Room-temperature heterodyne terahertz detection with quantum-level sensitivity

Our Universe is most radiant at terahertz frequencies (0.1-10.0 THz) (ref. (1)), providing critical information on the formation of the planets, stars and galaxies, as well as the atmospheric constituents of the planets, their moons, comets and asteroids(2-9). The detection of faint fluxes of photons at terahertz frequencies is crucial for many planetary, cosmological and astrophysical studies(10-14). For example, understanding the physics and molecular chemistry of the life cycle of stars and their relationship with the interstellar medium in galaxies requires heterodyne detectors with noise temperatures close to the quantum limit(15). Near-quantum-limited heterodyne terahertz detection has so far been possible only through the use of cryogenically cooled superconducting mixers as frequency downconverters(15-18). Here we introduce a heterodyne terahertz detection scheme that uses plasmonic photomixing for frequency downconversion to offer quantum-level sensitivities at room temperature. Frequency downconversion is achieved by mixing terahertz radiation and a heterodyning optical beam with a terahertz beat frequency in a plasmonics-enhanced semiconductor active region. We demonstrate terahertz detection sensitivities down to three times the quantum limit at room temperature. With a versatile design capable of broadband operation over a 0.1-5.0 THz bandwidth, this plasmonic photomixer has broad applicability to astronomy, cosmology, atmospheric studies, gas sensing and quantum optics.