Detecting extremely weak THz radiation
Thermography is one example of a passive method, but its application has been limited because of restricted spectral and spatial resolutions. In general, passive microscopy has so far been relatively unexplored despite its potential importance. This is because the radiation to be detected can be extremely weak and sufficiently sensitive microscopes were not readily available. Recently, however, the situation is beginning to change. While the development of superconducting sensors is expanding the spectral range of photon counting towards the infrared region, photon counting has been realized in the THz/GHz region with semiconductor quantum-dot (QD) detectors. Highly sensitive microscopy in the THz region is hence possible if one successfully combines those QD detectors with appropriately designed microscopes. A spectral resolution of Δλ/λ < 0.05 is obtained by a narrow bandwidth of the detectors, where the wavelength of the maximum sensitivity is magnetically tunable over λ = 120–170 μm. The highest sensitivity achieved in this actual measurement is about 0.1 count per second at λ ∼ 130 μm. This corresponds to the incidence of about one photon per second, or 10^{−21} watt, on the detector, and is roughly about 1000 times more sensitive than the level of present astronomy.