Probing strong light-matter interaction in the optically thin limit

Sharp and isolated resonances in condensed-matter systems yield pronounced signatures in dielectric function, which, in turn, are manifest in the reflectance and transmittance spectra. A canonical example is the Reststrahlen band, extended spectral bands with near-unity reflectance, known to occur in ionic crystals near their optical phonon frequencies in the mid-infrared. Although Reststrahlen bands in bulk materials have been extensively studied, their behavior in the optically thin limit has not been examined systematically. With the advent of 2D van der Waals materials, it has become possible to probe materials with precise thicknesses across several orders of magnitudes, with essentially no change to the dielectric function down to the few- or monolayer limit. Here we present a systematic study of Reststrahlen-like reflection bands when the material transitions from optically thick to optically thin. We describe, experimentally and theoretically, the evolution of the reflectance spectra and their relation to radiative and nonradiative rates.