Optical and acoustic intersubband plasmons

Intersubband plasmon modes in a tunneling coupled bilayer system based on a 40nm wide GaAs/Al(0.3)Ga(0.7)As double heterostructure are studied by grating-coupler-assisted far-infrared spectroscopy. Both the charge density and the space symmetry of the bilayer can be tuned via front gate and are determined by means of magnetotransport measurements. We observe a strong-coupling behavior of the bilayer intersubband plasmon branches, resulting in the novel feature of optical and acoustic intersubband modes. The transmission spectra are excellently described by the self-consistently calculated absorption of the bilayer electron gas. Our theory in the framework of the current-response scheme takes into account the optics of the grating coupler and includes the interaction between the quasi-two-dimensional plasmon excitations.