On-chip multimode vibrational strong coupling of polar dielectrics within hybrid metal–dielectric nanocavities

Abstract Vibrational strong coupling (VSC) provides a powerful route to manipulate energy exchange and polaritonic energy landscapes through the hybridization of vibrational modes with optical resonances, offering significant potential for ultrasensitive infrared spectroscopy, biochemical sensing, and the control of chemical reactivity. Plasmonic nanocavities can achieve strong confinement, but their substantial ohmic losses often broaden the optical resonance and obscure the intrinsic evolution of hybrid branches. In contrast, low-loss dielectric platforms generally suffer from insufficient field overlap with the vibrational medium. Moreover, in dual-channel or mode-selective architectures, coupling behavior is highly sensitive to localized hotspots and the precise placement of the active layer. Consequently, achieving simultaneous and robust access to both transverse-optical (TO) and longitudinal-optical (LO) channels remains challenging, especially within systems that require continuous tunability and fabrication simplicity. Here, we experimentally demonstrate multimode VSC in a hybrid metal–dielectric nanocavity, where the optical field is strongly confined inside the SiO 2 layer. Owing to the coexistence of in-plane and out-of-plane electric field components in the vibrational layer, the same photonic mode can simultaneously couple to both the TO and LO phonon responses of SiO 2 . The measured Fourier transform infrared spectra agree well with full-wave simulations and are quantitatively reproduced by an effective three-oscillator model. By varying the thickness of SiO 2 , we further reveal the relationship between the coupling strength, the polar molecular layer thickness, and the effective mode volume. The proposed architecture also shows good robustness against the fabrication distortions, establishing a low-loss and experimentally feasible platform for multimode VSC in polar dielectrics.