Using two most abundant materials in nature: silicon and aluminum, spectral selective perfect light absorption in single layer silicon films on aluminum surface is demonstrated. Perfect light absorption is achieved due to the critical coupling of incident optical wave to the second order resonance mode of the optical cavity made of a thin silicon film on aluminum surface. Spectral selective perfect light absorption results in different optical colors corresponding to different thicknesses of silicon films. The device colors do not change when viewing from large angles with respect to the surface normal. Perfect absorption wavelength can be tuned over a wide wavelength range over 70 nm by thermal annealing. This new technology, which is low cost and compatible with silicon technology platform, paves the way for many applications such as optical color filters and wavelength selective photodetectors.

Methods for spectrally controlling light absorption in optoelectronic devices have attracted considerable attention in recent years. It is now well known that a Fabry-Perot nanocavity comprising thin semiconductor and metal films can be used to absorb light at selected wavelengths. The absorption wavelength is controlled by tailoring the thickness of the nanocavity and also by nanostructure patterning. However, the realization of dynamically tuning the absorption wavelength without changing the structural geometry remains a great challenge in optoelectronic device development. Here it is shown how an ultrathin n-type doped indium antimonide integrated into a subwavelengththick optical nanocavity can result in an electrically tunable perfect light absorber in the visible and near infrared range. These absorbers require simple thin-film fabrication processes and are cost effective for large-area devices without resorting to sophisticated nanopatterning techniques. In the visible range, a 40 nm spectral shift can be attained by applying a reasonable bias voltage to effect the color change. It is also shown that these electrically tunable absorbers may be used as optical modulators in the infrared. The predicted (up to) 95.3% change in reflectance, transforming the device from perfectly absorbing to highly reflective, should make this technology attractive to the telecommunication (switching) industry.Thin-film light absorbers have recently received considerable attention due to their straightforward fabrication, low cost, and wide range of potential applications, however they have been restricted to the near infrared range or they are not tunable, or they are not perfect absorbers. Tunable perfect light absorbers functioning in the visible range as discussed in this paper comprise a Fabry-Perot nanocavity made of thin metal and semiconductor films that absorb light completely over selected wavelength ranges in the visible and infrared. The fundamental absorption wavelength is determined by the thickness of the nanocavity and tunability is bias voltage controlled. This newest generation of perfect light absorbers would have many interdisciplinary applications in chemical and biological sensing 1-7 , solar energy harvesting 8-11 , photodetectors 12,13 , gas sensors 14 , structural color printing [15][16][17][18] , and color filters [19][20][21][22][23][24][25][26] . However, if the absorption could be controlled in real time, then multiple new applications can be envisioned such as high speed, high resolution, high grey scale displays, smart windows, and a variety of telecommunication devices to compete with those currently available. To ascertain the possibilities, a theoretical investigation into ultrathin spectrally selective perfect light absorption in a nanocavity structure made of an epsilon-near-zero (ENZ) material (as the active layer) is summarized here. The metal/ENZ/dielectric/metal structure is modeled as a nanocavity, allowing the enhancement of light absorption at the resonant wavelength to be explored....

We demonstrated perfect light absorption in optical nanocavities made of ultra-thin percolation aluminum and silicon films deposited on an aluminum surface. The total layer thickness of the aluminum and silicon films is one order of magnitude less than perfect absorption wavelength in the visible spectral range. The ratio of silicon cavity layer thickness to perfect absorption wavelength decreases as wavelength decreases due to the increased phase delays at silicon-aluminum boundaries at shorter wavelengths. It is explained that perfect light absorption is due to critical coupling of incident wave to the fundamental Fabry-Perot resonance mode of the structure where the round trip phase delay is zero. Simulations were performed and the results agree well with the measurement results.

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