Backward Electro-Optic Detection for Narrowband Terahertz Time-Domain Spectroscopy

A new approach to electro-optic (EO) detection is demonstrated through a novel geometry, whereby the probe and terahertz electric fields propagate in opposite directions. Theoretical analyses and experimental measurements are performed using a representative ZnGeP<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> crystal implemented within this backward EO detection configuration. The backward EO detection signals encompass frequencies within a narrow spectral band of a few hundred gigahertz, tunable from ∼0.6–2.4 THz through adjustment of the incidence angle of the electric fields relative to the EO crystal. This novel arrangement provides EO detection with new phase-matching possibilities and offers a powerful tool for narrow-band spectroscopy.