Determination of the refractive indices of highly biaxialanisotropic coatings using guided modes

Precise determination of the main refractive indices of biaxial anisotropic coatings is essential for the calculation of their optical performance. Earlier measurements based on wave guiding methods have proved to yield excellent results with obliquely deposited and hence anisotropic layers, deposited under an angle of 32 deg or less. But with higher deposition angles, new problems arise due to the less defined microstructure and the increasing influence of the accuracy of the column inclination angle on the resulting indices. In a theoretical study, these effects are shown to limit the precision of the method. Experimental results at lambda = 633 nm on HfO2 coatings, deposited at angles of 0 deg, 30 deg, 45 deg and 60 deg are given. All three main refractive indices were found to decrease with an increasing deposition angle, while anisotropy increases almost linearly.