Brewster reflection-enhanced electro-optic sampling for high-sensitivity terahertz time-domain spectroscopy

We report an electro-optic sampling (EOS) scheme that uses the reflection Brewster effect from a single ZnSe window surface to enhance EOS sensitivity. By directing the probe beam near its Brewster angle, the terahertz (THz)-induced ellipticity is amplified while the strong probe background is suppressed prior to balanced detection. This reflection geometry eliminates the birefringence and scattering noises associated with window transmission in previously reported configurations. Using GaP as the EO medium, we achieve a tenfold improvement in signal-to-noise ratio (SNR), reaching the fundamental polarization noise and enabling 0.1 V/m sensitivity (1 s acquisition) over a 4.5 THz bandwidth. Our theoretical analysis extracts key polarization noise parameters and provides explicit criteria for optimizing SNR. This approach is readily extendable to other polarization-sensitive ultrafast measurements.