Definitive molecular strong-coupling classification

Determine whether the coupling between the hybridized plasmonic mode and the BPT molecular vibrational mode is in the weak- or strong-coupling regime, including whether it lies at the plasmonic-molecular exceptional point, given the reported experimental uncertainties.

Background

The paper models the biphenyl-4-thiol (BPT) molecular vibration as a third resonator coupled to the hybridized dimer–waveguide plasmonic system. It defines a molecular exceptional point at ν=βm\nu=\beta_m, where ν\nu is the plasmonic–molecular coupling amplitude and βm\beta_m is the molecular dephasing contrast.

Although the extracted coupling satisfies ν>βm\nu>\beta_m using the central estimates, the stated uncertainties permit both weak and strong coupling and include the exceptional-point condition. Consequently, the paper does not resolve the definitive coupling regime of the plasmonic–BPT molecular system.

References

We find $\beta_m=2.81 \pm 0.50$ and so $\nu>\beta_m$. However, within the uncertainties, both the weak and strong regimes can be reached, along with the plasmonic-molecular exceptional point. Therefore, a definitive claim of strong coupling to the molecule cannot be made.

— Revealing antenna-molecule coupling using virtual gain from synthetic complex frequency analysis  (2608.30146 - Millard et al., 31 Aug 2026) in Section “Molecular coupling” (Results and discussion), paragraph following Eq. (main_CM_Eq_moleculeEigenvalues)