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.
Highlights
Every year the interest in the terahertz (THz) frequency range is stimulated by the new ideas on its various applications
We experimentally and numerically investigate the double-pulse excitation technique to enhance the generation of THz radiation in case of laser single-color filamentation in water jet
We study experimentally the enhancement of the THz radiation energy, generated during single-color laser filamentation, using double pulse excitation of a flat water jet (Fig. 2)
Summary
Every year the interest in the terahertz (THz) frequency range is stimulated by the new ideas on its various applications. Most of the works are devoted to the study of air filamentation [3,4], but it was demonstrated that the plasma channel induced in media with higher density (clusters [5], liquids [6]) can be the source of more powerful THz fields. This opens up a search for the new methods of enhancing the optical-to-terahertz conversion efficiency One of such methods can be double-pump medium excitation, which is often used for spectral analysis, X-ray generation and plasma studies [11,12,13]. We experimentally and numerically investigate the double-pulse excitation technique to enhance the generation of THz radiation in case of laser single-color filamentation in water jet. Our results pave the way for further development of the high-power THz field generation technology important for both fundamental science and practical application
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