  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-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 mid-infrared. We infer
carrier-induced changes in the permittivity required for useful tunability (~
40 cm$^{-1}$) in nanoscale resonators, providing a direct avenue towards the
realization of actively tunable nanophotonic devices in the mid-infrared to
terahertz spectral range.
