Aluminum and indium plasmonic nanoantennas in the ultraviolet

We use classical electrodynamics calculations to investigate the versatility and capability of aluminum and indium dimers with small gaps as ultraviolet plasmonic nanoantennas, focusing on the particle size and wavelength range that gives optimum near-field enhancement. We find that Al and In are highly capable plasmonic materials in the ultraviolet, even with the incorporation of Al₂O₃ shells on the Al spheres; however, Al is strongly influenced by quadrupole modes while In is not. Al is the optimal material in the deep-UV, while In is ideal in the near-UV and near-visible spectral regions. Unlike Au and Ag, Al and In are most effective with the lowest refractive index background media possible, with vacuum being ideal. Ag outperforms both Al and In red of ̃320 nm, but optimal surface-enhanced Raman spectroscopy enhancement factors are still substantial for Al and In, with peak |E|⁴ values (for dimers in vacuum with a 1 nm gap) determined to be: Al, 2.0 × 10⁹ (at 204 nm); In, 1.2 × 10 ⁹ (at 359 nm); Al/Al₂O₃, 1.2 × 10 ⁷ (at 218 nm). For comparison, the optimal |E|⁴ for Au dimers is 2.8 × 10¹¹ (at 723 nm) and for Ag is 1.3 × 10¹² (at 794 nm), with background indices of 1.50 and 2.25, respectively. These data suggest that the continued exploration of Al and In as plasmonic materials could provide powerful opportunities in ultraviolet spectroscopic enhancement, fluorescence quenching, and cellular imaging.