Materials with a negative dielectric permittivity (e.g., metals) display high reflectance and can be shaped into nanoscale optical resonators exhibiting extreme mode confinement, a central theme of nanophotonics. However, the ability to actively tune these effects remains elusive. By photoexciting free carriers in $4H-\mathrm{SiC}$, we induce dramatic changes in reflectance near the “reststrahlen band” where the permittivity is negative due to charge oscillations of the polar optical phonons in the midinfrared. We infer carrier-induced changes in the permittivity required for useful tunability $(\sim 40\mathrm{c}{\mathrm{m}}^{-1})$ in nanoscale resonators, providing a direct avenue towards the realization of actively tunable nanophotonic devices in the midinfrared to terahertz spectral range.

• Received 16 June 2015
• Revised 22 January 2016

DOI:https://doi.org/10.1103/PhysRevB.93.085205