows that nonlinear refractive index coefficient in water is positive and can be as large as 7×10−10 cm2/W in the THz frequency range, which exceeds the values for the visible and NIR ranges by 6 orders of magnitude. To estimate n2, we use the theoretical model that takes into account ionic vibrational contribution to the third-order susceptibility. We show that the origins of the nonlinearity observed are the anharmonicity of molecular vibrations.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement