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We report a systematic investigation into the processes behind a near hundred-fold enhanced second harmonic wave generated from a laser-induced air plasma, by examining the temporal dynamics of the frequency conversion processes, and the polarization of the emitted second harmonic beam. Contrary to typical nonlinear optical processes, the enhanced second harmonic generation efficiency is only observed within a sub-picosecond time window and found to be nearly constant across fundamental pulse durations spanning from 0.1 ps to over 2 ps. We further demonstrate that with the adopted orthogonal pump–probe configuration, the polarization of the second harmonic field exhibits a complex dependence on the polarization of both input fundamental beams, contrasting with most of the previous experiments with a single-beam geometry.

 

Broadband terahertz (THz) wave emission from flowing liquid targets has been demonstrated under short optical pulse excitation. Observations have been reported by using liquid THz sources, including optimal angle of incidence, preference of subpicosecond pulse excitation, and strong sideway emission. Compared with solid targets, the fluidity of liquid allows each laser pulse to interact with a fresh area, which makes it possible to use a table-top laser with a high repetition rate for excitation. Liquids with a comparable material density to solids make them promising candidates for the study of high-density plasma and bright THz sources. In this paper, we review recent progress, challenges, and opportunities of THz emission from liquids. This topic may offer new possibilities in the exploration of THz liquid photonics and may play an indispensable role in the study of laser-liquid interaction.

 

We characterize a terahertz (THz) source based on plasma in liquid gallium. The dependence of the emitted THz pulse energy on second-order phase, pump pulse energy, and polarization of the short laser pulse is demonstrated. Our study suggests that the THz emission mechanism is due to the ponderomotive force and is aided by a direct-field driven term. The proposed source and accompanying generation mechanism are studied under a non-relativistic regime ($10^{15}<<\#comment/>\mathrm{I}<<\#comment/>10^{18}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$) for forward directed THz under a single pump excitation scheme.

 

Matters are generally classified within four states: solid, liquid, gas, and plasma. Three of the four states of matter (solid, gas, and plasma) have been used for THz wave generation with short laser pulse excitation for decades, including the recent vigorous development of THz photonics in gases (air plasma). However, the demonstration of THz generation from liquids was conspicuously absent. It is well known that water, the most common liquid, is a strong absorber in the far infrared range. Therefore, liquid water has historically been sworn off as a source for THz radiation. Recently, broadband THz wave generation from a flowing liquid target has been experimentally demonstrated through laser-induced microplasma. The liquid target as the THz source presents unique properties. Specifically, liquids have the comparable material density to that of solids, meaning that laser pulses over a certain area will interact with three orders more molecules than an equivalent cross-section of gases. In contrast with solid targets, the fluidity of liquid allows every laser pulse to interact with a fresh area on the target, meaning that material damage or degradation is not an issue with the high-repetition rate intense laser pulses. These make liquids very promising candidates for the investigation of high-energy-density plasma, as well as the possibility of being the next generation of THz sources. 

We experimentally show the spectrally averaged nonlinear refractive index and absorption coefficient for liquid water, water vapor,$\mathrm{\alpha <\#comment/>}$-pinene, and Si using a full-phase analysis in the terahertz regime through a standard time-domain spectrometer. Our results confirm that the nonlinear index of refraction of the liquid samples in this regime exceeds the near-infrared optical nonlinear index by six orders of magnitude. In the case of liquid water and water vapor at atmospheric pressure, we find a nonlinear index of$7.8\times <\#comment/>{10}^{-<\#comment/>10}{\mathrm{c}\mathrm{m}}^2/\mathrm{W}$and$6\times <\#comment/>{10}^{-<\#comment/>11}{\mathrm{c}\mathrm{m}}^2/\mathrm{W}$, respectively, which are both much larger than expected.