Abstract
Ultra-highly sensitive terahertz detectors, called charge-sensitive infrared phototransistors (CSIPs), are
described. The detectors are fabricated in GaAs/AlGaAs double quantum well (QW) structures, where
electrons in an isolated upper QW (UQW) are photoexcited via intersubband transition, corresponding to
a wavelength in a range of 8 μm‒46 μm. Photoexcited electrons immediately tunnel out of UQW,
causing the UQW to positively charge up. The pile-up positive charge on the UQW (acting as a
photosensitive floating gate) is sensed by a conductance change in the lower QW (acting as the sourcedrain
channel). Unprecedented sensitivity has been achieved, leading to the first realization of passive
terahertz near-field microscopy. Described here are recent efforts of improving the performance of
CSIP: One is the careful design of photo-couplers for better efficiency of intersubband excitation
utilizing plasmonic antenna, and the other is the improvement of spectral performance, including the
development of multicolor detection with CSIP.
