Efficient THz-to-NIR upconversion detection for quantum-level THz detection

Abstract

We demonstrate room-temperature terahertz (THz) detection schemes that combine parametric upconversion and engineered photonic structures to achieve ultrahigh sensitivity. Using a GaP nonlinear crystal, we detect THz pulses with energies as low as 1.4 zeptojoules (~1.5 photons per pulse) at 2 THz when averaged over just 50k pulses. Replacing GaP with the organic crystal BNA extends the upconversion bandwidth to 1–7.5 THz, achieving ~2% SFG photon-conversion efficiency and an ability to resolve individual THz pulses containing ~60 photons with a 50% click probability. We also demonstrate that the implementation of a microstructured device integrating a photonic-crystal waveguide with a plasmonic bull’s‑eye antenna further enhances THz detection sensitivity by more than one order of magnitude. Together, these approaches offer compact, broadband, and high-performance THz detection for quantum optics, secure wireless communication, and low-photon-flux spectroscopy.