Photonics and Quantum Detection

We report a quantitative determination of wavelength-dependent, Kramers–Kronig (KK) consistent magneto-optical (MO) coupling constants in doped iron garnet thin films using vector MO generalized ellipsometry combined with a full 4×4 Müller-matrix optical model for back-scattered light. Two few micrometer thick, liquid-phase-epitaxy grown garnet films, (Dy1.97Lu0.64Bi0.39)3(Fe3.80Ga1.20)5O12 (sample S1) and (Dy2.47Lu0.47Bi0.06)3(Fe3.98Ga1.02)5O12 (sample S2), were investigated over the 400–1000nm spectral range. The complex MO coupling constant Q(λ) was extracted from differential Müller-matrix elements under magnetic saturation, yielding both real and imaginary components that satisfy KK relations. The analysis demonstrates that diagonal dielectric functions and off-diagonal MO response can be treated within a unified, physically consistent framework, even in the presence of thickness-fringe interference in transparent films. Comparative analysis of the two compositions reveals pronounced differences in magnetic and MO response arising from variations in effective Fe sublattice occupancy, rare-earth substitution, and Bi incorporation, without implying general materials tunability. The presented methodology provides a robust and general approach for the KK-consistent characterization of MO materials and multilayer systems relevant to sensing and photonic applications.