We report an experimental method to control large-area air-gaps in the nanometer range for evanescent coupling of light to surface waves such as surface plasmon polaritons or surface phonon polaritons. With the help of spectrally resolved white-light interferometry, we are able to stabilize and tune the gap with nanometer precision and high parallelism. Our technique is non-invasive and leaves the coupling area unobstructed, and the setup delivers reference-free real-time readout up to a distance of 150 μm between the coupling prism and the sample. Furthermore, we demonstrate the application to prism coupled surface polariton excitation. The active gap control is used to determine the dispersion of a critically coupled surface phonon polariton over a wide spectral range in the mid infrared region.

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