## 3 Results and Discussion
### Optical Characterization  
Spectroscopic ellipsometry (SE) is a technique used for the characterization of thin films, since it allows for estimating electrical/optical changes associated to both film surface and bulk phase modifications. Experimental SE parameters are \(\Psi\) and \(\Delta\) angles, which are related with differential changes in amplitude and phase between light waves according to Equation (1) [38]. Figure 3 shows a comparison of wavelength dependence of measured parameters (tan\(\Psi\) in Figure 3a and cos\(\Delta\) in Figure 3b) for the different samples, where slight differences depending on geometrical parameters for symmetric samples and surface geometry in the case of the asymmetric one were obtained. It should be pointed out that the oscillatory character of cos(\(\Delta\)) has already been reported in the literature for anodized nanoporous alumina films, being the increase of oscillatory cycles related with pore size reduction, whereas sample impurities and roughness may also affect these values [50]. In the case of cos(\(\Delta\)) values, differences depending on sample structure and optical region (visible or near infrared) seem also to exist.  
From experimental tan(\(\Psi\)) and cos(\(\Delta\)) values, the refraction index (n) and the extinction coefficient (k) for the studied samples can be determined [50; 51]. Figure 4 shows the dependence of the refraction index, n, and the extinction coefficient, k, with the  
The average values of the refraction index for the whole range of frequencies as well as for frequencies ranging between 250-800 nm (visible) and 800-1700 nm (near infrared) regions for the studied samples are indicated in Table 3. The reduction sequence obtained for <n> values is in agreement with the increase of sample porosity, that is, with the higher air content of the samples. However, although <n> values for NPAS-Sf and NPAS-Ph samples were practically independent on wavelength and optical region, slight differences were observed in the visible region between both surfaces of the NPAS-And sample. The effect of porosity on other optical properties of NPASs such as solar absorptivity has already been reported [32]. The obtained values for the refraction index of NPAS-Sf and NPAS-Ph samples were in agreement with those reported for other thick, low porosity alumina nanoporous samples (15 nm pore radii, 9% porosity and thickness ranging between 9 \(\upmu\)m and 45 \(\upmu\)m) obtained by optical transmission spectra, ranging in such cases with n values between 1.52 and 1.62 [15].  
In the case of the extinction coefficient, k, curve shapes and values seem to depend on both pore size and optical region. Its average value, <k>, in the visible region for samples NPAS-Ph, NPAS-And(A), and NPAS-And(B) were 0.18 \(\pm\) 0.02, 0.08 \(\pm\) 0.01, and 0.06 \(\pm\) 0.04, respectively; this value slightly increases to <k> = 0.26 \(\pm\) 0.08 for sample NPAS-Ph in the NIR region. However, the NPAS-Sf sample exhibits oscillating behavior for the whole range of wavelengths, while values obtained for both sides of the NPAS-And in the NIR region also show similar tendencies, which would indicate the higher influence of lower pore size in that optical region.