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Polaritonic-to-Plasmonic Transition in Optically Resonant Bismuth Nanospheres for High-Contrast Switchable Ultraviolet Meta-Filters

In the quest aimed at unveiling alternative plasmonic elements overcoming noble metals for selected applications in photonics, we investigate by numerical simulations the near ultraviolet-to-near infrared optical response of solid and liquid Bi nanospheres embedded in a dielectric matrix. We also determine the resulting transmission contrast upon reversible solid-liquid phase transition to evaluate their potential for switchable optical filtering. The optical response of the solid (liquid) Bi nanospheres is ruled by localized polaritonic (plasmonic) resonances tunable by controlling the diameter. For a selected diameter between 20 nm and 50 nm, both solid and liquid nanospheres present a dipolar resonance inducing a strong peak extinction in the near ultraviolet, however at different photon energies. This enables a high transmission contrast at selected near ultraviolet photon energies. It is estimated that a two-dimensional assembly of 30 nm solid Bi nanospheres with a surface coverage of 32% will almost totally extinct (transmission of 2%) a near ultraviolet 3.45 eV (359 nm) light beam, whereas upon phase transition the resulting liquid Bi nanospheres will show a transmission of 30%. This work appeals to the fabrication of locally reconfigurable optical metamaterials for integrated switchable near ultraviolet optics.