a wavelength causes destructive interference of light and the light is absorbed at the metal surface.Along with current trends to eversmaller, more versatile devices, thin and reconfigurable infrared absorbers which do not require cumbersome nanofabrication are desirable, for example, to fabricate ultra-thin, effective light detectors. This is especially true for the mid-infrared spectral range, where optical imaging devices are important for many technological applications in key areas like thermography, surveillance, automotive safety, and astronomy. [8] It has been shown that ultra-thin absorbers can consist of just an imperfect conductor as a reflector and a very thin, lossy dielectric as the absorbing film because of non-trivial phase shifts at the interfaces. [13] Originally, the absorber functionalities are fixed after fabrication, limiting their application range. To achieve more versatile absorbers, research has been directed towards realizing tunable absorption functionality. [8,[14][15][16][17][18][19][20][21][22] Phase-change materials (PCMs) are especially interesting for this because they offer non-volatile tuning (in contrast to the volatile tuning with transparent conducting oxides or phasetransition materials) by switching between their amorphous and crystalline structural phases whose optical properties differ significantly, because of a unique bonding mechanism, referred to as metavalent bonding. [23][24][25][26] They can be switched between their phases by thermal, [27] electrical, [28] or optical heating [29] on time-scales down to a few nanoseconds [30] and they have already been successfully commercialized in products like re-writable DVDs [29] and PC-RAM. [31] Crystallization leads to an approximately twofold increase of the refractive index in the infrared spectral range for many PCMs [32,33] like Ge 2 Sb 2 Te 5 and its stoichiometric neighbors Ge 3 Sb 2 Te 6 and Ge 8 Sb 2 Te 11 . In addition, these PCMs feature low losses in the infrared, which makes them suitable for many nanophotonics applications [34][35][36] like waveguides, [37,38] photonic memory, [39,40] polaritonics, [41][42][43][44] tunable metasurfaces, [45][46][47][48][49][50][51][52][53] and tunable metasurface absorbers. [8,14,18,[54][55][56][57] But the same low-loss optical properties become disadvantageous for constructing ultra-thin absorbers with thicknesses well beyond the quarterwavelength limit. While their use is still viable in the visible and near-infrared spectral range [58][59][60] where they have significant optical losses, they are no longer suitable at infrared frequencies.
Absorbersfor 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 (Ge 3 Sb ...