Abstract
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.
Highlights
The principal challenge for achieving reconfigurable optical antennas and metasurfaces is the need to generate continuous and large tunability of subwavelength, low-Q resonators
Further we experimentally demonstrate thermally driven steady-state resonant shifts (ΔλR ~ λFWHM) of dielectric (i-InSb) antenna resonators strongly coupled to highly doped (n-InSb) epsilon-near-zero (ENZ) substrates
Through moderate doping (1016–1019), the infrared permittivity of InSb can be continuously engineered between high refractive index and plasmonic regimes according to simple Drude models[28]
Summary
The principal challenge for achieving reconfigurable optical antennas and metasurfaces is the need to generate continuous and large tunability of subwavelength, low-Q resonators. 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. Reconfigurable devices require shifting resonances (ΔλR) by at least one line-width[15] (λFWHM) to extract sufficiently large amplitude[16] or phase[17] shifts Achieving such tunability in resonators with inherently subwavelength dimensions and modest Qs4, 18, has motivated new approaches and designs for large-magnitude order-unity refractive index tuning (Δn ≥ 1)[19]. Electrical depletion[23,24,25] based devices can only achieve large index modulation over a few nanometers due to inherent tradeoffs between depletion width and carrier density As a result, such schemes have only demonstrated small resonance shifts (ΔλR
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