Near-Field Detection of Strong-to-Weak Coupling Driven by Small Extracellular Vesicles

Strong light–matter coupling arises when the coherent energy exchange rate between an electromagnetic mode and material excitation exceeds their combined losses. This regime supports hybrid polaritonic modes with pronounced Rabi splitting and steep dispersion, which enhance the terahertz response to nanoscale dielectric changes for label-free biosensing. Here, we design a ribbon-cavity resonator operating in the strong-coupling regime and integrate graphene pads at the ribbon ends to capture tumor-derived small extracellular vesicles (sEVs) at localized near-field hotspots. A clear hybrid-mode doublet in the bare device collapses into a single peak upon sEVs adsorption, as revealed by terahertz near-field nanospectroscopy on individual ribbon-cavity resonators, signaling a transition to the weak-coupling regime. Notably, with increasing sEV loading, the spectrum evolves from a nearly symmetric peak to distinctly asymmetric, Fano-like line shape and ultimately to a striking peak-dip inversion. To unravel the underlying light–matter interactions, we develop a three-oscillator model that couples the tip, ribbon, and cavity and quantitatively reproduces the line-shape evolution driven by sEV-induced changes in coupling, losses, and drive imbalance, in good agreement with the experiment. Our results demonstrate that a single-resonator terahertz platform can sensitively resolve sEV loading and surface coverage, highlighting its potential for chip-integrated liquid-biopsy workflows.