Open Access
Add to BibSonomy
Abstract
Water-based coherent detection of broadband terahertz (THz) wave has been recently proposed with superior performances, which can alleviate the limited detection bandwidth and high probe laser energy requirement in the solid- and air-based detection schemes, respectively. Here, we demonstrate that the water-based detection method can be extended to the aqueous salt solutions and the sensitivity can be significantly enhanced. The THz coherent detection signal intensity scales linearly with the third-order nonlinear susceptibility χ(3) or quadratically with the linear refractive index η0 of the aqueous salt solutions, while the incoherent detection signal intensity scales quadratically with χ(3) or quartically with η0, proving the underlying mechanism is the four-wave mixing. Both the coherent and incoherent detection signal intensities appear positive correlation with the solution concentration. These results imply that the liquid-based THz detection scheme could provide a new technique to measure χ(3) and further investigate the physicochemical properties in the THz band for various liquids.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Efficient detection of broadband terahertz (THz) wave has various applications such as physical chemistry research, medical diagnosis, and environmental monitoring. Both solids [1,2] and gases [3] have been applied as the media for coherent detection of THz electric fields. The solid-based detection techniques have been widely adopted in THz time-domain spectroscopy with high efficiency [4], but the detection bandwidths are significantly limited by the non-instantaneous response of carriers in semiconductor substrates of photoconductive antennas or the phonon absorption in electro-optic crystals [5–7]. The gas-based coherent detection methods could achieve broad bandwidth [8], but a relatively high probe laser energy (typically at the scale of hundreds of µJs) is required to ionize the gas media to form plasma. The liquid, especially water, usually has strong absorption in the THz range [9,10] and had long been considered impossible for detection of THz wave. Very recently, a water-based THz coherent detection scheme via the four-wave mixing mechanism has been firstly demonstrated [11].
The water-based coherent detection exhibits superior performances and can alleviate the existing problems in the methods based on the solids and gases. Without the restriction of Restrahlen bands, the water-based detection has demonstrated to be coherently sensitive to THz pulses over a broad frequency range (0.1–18 THz), overcoming the limited detection bandwidth in the solid-based method. Moreover, the fluidity of water allows each laser pulse to interact with a fresh water area, which can avoid the material degradation or damage problem in the solid target induced by intense probe laser beam with high repetition rate. Comparing to the gases, water has a smaller threshold for photo-ionization, and therefore the probe laser energy can be reduced by 1–2 orders of magnitude [11]. Furthermore, the density of water is comparable to that of solid, which means that the THz wave and probe laser could interact with 3 orders of magnitude more molecules than the gas-based method [12]. The newly proposed water-based method sharply reduces the probe laser energy, improves the sensitivity, and provides reliable coherent detection for broadband THz fields. Whether this water-based scheme can be extended to other liquids, in particular aqueous salt solutions, and further improve the detection performance demands to clarify pressingly. Basically, because the water-based detection is attributed to the four-wave mixing mechanism, an enhanced detection sensitivity in the aqueous salt solution is highly expected due to the commonly larger third-order nonlinear susceptibility ${\chi ^{(3 )}}$ than pure water [13,14].
On the other hand, the liquid-based THz detection scheme provides a possibility to reveal ${\chi ^{(3 )}}$ or the refractive index of various liquids in the THz range. Different kinds of aqueous salt solutions and pure water have great differences in the physicochemical properties such as viscosity [15,16], melting [17], boiling point [18], etc. Many techniques such as THz spectroscopy [19,20], ultrafast infrared spectroscopy [21,22], low-frequency Raman spectroscopy [23,24], and THz Kerr effect spectroscopy [25–28], etc., have been employed to study the nonlinear susceptibility or refractive index in the THz range for pure water and some aqueous salt solutions (such as NaCl, KCl, NaI, etc.). Due to the varieties of salt compounds, some other kinds of aqueous salt solutions have not been systematically observed in the THz region. Furthermore, each of previously used spectroscopy techniques has its pros and cons, the liquid-based THz detection could also offer valuable and supplementary information for the adopted aqueous salt solutions.
