Broadband Total Absorption of Light

We report a new light absorption device that - in principle - can absorb light completely across the entire electromagnetic spectrum by utilizing its spatial coherence and we provide proof-of-concept experimental demonstration of such a device. Complete absorption of broadband light is key for several technologies including photovoltaics, energy conversion, quantum and stealth technologies.<br/><br/>We demonstrated <i>broadband</i> coherent perfect absorption through experimental results. Constructive interference of counterpropagating waves on a thin film with appropriate optical properties (e.g. chromium or graphene film of appropriate thickness with 50% absorption) enables complete (deterministic) absorption of light, down to the single-photon level. We, recently, succeeded in demonstrating experimentally how this discovery can be utilized to realize a compact device that perfectly absorbs broadband spectrum of light simultaneously- by broadband constructive interference of light we achieved perfect absorption in nanometer scale thin films. When the optical path lengths were not matched, the reflectivity spectrum oscillated between coherent absorption and coherent transmission. With alignment, the bandwidth of coherent absorption increased, and became very large when the path lengths were matched. In this case, we observed only 9% reflectivity across the studied wavelength range of 1540 to 1620 nm, implying &gt;90% absorption. Changing one effective optical path length by about half a wavelength switched between coherent absorption and coherent transmission of the Cr film, i.e., between a perfect absorber and a mirror. We also note that perfect absorption of s-polarization coincides with perfect transmission of p-polarization and vice versa.<br/><br/>The thin film can be made photoactive (using 2D transition-metal dichalcogenides, semiconductors thin films, or organic-inorganic mixed films like perovskites) so that it not only absorbs all the light but also produces electricity/detects. Several permutations of optical designs including 2D and 3D structures, tessellated arrays, scaling, broadband light sources, polarizers, and various absorbing thin film materials are being investigated to develop the demonstration further and to optimize the concept for specific applications.<br/><br/>The recent results are noteworthy as the first demonstration in the field – of <i>broadband</i> coherent perfect absorption in a single compact device. This demonstration will help pave the way forward to go beyond the ergodic light trapping limit and the Shockley–Queisser limit to create highly efficient ultrathin absorbers, solar cells, and detectors; and forms building blocks for future adaptive nanophotonics as well.