# Second Harmonic Generation from Critically Coupled Surface Phonon Polaritons

## TL;DR

This paper demonstrates how critically coupled surface phonon polaritons at a 6H-SiC-air interface can significantly enhance second harmonic generation in the mid-infrared, with precise control and detailed analysis of the process.

## Contribution

It provides the first experimental demonstration of second harmonic generation from critically coupled surface phonon polaritons with tunable excitation conditions and detailed theoretical modeling.

## Key findings

- High-accuracy match between experimental and theoretical spectra
- Quantification of optical field enhancement
- Identification of low out-coupling efficiency mechanism

## Abstract

Mid-infrared nanophotonics can be realized using sub-diffractional light localization and field enhancement with surface phonon polaritons in polar dielectric materials. We experimentally demonstrate second harmonic generation due to the optical field enhancement from critically coupled surface phonon polaritons at the 6H-SiC-air interface, employing an infrared free-electron laser for intense, tunable, and narrowband mid-infrared excitation. Critical coupling to the surface polaritons is achieved using a prism in the Otto geometry with adjustable width of the air gap, providing full control over the excitation conditions along the polariton dispersion. The calculated reflectivity and second harmonic spectra reproduce the full experimental data set with high accuracy, allowing for a quantification of the optical field enhancement. We also reveal the mechanism for low out-coupling efficiency of the second harmonic light in the Otto geometry. Perspectives on surface phonon polariton-based nonlinear sensing and nonlinear waveguide coupling are discussed.