Here, we investigate the THz coherent detection based on different aqueous salt solutions and find that the detection sensitivity can be significantly improved in contrast to the pure water. In addition to maintaining the low probe laser energy requirement to form plasma as in the pure water case, the aqueous salt solution can produce a stronger detection signal, thus resulting in higher detection sensitivity. For eleven kinds of aqueous salt solutions adopted in our experiments, we observe that the THz coherent detection signal intensity is linearly proportional to ${\chi ^{(3 )}}$ or quadratically dependent on the linear refractive index ${\eta _0}$ of the aqueous salt solutions while the signal intensity of incoherent detection is linearly proportional to ${({\chi ^{(3 )}})^2}$ or ${({\eta _0})^4}$, which is in good agreement with the four-wave mixing theory. The liquid-based THz detection method could be applied as a new tool to measure ${\chi ^{(3 )}}$ in the THz band and further characterize the aqueous salt solutions via the strong correlation between ${\chi ^{(3 )}}$ and the physicochemical properties.
2. Experimental results and theoretical analysis
2.1 Experimental setup
The schematic of the experimental system is illustrated in Fig. 1(a). The organic 4-N,N-dimethylamino-4'-N'-methylstilbazoliumtosylate (DAST) crystal irradiated by a 1550 nm laser beam is used to generate the strong THz pulse with a maximum electric field intensity of 5 MV/cm. The THz pulse and 800 nm fundamental probe beam (vertical polarization) with an energy of $10\mathrm{\;\ \mu J}$ are co-focused on a forced-flowing liquid film to form a liquid plasma. The 400 nm second harmonic (SH) beam induced by the vertically polarized THz pulse is then collected by a photomultiplier tube (PMT) through a 400 nm narrowband filter. When a Type-I β-barium borate (BBO) crystal with 100-µm-thickness is added in the 800 nm probe beam path, a control second harmonic (CSH) beam is produced and transmitted collinearly with the fundamental beam. When the CSH beam is with vertical polarization, the interference between the THz-induced second harmonic (TISH) beam and CSH beam occurs and a linear component in positive correlation with THz field is observed in the measured signal.
Fig. 1. Coherent detection of THz pulse with different liquid films. (a) Diagram of the experimental system. (b) Measured corresponding spectra, where the blue and red lines in correspond to the results detected in the pure water and 2 mol/L CsI solution, respectively. THz time-domain waveforms detected in (c) aqueous iodide solutions (LiI, NaI, KI, CsI), (d) aqueous bromide solutions (LiBr, NaBr, KBr), and (e) aqueous chloride solutions (LiCl, NaCl, KCl, CsCl). The concentrations for all aqueous salt solutions are 4 mol/L except that the CsI solution concentration is set as its saturation value of 2 mol/L. The waveform detected in the pure water is presented in each plot for comparison.
A liquid film device is adopted to produce a stable 50 ± 4 µm thick flowing film with 3 mm width, which is larger than the spot size of the THz beam. Pressurized by a micro self-priming peristaltic pump, the liquid is ejected through a flat needle at rate of ∼80 mL/min. The viscosity changes of different solutions can be ignored and the thicknesses keep constant using our forced-flowing liquid film device. On the femtosecond timescale involved in the experiment, the flowing liquid film can be treated as the stationary isotropic medium during the process of ionization. All the measurements are performed at room temperature of 22 ± 1 °C.
2.2 Enhanced coherent detection sensitivity with aqueous salt solutions
We conduct the coherent THz detection experiments using eleven kinds of aqueous salt solutions. The concentrations of all the aqueous salt solutions are set as the same value of 4 mol/L, except for the CsI solution with the saturation concentration of 2 mol/L. The other experimental conditions are exactly the same for different aqueous salt solutions and pure water. The detected time-domain waveforms of THz fields in aqueous iodide solutions of CsI, LiI, NaI, KI are shown in Fig. 1(c). Compared with the observed signal in the pure water (the blue curve), the detection sensitivities are much higher in all the four aqueous iodide solutions. The detected signal enhancement can be attributed to the addition of salt ions. One can observe a clear sequence of the detection sensitivity, that is CsI > LiI > NaI > KI > water. In particular, the sensitivity is enhanced by 2.3 times in CsI solution compared to the pure water. The results demonstrate that the detection sensitivities of aqueous iodide solutions are not arranged in the ascending order of the atomic weights of ions except that cesium (Cs+) is the largest. The results for the aqueous bromide and chloride solutions are shown in Figs. 1(d) and 1(e), respectively, where CsBr solution is excluded in the experiment due to its high toxicity. Similarly, the detection sensitivities in the aqueous bromide and chloride solutions are basically higher than that in the pure water.
