Ultrabroadband THz Time-Domain Spectroscopy of a Free-Flowing Water Film

Abstract

We demonstrate quantitative ultrabroadband THz time-domain spectroscopy (THz-TDS) of water by application of a 17-m thick gravity-driven wire-guided flow jet of water. The thickness and stability of the water film is accurately measured by an optical intensity crosscorrelator, and the standard deviation of the film thickness is less than 500 nm. The cross section of the water film is found to have a biconcave cylindrical lens shape. By transmitting through such a thin film, we perform the first ultrabroadband (0.2-30 THz) THz-TDS across the strongest absorbing part of the infrared spectrum of liquid water using two different THz-TDS setups. The extracted absorption coefficient and refractive index of water are in agreement with previous results reported in the literature. With this we show that the thin free-flowing liquid film is a versatile tool for windowless, ultrabroadband THz-TDS with sub-100-femtosecond time resolution of aqueous solutions in transmission mode in the important cross-over region between vibrational and relaxational dynamics.Index Terms-Air plasma, photoconductive antenna, THz spectroscopy, ultrafast nonlinear optics, water hydrogen bond network.

Design Considerations for Integration of Terahertz Time-Domain Spectroscopy in Microfluidic Platforms

“…(This fundamental equation was initially reported by Jepsen and Fischer [34] and its derivation is given in the Appendix.) A piece-wise equation for the refractive index of water is fit to data reported by Wang et al [38]. The piece-wise function for refractive index is n = −1.8f + 3.1, for f < 0.5 THz with respective slope and intercept uncertainties of 10% and 1%; and n = −0.1f + 2.3, for f > 0.5 THz with respective slope and intercept uncertainties of 7% and 1%.…”

“…Figure 3a shows the maximum measurable absorption coefficient for the dynamic range values of DR = 5.8 × 10 3 , 5.7 × 10 3 , 2.4 × 10 3 , 3.0 × 10 2 , 2.8 × 10 1 , and 2.8 × 10 0 and a constant sample thickness of d = 120 µm, plotted as red, yellow, green, blue, indigo, and violet solid lines, respectively. The absorption coefficient of water, α water = 121f + 109 cm −1 , is quantified by fitting data from Wang et al [38] to a linear fit of R 2 = 0.978 with both slope and intercept uncertainties being 4%, and is plotted as a solid black line. The intersection of the α water curve with the maximum measurable absorption coefficient curves defines each of the maximum measurable frequencies of f 0-5 , which correspond to the respective DR values of 5.8 × 10 3 , 5.7 × 10 3 , 2.4 × 10 3 , 3.0 × 10 2 , 2.8 × 10 1 , and 2.8 × 10 0 .…”

Time resolved broadband terahertz relaxation dynamics of electron in water

“…TRTS is not only useful for characterizing charge carriers under steady state conditions, but is also qualified for nonequilibrium measurements. In the experiment we use air photonics terahertz time domain spectroscopy (THz-TDS) setup [6], which gives usable bandwidth up to 12 THz. The electron in water is injected by photodetachment from K 4 Fe(CN) 6 using UV pump at 267 nm.…”

“…In the experiment we use air photonics terahertz time domain spectroscopy (THz-TDS) setup [6], which gives usable bandwidth up to 12 THz. The electron in water is injected by photodetachment from K 4 Fe(CN) 6 using UV pump at 267 nm. The anion [Fe(CN) 6 ] 4-has a high quantum yield with one photon absorption at 267 nm.…”

“…The electron in water is injected by photodetachment from K 4 Fe(CN) 6 using UV pump at 267 nm. The anion [Fe(CN) 6 ] 4-has a high quantum yield with one photon absorption at 267 nm. The 267 nm UV pulse with pulse energy of 30 µJ is generated by frequency tripling part of the fundamental 800 nm pulse.…”