  Intersubband optoelectronic devices rely on transitions between
quantum-confined electron levels in semiconductor heterostructures, which
enables infrared (IR) photodetection in the 1-30 $\mu$m wavelength window with
picosecond response times. Incorporating nanowires as active media could enable
an independent control over the electrical cross-section of the device and the
optical absorption cross-section. Furthermore, the three-dimensional carrier
confinement in nanowire heterostructures opens new possibilities to tune the
carrier relaxation time. However, the generation of structural defects and the
surface sensitivity of GaAs nanowires have so far hindered the fabrication of
nanowire intersubband devices. Here, we report the first demonstration of
intersubband photodetection in a nanowire, using GaN nanowires containing a
GaN/AlN superlattice absorbing at 1.55 $\mu$m. The combination of spectral
photocurrent measurements with 8-band k$\cdot$p calculations of the electronic
structure supports the interpretation of the result as intersubband
photodetection in these extremely short-period superlattices. We observe a
linear dependence of the photocurrent with the incident illumination power,
which confirms the insensitivity of the intersubband process to surface states
and highlights how architectures featuring large surface-to-volume ratios are
suitable as intersubband photodetectors. Our analysis of the photocurrent
characteristics points out routes for an improvement of the device performance.
This first nanowire based intersubband photodetector represents a technological
breakthrough that paves the way to a powerful device platform with potential
for ultrafast, ultrasensitive photodetectors and highly-efficient quantum
cascade emitters with improved thermal stability.
