Mapping Phonon Polaritons with Visible Light

Phonon polaritons (PhPs) are hybrid light-matter waves which enable strong light-matter interactions and subdiffractional confinement, potentially empowering nanophotonic applications in sensing, nonlinear optics and nanoscale energy manipulation. Proposals to electrically inject PhPs are limited by the presumed weak coupling between the PhP modes and bulk phonons. Further, the multiple interactions between phonons, electrons, and PhPs have a dramatic impact on the electromagnetic response in nanostructures, but remain difficult to investigate due to a lack of experimental techniques to map electromagnetic eigenstates in all three dimensions which are inexpensive, easy, and non-destructive. In this work, we use confocal Raman microscopy combined with a geometric eigenmode expansion to map the full 3D eigenstates of PhP modes in Indium Phosphide (InP) nanopillars and 4H-Silicon Carbide (SiC) gratings. Our results indicate that, contrary to expectation, the bulk phonon modes of InP and 4H-SiC couple strongly to the localized PhP modes without the need for zone folding. Further, we confirm that polarizability selection rules form the predominant coupling method between phonons and PhP modes, with electron-phonon coupling playing an important role only for certain phonon modes (A1(LO) and E1(TO) in 4H-SiC). These observations further provide a method for extending Raman studies of PhP modes to achieve full 3D reconstruction of the PhP eigenmodes and thus enable applications in nanophotonics.