  Surface-phonon polaritons (SPhPs) are attractive alternatives to far-infrared
plasmonics for sub-diffractional confinement of light. Localized SPhP
resonances in semiconductor nanoresonators are very narrow, but that linewidth
and the limited extent of the Reststrahlen band inherently limit spectral
coverage. To address this limitation, we report active tuning of SPhP
resonances in InP and 4H-SiC by photoinjecting free carriers into the
nanoresonators, taking advantage of the coupling between the carrier plasma and
optical phonons to blue-shift SPhP resonances. We demonstrate state-of-the-art
tuning figures of merit upon continuous-wave (CW) excitation (in InP) or pulsed
excitation (in 4H-SiC). Lifetime effects cause the tuning to saturate in InP,
and carrier-redistribution leads to rapid (<50 ps) recovery of the tuning in
4H-SiC. This work opens the path toward actively tuned nanophotonic devices,
such as modulators and beacons, in the infrared and identifies important
implications of coupling between electronic and photonic excitations.
