  The modified rigorous coupled-wave analysis technique is developed to
describe the optical characteristics of the plasmonic structures with the
grating-gated delta-thin conductive channel in the far- and near-field zones of
electromagnetic waves. The technique was applied for analysis of the resonant
properties of AlGaN/GaN heterostructures combined with deeply-subwavelength
metallic grating which facilitates the excitation of the two-dimensional
plasmons in the THz frequency range. The convergence of the calculations at the
frequencies near the plasmon resonances is discussed. The impact of the
grating's parameters, including filling factor and thickness of the grating, on
resonant absorption of the structure was investigated in details. The spatial
distributions of electromagnetic field in a near-field zone were used for the
evaluation of total absorption of the plasmonic structures separating
contributions of the grating-gated two-dimensional electron gas and the grating
coupler.
