The field of nanophotonics aims at understanding and harnessing light-matter interaction in structures of dimensions far below the wavelength, enabling applications such as highly efficient sensing or all-optical integrated circuitry. The fundamental excitation driving nanophotonics is the surface polariton, arising in different types depending on the supporting material. A promising candidate for applications at infrared frequencies is the surface phonon polariton (SPhP) supported by polar crystals. However, a SPhP on a single polar crystal possesses several limitations that hinder the application in nanophotonic technologies.This work implements layered heterostructures built from various materials as a versatile platform for phonon polariton nanophotonics, overcoming the limitations of a conventional SPhP. By studying a variety of different polar crystal heterostructures, novel polariton modes with intriguing characteristics are discovered, such as ultra-thin film modes with immense field enhancements, strongly coupled polaritons at epsilon-near-zero frequencies, and waveguide modes with polariton-like properties.
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