  As high-average power ultrafast lasers become increasingly available for
nonlinear conversion, the temperature dependence of the material properties of
nonlinear crystals becomes increasingly relevant. Here, we present
temperature-dependent THz complex refractive index measurements of the organic
crystal BNA over a wide range of temperatures from 300 K down to 80 K for THz
frequencies up to 4 THz for the first time. Our measurements show that whereas
the temperature-dependent refractive index has only minor deviation from room
temperature values, the temperature-dependent absorption coefficient decreases
at low temperature. We additionally compare these measurements with conversion
efficiency and spectra observed during THz generation experiments in the same
temperature range and using the same crystal, using an ultrafast Yb-laser for
excitation. Surprisingly, the damage threshold of the material does not improve
significantly upon cooling, pointing to a nonlinear absorption mechanism being
responsible for damage. However, we observe a significant increase in THz yield
at lower temperatures, which is most likely due to the reduced THz absorption.
These findings will be useful for future designs of high average power pumped
organic-crystal based THz-TDS systems.