To further investigate the detection sensitivity dependent parameters, we plot the peak signal intensity as a function of the linear refractive index of solid salts for different solutions, as illustrated in Fig. 2(a). One can expect that the linear refractive index of the aqueous salt solution is proportional to the linear refractive index of the dissolved solid salt and its concentration, which will be demonstrated by Eq. (2) in the following section. Here, we take the refractive indices of eight kinds of solid salts [29] at the frequency of 9 THz, which is within the frequency range of the THz source used in our experiments. To the best of our knowledge, the refractive indices of the other three kinds of salts (LiCl, LiBr, and LiI) at 9 THz were not reported in the literature. The most reasonable data for reference are as follows: the index is 1.2944 for LiCl at 18 THz, 1.338 for LiBr at 15 THz, and 1.6605 for LiI at 12 THz [29]. In Figs. 2(a) and 3(a), we still match the experimental results with the listed refractive indices of these 3 kinds of salts, but they are shown by hollow squares to distinguish. It is obvious that the detection signal intensity increases monotonically with the linear refractive index of different solutions. The quadratic fitting results is also demonstrated in Fig. 2(a) and the specific scaling law of coherent detection signal intensity on the linear refractive index will be illustrated in detail in the following section. Figures 2(b)–2(d) show that the signal intensities increase with the concentration increments for different aqueous salt solutions. We attribute the detection sensitivity enhancement to the refractive index increase of solution with higher concentration.
Fig. 2. Coherent detection using different aqueous salt solutions. (a) Measured peak signal intensities (squares) as a function of the linear refractive indices of solid salts for different solutions, where a quadratic fitting curve is presented by the red dotted line. The concentrations for all aqueous salt solutions are 4 mol/L except that the CsI solution concentration is set as its saturation value of 2 mol/L. The dependence of the measured peak signal intensity on the concentration of (b) aqueous iodide solutions, (c) aqueous bromide solutions, and (d) aqueous chloride solutions, respectively.
Fig. 3. Incoherent detection using different aqueous salt solutions. The TISH energy signals detected in (a) aqueous iodide solutions (LiI, NaI, KI, CsI), (b) aqueous bromide solutions (LiBr, NaBr, KBr), (c) aqueous chloride solutions (LiCl, NaCl, KCl, CsCl), the TISH energy signal detected in the pure water is presented for comparison. The concentrations for all aqueous salt solutions are 4 mol/L except that the CsI solution concentration is set as its saturation value of 2 mol/L. (d) Measured peak TISH energy signal intensities (squares) as a function of the linear refractive indices of solid salts for different solutions, where a quartic fitting curve is presented by a red dotted curve. The concentrations for all aqueous salt solutions are 4 mol/L except that the CsI solution concentration is set as its saturation value of 2 mol/L. (e) The dependence of measured peak TISH energy signal intensities on the concentrations for six representative solutions.
