  Semiconductor nanowire field-effect transistors represent a promising
platform for the development of room-temperature (RT) terahertz (THz) frequency
light detectors due to the strong nonlinearity of their transfer
characteristics and their remarkable combination of low noise-equivalent powers
(< 1 nW/Hz$^{1/2}$) and high responsivities (> 100 V/W). Nano-engineering a NW
photodetector combining high sensitivity with high speed (sub-ns) in the THz
regime at RT is highly desirable for many frontier applications in quantum
optics and nanophotonics, but this requires a clear understanding of the origin
of the photo-response. Conventional electrical and optical measurements,
however, cannot unambiguously determine the dominant detection mechanism due to
inherent device asymmetry that allows different processes to be simultaneously
activated. Here, we innovatively capture snapshots of the photo-response of
individual InAs nanowires via high spatial resolution (35 nm) THz photocurrent
nanoscopy. By coupling a THz quantum cascade laser to scattering-type scanning
near-field optical microscopy (s-SNOM) and monitoring both electrical and
optical readouts, we simultaneously measure transport and scattering
properties. The spatially resolved electric response provides unambiguous
signatures of photo-thermoelectric or bolometric currents whose interplay is
discussed as a function of photon density and material doping, therefore
providing a route to engineer photo-responses by design.
