Freeze-out of difference-phonon modes in ZnTe and its application in detection of THz pulses

Generation by optical rectification and detection by electro-optic sampling of freely propagating subpicosecond electromagnetic pulses (terahertz pulses) in ZnTe is limited by two broad difference-phonon absorption bands in the spectral region below the transverse optical resonance at 5.32 THz. In this letter, we show that, at temperatures below 80 K, these difference modes are frozen out and consequently ZnTe becomes completely transparent within our detection bandwidth. We utilize this effect to obtain a three-fold increase in detection efficiency at frequencies above 3 THz in a thin ZnTe detector operated at 80 K, compared to the performance at room temperature. (C) 2000 American Institute of Physics. [S0003-6951(00)02944-2].