2.3 Scaling law of coherent detection signal intensity
Comparing the experimental results with the quadratic fitting curve in Fig. 2(a), the signal intensity scales quadratically in approximation with the refractive index of solid salt in the aqueous salt solution. The quadratic scaling law can be attributed to the four-wave mixing model, because the coherent detection signal ${S_{2\omega }}$ is linearly dependent on the third-order nonlinear susceptibility ${\chi ^{(3)}}$ and the latter is quadratically proportional to the linear refractive index ${\eta _0}$ of the solution, i.e.,
where this equation will be proved by Eqs. (4) and (5) below.According to Refs. [13] and [30], ${\chi ^{(3)}}$ increases linearly with the solution concentration C, i.e., ${\chi ^{(3)}} \propto C$, which can get
The linear refractive index ${\eta _\textrm{s}}$ of a solid salt is a constant for a given frequency. One can set $\eta _0^2 \propto C\eta _s^2 + \alpha $, and combine Eqs. (1) and (2), and then obtain $C\eta _s^m \propto C\eta _s^2 + \alpha $, where m and α are constants. By comparing these equations, the following relation can be easily obtained. Equation (3) can well explain the signal intensity scaling with $\eta _s^2$ observed in Fig. 2(a). This equation can also explain Figs. 2(b)–2(d), which illustrate that the coherent detection signal intensity rises with the increase of the solution concentration C. The increasing slope of the signal intensity with C in Figs. 2(b)–2(d) is larger for the aqueous salt solution with higher refractive index. For the aqueous iodide solutions with relatively large sensitivity increments in Fig. 2(b), the signal of CsI solution gets saturated and remains unchanged when the concentration is higher than 2 mol/L. For the NaI and KI solutions, the linear growth of the signal intensity with C holds in lower concentrations, but the growth becomes slow in higher concentrations as the saturation is approaching.Next, we deduce and prove Eq. (1), which is the key to derive Eq. (3). According to Ref. [11], the liquid-based detection mechanism is based on the TISH generation via the four-wave mixing process, where two fundamental laser beams and a THz beam are superimposed to produce a second harmonic beam. Based on the four-wave mixing model, the dependence relation of the TISH field ${E_{TISH}}$ on the THz field ${E_{TH\textrm{z}}}$ and the fundamental probe laser field ${E_\omega }$ is shown as ${E_{TISH}} \propto {\chi ^{(3)}}E_\omega ^2{E_{THz}}$. To achieve coherent detection, a CSH field is needed to spatiotemporally overlap with the TISH field. At a time point ${t_{THz}}$, the detection signal can be represented as the interference of the two SH fields. Considering that the pulse durations of the fundamental laser beam and the SH beam are at the scale of tens of femtoseconds, which is much smaller than the integration time of the detector, the signal obtained by PMT in the experiment is given by:
Obviously, the coherent detection signal intensity is linearly proportional to ${\chi ^{(3)}}$ which strongly depends on the refractive index of the solution. The refractive index $\eta $ can be defined as $\eta = {\eta _0} + {\eta _2}I$, where ${\eta _0}$ is the linear refractive index, ${\eta _2}$ is the nonlinear refractive index, I is the light intensity. The relation between ${\eta _2}$ and ${\chi ^{(3)}}$ is given by Ref. [31]
where ${\eta _2}$ is with the unit of ${m^2}\textrm{/W}$ and ${\chi ^{(3)}}$ is with ${m^2}\textrm{/}{\textrm{V}^2}$. In our study, ${\eta _2}$ is around $2.5 \times {10^{ - 20}}{m^2}\textrm{/W}$ in terms of the experimental data and changes much more slightly than ${\eta _0}$ for different aqueous salt solutions. Therefore, ${\eta _2}$ can be considered as a constant in our case. Then, the scaling law of the coherent detection signal intensity with the linear refractive index of the solution can be derived in approximation and shown as in the above Eq. (1).2.4 Scaling law of incoherent detection signal intensity
The first term of Eq. (4) corresponds to the incoherent detection component, which can be measured separately in the condition of ${|{{\chi^{(3 )}}E_\omega^2{E_{THz}}} |^2} \gg |{\chi ^{(3 )}}E_\omega ^2E_{CSH}^{\ast }{E_{THz}}|$. The incoherent detection can be easily implemented if the CSH beam is not adopted in the experiment. In this case, the detected signal only contains the TISH beam, i.e.,
2.5 THz time-domain waveform comparison in coherent detection
The normalized THz time-domain waveforms by coherent detection in different aqueous salt solutions are presented in Figs. 4(a)–4(c). Comparing to the pure water (the blue curve), the waveform in the first half cycle maintains the same shape while the waveform in the second half cycle (or the first negative half cycle) appears apparent attenuation in the aqueous salt solutions. We calculate the differences of the THz filed amplitude in the second half cycle between the aqueous salt solutions and pure water cases. The corresponding dependence on the linear refractive index of solid salt in the solution is shown Fig. 4(d). The results reveal that the attenuation is positively correlated to the refractive index. Note that the TISH field is generated as the liquid plasma is formed in our experiment [11]. The ionization excited by the probe laser beam mainly appears in the first half cycle of THz pulse since the laser duration is about 50 fs. The interactions between the THz pulse and the formed plasma occurs mainly after the first half cycle of the THz pulse, which results in the apparent attenuation appearing in the second half cycle of THz time-domain waveform. In addition, in the aqueous salt solution with a higher refractive index (i.e. higher third-order nonlinear susceptibility), the laser beam tends to form a stronger plasma filament which then causes larger attenuation to the transmitted THz field.
