  Semiconductor nanowires offer great potential for realizing broadband
photodetectors that are compatible with silicon technology. However, the
spectral range of such detectors has so far been limited to selected regions in
the ultraviolet, visible and near infrared. Here, we report on broadband
nanowire heterostructure array photodetectors exhibiting a photoresponse from
the visible to long-wavelength infrared. In particular, the infrared response
from 3-20 um is enabled by normal incidence excitation of intersubband
transitions in low-bandgap InAsP quantum discs synthesized axially within InP
nanowires. The optical characteristics are explained by the excitation of the
longitudinal component of optical modes in the photonic crystal formed by the
nanostructured portion of the detectors, combined with a non-symmetric
potential profile of the discs resulting from synthesis. Our results provide a
generalizable insight into how broadband nanowire photodetectors may be
designed, and how engineered nanowire heterostructures open up new fascinating
opportunities for optoelectronics.
