The Reststrahlen effect in the optically-thin limit

The Reststrahlen effect of extended spectral bands in the mid-infrared with near-unity reflectance has been known for many bulk materials since the 19th century. The case for finite thickness, however, has not been treated systematically. Here we describe the evolution of reflectance spectra near the phonon resonance in hexagonal boron nitride (hBN) across more than 5 orders of magnitude in thickness, down to a monolayer. We show that the Reststrahlen band evolves to a single peak in the optically-thin limit, within which two regimes can be identified: a strong-response regime dominated by coherent radiative decay and a weak-response regime dominated by damping. This evolution clearly links fast radiative decay with strong coherent optical response and represents an extended-state analog of superradiance.