Fig. 4. Normalized THz time-domain waveforms by coherent detection in (a) aqueous iodide solutions (LiI, NaI, KI, CsI), (b) aqueous bromide solutions (LiBr, NaBr, KBr), and (c) aqueous chloride solutions (LiCl, NaCl, KCl, CsCl). The concentrations for all aqueous salt solutions are 4 mol/L except that the CsI solution concentration is set as its saturation value of 2 mol/L. The waveform detected in the pure water is presented in each plot for comparison. (d) Differences of the THz filed amplitude in the second half cycle between the aqueous salt solutions and pure water cases.
3. Conclusion
In conclusion, we have experimentally found that the water-based coherent detection scheme can be extended to other liquids and the detection sensitivity for broadband THz pulses can be significantly improved by replacing water with aqueous salt solutions. The detection mechanism is based on the four-wave mixing and the improved sensitivity can be attributed to the enhanced third-order nonlinear susceptibility ${\chi ^{(3 )}}$ or the linear refractive index ${\eta _0}$ by adding salt ions in the water. We have observed that the THz coherent detection signal intensity in the aqueous salt solutions scales linearly with ${\chi ^{(3 )}}$ or quadratically with ${\eta _0}$, while the incoherent detection signal intensity scales quadratically with ${\chi ^{(3 )}}$ or quartically with ${\eta _0}$. Moreover, since ${\chi ^{(3 )}}$ and ${\eta _0}$ basically increases with the increase of the solution concentration C, the increasing rate of coherent or inherent detection signal intensity shows a positive correlation with $\textrm{d}{\chi ^{(3 )}}/\textrm{d}C$.
Our work provides not only a scheme to enhance the sensitivity of THz coherent detection in liquids, but also a feasible route to explore the physicochemical properties of various liquids in the THz band. The correlation of the THz detection signal with ${\chi ^{(3 )}}$ or ${\eta _0}$ of aqueous salt solution has been established, which can be extended to more kinds of liquids and regarded as a simple rule of thumb to assess the nonlinear susceptibility of various liquids. Furthermore, as the rotation and vibrational energy levels of many biomacromolecules locate in the THz range, we believe the liquid-based THz detection technique is of great potential in measuring and even controlling the characteristics of biomacromolecular solutions [32,33].
Funding
National Natural Science Foundation of China (12074272, 61905271); National Defense Science and Technology Innovation Fund of the Chinese Academy of Sciences (20-163-02-ZT-008-009-01); Strategic Priority Research Program of Chinese Academy of Sciences (XDA25050300); National Key Research and Development Program of China (2018YFA0404801); Basic and Applied Basic Research Foundation of Guangdong Province (2020A1515011083); Air Force Office of Scientific Research (FA9550-21-1-0389), (FA9550-21-1-0300); National Science Foundation (ECCS-2152081).
Acknowledgments
We would like to thank Dr. Yong Tan for helpful discussion.
Disclosures
The authors declare no conflicts of interest.
Data availability
The data underlying the results presented herein are not publicly available currently but can be obtained from the authors upon reasonable request.
