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...

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