Two-Photon Driven Tip-Enhanced Strong Coupling in MAPbBr₃ Perovskite Quantum Dots

Understanding and controlling light-matter interactions at the nanoscale is central to the development of next-generation photonic, optoelectronic, and quantum technologies. Plexcitons, hybrid quasiparticles arising from the strong coupling between localized surface plasmons and excitons, offer a unique platform to achieve this control. Here, we demonstrate strong plasmon-exciton coupling in a nanocavity formed between a gold AFM tip and a gold substrate driven by two-photon excitation (TPE). Near-infrared femtosecond excitation generates excitons in perovskite MAPbBr₃ quantum dots (QDs) via TPE precisely within the tip-substrate nanogap. This excitation scheme maximizes exciton density while minimizing far-field background and sample heating. We observed strong Rabi splitting exceeding 200 meV at room temperature. The coupling strength can be controlled by the tuning nanocavity gap via vertical control tip-sample distance, and vanishes when the separation exceeds 4 nm. Additional factors, such as QD size and excitation laser polarization, also influence the coupling behavior. Our results establish perovskite QDs as a robust platform for two-photon, tip-enhanced polaritonic at ambient conditions, opening routes to ultrafast, low-fluence control of nanoscale quantum states for sensing and plexciton qubits.