Double-pump technique – one step closer towards efficient liquid-based THz sources

Abstract

By irradiating a water jet with double pulses, we demonstrate 4-fold higher THz wave generation than for a single pump pulse. The dependence of the enhanced THz signal on the temporal delay between two collinear pulses reveals the optimal time for launching signal pulse is near 2-4 ps, which corresponds to the time needed to create the complete pre-ionization state when sufficient electron density is already induced, and there is no plasma reflection of the pump pulse radiation. The increase in THz waves generation efficiency corresponds to the case of water jet excitation by the pulses with an optimal duration for a certain jet thickness, which is determined by the spatial pulse size. Using a theoretical model of the interaction of a high-intensity sub-picosecond pulse with an isotropic medium, we held a numerical simulation, which well describes the experimental results when using 3 ps value of population relaxation time. Thus, in this work, double pump method allows not only to increase the energy of the generated THz waves, but also to determine the characteristic excited state lifetime of liquid water. The optical-to-terahertz conversion efficiency in case of double pulse excitation of water column is of the order of 0.5·10 −3 , which exceeds the typical values for THz waves generation during two-color filamentation in air and comparable with the achievable values due to the optical rectification in some crystals.

Varying pre-plasma properties to boost terahertz wave generation in liquids

“…2b). Moreover, in this case, we obtain a 14-fold enhancement compared to a 4-fold one from the previous results 14 .…”

“…Femtosecond p-polarized laser radiation with a central wavelength of 800 nm and a 1 kHz pulse repetition rate is divided by a beam splitter (BS1) into the reference and signal beams. In contrast to work 14 , where a Michelson interferometer was used, in this research a scheme with a Mach-Zehnder interferometer is presented (Fig. 1).…”

“…Firstly, we compare the optimal value for the pump pulse duration in the case of single-and double-pulse excitation. We conduct experiments to study the terahertz field enhancement considering 2 ps temporal delay 14 and varying pre-pulse and signal pulse durations from 60 to 250 fs ( Fig. 2).…”

“…In the present work, we investigate the modification of the experimental scheme due to the introduction of double-pulse liquid jet irradiation and bring out the features that distinguish this approach from the single-pulse configuration. Although the double-pump technique itself is not a recent development 14,15 , the induced pre-plasma effect that manipulates the output energy of the generated terahertz radiation is of considerable interest, motivating further laboratory and theoretical experiments with various input conditions. Thereby, in this study we provide a comprehensive analysis, considering both polar and nonpolar liquid media together with varying pre-pulse and signal pulse duration with their mutual ratio.…”

Giant Enhancement of THz Wave Emission under Double-Pulse Excitation of Thin Water Flow

“…Under double-pulse excitation of Ar gas at delay times of 500 ps, THz emission increases 10times as compared with single-pulse excitation [37]. In the case of flat water flow, when the thickness of jet was 100-240 µm at the delay time of ∼6 ps, THz emission increases 4-times [39], while in the case of water jet of 210 µm diameter at the delay time of 50 ps, THz pulse energy increases more than 8-times [38]. For thinner flat water flow with 20 µm thickness, the enhancement was about 100-times in |E| 2 estimated from TDS signal peak intensities at the nanosecond delays [32].…”