Ultra‐Thin Switchable Absorbers Based on Lossy Phase‐Change Materials

Abstract Absorbers for infrared light are important optical components in key areas like biosensing, infrared imaging, and (thermal) light emission, with special need for thin and reconfigurable devices. Here, the authors demonstrate ultra‐thin, switchable infrared absorbers based on thin layers of chalcogenide phase‐change materials (PCMs) with high optical contrast between a lossless amorphous and an exceptionally lossy crystalline phase (Ge₃Sb₂Te₆, Ge₂Sb₂Te₄, Ge₂SbTe₄, Ag₄In₃Sb₆₇Te₂₆, and GeTe) on top of polar substrates (SiC, Al₂O₃, and SiO₂). It is found that light is mainly absorbed in the substrate for amorphous PCMs, and in the thin layer for crystalline PCMs. Using the concept of admittance matching, the authors demonstrate dramatic layer thickness reduction by a factor f = πκ of up to 14 for high PCM extinction coefficients κ compared to classic λ/4 anti‐reflection coatings. The authors show continuous tuning of the maximum absorption wavelength by up to 2.5 µm in the epsilon‐near‐zero ranges of the substrates via annealing on a hot plate and optical switching. By selecting a suitable PCM‐substrate combination, the tuning range and its size can be shifted through the whole infrared range. The results demonstrate that exceptionally lossy PCMs show great potential for ultra‐thin, reconfigurable nanophotonic devices.