Real-time 3D mapping of thickness, interphase, and optical properties in UV-exposed photochromic YHO thin films: Part I

This work introduces a real-time, spatially resolved spectroscopic ellipsometry (SE) mapping methodology for investigating UV-induced optical and structural modifications in photochromic yttrium oxyhydride YHO thin films, enabling the dynamic and depth-dependent characterization of interfacial and optical changes under external stimuli — offering insights beyond conventional post-irradiation and “single spatial point” techniques. A (1 × 1) cm YHO thin film area is irradiated with a UV lamp (385–410 nm) for up to 480 min, while real-time SE measurements are acquired over a spectral range of 0.7–5.9 eV. UV exposure results in film thickness reduction, attributed to lattice contraction, refractive index n and extinction coefficient k variation along with an increase in interphase thickness and film inhomogeneity. Surface modification under UV irradiation is further reflected by a reduction in the EMA-defined water/hydroxyl-related contribution, which is interpreted as an optically effective change possibly linked to vacancy charge-state conversion and surface-affinity changes. In-situ SE provides time-resolved information on optical, structural, interfacial, and surface-related changes in YHO during UV irradiation. This approach offers a non-destructive route for monitoring dynamic modifications in photoactive thin films.