Surface plasmon resonances (SPRs) are crucial for confining light beyond the diffraction limit, yet metal losses often limit their spectral localization. Here, we propose a practical strategy for enabling the spectral localization of single-nanoparticle SPRs through photonic substrate engineering, which creates distinct optical pathways (OPs) to tailor the electromagnetic environments. By analyzing the multiplication factor spectrum of the projected local density of states, we can accurately trace these OPs, achieving high spatial and spectral localization of single-nanoparticle SPRs. Simulations show that using a photonic substrate can reduce the mode volume by a factor of 5 and enhance the quality factor by more than 80 times compared to a bare metal nanoparticle on a dielectric substrate. Subsequently, proof-of-concept experiments demonstrate the manipulation of single-nanoparticle SPRs by engineering OPs in both “open” and “closed” states, using two fabricated leaking Fabry-Pérot photonic substrates. This multidimensional photonic substrate engineering establishes a customizable platform for single-nanoparticle plasmonics, potentially transforming applications that were previously limited by spectral localization.

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• Photonics
• Surface plasmons
• Mode coupling theory
• Optical spectroscopy