OPTICAL-RESPONSE OF GRATING-COUPLER-INDUCED INTERSUBBAND-RESONANCES -THE ROLE OF WOODS ANOMALIES

Grating-coupler-induced collective intersubband transitions in a quasi -two-dimensional electron system are investigated both experimentally and theoretically. Far-infrared transmission experiments are performed on samples containing a quasi-two-dimensional electron gas quantum co nfined in a parabolic quantum well. For rectangular-shaped grating cou plers of different periods we observe a strong dependence of the trans mission line shape and peak height on the period of the grating, i.e., on the wave-vector transfer from the diffracted beams to the collecti ve intersubband resonance. It is shown that the line shape transforms with increasing grating period from a Lorentzian into a strongly asymm etric line shape. Theoretically, we treat the problem by using the tra nsfer-matrix method of local optics and apply the modal-expansion meth od to calculate the influence of the grating. The optically uniaxial q uasi-two-dimensional electron gas is described in the long-wavelength limit of the random-phase approximation by a local dielectric tensor, which includes size quantization effects. Our theory reproduces excell ently the experimental line shapes. The deformation of the transmissio n line shapes we explain by the occurrence of both types of Wood's ano malies.