We describe an inexpensive, open-source rotating polariser, rotating analyser variable angle reflective ellipsometer (VARE), suitable for measuring the generalised ellipsometric angles of anisotropic materials. With a build cost of $800 and substantial scope for customisation, the instrument offers functionality typically associated with far costlier commercial instruments. Phase alignment of the rotating polarisers is achieved by a phase-locked loop, while the ellipsometric angles are determined by Fourier analysis of the reflected signal, using an integrated microcontroller-based lock-in amplifier. The performance of the instrument is compared against a high-end commercial instrument.
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