Modulated orientation Sensitive THz Spectroscopy (MOSTS)

The far infrared spectroscopy of molecular crystals reveals both intra-molecular and intermolecular vibrationalmodes. However with the significant increase in complexity of crystal structures(e.g. protein), mode overlap also increases. We are developing a new technique called MOSTS (Modulated Orientation Sensitive THz Spectroscopy) to suppress the relaxationalmode from THz dielectric response in molecular crystals. We achieve high sensitivity and mode separation by using single crystal and rapid modulation of the relative alignment of the terahertz polarization and the crystal axes by rotating the sample.  By locking into the signal at the rotation frequency we determine the polarization sensitive signal as a function of phase and map out the optically active vibrationalresonances for different orientations of the crystal. The method is only sensitive to the anisotropic part of the spectroscopic response.

In a standard THz-TDS a photoconductive GaASantenna is AC biased and  excited by 800nm femto-second laser pulse. The bias is used as modulation frequency for lockindetection. In MOSTS the generating antenna is DC biased and the sample is rotated. The orientation of the crystal axes, not the bias of the antenna, is used as modulation for lockin detection.