We have implemented a hyperbolic metamaterial covering a spectral bandwidth of 2000 inverse cm for wavelengths above 4.7 µm. A stack of intercalated heavily-doped InAs and undoped InAs epilayers was grown by molecular beam epitaxy with tellurium as the n-type dopant for obtaining electron concentrations of ~8e19 per cubic cm. The Type II hyperbolicity was determined through the effective optical constants deduced from infrared ellipsometry measurements of the stacks. The materials were then dry etched to form ID gratings with features ranging from 1 to 5 µm. The effective optical constants were used to model the grating’s optical response by finite element electromagnetic calculations. The models showed the formation of hyperbolic plasmon polaritons at the same frequencies where experimental features were observed.
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Interrogating surface phonon polariton (SPhP) modes has mostly been pursued by measuring the far field behavior of resonant modes, 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 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.
Here, confocal Raman microscopy is used to obtain the spatial distribution of phonon modes in nanostructured polar materials. 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.
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