Transdimensional material platforms for tunable metasurface design
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Introduction Surface plasmons (SPs), which are optically induced oscillations of the electron cloud excited at metal–dielectric interfaces, enable strong light confinement beyond the diffraction limit and highly localized electromagnetic fields.1,2 Plasmonic nanostructures uniquely allow for light manipulation at the nanoscale and have been instrumental in advancing a range of applications,3 including photovoltaics,4,5 optical circuitry,6–8 and imaging.9,10 Advancements in nanofabrication techniques have allowed for precise optimization of the optical response of plasmonic devices by controlling the properties of the material building blocks, as well as the structure dimensions and geometry at the nanoscale.11 Moreover, dynamic control of plasmon resonances has been of great interest to produce tunable nanophotonic devices for optical switches,12,13 sensing,14–16 and modulators.17,18 Several techniques to realize dynamically tunable metasurfaces have been proposed,19 such as thermal tuning with phase-change materials,20–23 mechanical control with stretchable substrates,24,25 and free-carrier excitation.26–30 Of these
methods, the excitation of free carriers through electrical and optical means provides a considerably fast and large magnitude of modulation. This free-carrier-assisted modulation depends on an effect called Drude dispersion, which involves changing the dielectric permittivity of a light-channeling medium by the injection, creation, or depletion of free carriers. This can be achieved by electrically or optically doping the medium with free carriers, by interband generation of free carriers,12,31 or via the intraband excitation of free carriers.13,32 The injected/excited free carriers alter the dielectric permittivity of the material through Drude dispersion, changing the refractive index and the optical losses. Plasmonic components are typically made up of metals, such as gold or silver, due to their low losses and high DC conductivity.33 However, because of the large charge densities of these metals, the penetration of electromagnetic fields inside metals is low. This low penetration depth, coupled with high ohmic losses, makes appreciable optical tuning using free-carrier-induced dispersion difficult with bulk metals.34,35 In contrast, emerging plasmonic materials, such as transparent
Deesha Shah, School of Electrical and Computer Engineering and Birck Nanotechnology Center, Purdue University, USA; [email protected] Zhaxylyk A. Kudyshev, School of Electrical and Computer Engineering, Birck Nanotechnology Center, and Center for Science of Information, Purdue University, USA; [email protected] Soham Saha, School of Electrical and Computer Engineering and Birck Nanotechnology Center, Purdue University, USA; [email protected] Vladimir M. Shalaev, School of Electrical and Computer Engineering and Birck Nanotechnology Center, Purdue University, USA; [email protected] Alexandra Boltasseva, School of Electrical and Computer Engineering and Birck Nanotechnology Center, Purdue University, USA; aeb@purdue
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