Metasurfaces exploit the ability to engineer the optical phase, amplitude and polarization at subwavelength dimensions providing unprecedented control of light. The realization of the all dielectric approach to metasurfaces has led to the demonstration of extensive flat optical elements and functionalities with low losses. However, to reach their ultimate potential, metasurfaces must move beyond static operation and incorporate active tunability and reconfigurable functions. The central challenge is achieving large tunability in subwavelength resonator elements which require large optical effects in response to external stimuli. Here we study the thermal tunability of high-index silicon and germanium semiconductor resonators over a large temperature range. We demonstrate thermal tuning of Mie resonances due to the normal positive thermo-optic effect (dn/dT >0) over a wide infrared range. We show that at higher temperatures and long wavelengths the sign of the thermo-optic coefficient is reversed (dn/dT<0) culminating in a negative induced index due to thermal excitation of free carriers. We also demonstrate the tuning of high order Mie resonances by several linewidths with a temperature swing of {Delta}T<100K. Finally, we exploit the larger thermo-optic coefficient at NIR wavelengths in Si metasurfaces to realize optical switching and tunable metafilters.
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