Plasmon absorption in nanospheres: a comparison of sodium, potassium, aluminium, silver and gold

The optical absorption for nanospheres made from Na, K, Al, Ag and Au are compared as a precursor to choosing the ideal metal for use in a negative permittivity (NP) near-field superlens. The relationship between optical absorption of the metal nanosphere and the ability of the NP lens to reconstruct the near-field is one to one. Metals with low dielectric losses have large plasmon absorption cross-sections and absorb over a very narrow wavelength range; they are consequently excellent materials for superlenses. Numerical solutions to Mie theory were used to calculate the absorption efficiency, Qabs, for nanospheres varying in radius between 5 nm and 100 nm in vacuum. We show that, although silver is the most commonly used material for superlensing, its absorption efficiency, as a nanosphere, at the plasmon resonance is not as strong as materials such as the alkali metals. Of all these materials, potassium spheres with a radius of 21 nm have an optimum absorption efficiency of 14.7, resulting in the ability of a film with thickness of 40 nm to reconstruct a grating with a period of 57 nm.