THz properties of grating-gate plasmonic crystals crystals

This study reviews recent advances in the modern field of terahertz plasmonics concerning the control of resonant properties of grating-gate plasmonic crystal structures. Particularly, we conducted both experimental and theoretical investigations of AlGaN/GaN grating-gate structures with a focus on investigations of the resonant structure of transmission spectra associated with plasmon excitations in two-dimensional electron gas at different modulation degree of concentration profiles. Two distinct resonant phases of the plasmonic crystal structure were analyzed. The first one, the delocalized phase, is observed in the case of a small modulation degree of electron gas. In this phase, we found that plasmonic resonant absorption of incident radiation occurs across the entire grating-gate structure, with domination in the gated regions of the electron gas. In contrast, the second phase, the localized one, is realized at a strong modulation of the electron concentration profiles when the gated regions of the electron gas are completely depleted. Here, plasmon resonances are characterized by the spatial localization of absorption of incident radiation exclusively within the ungated regions of the electron gas. Moreover, in the localized phase, we observed the unexpected blue shift of plasmon resonant frequency with an increase of gate voltage. This observation was explained by the result of ‘edge gating effect’ and additional shrinking of the concentration profile of the electron gas in the ungated region. We demonstrate that the correct description of both phases requires rigorous electrodynamic simulations and cannot be achieved solely in the frameworks of simplified single-mode or single-cavity models.