Ultrathin Perfect Light Absorbers as Color Filters and Modulator

Methods for spectrally controlling light absorption in optoelectronic devices have attracted considerable attention in recent years. The principle objectives of this research are to experimentally demonstrate spectrally selective perfect light absorption in static thin-film layer structures in the visible and near infrared and provide a path toward dynamically controlling this absorption. The absorber structure comprises a Fabry-Perot nanocavity made of ultrathin semiconductor and metal films. The absorption wavelength is controlled by either tailoring the thickness of the cavity, applying thermal annealing, altering the metal layer, or by applying an electrical voltage. Investigating static spectrally selective light absorption in the visible range resulted in fabricated devices that appear in different colors. These devices are substantially angle insensitive as well; their colors do not change when viewed at different angles. More than 90% of the incident light is absorbed even at large incident angles, up to +/-70°. Perfect light absorption was experimentally observed for deep sub-wavelength thick silicon films as thin as 1/27 of the absorption wavelength. Simulations were then undertaken and they agreed very well with real measurements. Dynamically tuning the absorption wavelength (changing the device color without tailoring the structural geometry) however was the ultimate goal in this optoelectronic device development effort. To that end, it is also shown in this work how an ultrathin indium antimonide (InSb) film integrated into a subwavelength-thick optical nanocavity can be used to create electrically tunable perfect light absorbers in the visible range; the color of the device can be changed in real time by applying an electrical voltage. It is predicted that these electrically tunable absorbers may also be used as optical modulators in the infrared. The projected 95.3% change in reflectance transforms the device from perfectly absorbing to highly reflective, which should make this technology attractive to the telecommunication (switching) industry.;This new technology paves the way for many applications such as color filters, wavelength selective photodetectors, biosensors, high resolution ultra-fast displays, solar cells, smart windows, and high-speed telecommunication switching. These absorbers require only simple thin-film fabrication processes, making them cost effective for large-area devices without resorting to sophisticated nanopatterning techniques.