A new method to correlate prism and substrate refractive indices is presented and applied to the determination of rutile refractive indices, both ordinary and extraordinary, at different wavelengths in the visible and the near infrared. The method exploits radiation modes and, when the sample structure allows it, hybrid modes. An accuracy of approximately 2 × 10-4 was achieved without the need of any specific preparation of the samples. The method exhibits a good versatility since it can be exploited for the determination of the refractive index of a coupling prism when the substrate index is known and vice versa.
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