Optical Access to Dark Excitons Via Spin-Phonon Fano Interference

Spin-dark excitons in semiconductor nanostructures are inherently optically silent: angular-momentum selection rules forbid their radiative recombination, granting long lifetimes attractive for quantum information storage and excitonic condensation, but placing their energy structure and bright-dark coupling beyond standard optical spectroscopy. Established access routes rely on symmetry-lowering perturbations, dedicated single-emitter device fabrication, or near-field methods, each of which perturbs the states under study or excludes ensembles. Here, we show that a Fano-type quantum interference between two acoustic-phonon scattering pathways, consisting of a resonant spin-flip process coupling bright and dark excitons and a non-resonant inelastic process sharing the same acoustic phonon mode, opens an optical window onto the dark exciton in unperturbed ensembles of (In,Ga)As/GaAs quantum dots. The interference manifests as antiresonance and bipolar spectral profiles whose shape and position encode the bright-dark exciton coupling, are tunable through magnetic-field orientation, optical pumping intensity, and temperature, and persist to about 40 K, with a characteristic thermal scale of about 5 meV. The same lineshape analysis reveals a sign reversal of the exciton exchange energy at excited quantum-dot shells. Relying only on exchange-split exciton states and acoustic phonons, the mechanism should transfer to two-dimensional transition metal dichalcogenides and perovskite nanocrystals, where dark excitons play a central role.