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Ultra-sensitive, room-temperature THz detection based on parametric upconversion by using a pulsed 1550nm optical source

We demonstrate ultra-sensitive optical detection of terahertz by using nonlinear parametric upconversion. Terahertz radiation is mixed with pump light at 1550 nm in a nonlinear crystal to generate an optical sideband or idler wave. The idler signal is separated from the optical pump, coupled into an optical fiber and detected using a Geiger-mode avalanche photo-diode. Our scheme to detect THz waves leverages mature technology at 1550 nm developed for telecommunications to enable ultra-sensitive detection at room-temperature. We have fabricated a diffusion-bonded, quasi phasematched GaAs crystal, a χ(2) nonlinear material, that is pumped with a readily obtainable erbium doped fiber amplifier to perform the parametric conversion. We demonstrate efficient upconversion of terahertz radiation using both a continuous-wave THz source operating at 0.82 THz and a pulsed sub-picosecond THz source with spectral coverage from 0.5 THz to 1.5 THz. The resulting THz detector has a noise equivalent power of 78 fW/Hz1/2 with a timing resolution of 1 ns. χ(2) nonlinear interactions are intrinsically very fast; our temporal bandwidth is limited by the optical detector. Additionally, the THz detector demonstrates a broadband response with a phase-matching bandwidth exceeding 1 THz. This noise equivalent power of 78 fW/Hz1/2 and the corresponding power conversion efficiency of 1.2× 10-3 are the best reported, to our knowledge. This paper presents both theoretical and experimental results.