Excitation of the Tunable Longitudinal Higher-Order Multipole SPR Modes by Strong Coupling in Large-Area Metal Sub-10 nm-Gap Array Structures and Its Application

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The excitation of the higher-order multipole localized surface plasmon resonance (LSPR) modes in sub-10 nm-gap plasmonic structure is desired in many plasmon enhancement applications. However, unfortunately, the higher-order multipole modes are either hard to be excited by light or make a very small contribution to electromagnetic enhancement, generally. In this paper, we demonstrate that tunable longitudinal higher-order multipole LSPR modes can be excited in large-area metal sub-10 nm-gap structures based on mature PS@Au core-halfshell ordered array plasmonic nanostructure by experiment and numerical simulation. Our results show that the longitudinal higher-order multipole SPR modes can be regarded as the like-cavity modes and can be excited effectively in strong coupling sub-10 nm-gap array structures and, furthermore, make a major contribution to the huge electromagnetic enhancement. Additionally, the longitudinal higher-order multipole SPR modes can be tuned easily by changing the longitudinal size of the plasmonic structure. As a benefit from this, we fabricated the optimized ultrasensitive surface-enhanced Raman scattering (SERS) substrate successfully. Our study provides a new view for design of surface plasmon resonance enhanced devices.