The field of nanophotonics aims at understanding and harnessing light-matter interaction in structures of dimensions far below the wavelength. By squeezing light into nanostructures, the local electric fields can be immensely enhanced, boosting the efficiency of applications such as solar cells or molecular sensing. Furthermore, nanophotonics facilitates the miniaturization of optical devices, pushing forward the development of modern communication technologies and 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 different 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.
Furthermore, a new experimental and theoretical methodology is developed, enabling the systematic, extensive study of phonon polaritons in layered heterostructures of arbitrarily anisotropic media. In the conducted experiments, the phonon polaritons are excited via prism coupling in a home-built Otto geometry, featuring full control over the excitation conditions with direct read-out of the characteristic air gap size between prism and sample. This unique setup allows to characterize a polariton dispersion at critical coupling conditions, yielding the discovery of new phonon polariton modes.
As an excitation source, the free electron laser at the Fritz Haber Institute is employed, allowing for non-linear optical spectroscopy measurements at infrared phonon-polariton frequencies. The results include the first observation of second harmonic generation from propagating SPhPs and from the ultra-thin film Berreman polariton, enabling experimental access to the field enhancement of the excited polariton mode.
The theoretical advances of this work comprise a transfer matrix formalism for the calculation of light-matter interaction in arbitrarily anisotropic layered heterostructures. The developed versatile and robust framework enables the simulation, analysis and prediction of polaritons and their properties in any multilayer system, constituting a significant contribution to the field of polaritonic nanophotonics. The formalism is implemented in an open-access computer program, facilitating future studies towards phonon polariton-based technologies.
By discovering various new phenomena and implementing a new experimental and theoretical methodology with great potential for future investigations, this work constitutes a comprehensive study of phonon polaritons in polar dielectric heterostructures. Furthermore, this thesis lays out perspectives on how to use the developed experimental and theoretical methods for a number of future studies, bearing great potential to further advance the field of infrared nanophotonics.