*4.1. Linear Optical Constants from Spectrophotometry and Their Parametrization*

The linear optical constants of the titanium dioxide samples were fitted by means of Equation (2). In Figure 5, the measured and modeled transmittance and reflectance for sample 3 at near-normal incidence and at 60◦ for s- and p-polarization and the corresponding optical constants are shown. The complete set of calculated model parameters is summarized in Table 4.

**Table 4.** Here, beta-distributed oscillator (ß\_do) parameters of all samples obtained from the spectra fitting.


In order to verify the film elementary composition, EDX measurements were performed. Uncorrected (raw) data on the elementary composition contained a significant substrate contribution, which increased with increasing acceleration voltage. In order to eliminate the substrate contribution, each sample was measured with two acceleration voltages, and the (corrected) elementary composition of the film was estimated using Equation (6):

$$\begin{array}{l} N\_{\text{corrected},\text{Ti}} = \frac{N\_{\text{raw},\text{Si},16}N\_{\text{raw},\text{Ti},10} - N\_{\text{raw},\text{Si},10}N\_{\text{raw},\text{Ti},16}}{N\_{\text{raw},\text{Si},16} - N\_{\text{raw},\text{Si},10}}\\ N\_{\text{corrected},\text{ng}} = \frac{1}{2} \left[ \frac{N\_{\text{raw},\text{ng},16}}{N\_{\text{raw},\text{Ti},16}} + \frac{N\_{\text{raw},\text{ng},10}}{N\_{\text{raw},\text{Ti},10}} \right] N\_{\text{corrected},\text{Ti}} \\ N\_{\text{corrected},\text{O}} = 1 - N\_{\text{corrected},\text{Ti}} - N\_{\text{corrected},\text{ng}} \end{array} \tag{6}$$

Generally, *N* denotes the atomic concentration, whereas "raw" and "corrected" subscript indicate raw and corrected values, respectively. Accordingly, "10" or "16" subscripts indicate the raw values obtained at 10 and 16 kV of acceleration voltage, respectively. Here, ng stands for the noble gas used, i.e., argon or xenon.

**Figure 5.** Modeled (solid line) and measured transmittance (up triangles) and reflectance (down triangles) at 60◦ for s- (**top left**) and p-polarizations (**top right**) and at near-normal incidence (**bottom left**) for sample 3 and modeled optical constants (**bottom right**, refractive index: Solid line, left axes; extinction coefficient: Dotted line, right axes).

The final calculated atomic concentrations are summarized in Table 5.

**Table 5.** Energy-dispersive X-ray spectroscopy (EDX) results and linear refractive index of the PIAD films.


Note that sample 1 had the highest refractive index, such that we expected the largest density for that sample. This was in agreement with the EDX results, indicating a correct stoichiometric relation between titanium and oxygen atoms. Samples 2 and 3 showed some more oxygen in the EDX analysis, indicating a somewhat porous layer structure with some incorporated water molecules. Correspondingly, the refractive index turned out to be smaller. Noble gas impurities from the used inert gas of the plasma source (Table 3) were of the order of 3 to 4 at % in all samples.
