Highly efficient coherent detection of terahertz pulses based on ethanol

Abstract

Water-based terahertz (THz) coherent detection scheme has been recently proposed, which overcomes the bandwidth limitation or high probe laser power demand in solid-/gas-based schemes. Here, we report a highly efficient THz coherent detection method using ethanol with superior performances under the mechanism of four-wave mixing. We systematically investigate the energy ratios and relative polarizations between the THz-induced second harmonic and control second harmonic (CSH) beams and reveal that ethanol always exhibits significantly higher detection sensitivity than water. The coherent, incoherent, or hybrid detection mode can be flexibly switched by changing the CSH beam polarization. The enhanced sensitivity derives from the much larger third-order nonlinear coefficient and lower ionization potential of ethanol. In addition, for the ethanol–water mixtures with various concentrations, the THz coherent detection signals can always be decomposed into the linear superposition of those from pure ethanol and neat water at the sub-picosecond timescale, indicating the synergistic contribution of ethanol and water molecules in the mixture during the detection process. This work provides a valid method to significantly improve the sensitivity of the liquid-based coherent detection scheme and a research perspective for exploring the solute–solvent molecular interactions.