Remote detection of biomolecules in the THz region

Summary form only given. Recent measurements of the electromagnetic transmission through certain bioparticles such as bacillus subtillus (BS), have revealed absorption resonances in the THz region. The physical origin of these resonances, be it nucleic acids, proteins or other biomolecules embedded in the particle, is not yet clear. But the resonances are repeatable and strong enough that one can estimate their detectability in a remote sensor. This paper summarizes calculations of the detectability of BS by four sensor modalities, including (1) passive incoherent (i.e., direct detection), (2) passive coherent (i.e., heterodyne radiometry), (3) active incoherent differential absorption spectroscopy, and (4) active coherent radar. Because of the small absorption cross section of the typical THz resonance (~10^-11 cm²), the small concentrations expected in the environment (<<10⁵ cm^-3), and the large background temperature at THz frequencies, passive techniques lack the sensitivity, even when implementing liquid-helium-cooled electronics. An active differential absorption spectrometer (DAS) is more promising using either a monostatic (i.e., retrodirective mirror) or bistatic (i.e., opposed transmitter Tx and receiver Rx) architecture. For example, a two-frequency DAS sensor operating at the absorption center (20.65 cm^-1 or 619.5 GHz) and the absorption edge (20.0 cm^-1 or 600 GHz) with 1 mW of transmit power and a room-temperature direct detector can obtain a receiver-noise-limited S/N ratio of unity through a 20-m cloud containing 10⁵ bioparticles/cm³ (minimum detectable concentration MDC) at a Tx-Rx separation of 1 km and Tx/Rx apertures of 100 cm²