Light absorber based on nano-spheres on a substrate reflector

We systematically study a type of plasmonic light absorber based on a monolayer of gold nano-spheres with less than 30 nm in diameters deposited on top of a continuous gold substrate. The influences of particle size, inter-particle distance, particle-substrate spacer size etc on the resonance are studied thoroughly with a 3D finite-element method. We identified that the high-absorption resonance is mainly due to gap plasmon (coupled through particle bodies) when the separation between neighboring nano-spheres is small enough, such as close to 1 nm; at larger particle separations, the resonance is dominated by particle dipoles (coupled through the host dielectric). Experimentally, an absorber was fabricated based on chemically-synthesized gold nanoparticles coated with silica shell. The absorber shows a characteristic absorption band around 810 nm with a maximum absorbance of approximately 90%, which agrees reasonably well with our numerical calculation. The fabrication technique can be easily adapted for devising efficient light absorbers of large areas.