We report broadband high-precision measurements of the complex dielectric function of near-optimally doped DyBa₂Cu₃O_7-𝛿 [1] and NdFeAsO_1-xF_x [2] thin films between 1 meV and 1 eV by combining submm-microwave interferometry, THz time-domain spectroscopy, and IR ellipsometry. By applying a Kramers-Kronig consistency analysis to the directly measured optical conductivity and dielectric permittivity spectra, we find that the Ferrell-Glover-Tinkham sum rule is satisfied with unprecedented accuracy of 1–2 % below 0.6 eV for the cuprate and 0.1 eV for the iron pnictide superconductor. We attribute specific features in the optical conductivity of DyBa₂Cu₃O_7-𝛿 to the known resonance energy and spectrum of collective antiferromagnetic spin fluctuations. For NdFeAsO_1-xF_x, we achieve quantitative agreement with the measured optical conductivity spectra by employing a multiband Eliashberg approach with extended s_+- strong coupling to a bosonic pairing mediator, consistent with our earlier optical studies of 122 iron pnictides [3]. The coupling of characteristic spin-fluctuation energy scales appearing in the superconductivity-induced optical conductivity spectra is compatible with a picture of antiferromagnetic spin fluctuations as the dominant pairing mechanism in both families of high-temperature superconductors.

[1] D. Dawson et al., Phys. Rev. B 108, 104501 (2023); [2] A.V. Boris et al., in preparation; [3] A. Charnukha et al., Phys. Rev. B 84, 174511 (2011).