Nonlinear THz spectroscopy extends the full-phase analysis of traditional THz-TDS onto the high-intensity regime. With THz sources reaching peak electric fields in excess of hundreds of kV/cm, it is now possible to induce large index changes in some materials. In the case of liquid water, a nonlinear index of 7.8x10-10 cm2/W is confirmed. This value is large enough to break the perturbative regime when sources with peak electric fields in the MV/cm range are used. In addition, the spectrally resolved nonlinear index dispersion can also be extracted.
Nanoparticles are a favorable way to enhance ionization for ultrashort laser pulse focusing on liquid targets. We experimentally investigate how nanoparticles affect the THz generation from deionized water for different concentrations. However, no obvious enhancement is observed comparing with the signal from deionized water under the same excitation condition. Thus, the concentration of nanoparticles is not a crucial factor to enhance THz wave generation within the order of 10^8~10^11 particles/ml. We believe our results provide useful information on enhancing the liquid THz emission source by nanoparticles
THz liquid photonics is a new research frontier in laser-matter interaction community. We have successfully demonstrated THz wave generation from ionized liquids, including from liquid water, liquid nitrogen, and liquid gallium. Preferable to general targets, a flowing liquid line provides a fresh area for each excitation pulse, so the chaos and debris caused by the previous pulse will not influence the next one. This makes it possible of using a kHz repetition rate laser for excitation. THz wave generation from ionized liquids presents photoionization processes that are different from those in gases.