The material properties of silicon nitride play an important role in the performance of SIN membranes used in opto-mechanical setups. Accurate modeling of the reflectivities of sub-wavelength diffraction gratings is especially dependent on knowledge of the complex index of refraction. Here we describe a cavity enhanced method to measure the complex index of refraction along with the material thickness of dielectric samples with small, but non-zero absorption coefficients. The method relies on the enhanced sensitivity of the cavity optics to a dieletric film inserted into the cavity. By determining Brewster's angle and a second interference angle, both the real part of the index and thickness can be measured, while measurements of the losses in the empty cavity and cavity with dielectric provide a measurement of the absorption of the dielectric.
thin film, absorption, silicon nitride, optics, cavity, index of refraction