  We study a "strongly-coupled" (SC) polariton system formed between the
atom-like intersubband transitions in a semiconductor nanostructure and the THz
optical modes that are localised at the edges of a gold aperture. The
polaritons can be excited optically, by incoherent excitation with bandgap
radiation, and we find that they also coherently scatter the same input laser,
to give strikingly sharp "sideband" (SB) spectral peaks in the backscattered
spectrum. The SB intensity is a sensitive track of the polariton density and
they can be detected down to a quantum noise floor that is more than 2500 times
lower than the excitation thresholds of comparable quantum cascade laser
diodes. Compared with other coherent scattering mechanisms, higher order SB
scattering events are readily observable, and we speculate that the effect may
find utility as a passive all-optical wavelength shifting mechanism in
telecommunications systems.
