Exciton dispersion fine structure and deep ultraviolet optical conductivity of freestanding two-dimensional h-BN

Excitons govern the light-matter interaction in 2D gapped materials with intrinsically large binding energies. In spite of plentiful optical measurements in the visible for semiconducting transition-metal dichalcogenides, we still lack optical-absorption studies of the exciton structure of insulating 2D materials that requires ultraviolet (UV) light. Moreover, measurements of the momentum dispersion of excitons in the vicinity of optical limit are rare owing to low resolutions but hold the key to reveal quasiparticle interactions. To close this gap, we employ high momentum resolution electron energy loss spectroscopy (q-EELS) to explore exciton dispersions of mono- and few-layer hexagonal boron nitride. Surprisingly, we reveal a fine structure of the first bright exciton dispersion band composed by two features (A and $A^{\text{'}}$ ), visible only at small momentum, not predicted by frozen-nuclei Bethe-Salpeter calculations. Introducing an optical conductivity approximation (OCA), we extract from the experimental q-EELS spectra deep-UV optical conductivity at zero momentum, σ(ω), and discuss the exciton fine structure in σ(ω), consistent with previous photoluminescence observations. Our findings establish a general methodology to probe the fine structure of exciton dispersions, providing new insights into exciton-phonon sidebands and eventually polarons in low-dimensional materials.