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To better understand the effect of
water adsorption on the optical property variations, theoretical simulations had been conducted using
the software TFCalc. The refractive indexes of solid SiO2 and TiO2 layers are assumed to be 1.50 and
2.30, respectively. Given the same amount of structural defect in both SiO2 and TiO2 layers, the changes of the effective
refractive index are calculated as the function of air volume fraction in the films. The right
figure shows that the decreasing rate of the effective refractive index in the
TiO2 layer is greater than that of the SiO2 layer, resulting in a reduced index contrast.
Accordingly, the variations of the first-stop-band position and the maximum reflectance
were also simulated based on the calculated effective refractive
indexes at different defect levels (left figure).