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

## TL;DR

This paper introduces a non-invasive white-light interferometry technique for precise, real-time control of nanoscale air-gaps in surface polariton coupling, enabling detailed dispersion measurements.

## Contribution

The study presents a novel spectrally resolved white-light interferometry method for stabilizing and tuning nanometer-scale air-gaps in surface polariton experiments.

## Key findings

- Achieved nanometer precision in gap control
- Demonstrated real-time, reference-free readout
- Measured dispersion of surface phonon polaritons

## Abstract

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.