The Reststrahlen Effect in the Optically Thin Limit: A Framework for Resonant Response in Thin Media

Area of Science:

• Condensed Matter Physics
• Materials Science
• Optics

Background:

• Sharp resonances significantly alter a material's electromagnetic response.
• The Reststrahlen effect, characterized by high mid-infrared reflectivity in polar crystals near optical phonon resonances, is well-studied in bulk but challenging for finite thicknesses.

Purpose of the Study:

• To systematically investigate the Reststrahlen response in hexagonal boron nitride (hBN) across a wide range of thicknesses, from bulk to monolayer.
• To understand the evolution of the Reststrahlen effect as material thickness decreases into the optically thin limit.

Main Methods:

• Experimental measurements of Reststrahlen response in hexagonal boron nitride.
• Theoretical modeling to explain the observed phenomena.
• Investigation across over five orders of magnitude in material thickness.

Main Results:

• The high reflectivity plateau characteristic of the Reststrahlen band transforms into a single peak in the optically thin limit.
• Two distinct regimes were identified in the thin material limit: a strong-response regime governed by coherent radiative decay and a weak-response regime dominated by damping.
• A simple two-dimensional sheet model successfully explains the observed thickness-dependent evolution of the Reststrahlen response.

Conclusions:

• The Reststrahlen response in hexagonal boron nitride exhibits unique behavior at the nanoscale.
• The findings are generalizable, with the proposed 2D sheet model applicable to various thin media exhibiting similar resonant phenomena.
• This work provides a fundamental understanding of light-matter interactions in atomically thin materials.