Dynamic reflectometry near the critical angle for high-resolution sensing of the index of refraction

We investigate, theoretically and experimentally, a simple technique for se nsing very small changes on the index of refraction. The technique consists on measuring the derivative of the reflectivity near the critical angle by modulating the angle of incidence and measuring the AC amplitude of the re flectivity variations. Analytical results on the reflection of a Gaussian b eam near the critical angle are used to derive simple formulas for the sens itivity, dynamic range, and the optical-limit to the resolution. Experiment al results validating the theory are presented. We found an experimental re solution of about 1 x 10(-6), while the ultimate theoretical resolution cal culated for our experimental setup was 5.7 x 10(-8). This technique may be a useful alternative in designing a refractive index probe for some applica tions. (C) 1998 Elsevier Science S.A. All rights reserved.