  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.
