Enhancing the Amplitude of Terahertz Transients Using Lock-In Amplification and a Spinning Nonlinear Crystal

A terahertz time-domain spectroscopy experiment is discussed where the few-cycle terahertz pulses are generated through optical rectification and detected using electro-optic sampling and a lock-in technique. It is demonstrated theoretically and experimentally that the measured amplitude may be enhanced if the terahertz signal is modulated by spinning the source crystal instead of the usual on/off chopping of the pump laser pulse.