Detection of Zeptojoule Terahertz Pulses via Parametric Upconversion

We demonstrate a high-sensitivity room-temperature detection scheme for terahertz (THz) radiation based on parametric frequency upconversion. The upconverted photons generated by the mixing of a THz pulse with a near-infrared (NIR) pulse in a (110)-oriented GaP crystal are spectrally resolved using a monochromator and a commercial single-photon detector sensitive in the NIR. We detect THz pulses with energies as low as 1.4 zJ (or about 1.5 photons per pulse) at a frequency of 2 THz when averaged over only 50k pulses.