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

We demonstrate quantitative ultrabroadband THz time-domain spectroscopy (THz-TDS) of water by application of a 17- μm thick gravity-driven wire-guided flow jet of water. The thickness and stability of the water film is accurately measured by an optical intensity cross-correlator, 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.