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.