Retardation effects on intra- and intersubband plasmons in quantum wells and their manifestations in grating-coupler-assisted optical transmission

The dispersion relations of intra- and intersubband plasmon polaritons in quantum wells are investigated in the small wave-vector region. It is shown that the intrasubband plasmon polariton is a normal mode of the layered semiconductor microstructure for all wave vectors. The intersubband plasmon polariton, however, appears in two branches. The higher-frequency branch is a radiative virtual mode and the lower-frequency branch is a normal mode. It is shown that the normal modes appear as maxima and the virtual mode as a minimum in the relative transmission spectra of samples with a grating coupler above the electron system. Thus, the well-known deformation of the transmission line shape of the intersubband plasmon mode from a maximum for smaller grating periods to a minimum for larger grating periods is explained by the excitation of the radiative virtual intersubband plasmon polariton above the onset of the Rayleigh anomaly.