  We present a systematic study of top-down processed GaN/AlN heterostructures
for intersubband optoelectronic applications. Samples containing quantum well
superlattices that display either near- or mid-infrared intersubband absorption
were etched into nano- and micropillar arrays in an inductively coupled plasma.
We investigate the influence of this process on the structure and strain-state,
on the interband emission and on the intersubband absorption. Notably, for
pillar spacings significantly smaller ($\leq1/3$) than the intersubband
wavelength, the magnitude of the intersubband absorption is not reduced even
when 90\% of the material is etched away and a similar linewidth is obtained.
The same holds for the interband emission. In contrast, for pillar spacings on
the order of the intersubband absorption wavelength, the intersubband
absorption is masked by refraction effects and photonic crystal modes. The
presented results are a first step towards micro- and nanostructured group-III
nitride devices relying on intersubband transitions.
