Interaction of sub-terahertz radiation with low-doped grating-based AlGaN/GaN plasmonic structures. Time-domain spectroscopy measurements and electrodynamic modeling

Abstract

We have presented the results of terahertz time-domain spectroscopy measurements and a rigorous electrodynamic modeling of the optical characteristics of grating-based AlGaN/GaN plasmonic structures with low-doped two-dimensional electron gas in the frequency range of 0.1…1.5 THz. Two samples with grating aspect ratios (strip width/period) of 2.4/3 and 1.2/1.5 µm have been investigated. The measured transmission spectra are reconstructed in the calculations with high accuracy. The transmission spectra for p-polarized incident radiation exhibits Fabri–Pérot oscillation behavior due to the optically-thick substrate. The specific values of amplitude and spectral position of the transmission maxima are associated with the coupling of terahertz radiation with 2D electron gas due to plasmon excitations. Both calculations and transmission/reflection measurements demonstrate that plasmonic structures with micro-scaled metallic grating have three-fold increase of non-resonant absorption of terahertz radiation in comparison with the bare heterostructure. The polarization measurements of the transmission spectra of the plasmonic structures well agree with calculations and indicate a well-pronounced filtering effect of the grating for the s-component of the incident electromagnetic wave. The obtained values of the transmission for p-and s-polarized incident radiation demonstrate the high quality of deposited metallic grating with the extinction ratio higher than 80:1 for sub-and few THz frequency range.