We show that perfect absorption can be achieved in a system comprising a single lossy dielectric layer of thickness much smaller than the incident wavelength on an opaque substrate by utilizing the nontrivial phase shifts at interfaces between lossy media. This design is implemented with an ultra-thin (similar to lambda/65) vanadium dioxide (VO2) layer on sapphire, temperature tuned in the vicinity of the VO2 insulator-to-metal phase transition, leading to 99.75% absorption at lambda = 11.6 mu m. The structural simplicity and large tuning range (from similar to 80% to 0.25% in reflectivity) are promising for thermal emitters, modulators, and bolometers.
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