Admittance matching analysis of perfect absorption in unpatterned thin films

We perform a detailed analysis of perfect absorption in unpatterned thin films by invoking admittance matching conditions. We consider a single, absorptive layer coated on a reflective substrate and investigate perfect absorption conditions for normal and oblique incidences of light. For normal incidence, it is found that we need larger optical losses as the film gets thinner. But, for oblique incidence, perfect absorption can be achieved even for very small losses especially around the epsilon-near-zero frequency of the film. We consider the oblique incidence case in two different configurations (termed Berreman and ATR configurations). In both cases, we show that perfect absorption involves critical coupling to TM modes existing in thin films. We also evaluate actual light absorption and the electric field distribution in ITO films using the transfer matrix method, and show that it agrees well with our analytic theory. Finally, we present a versatile diagram that visualizes the admittance matching process in thin film structures. Our work presents an in-depth analysis of light absorption in thin films and provides design principles for various thin film devices, such as sensors, optical filters, optical modulators, and thermal emitters.