Fr\"ohlich-type Polarons in Isotopically Enriched Hexagonal Boron Nitride

TL;DR

This study quantitatively characterizes Fr"ohlich-type exciton-phonon interactions in isotopically enriched hexagonal boron nitride, revealing larger exciton binding energies and polaron behaviors influenced by isotope enrichment.

Contribution

It provides the first direct quantitative analysis of exciton-phonon coupling in isotopically enriched hBN, highlighting the effects of isotope enrichment on excitonic properties.

Findings

Identified the Fr"ohlich exciton-phonon coupling with a coupling constant of 0.159.

Measured an exciton binding energy of 161 meV, larger than natural hBN.

Deduced a polaron radius larger than the lattice constant, indicating large-polaron behavior.

Abstract

Exciton-phonon interactions play a central role in defining the optical response of hexagonal boron nitride (hBN), yet their quantitative determination has remained incomplete. Here, we reveal the Fr\"ohlich-type exciton-phonon coupling in boron-10-enriched hBN using low-temperature cathodoluminescence. We resolve the indirect exciton 5.95$\pm$0.02 eV together with its longitudinal optical (LO) phonon replica detuned by 184$\pm$56 meV, enabling the extraction of a Fr\"ohlich coupling constant $\alpha$=0.159 and a larger exciton binding energy of 161 meV, larger than previously reported values for natural-abundance hBN, which is attributed to isotope enrichment. The inferred polaron radius exceeds the lattice constant, indicating large-polaron behavior. We deduced an exciton scattering time ~of 97 fs, corresponding to a homogeneous linewidth of ~6.76 meV. We further obtain a polaron binding energy of ~48 meV and an effective mass of 1.045 $m_0$. These results provide a direct quantitative characterization of exciton-phonon coupling in isotopically engineered hBN and establish a foundation for tailoring its phonon-polaritonic and quantum-optical properties.