Ultrawide thermal free-carrier tuning of dielectric antennas coupled to epsilon-near-zero substrates.

The principal challenge for achieving reconfigurable optical antennas and metasurfaces is the need to generate continuous and large tunability of subwavelength, low-Q resonators. We demonstrate continuous and steady-state refractive index tuning at mid-infrared wavelengths using temperature-dependent control over the low-loss plasma frequency in III-V semiconductors. In doped InSb we demonstrate nearly two-fold increase in the electron effective mass leading to a positive refractive index shift (Δn > 1.5) that is an order of magnitude greater than conventional thermo-optic effects. In undoped films we demonstrate more than 10-fold change in the thermal free-carrier concentration producing a near-unity negative refractive index shift. Exploiting both effects within a single resonator system-intrinsic InSb wires on a heavily doped (epsilon-near-zero) InSb substrate-we demonstrate dynamically steady-state tunable Mie resonances. The observed line-width resonance shifts (Δλ > 1.7 μm) suggest new avenues for highly tunable and steady-state mid-infrared semiconductor antennas.Achieving large tunability of subwavelength resonators is a central challenge in nanophotonics. Here the authors demonstrate refractive index tuning at mid-infrared wavelengths using temperature-dependent control over the low loss plasma frequency in III-V semiconductors.