  Context: Reliable, directly measured optical properties of astrophysical ice
analogs in the infrared (IR) and terahertz (THz) range are missing. These
parameters are of great importance to model the dust continuum radiative
transfer in dense and cold regions, here thick ice mantles are present, and are
necessary for the interpretation of future observations planned in the far-IR
region. Aims: Coherent THz radiation allows direct measurement of the complex
dielectric function (refractive index) of astrophysically relevant ice species
in the THz range. Methods: The time-domain waveforms and the frequency-domain
spectra of reference samples of CO ice, deposited at a temperature of 28.5 K
and annealed to 33 K at different thicknesses, have been recorded. A new
algorithm is developed to reconstruct the real and imaginary parts of the
refractive index from the time-domain THz data. Results: The complex refractive
index in the wavelength range of 1 mm - 150 ${\mu}$m (0.3 - 2.0 THz) has been
determined for the studied ice samples, and compared with available data found
in the literature. Conclusions: The developed algorithm of reconstructing the
real and imaginary parts of the refractive index from the time-domain THz data
enables, for the first time, the determination of optical properties of
astrophysical ice analogs without using the Kramers-Kronig relations. The
obtained data provide a benchmark to interpret the observational data from
current ground based facilities as well as future space telescope missions, and
have been used to estimate the opacities of the dust grains in presence of CO
ice mantles.
