General approach to perfect and broadband absorption in thin films

Absorption of light in thin films is normally probabilistic. Here, we show that a film of any lossy material, with appropriate thickness, can achieve perfect absorption at any chosen wavelength. We derive the dispersion required to maintain perfect absorption across all wavelengths, show that real materials come close and demonstrate that even non-dispersive lossy materials can typically maintain >95% absorption across a spectral octave or more. This is achieved by coating a dielectric prism, where full absorption of linearly polarized and unpolarized light can be achieved with one and two coatings, respectively. Proof-of-principle experiments with chromium- and nickel-coated glass prisms show absorption exceeding 95% throughout the 480-1700 nm spectral range. We believe that the proposed method provides opportunities from photovoltaics and light detection to reflection-prevention and stealth technologies.