Fig. 1. Schematic of TISH measurement system in liquid water^[28] (BBO: barium metaborate crystal; TPs: terahertz polarizer; PMT: photomultiplier tube)

Fig. 2. Intensity of SH in the supercontinuum radiation of air and water plasmas versus probe energy^[28]. (a) Air plasma; (b) water plasma

Fig. 3. Time-domain waveform of TISH in water plasma^[28], where the red line is the fitting result based on the four-wave mixing mechanism, and the dashed line represents the intensity of terahertz wave

Fig. 4. Dependence of TISH intensity on terahertz electric field intensity^[28]

Fig. 5. Schematic of the water-based terahertz wave coherence detection experimental setup and measured terahertz waveforms and the corresponding frequency spectra^[29]. (a) Schematic of experimental setup; (b)(c) measured terahertz waveforms and the corresponding frequency spectra

Fig. 6. Terahertz signals vary with time delay and phase difference when the bandwidth is 2, 3, 6, and 18 THz, respectively^[28]. (a)‒(d) Recorded results; (e)‒(h) corresponding simulation results

Fig. 7. TISH energy as a function of the relative polarization angle between the probe laser and terahertz fields^[29]. (a) TISH energy as a function of the relative polarization angle in the absence of CSH, where the crosses and circles correspond to the vertical and horizontal components, respectively; (b) TISH energy as a function of the relative polarization angle in the case of CSH

Fig. 8. Comparison of the water- and air-based detection methods^[29]. (a) Terahertz time-domain waveforms and the corresponding spectra; (b) required probe beam energy to generate the TISH energy for the given terahertz field of 1 MV/cm

Fig. 9. Dependence of TISH energy on probe laser energy when THz field intensity is 0.3, 1, and 3 MV/cm, respectively^[28]. (a) Air-based detection method; (b) water-based detection method

Fig. 10. Terahertz coherence detection in saline solutions^[50]. (a) Iodide aqueous solutions (LiI, NaI, KI, and CsI); (b) bromide aqueous solutions (LiBr, NaBr, and KBr); (c) chloride aqueous solutions (LiCl, NaCl, KCl, and CsCl)

Fig. 11. Detection signal intensity varies with solution refractive index and concentration^[50]. (a) Detection signal intensity as a function of solution refractive index, where the dashed line represents the quadratic fitting curve; (b) detection signal intensity as a function of the aqueous iodide solution concentration; (c) detection signal intensity as a function of the aqueous bromide solution concentration; (d) detection signal intensity as a function of the aqueous chloride solution concentration

Fig. 12. Incoherent detection using different aqueous salt solutions^[50]. (a) Comparison of TISH signal detected in aqueous iodide (LiI, NaI, KI, and CsI) solutions and pure water; (b) comparison of TISH signal detected in aqueous bromide (LiBr, NaBr, and KBr) solutions and pure water; (c) comparison TISH signal detected in aqueous chloride (LiCl, NaCl, KCl, and CsCl) solutions and pure water; (d) measured TISH energy as a function of the refractive index of different solutions; (e) the dependence of measured TISH energy on the solution concentration

Fig. 13. Normalized terahertz time-domain waveforms detected in different solutions and differences of the amplitude in the second half-cycle between aqueous salt solutions and pure water^[50]. (a) Normalized terahertz time-domain waveforms detected in aqueous iodide (LiI, NaI, KI, and CsI) solutions; (b) normalized terahertz time-domain waveforms detected in aqueous bromide (LiBr, NaBr, and KBr) solutions; (c) normalized terahertz time-domain waveforms detected in aqueous chloride (LiCl, NaCl, KCl, and CsCl) solutions; (d) differences of the amplitude in the second half-cycle between aqueous salt solutions and pure water versus refractive index of saline solutions

Fig. 14. Coherent detection of terahertz wave by liquid film^[27]. (a) Terahertz waveforms detected by ethanol, pure water, and air; (b) the corresponding frequency spectra

Fig. 15. Terahertz coherent detection based on liquid film^[27]. (a) Terahertz time-domain waveforms detected by ethanol (solid lines) and water (dotted lines) at different terahertz electric field intensities, and the linear fitting results represent the ethanol and water detection signal peaks; (b) waveforms detected by ethanol (solid lines) and pure water (dotted lines) under probe laser energy from 5 µJ to 30 µJ

Fig. 16. Measured waveforms at different BBO angle values^[27]. When α=0°(180°), CSH is perpendicular polarized to TISH, leads to incoherent detection. When α=90°(270°), the polarizations of CSH and TISH are parallel, which presents as coherent detection. When α=45°(225°), a hybrid detection is introduced. The black dots and gray dashed line are the measured and theoretical CSH energy, respectively

Fig. 17. Coherent detection using ethanol-water mixture^[27]. (a) Measured terahertz time-domain waveforms and fitting result of coherent detection by ethanol-water mixtures; (b) coefficients of water and ethanol