An ultra-high-resolution technique for detection of terahertz pulses

We present a novel technique for the detection terahertz pulses generated by femtosecond lasers. It features a Hz-level resolution and does not require a mechanical delay stage or a second femtosecond laser. It works by opto-electronically downconverting the individual frequency components composing the terahertz pulse. Unlike other techniques, it does not require an external spectrum analyzer or a lock-in amplifier for its implementation, although it can also be implemented using them. We believe this technique is a valuable tool to study the ultimate frequency stability of terahertz pulsed systems and the phase noise of the femtosecond lasers that drives them.