Theoretical Design of Effective Multilayer Optical Coatings Using Oxyhydride Thin Films

TL;DR

The paper investigates how partial oxidation in yttrium oxyhydrides modifies optical properties by comparing low-oxidized Y4H10O and high-oxidized YHO using ab initio hybrid-DFT, deriving the frequency-dependent dielectric function $\epsilon(\omega)$ and related spectra. It demonstrates that oxygen content drives blue shifts in absorption edges, enlarges dielectric gaps, and alters refraction in the visible, enabling modelling of bi-phase multilayer coatings such as a low-emissivity UV/IR-blocking stack and a broadband light-absorbing coating. Furthermore, it shows that a noncentrosymmetric Y4H10O phase can exhibit sizeable second-order nonlinear susceptibility $\chi_{xyz}(2\omega,\omega,\omega)$ with values up to $82.7$ pm/V, suggesting potential for integrated nonlinear optics. The work provides a computational framework linking crystal structure, electronic structure, and macroscopic optical behavior to guide the design of oxyhydride-based coatings with tunable transmission, reflection, absorption, and nonlinear responses.

Abstract

Rare-earth metal oxyhydride compositions are currently attracting increasing attention to develop materials with unusual optical responses. Herein, using computer simulations of the electronic and optical properties, the optical responses of two stable yttrium oxyhydride compounds, Y4H10O and YHO, are studied for the visible light range. The emphasis is on modeling macroscopic optical characteristics, which are numerically derived within a conventional scheme using refractive indices, and absorption, transmittance, and reflection spectra. The main goal is twofold: first, to simulate spectral behavior of different single-phase and two-phase oxyhydride compositions and second, to conduct a comparative analysis that could explain the features of the transmission spectra measured for different samples. Based on the obtained results, models of new optical coatings are proposed in which yttrium oxyhydrides play the key role. In the context of nonlinear optics, the frequency profile of the second-order susceptibility for the noncentrosymmetric cubic structure of Y4H10O is evaluated and it is shown that this system could exhibit large optical nonlinearity.

Figures (6)

• Figure 1: Spectral behavior of optical characteristics of yttrium oxyhydrides, Y4H10O and YHO, simulated in terms of complex dielectric function $\epsilon$, refractive index $n$, reflectance $R$ and transmittance $T$ spectra.
• Figure 2: Design and optical properties of the proposed bi-phase composite system. Experimental work and measurement data are presented in Ref. Baba2020. Part (b) -- transmittance spectra for the transparent state, Part (c) -- transmittance spectra for the photodarkened (opaque) state.
• Figure 3: Design and optical properties of an oxyhydride model of a low-emissivity coating.
• Figure 4: Design and optical properties of an oxyhydride model of a light-absorbing non-reflective coating.
• Figure 5: a) Angle dependence of the optical absorption for the oxyhydride model of the light-absorbing non-reflective coating. The absorption values were averaged over the $375$-$550$ nm wavelength range. b) Absorbance and reflectance spectra calculated for three angles of incident light: $0$, $30$ and $50$ degrees.