Abstract
In this paper we report an investigation of the temperature dependence oí the far-infrared spectrum of liquid water in the range from 2 to 40 cm-1. To do this we have used a new experimental technique based on ultrashort electromagnetic pulses (terahertz pulses) radiated from small semiconductor antennas driven by femtosecond laser pulses.’ The bandwidth of the terahertz pulses is very broad, extending from 50 GHz to several terahertz (1 THz = 33.356 cm-1). By reflecting these pulses off a silicon-water interface and measuring the frequency-dependent change in phase and amplitude of the pulses, the optical constants, the index of refraction, and the absorption coefficient of liquid water can be obtained if knowledge of the optical constants of silicon is assumed. The pulse sequence obtained from the terahertz timedomain reflection spectrometer is shown in Fig. 1. By Fourier-transforming the two pulses and taking their complex ratio, one directly obtains the optical constants for water, as shown in Fig. 2. We observe good agreement with the literature for both index and absorption at room temperature.
© 1995 Optical Society of America
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