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Polaritonic quantisation in nonlocal polar material

In the Reststrahlen region, between the transverse and longitudinal phonon frequencies, polar dielectric materials respond metallically to light and the resulting strong light-matter interactions can lead to the formation of hybrid quasiparticles termed surface phonon polaritons. Recent works have demonstrated that when an optical system contains nanoscale polar elements these excitations can acquire a longitudinal field component as a result of the material dispersion of the lattice, leading to the formation of secondary quasiparticles termed longitudinal-transverse polaritons. In this work we build on previous macroscopic electromagnetic theories developing a full second-quantised theory of longitudinal-transverse polaritons. Beginning from the Hamiltonian of the light-matter system we treat distortion to the lattice introducing an elastic free energy. We then diagonalise the Hamiltonian, demonstrating the equations of motion for the polariton are equivalent to the those of macroscopic electromagnetism and quantise the nonlocal operators. Finally we demonstrate how to reconstruct the electromagnetic fields in terms of the polariton states and explore polariton induced enhancements of the Purcell factor. These results demonstrate how nonlocality can narrow, enhance and spectrally tune near field emission with applications in mid-infrared sensing.