References
1. Y. Cai, I. Brener, J. Lopata, J. Wynn, L. Pfeiffer, J. B. Stark, Q. Wu, X. C. Zhang, and J. F. Federici, “Coherent terahertz radiation detection: direct comparison between free-space electro-optic sampling and antenna detection,” Appl. Phys. Lett. 73(4), 444–446 (1998). [CrossRef]
2. T. Harter, “Silicon-plasmonic integrated circuits for terahertz signal generation and coherent detection,” Nat. Photonics 12(10), 625–633 (2018). [CrossRef]
3. J. Dai, X. Xie, and X.-C. Zhang, “Detection of broadband terahertz waves with a laser-induced plasma in gases,” Phys. Rev. Lett. 97(10), 103903 (2006). [CrossRef]
4. A. Nahata, D. H. Auston, T. F. Heinz, and C. Wu, “Coherent detection of freely propagating terahertz radiation by electro-optic sampling,” Appl. Phys. Lett. 68(2), 150–152 (1996). [CrossRef]
5. T. Löffler, T. Hahn, M. Thomson, F. Jacob, and H. G. Roskos, “Large-area electro-optic ZnTe terahertz emitters,” Opt. Express 13(14), 5353 (2005). [CrossRef]
6. B. Ferguson and X.-C. Zhang, “Materials for terahertz science and technology,” Nat. Mater. 1(1), 26–33 (2002). [CrossRef]
7. G. Gallot, J. Zhang, R. W. McGowan, T.-I. Jeon, and D. Grischkowsky, “Measurements of the THz absorption and dispersion of ZnTe and their relevance to the electro-optic detection of THz radiation,” Appl. Phys. Lett. 74(23), 3450–3452 (1999). [CrossRef]
8. X. Lu, N. Karpowicz, Y. Chen, and X.-C. Zhang, “Systematic study of broadband terahertz gas sensor,” Appl. Phys. Lett. 93(26), 261106 (2008). [CrossRef]
9. C. Rønne, L. Thrane, P.-O. Åstrand, A. Wallqvist, K. V. Mikkelsen, and S. R. Keiding, “Investigation of the temperature dependence of dielectric relaxation in liquid water by THz reflection spectroscopy and molecular dynamics simulation,” J. Chem. Phys. 107(14), 5319–5331 (1997). [CrossRef]
10. J. K. Vij, D. R. J. Simpson, and O. E. Panarina, “Far infrared spectroscopy of water at different temperatures: GHz to THz dielectric spectroscopy of water,” J. Mol. Liq. 112(3), 125–135 (2004). [CrossRef]
11. Y. Tan, H. Zhao, W.-M. Wang, R. Zhang, Y.-J. Zhao, C.-L. Zhang, X.-C. Zhang, and L.-L. Zhang, “Water-based coherent detection of broadband terahertz pulses,” Phys. Rev. Lett. 128(9), 093902 (2022). [CrossRef]
12. Y. E. L. Zhang, A. Tcypkin, S. Kozlov, C. Zhang, and X.-C. Zhang, “Broadband THz sources from gases to liquids,” Ultrafast Sci. 2021, 1–7 (2021). [CrossRef]
13. D. Débarre and E. Beaurepaire, “Quantitative characterization of biological liquids for third-harmonic generation microscopy,” Biophys. J. 92(2), 603–612 (2007). [CrossRef]
14. W. E. Kucia, G. Sharma, C. S. Joseph, S. Sarbak, C. Oliver, A. Dobek, and R. H. Giles, “Optical Kerr effect of tRNA solution induced by femtosecond laser pulses,” Chem. Phys. Lett. 662, 132–136 (2016). [CrossRef]
15. M. Kaminsky, “Ion-solvent interaction and the viscosity of strong-electrolyte solutions,” Discuss. Faraday Soc. 24, 171–179 (1957). [CrossRef]
16. R. A. McAllister, “The viscosity of liquid mixtures,” AIChE J. 6(3), 427–431 (1960). [CrossRef]
