Tailoring Optical and Plasmon Resonances in Core-shell and Core-multishell Nanowires for Visible Range Negative Refraction and Plasmonic Light Harvesting: A Review

Semiconductor nanowires (NWs) are sub-wavelength structures which exhibit strong optical (Mie) resonances in the visible range. In addition to such optical resonances, the localized surface plasmon resonances (LSPRs) in metal-semiconductor core-shell (CS) and core-multishell (CMS) NWs can be tailored to achieve novel negative-index metamaterials (NIM), extreme absorbers, invisibility cloaks and sensors. Particularly, in this review, we focus on our recent theoretical studies which highlight the versatility of CS and CMS NWs for: 1) the design of negative-index metamaterials in the visible range and 2) plasmonic light harvesting in ultrathin photocatalyst layers for water splitting. Utilizing the LSPR in the metal layer and the magnetic dipole (Mie) resonance in the semiconductor shell under transverse electric (TE) polarization, semiconductor-metal-semiconductor CMS NWs can be designed to exhibit spectrally overlapping electric and magnetic resonances in the visible range. NWs exhibiting such double resonances can be considered as meta-atoms and arrayed to form polarization dependent, low-loss NIM. Alternatively, by tuning the LSPR in the TE polarization and the optical resonance in the transverse magnetic (TM) polarization of metal-photocatalyst CS and semiconductor-metal-photocatalyst CMS NWs, the absorption within ultrathin (sub-50 nm) photocatalyst layers can be substantially enhanced. Notably, aluminum and copper based NWs provide absorption enhancement remarkably close to silver and gold based NWs, respectively. Further, such absorption is polarization independent and remains high over a large range of incidence angles and permittivity of the medium. Therefore, due to the tunability of their optical properties, CS and CMS NWs are expected to be vital components for the design of nanophotonic devices.