Lithography-free broadband visible light absorber based on a mono-layer of gold nanoparticles

We experimentally demonstrate a large area, optically opaque plasmonic absorber which can absorb 95% of visible light with an effective thickness of less than 150 nm. The absorber comprises, from top to bottom, a mono-layer of random gold nanoparticles, a dielectric spacer, and a bottom gold reflector. Reflectometry analyses show that its absorption is insensitive to the incidence's polarization or angle when the incident angle is less than 50 degrees. At a larger incident angle, reflection increases and absorption spectra differ for two polarizations. Numerical simulations based on a 3D finite-element method suggest that the high absorbance is due to collective efforts of dipolar particle resonances, most often strongly coupled and forming chain resonances, as well as coupling of light to the surface plasmon polariton, irrespective of the incidence's polarization, through the top-layer particles. Similar high absorptivity is also demonstrated with silver or aluminum as the bottom reflector. These highly efficient visible light absorbers can be potential candidates for a range of passive and active photonic applications, including solar-energy harvesting as well as producing artificial colors on a large scale.