Linewidths of plasmonic cavities play an important role in strong light-matter coupling. Here, we report an extensive study of reducing the linewidths of various plasmonic cavities via substrate engineering. The engineered substrate was fabricated by depositing a 2 nm SiO \(_2\) film on a 100 nm silver film, which was itself deposited on the SiO \(_2\) /Si wafer substrate. The plasmonic bowtie cavities show a linewidth of 340 meV on SiO \(_2\) /Si substrate, which gets reduced to 115 meV on the engineered insulator-metal-insulator (IMI) substrate. Further, silver nanocubes and gold nanospheres exhibit a reduction in linewidths from \(\approx \) 490 to \(\approx \) 157 meV and from \(\approx \) 409 to \(\approx \) 218 meV, respectively. Our study reveals that silver nanoparticles exhibit a larger reduction in the linewidths compared to gold nanoparticles. Further, within silver nanoparticles, the dimer structure exhibits similar linewidth reduction as single nanoparticles. Using these reduced linewidths, we further demonstrated strong light-matter coupling by integrating these plasmonic nanoparticles on the engineered IMI substrate with the semiconductor quantum dots (QDs). The dark-field scattering spectra of the QD-coupled system exhibit Rabi splitting \(\approx \) 277 meV for silver nanocubes and \(\approx \) 156 meV for gold nanospheres.
