The interaction between light and matter at the nanoscale frequently exhibits complex behaviour, particularly when multiple excited states become involved. Fumero, Paul, Wahlstrand, and Bristow, researchers at the National Institute of Standards and Technology, investigate the coupling between biexcitons – bound states of two electron-hole pairs – and polaritons, hybrid light-matter quasiparticles formed from the strong coupling of excitons (electron-hole pairs) and photons. Their work centres on semiconductor microcavities, structures designed to confine light and enhance these interactions. Using multidimensional coherent spectroscopy, they demonstrate that biexciton formation proceeds via ‘dark’ excitons – states that do not directly interact with light – which then couple to the bright, light-active component of the polaritons through Coulombic interactions. This indirect coupling model resolves discrepancies observed in previous studies and offers a pathway to probe and utilise these typically elusive dark excitons through multi-exciton correlations within strongly coupled systems.
## Biexciton-Polariton Coupling Mediated by Dark States
Recent research details the complex interaction between exciton-polaritons and biexcitons within a semiconductor microcavity. The study, employing multidimensional coherent spectroscopy (MDCS) in both collinear and non-collinear geometries, resolves the long-standing question of how biexcitons couple to the intracavity field. Results demonstrate that the biexciton response is largely independent of excitation wavevector and is modified by the cavity, indicating an interaction with polariton modes.
The investigation utilises a GaAs/AlAs microcavity containing an InGaAs quantum well. By analysing polarization-dependent selection rules, researchers isolated and characterised biexciton features as a function of cavity detuning – the energy difference between the cavity mode and the exciton resonance. Comparable detuning-dependent signatures were obtained regardless of whether excitation was achieved with a single or multiple wavevectors, confirming the excitation geometry does not significantly influence the biexciton’s coherent response.
Analysis of the observed energy dispersion and cavity enhancement rules out a direct coupling mechanism, such as a bipolariton picture. Instead, the data support a model wherein biexciton formation arises from dark excitons – excitons that do not directly interact with light – which then Coulomb couple to the bright-exciton fraction of the polaritons. This indirect coupling provides a plausible explanation for previously inconsistent observations of biexciton behaviour in similar microcavity systems.
The findings suggest a pathway to access and manipulate long-lived dark excitons through multi-exciton correlations in strongly coupled systems. Future work could focus on exploring the potential of this mechanism for applications in nanophotonics and quantum photon sources, as well as further characterising the properties of the dark excitons involved and refining the theoretical model to account for more complex interactions within the microcavity. This research provides a significant advancement in understanding many-body interactions in semiconductor nanostructures and opens new avenues for controlling light-matter interactions at the nanoscale.
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Biexciton-polariton coupling mediated by dark states
DOI: https://doi.org/10.48550/arXiv.2507.07363
