We probe the density-wave transition of the trilayer nickelate ${\mathrm{La}}_4{\text{Ni}}_3{\mathrm{O}}_{10}$ with polarization-resolved infrared spectroscopy. The low-energy electrodynamics is strongly anisotropic, with metallic in-plane and insulating out-of-plane character. In the ordered phase, the anisotropy grows more than an order of magnitude as the out-of-plane conductivity is sharply suppressed. We interpret this enhancement as an effective electronic decoupling of the Ni-O layers driven by a spin-density-wave-induced redistribution of $\mathrm{Ni}\text{−}{d}_{z^2}$ occupation within the trilayers. This electronic response is accompanied by clearly shifting and splitting out-of-plane phonons, compatible with a density-wave instability of electronic origin.

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• Charge density waves
• Charge order
• Optical conductivity
• Spin density waves
• Nickelates
• Strongly correlated systems
• Density functional theory
• Fourier transform infrared spectroscopy
• Terahertz spectroscopy