Controlling Nanoscale Air-Gaps for Critically Coupled Surface Polaritons by Means of Non-Invasive White-Light Interferometry

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, leaves the coupling area unobstructed, and the setup delivers reference-free real-time readout up to 150 \mu m distance between the coupling prism and 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.