Water, especially liquid water, strongly absorbs terahertz (THz) waves. Generating or detecting THz waves with liquid water has long been thought impossible. Some recent literatures have reported the successful radiation of THz waves from liquid water, which brings new opportunities for the development of THz-related devices based on liquid media. However, the radiation mechanism has not been well elucidated, and the generation efficiency needs to be further improved. We experimentally show that the application of liquid water lines instead of liquid films as THz radiation sources can effectively enhance THz signals. Generally, the generation and detection of THz waves are considered to be "reversible" physical processes. In view of this, we realized the coherent detection of THz waves for the first time based on plasma in liquid water, filling the gap in the field of coherent detection of THz waves in liquid media. Meanwhile, the THz Kerr effect technique for THz-driven liquid water, as a novel tool, is expected to help explore the low-frequency molecular dynamics associated with hydrogen bonding in liquid water.
We demonstrate terahertz (THz) coherent detection using ethanol and ethanol-water mixtures. Combined with the THz-induced second harmonic (TISH) radiated from the liquid plasma and the control second harmonic (CSH) generated by BBO crystal, when the polarization directions of the CSH and TISH beams are parallel, a measured time-resolved waveform of the THz field is obtained due to the four-wave mixing mechanism. Since the third-order nonlinear coefficient of ethanol is larger than that of pure water, our scheme further enhances the sensitivity and signal-to-noise ratio of coherent detection. Meanwhile, the amplitude of THz wave detected by ethanol-water mixtures were increased with the increase of the ethanol concentration. This work provides a new perspective for exploring the solute-solvent molecular interaction and lays an experimental foundation for the theoretical analysis of THz wave coherent detection of classical liquid containing hydrogen bonds.
This research uses a two-dimensional photocurrent model to propose and theoretically demonstrate an effective method for generating polarization-controllable terahertz wave from gaseous plasma according to experimental conditions. The simulation results show that in a combined field composed of a linear two-color laser field of 1600 nm+800 nm and a third circularly polarized laser pulse, the phase of the third pump laser is the key factor to control the polarization of the THz wave generated by the emitted air plasma. Theoretical simulation also shows that based on the two-color field, by selecting the appropriate available laser wavelength for the third pulse, the terahertz output can be effectively increased, and the terahertz conversion efficiency is significantly improved, which improves the production efficiency of the lasers. For such an incommensurate three-color femtosecond laser field, the intensity ratios of the fundamental beam and the third laser can also be used to control the polarization state and the direction of the terahertz radiation. Using current laser technology, our scheme can be implemented in experiments.
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