Fig. 3: Computation of the light absorptance due to multiple internal reflections in the MOx layer based on measured values of the refractive index and the extinction coefficient.

a Measured optical constants, refractive indices \(n\) (blue curves), and extinction coefficients \(\kappa\) (red curves) as functions of wavelength for I Si, II TiO₂ (anatase), and III ZnO for computation of the Fresnel coefficients in Eq. 8. The dashed vertical line indicates the position of the 365 nm wavelength. b Schematic illustration of the reflection \(r\) and transmission coefficients \(t\) from Eq. 8 for an incident wave with electric field amplitude \({E}_{0}\). The indices \(j\) of the optical constants \({\widetilde{n}}_{j}\) for the three media refer to air (index 1), MOx with thickness \(d\) (index 2), and the Si substrate (index 3). c The absorptance as a function of the MOx layer thickness \(d\) for illumination with UV light at 365 nm wavelength for TiO₂/Si (I) and ZnO/Si (II). The curves are computed with Eq. 9. The vertical dashed lines indicate the positions in MOx thickness yielding the first maxima in absorptance for the two heterojunctions. (See also Supplementary Fig. 3 for computed absorptance at other wavelengths).