Periodic metal nanoparticle (NP) arrays support narrow lattice plasmon resonances that can be tuned by changing the localized surface plasmons of the individual NPs in the array, NP periodicity, and dielectric environment. The high quality factors (100-200) of lattice plasmon resonances enabled NP arrays to function as nanocavities for surface-emitting lasers. We have developed optical lithography tools to fabricate hierarchical Au NP arrays over cm^2 areas, where finite arrays of NPs (patches) are organized into arrays with larger periodicities. We found superlattice plasmons are supported by hierarchical Au NP arrays and can be described by the coupling of single-patch lattice plasmons and Bragg modes defined by patch periodicity. Superlattice plasmons resonances are often significantly narrower than that of single-patch lattice plasmon resonances and exhibit stronger local peak fields. By varying patch periodicity, we found that the number and spectral location of superlattice plasmon resonances can be tailored in hierarchical NP arrays. These narrow superlattice plasmon resonances in our hierarchical NP arrays system open prospects in ultrasensitive sensing and energy transfer and plasmon amplification in plasmonic cavities.
