Evolution of Reststrahlen-like reflection in the optically thin limit

Reststrahlen bands, extended spectral bands with near-unity reflectance from the surface of a bulk material, are known to occur in ionic crystals near their optical phonon frequencies in the mid-infrared. They are manifestations of strong light-matter interaction near sharp and isolated resonances in solids. Although Reststrahlen bands in bulk materials have been extensively studied, their behavior in the optically thin limit has not been examined systematically. Here we present a study of Reststrahlen-like reflection bands when the material transitions from optically thick to optically thin. We take advantage of the unique layered structure of 2D van der Waals materials, which allows preparation of films with precise thicknesses across several orders of magnitudes, with essentially no change to the dielectric function, down to few atomic layers. We describe, experimentally and theoretically, the evolution of the reflectance spectra and their relation to radiative and nonradiative rates, considering hexagonal boron nitride as a test case.