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The high optical losses of metal-based plasmonic materials have driven an extensive search for alternative lower-loss materials that can support plasmonic-like effects, such as sub-diffraction confinement of optical fields. One such alternative employs phonon-mediated collective-charge oscillations (surface phonon polaritons, SPhPs) that can be optically excited in nanostructured polar dielectric materials. Similar to plasmonics, tailoring the geometry of polar-dielectric resonators results in resonances that can be spectrally tuned throughout the spectral range between the LO and TO phonons. However, generally, the spectral position and amplitude of these resonances remain fixed after sample fabrication. In this presentation, we discuss recent advancements made by our group in achieving actively tunable localized SPhP resonances in the long-wave- and far-infrared spectral regimes. In particular, we focus on three experiments that demonstrate active modulation of resonances. The first and second experiments focus on tuning the spectral position of localized SPhP resonances in cylindrical nanopillars that are etched into indium phosphide and silicon carbide substrates. In both of these cases we are able to induce resonance shifts as large as 15 cm-1 by optically injecting free-carriers into the pillars. The optical injection introduces a reversible, free-carrier perturbation to the dielectric permittivity that results in a spectral shift of the resonances. While the effects investigated for both the InP and SiC systems are similar, each material allows us to explore a different aspect of the phenomena. For InP we investigate the effects in the far-infrared (303-344 cm-1) with steady-state carrier photoinjection, while for SiC we investigate the dynamics of frequency modulated resonances in the long-wave infrared (797-972 cm-1) via transient reflection spectroscopy. Lastly, in the third experiment we demonstrate the ability to modulate the amplitude of resonances by coating SiC nanopillars with vanadium dioxide, a well-known phase change material that undergoes a metal-to-insulator transition near a temperature of 70 C. As such, we show that by exploiting this phase change we are able to modulate the reflectance and thermal emission of nanopillar arrays. The results described in this work may open the door to tunable, narrow-band thermal sources that operate in the long-wave to far-infrared spectral regimes.
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