Theoretical analysis of plasmonic black gold: periodic arrays of ultra-sharp grooves

Abstract

Periodic arrays of tapered grooves in gold surfaces are theoretically considered for turning high-reflectivity (shiny) gold surfaces into broadband low-reflectivity (black) surfaces, when illuminated by light polarized perpendicular to the groove direction, by making use of nanofocusing and subsequent absorption of gap-plasmon modes excited in the tapered grooves. The importance of realization of the adiabatic regime of nanofocusing, i.e. without reflection of plasmonic modes by tapered walls, is emphasized, and the taper angle that can be used for a given groove width without causing significant reflection is quantified. It is shown that nearly parallel groove walls at the groove bottom, i.e. ultra-sharp grooves, are required in order to sufficiently suppress the light reflection. Periodic arrays of V-grooves, and grooves with profiles described by a power-formula, are demonstrated to have much higher reflectivity levels than ultra-sharp groove arrays. Reflectivity spectra of ultra-sharp groove arrays are presented for a wide range of parameters, including the groove depth, bottom groove width, period, width of a flat plateau, and the angle of light incidence. Similar surface geometries in other metals are shown to have higher or smaller reflectivity levels depending on whether the metals are more or less absorptive than gold.