17. J. A. Lazzús, “A group contribution method to predict the melting point of ionic liquids,” Fluid Phase Equilib. 313, 1–6 (2012). [CrossRef]
18. J.-P. Ryckaert and A. Bellemans, “Molecular dynamics of liquid N-butane near its boiling point,” Chem. Phys. Lett. 30(1), 123–125 (1975). [CrossRef]
19. S. Funkner, G. Niehues, D. A. Schmidt, M. Heyden, G. Schwaab, K. M. Callahan, D. J. Tobias, and M. Havenith, “Watching the low-frequency motions in aqueous salt solutions: the terahertz vibrational signatures of hydrated ions,” J. Am. Chem. Soc. 134(2), 1030–1035 (2012). [CrossRef]
20. F. Sebastiani, S. L. P. Wolf, B. Born, T. Q. Luong, H. Cölfen, D. Gebauer, and M. Havenith, “Water dynamics from THz spectroscopy reveal the locus of a liquid–liquid binodal limit in aqueous CaCO3 solutions,” Angew. Chem. Int. Ed. 56(2), 490–495 (2017). [CrossRef]
21. H. Ohtaki and T. Radnai, “Structure and dynamics of hydrated ions,” Chem. Rev. 93(3), 1157–1204 (1993). [CrossRef]
22. T. Dodo, M. Sugawa, E. Nonaka, H. Honda, and S. Ikawa, “Absorption of far-infrared radiation by alkali halide aqueous solutions,” J. Chem. Phys. 102(15), 6208–6211 (1995). [CrossRef]
23. S. E. M. Colaianni and O. F. Nielsen, “Low-frequency Raman spectroscopy,” J. Mol. Struct. 347, 267–283 (1995). [CrossRef]
24. K. Mizoguchi, Y. Hori, and Y. Tominaga, “Study on dynamical structure in water and heavy water by low-frequency Raman spectroscopy,” J. Chem. Phys. 97(3), 1961–1968 (1992). [CrossRef]
25. M. C. Hoffmann, N. C. Brandt, and H. Y. Hwang, “Terahertz Kerr effect,” Appl. Phys. Lett. 95(23), 231105 (2009). [CrossRef]
26. H. Zhao, Y. Tan, L. Zhang, R. Zhang, M. Shalaby, C. Zhang, Y. Zhao, and X.-C. Zhang, “Ultrafast hydrogen bond dynamics of liquid water revealed by terahertz-induced transient birefringence,” Light: Sci. Appl. 9(1), 136 (2020). [CrossRef]
27. H. Zhao, Y. Tan, R. Zhang, Y. Zhao, C. Zhang, and L. Zhang, “Anion–water hydrogen bond vibration revealed by the terahertz kerr effect,” Opt. Lett. 46(2), 230 (2021). [CrossRef]
28. Y. Tan, H. Zhao, R. Zhang, C. Zhang, Y. Zhao, and L. Zhang, “Ultrafast optical pulse polarization modulation based on the terahertz-induced Kerr effect in low-density polyethylene,” Opt. Express 28(23), 35330 (2020). [CrossRef]
29. H. H. Li, “Refractive index of alkali halides and its wavelength and temperature derivatives,” J. Phys. Chem. Ref. Data 5(2), 329–528 (1976). [CrossRef]
30. R. Tokarz, N. Cisek, U. Prent, V. Fekl, and Barzda, “Measuring the molecular second hyperpolarizability in absorptive solutions by the third harmonic generation ratio technique,” Anal. Chim. Acta 755, 86–92 (2012). [CrossRef]
31. W Nie, “Optical nonlinearity: phenomena, applications, and materials,” Adv. Mater. 5(7-8), 520–545 (1993). [CrossRef]
32. G. S. Ott, “The DNA melting transition in aqueous magnesium salt solutions,” Biochemistry 14(15), 3431–3438 (1975). [CrossRef]
33. H. I. Okur, J. Hladílková, K. B. Rembert, Y. Cho, J. Heyda, J. Dzubiella, P. S. Cremer, and P. Jungwirth, “Beyond the Hofmeister series: ion-specific effects on proteins and their biological functions,” J. Phys. Chem. B 121(9), 1997–2014 (2017). [CrossRef]
