The number of oscillations depends on the effective optical thickness (EOT) of the NAA layer, which is directly related with the refractive index of the NAA layer. On the other hand, the FI depends on the refractive index contrast between the NAA layer and the surrounding materials (the substrate and the incident medium in this case). Both the number of oscillations and their FI decrease with increasing t PW, INCB018424 purchase what indicates the consequent decrease in the NAA film effective refractive index. Figure 2 Reflectance spectra of samples with different t PW before gold deposition. Red symbols joined with solid red line represent experimentally measured reflectance spectra.

Solid black line represents the best least-square fit corresponding to simulation. (a) t PW = 0 min, (b) t PW = 6 min, (c) t PW = 12 min, and (d) t PW = 18 min. This analysis is performed more systematically by means of a fitting to a theoretical model of the sample. The same Figure  2 shows one calculated reflectance spectrum for each t PW. These spectra

are calculated assuming an optical model for the samples consisting of (i) the substrate (aluminum), (ii) the NAA porous layer, and (iii) the incident medium (air). The porous layer, in turn, is considered as a mixture of aluminum oxide and air, with thickness d = 1,620 nm. this website The Bruggeman effective medium approximation is used to obtain the refractive index of the porous layer (n eff) from the refractive index of the aluminum oxide [25] and that of air (n air = 1) taking into account the corresponding volume fractions: (1) These volume fractions are related to the porosity P of the porous layer, P being the volume fraction of air and 1 - P the volume fraction of aluminum oxide. The calculated reflectance spectra shown in Figure  2 correspond Celastrol to the best least-square fit obtained by varying the porosity of the layer. Table  2 summarizes the obtained results for the four t PW. Besides the porosity that gives the best fit of the model with the experimental measurements, Table  2

also reports the corresponding effective refractive index at 660 nm and the estimated pore KU-60019 cost diameter (D p) obtained from the porosity and the interpore distance D int (previously estimated from the SEM pictures). Assuming a perfect hexagonal arrangement, these magnitudes are related through the following expression: Table 2 Results from the optical characterization of the samples after the pore widening and before the deposition of gold Pore widening time (min) NAA film porosity, P(%) NAA film effective refractive index, n eff Estimated pore diameter, D p (nm) 0 14.3 1.65 38.6 6 23.1 1.58 51.2 12 44.6 1.41 72.3 18 71.2 1.20 90.9 (2) Comparing the D p obtained from this optical characterization method with the approximate estimation from SEM, it can be seen that both show an increasing trend but that pore size determinations are not very precise from image analysis of surface pictures.