Terahertz (THz) waves are electromagnetic waves with frequencies ranging from 0.1 to 10 THz, between microwave and infrared. With the development of femtosecond lasers, terahertz waves are gradually being widely used in imaging technology, communication technology, medical and health, biochemical technology, nondestructive testing, security inspection technology, and other fields. Currently, the energy required to generate terahertz waves is relatively low, and detection technologies with high sensitivity and bandwidth are urgently required. The most widely used techniques in the field of terahertz wave coherent detection are solid dielectric-based photoconductive sampling and electro-optic sampling. However, owing to the factors such as non-instantaneous response of dielectric carriers, phonon absorption, and the Reststrahlen frequency band, it is difficult for the detection bandwidth to cover the entire terahertz band. Gas media are not affected by these factors, and the coherent detection of sufficiently wideband terahertz waves can be achieved through air-biased and light field-biased coherent detection. However, to achieve high detection sensitivity, the femtosecond laser beam to be detected ionizes air into plasma. Owing to the high excitation threshold of plasma in air, the energy of the laser beam is usually several hundred microjoules (µJ). Solids and gases have been proven to be suitable media for detecting terahertz waves, and the potential use of liquids for the coherent detection of terahertz waves has been an important issue of interest for researchers in the terahertz field. Related research has confirmed that liquids can be used for the generation of terahertz waves, and the intensity of such generated terahertz waves is 1.8 times greater than that of air. Compared to gases, liquids have higher molecular densities and nonlinear coefficients, which results in higher free electron concentrations and lower ionization thresholds in liquid plasmas. Compared with solids, the fluidity of liquids increases their damage threshold and allows them to self-repair.
This article reviews the coherent detection of broadband terahertz pulses in pure water, salt solution, and ethanol. Water requires lower detection energy than air to generate the same level of terahertz induced second harmonic (TISH). The measurement results for a water film under 5 µJ probe light excitation for a terahertz wave with a frequency band of 18 THz and a field intensity of 1 MV/cm are shown in Fig. 8. When detecting in air, the energy of the detected light must be increased to 75 µJ to obtain a signal with the same signal-to-noise ratio level. These results indicate that it is necessary to increase the detection light energy by 1‒2 orders of magnitude to achieve the same TISH energy in air. Therefore, under the same experimental conditions, the sensitivity of liquid water detection is 1‒2 orders of magnitude greater than that of air (Figs. 8 and 9). Because of the high third-order nonlinear coefficient of salt solutions, the signal intensity of coherent detection increases with increasing solution concentration, and the slope of the signal intensity also changes accordingly. Salts with a higher refractive index have a higher signal amplitude; therefore, the improvement in detection sensitivity is attributed to the increase in the refractive index of high-concentration solutions (Figs. 10 and 11). Ethanol has a third-order nonlinear coefficient greater than that of water, making it easier to ionize. A lower detection energy is required to form liquid plasma, and ethanol has a higher molecular response than pure water in the terahertz band. When the detection light energy is fixed at 15 µJ, the sensitivity of ethanol in the terahertz band is higher than that of water. We compared the coherent detection signals of ethanol and pure water under different detection lenergies of 5‒30 µJ, and found that ethanol has a higher response than pure water under any detection light. Even if the detection energy is as low as 5 µJ, the time-domain waveform of ethanol still has a good signal-to-noise ratio, providing new research prospects for low-laser-energy terahertz coherent detection (Fig. 15). The liquid-based terahertz wave coherent detection scheme expands the variety of terahertz wave detectors, providing the possibility of revealing molecular interaction mechanisms in biological liquid environments.
Liquid detection of terahertz waves has unique advantages over gases and solids, providing a new perspective for the coherent detection of broadband terahertz pulses, which has great potential in terahertz time-domain spectral applications and remote sensing.
