Graphene as a Tunable Nonradiative Bath for Moiré Excitons

Abstract

A minimal theory of nonradiative energy transfer from a two-dimensional (2D) moiré exciton to a nearby graphene layer is presented. From Fermi's golden rule the transfer rate is the overlap of the exciton near-field spectrum with the dissipative density response of graphene, weighted by an exciton form factor, and it reproduces the established $\GET\propto z^{-4}$ law in the point-dipole limit. A finite exciton size filters out the high-momentum near field once the spacer thickness approaches the transition-polarization radius $R_X$, so the distance dependence of the rate---and of the photoluminescence (PL) quenching---probes the exciton size. A low-momentum expansion shows that, relative to the calibrated point-dipole response of the same bath, this leading correction is set by $R_X$ alone. In the ideal coherent-envelope limit the accompanying giant oscillator strength makes the rate non-monotonic in the exciton size, with a peak near $\lX\approx z$. Treating graphene as a gate-tunable bath, Pauli blocking suppresses the interband channel once $2|\muF|$ approaches $\hslash \omega$, partially restoring PL, and a full random-phase-approximation benchmark confirms the normalized interband distance dependence to within a few percent away from the threshold. Mapping the PL observables across the transition-metal dichalcogenide/hexagonal boron nitride/graphene parameter space, we find that a graphene gate acts not as a passive electrostatic element but as a tunable 2D electronic reservoir probed through exciton PL quenching.

18 pages, 6 figures

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