基于液体对太赫兹脉冲进行高灵敏探测

High-Sensitivity Detection of Terahertz Pulse Based on Liquids

由于液体(尤其是液态水)对太赫兹波的强吸收,以液体为介质的太赫兹波产生和探测长期以来被认为是无法实现的。本文综述了基于液体的太赫兹相干探测方法,此方法能在更低的探测激光能量下实现更高的探测灵敏度,可以解决目前基于固体和空气的探测方法遇到的探测频带受限、探测激光能量过高的问题。该探测方法的工作机理被归结为飞秒激光与太赫兹波在液体等离子体中的四波混频过程,因此太赫兹波与探测激光、产生的二次谐波间的场强、偏振具有简单的依赖关系,这使得此方法具有良好的稳定性,并可用于太赫兹波偏振敏感光谱的检测。液态水对太赫兹波的强吸收限制了检测灵敏度的进一步提高,可以利用其他液体或溶液替代纯水来降低太赫兹波的吸收,提高探测灵敏度,这对于该探测方法的进一步推广具有重要意义。

Significance 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.Progress 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 5μJ 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 75μJ 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 15μJ,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 5‒30μJ,and found that ethanol has a higher response than pure water under any detection light.Even if the detection energy is as low as 5μJ,the time-domain waveform of ethanol still has a good signalto-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.Conclusions and Prospects 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.