Design of Transparent Conducting Plasmonic Metasurfaces and Theory of Impedance-Matched Metastructures

Abstract

We propose and numerically demonstrate a low-loss deep-subwavelength plasmonic metasurface that exhibits strong optical transmission at near-infrared wavelengths and concomitantly high electrical conductivity. The efficient realization of these two contrasting functionalities without a severe trade-off establishes the proposed metasurface as a potential alternative for conventional transparent electrodes. In contrast to conventional strategies of focusing on free carrier concentration management, the present design decouples the optical and electrical functionalities to a large extent. While the requirements of low electrical contact resistance are facilitated by the presence of an unstructured noble metal thin film, the optical requirements are fulfilled by achieving a transmission resonance through judicious structural and dispersion engineering. The genesis of transmission resonance in our impedance-matched metastructure lies in the excitation of structure derived magnetic plasmon resonance and its coupling to the radiative mode of the underlying planar plasmonic multilayer system. Furthermore, we develop a generalized theoretical framework where we analytically derive the modal dispersion for the plasmonic structure and investigate for the existence of the aforementioned radiative modes. The analysis establishes the fact that for asymmetric multilayer plasmonic systems, the evanescent plasmonic mode can exhibit a mode cut-off and becomes a propagating mode in the high refractive index medium, thereby materializing the possibility of resonant transmission. This theoretical model will be instrumental in gaining crucial physical insights into the operation of plasmonic devices and help in identifying their domain of operation.