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Detection of faint fluxes of photons at terahertz frequencies is crucial for various applications including biosensing, medical diagnosis, chemical detection, atmospheric studies, space explorations, high-data-rate communication, and security screening. Heterodyne terahertz spectrometers based on cryogenically cooled superconducting mixers have so far been the only instruments that can provide high spectral resolution and near-quantum-limited sensitivity levels. The operation temperature, bandwidth constraints, and complexity of these terahertz spectrometers have restricted their use to mostly astronomy and atmospheric studies, limiting the overall impact and wide-spread use of terahertz technologies. Here we introduce a spectrometry scheme that uses plasmonic photomixing for frequency downconversion to offer quantum-level sensitivities at room temperature for the first time. 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 spectrometer sensitivities down to 3 times the quantum-limit at room temperature. Our presented spectrometry scheme can be applicable to resolve both the high-resolution spectra of gas molecules and mid-resolution spectra of condensed phase samples over a total operable bandwidth of 0.1-5 THz. As an example, we use the presented spectrometer to resolve the spectral information of ammonia, which has a number of narrowband absorption peaks over the 0.1-5 THz frequency range. With a versatile design capable of broadband spectrometry, this plasmonic photomixer has broad applicability to quantum optics, chemical sensing, biological studies, medical diagnosis, high data-rate communication, as well as astronomy and atmospheric studies.
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