  We present a development of a high precision, tunable far-infrared
(terahertz) frequency-domain dielectric spectrometer for studying the dynamics
of biomolecules in aqueous solutions in the gigahertz-to-terahertz frequency.
As a first application we report on the measurement of the absorption and
refractive index for liquid water in the frequency range from 5 GHz to 1.12 THz
(0.17 to 37.36 cm-1 or 0.268 to 60 mm). The system provides a coherent
radiation source with a power up to 20 mW in the gigahertz-to-terahertz region.
The power signal-to-noise ratio of our instrument reaches 1015 and the system
achieves a spectral resolution of less than 100 Hz. The temperature of samples
can be controlled precisely with an error bars of 0.02 oC from above 0 oC to 90
oC. Given these attributes, our spectrometer provides unique capabilities for
the accurate measurement of even very strongly absorbing materials such as
aqueous solutions.
