We report on optical studies of the Ruddlesden-Popper nickelates ${\mathrm{La}}_{n+1}{\mathrm{Ni}}_n{\mathrm{O}}_{3n+1}$, with $n=2$ (${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$), $n=3$ (${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$), and $n=\infty$ (${\mathrm{LaNiO}}_3$). As the number of the ${\mathrm{NiO}}_6$ octahedra layers $n$ grows, the ratio of the kinetic energy determined from the experimental optical conductivity and that from band theory $K_{\text{exp}}/K_{\text{band}}$ increases, suggesting a reduction of electronic correlations. While the bilayer ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ lies on the verge of the Mott insulating phase, the trilayer ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ and infinite-layer ${\mathrm{LaNiO}}_3$ fall into the regime of correlated metals. The evolution of the electronic correlations in ${\mathrm{La}}_{n+1}{\mathrm{Ni}}_n{\mathrm{O}}_{3n+1}$ is likely to be dominated by the Ni $d_{z^2}$ orbital. Our results provide important information for understanding the superconductivity in Ruddlesden-Popper nickelates.

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• Superconductivity
• High-temperature superconductors
• Strongly correlated systems
• Fourier transform infrared spectroscopy
• Infrared spectroscopy