An ultrathin film as near-perfect IR absorber

An ultrathin film as near-perfect IR absorber. Optical interference patterns, whether from an oily puddle, lens coating, or countless other thin films, are ubiquitous. They are most noticeable in materials that suffer little optical loss and whose thickness is on the order of the wavelength. But much thinner films can also generate strong interference effects. The trick, as researchers led by Harvard University’s Federico Capasso recently discovered, is to use a dielectric that’s strongly absorptive as well as ultrathin. Such “lossy” dielectrics can produce reflecting interfaces with phase shifts that substantially differ from 0 or π, making it possible to engineer an absorption resonance. Capasso’s group, in collaboration with researchers from four other universities, has now exploited that finding to develop a device that can absorb nearly all IR light that shines on it. Composed of a 180-nm-thick film of vanadium dioxide on a sapphire substrate, the device is simple and its reflectivity tunable. The researchers were able to reduce the reflectivity of 11.6-µm light from a high of 70–80% to 0.25% by raising (or lowering) the substrate temperature into VO₂’s metal–insulator transition until they reached a resonance close to 343 K. Their measured reflectivities around that temperature closely match their calculated values, shown here as a function of the real and imaginary components of the refractive index; the black line marks the trajectory of the complex index with temperature. The researchers envision the device as a component in various optical systems, including bolometers, modulators, and thermal emitters. (M. A. Kats et al., Appl. Phys. Lett. 101, 221101, 2012.)