Thresholdless coherent light scattering from subband polaritons in a strongly coupled microcavity

Abstract

We study a “strongly coupled” (SC) polariton system formed between the atomlike intersubband transitions in a semiconductor nanostructure and the terahertz optical modes that are localized at the edges of a gold aperture. The polaritons can be excited optically, by incoherent excitation with band-gap 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 if suitably optimized, the effect may find utility in a passive component capable of all-optical wavelength shifting for telecommunications systems.