View source for File:FSEOS of THz.png

In free-space electro-optic sampling, the THz waveform and a ultra-short "probe" pulse propagate collinearly through an electro-optic medium. The THz field induces birefringence in the EO medium through the Pockel effect, rotating the polarization of the "probe" beam. The imbalance in s- and p- polarization causes a current, proportional to the THz field and direction, to flow from the balanced photodoiodes.