Mapping Phonon Polaritons with Visible Light

Surface phonon polaritons (SPhPs) are surface electromagnetic modes resulting from the strong coupling of infrared light with optical phonons in polar materials. SPhPs show great promise for tailoring light-matter interactions at size-scales below the diffraction limit. Interrogating SPhP modes has mostly been pursued by measuring the far field behavior of resonant modes (i.e., eigenvalues), through which SPhPs can be investigated by looking at resonant frequencies and linewidths along with the strength of the resonances. In other instances, the study of SPhPs has been accomplished by mapping electromagnetic fields (i.e., eigenstates) solely at the surface of nanostructured resonators by atomic force microscopy assisted techniques and, in some limited cases, measuring the three-dimensional fields using electron scattering. Accurate knowledge of SPhPs has been hindered by the absence of experimental techniques to map eigenstates in three dimensions that are easy, cheap, and non-destructive.<br/><br/>Here, using Indium Phosphide (InP) nanopillars, we demonstrated direct three-dimensional measurements of SPhP modes. Confocal Raman microscopy is used to obtain the spatial distribution of phonon modes in the nanostructure. By comparing these maps with simulation results, we demonstrated that SPhPs couple to bulk Raman modes through the material's polarizability and, to a lesser extent, via electron-phonon coupling. These observations provide a new method for measuring SPhP modes in nanostructured materials and a novel way to investigate the physical phenomena involved in coupling bulk phonons to SPhPs.