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This paper reports how the spectral linewidths of plasmon resonances can be narrowed down to a few nanometers by optimizing the morphology, surface roughness, and crystallinity of metal nanoparticles (NPs) in two-dimensional (2D) lattices. We developed thermal annealing procedures to achieve ultranarrow surface lattice resonances (SLRs) with full-width at half-maxima linewidths as narrow as 4 nm from arrays of Au, Ag, Al, and Cu NPs. Besides annealing, we developed a chemical vapor deposition process to use Cu NPs as catalytic substrates for graphene growth. Graphene-encapsulated Cu NPs showed the narrowest SLR linewidths (2 nm) and were stable for months. These ultranarrow SLR nanocavity modes supported even narrower lasing emission spectra and high nonlinearity in the input–output light–light curves.