Double-resonance matching of fundamental and second-harmonic frequencies offers a powerful strategy to enhance second-harmonic generation (SHG). However, existing strategies often emphasize frequency alignment while overlooking the critical spatial overlap of the resonance field distributions, which is typically only partial. In this study, we proposed a new SHG enhancement mechanism leveraging ultrastrong coupling (USC) between plasmonic whispering-gallery and cavity modes in a spherical dielectric-metal core-shell four-layer nanostructure. This USC-based design enables simultaneous frequency matching via giant Rabi energy splitting and perfect spatial field overlap, facilitated by near-field interactions in the strong-coupling regime. These features lead to a three to four orders of magnitude enhancement of SHG efficiency across a broad spectral range (0.832–1.525 eV) compared with a standalone dielectric core under identical conditions. Our findings underscore the transformative potential of USC for advancing nonlinear optical processes and offer a robust framework for designing high-efficiency and compact nonlinear optical devices.

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• Light-matter interaction
• Nanophotonics
• Nonlinear optics
• Plasmonics
• Optical microcavities