Sensitive detection of terahertz pulses via parametrically upconverted near-infrared photons

We combine electro-optic sampling with single-photon counting technology to resolve weak terahertz (THz) radiation. A gallium-phosphide (GaP) crystal is used as the nonlinear medium for the parametric frequency conversion of broadband terahertz radiation to near-infrared (NIR) photons. The upconverted photons are measured using a monochromator combined with a single-photon counting module (SPCM). Results show good agreement of the measured upconverted photons with the theory of sum-frequency mixing [1]. We also demonstrate that our detection technique is 180 times faster than standard electro-optic sampling and can access selected spectral components of the THz field with a resolution of 170 GHz. At 2.5 THz, results show a minimum detectable THz pulse energy of $20\times 10^{-21}$ J, which corresponds to 3 photons/pulse. This THz detection technique based on a SPCM holds a strong potential for high-speed wireless THz communication systems as well as quantum sensing [2, 3].