Detecting terahertz radiation is a chore. Terahertz waves are of interest for a variety of imaging applications because they do not interact strongly with many materials, but if they are not absorbed, they cannot be detected. Now researchers in Japan have demonstrated a direct-detection superconducting tunnel junction terahertz detector that is sensitive and broadband and that can be scaled to an array format.Seiichiro Ariyoshi led the team at Riken in Wako in collaboration with scientists at Saitama University and at the National Astronomical Observatory of Japan in Tokyo to produce the proof-of-concept detector. The device incorporates an ~140-μm-diameter antenna structure that is responsive from 0.3 to 1.2 THz. A superconducting microstrip line is placed on the axis of symmetry of the antenna, along which are distributed six pairs of niobium superconducting tunnel junctions. The tunnel junctions are spaced to provide maximum sensitivity at 0.66 THz.Terahertz waves penetrate visually opaque objects. A high-sensitivity detector uses precisely spaced superconducting tunneling junctions to tailor the bandwidth to the application. Here, a railway payment integrated circuit card (a) and needles embedded in powdered milk (b) are imaged with the terahertz detector. Courtesy of Seiichiro Ariyoshi. Reprinted with permission. ©2006 American Institute of Physics.The currents from the individual tunnel junctions are summed to provide a broadband response extending from 0.60 THz to niobium’s superconductor gap at 0.72 THz. When operated at 0.3 K, the leakage current is less than 1 pA/μm2, resulting in a sensitivity of 10–16 W/√Hz. The dynamic range is 76 dB, more than 20 dB better than that of a semiconductor bolometer, and the response time is much less than the 1 ms of bolometers.To demonstrate the performance of the superconducting tunnel junction detector, the researchers imaged through visually opaque samples scanned across a focused beam of terahertz radiation. Although the data acquisition and translation speeds extended the acquisition time to about 40 minutes for a 50 × 50-mm area at 200-μm resolution, the signal-to-noise ratio of the one-pixel detector is high enough to enable much more rapid acquisition. With a 100-pixel array, a 6 × 6-mm area could be scanned in about 1 s, they note.The size and spacing of the superconducting tunnel junctions along the microstrip determine the detector’s peak spectral responsivity. Because the antenna has a broad spectral response, a multispectral array can be created by changing the size and location of the tunnel junctions, utilizing the intrinsic bandpass filtering of each element of the detector.The investigators are testing a 6 × 6 detector array fabricated on a 5 × 5-mm sapphire substrate 400 μm in thickness. To minimize detector noise, they also are working in parallel on a reliable and efficient refrigerator that can maintain the detector at 0.3 K. The sensitivity of the detector is important in low-flux conditions, such as exist for astronomical observations through the 0.66-THz atmospheric transmission window. The tunability of the detector will loosen restrictions on terahertz sources, facilitating the construction and application of terahertz imaging systems, Ariyoshi said.Applied Physics Letters, May 15, 2006, 203503.