Reconfigurable Semiconductor Phased-Array Metasurfaces

Phased-array metamaterial systems are enabling new classes of refractive and diffractive optical elements through spatial-phase engineering. In this article, we develop design principles for reconfigurable optical antennas and metasurfaces. We theoretically demonstrate the tunability of infrared scattering phase and radiation patterns in low-loss, high-index dielectric resonators using free carrier refraction. We demonstrate reconfigurable endfire antennas based on interference between multiple elements. Within single resonators, we demonstrate reconfigurable broadside antenna radiation lobes arising from interfering electric and magnetic dipole resonances. Extending this concept to infinite arrays, we design ideal Huygens metasurfaces with spectrally overlapping electric and magnetic dipole resonances. By introducing free charge carriers into these metasurfaces, we demonstrate continuously tunable transmission phase between 0 and 2π with less than 3 dB loss in intensity. Such tunable metasurfaces may form the basis for reconfigurable metadevices that enable unprecedented control over the electromagnetic wavefront

Reconfigurable Semiconductor Phased-Array Metasurfaces

Phased-array metamaterial systems are enabling new classes of refractive and diffractive optical elements through spatial-phase engineering. In this article, we develop design principles for reconfigurable optical antennas and metasurfaces. We theoretically demonstrate the tunability of infrared scattering phase and radiation patterns in low-loss, high-index dielectric resonators using free carrier refraction. We demonstrate reconfigurable endfire antennas based on interference between multiple elements. Within single resonators, we demonstrate reconfigurable broadside antenna radiation lobes arising from interfering electric and magnetic dipole resonances. Extending this concept to infinite arrays, we design ideal Huygens metasurfaces with spectrally overlapping electric and magnetic dipole resonances. By introducing free charge carriers into these metasurfaces, we demonstrate continuously tunable transmission phase between 0 and 2π with less than 3 dB loss in intensity. Such tunable metasurfaces may form the basis for reconfigurable metadevices that enable unprecedented control over the electromagnetic wavefront

Ultrawide Thermo-optic Tuning of PbTe Meta-Atoms

Subwavelength Mie resonators have enabled new classes of optical antenna and nanophotonic devices and can act as the basic meta-atom constituents of low-loss dielectric metasurfaces. In any application, tunable Mie resonances are key to achieving a dynamic and reconfigurable operation. However, the active tuning of these nanoantennas is still limited and usually results in sub-linewidth resonance tuning. Here, we demonstrate the ultrawide dynamic tuning of PbTe Mie resonators fabricated via both laser ablation and a novel solution-processing approach. Taking advantage of the extremely large thermo-optic (TO) coefficient and a high refractive index of PbTe, we demonstrate high-quality factor Mie resonances that are tuned by several linewidths with temperature modulations as small as Δ<i>T</i> ∼ 10 K. We reveal that the origin for this exceptional tunability is due to an increased TO coefficient of PbTe at low temperatures. When combined into metasurface arrays, these effects can be exploited in ultranarrow active notch filers and metasurface phase shifters that require only a few kelvin modulation. These findings demonstrate the enabling potential of PbTe as a versatile, solution-processable, and highly tunable nanophotonic material that suggests new possibilities for meta-atom paints, coatings, and 3D metamaterials fabrication

Widely Tunable Infrared Antennas Using Free Carrier Refraction

We demonstrate tuning of infrared Mie resonances by varying the carrier concentration in doped semiconductor antennas. We fabricate spherical silicon and germanium particles of varying sizes and doping concentrations. Single-particle infrared spectra reveal electric and magnetic dipole, quadrupole, and hexapole resonances. We subsequently demonstrate doping-dependent frequency shifts that follow simple Drude models, culminating in the emergence of plasmonic resonances at high doping levels and long wavelengths. These findings demonstrate the potential for actively tuning infrared Mie resonances by optically or electrically modulating charge carrier densities, thus providing an excellent platform for tunable metamaterials

Switchable Plasmonic–Dielectric Resonators with Metal–Insulator Transitions

Nanophotonic resonators offer the ability to design nanoscale optical elements and engineered materials with unconventional properties. Dielectric-based resonators intrinsically support a complete multipolar resonant response with low absorption, while metallic resonators provide extreme light confinement and enhanced photon–electron interactions. Here, we construct resonators out of a prototypical metal–insulator transition material, vanadium dioxide (VO₂), and demonstrate switching between dielectric and plasmonic resonances. We first characterize the temperature-dependent infrared optical constants of VO₂ single crystals and thin-films. We then fabricate VO₂ wire arrays and disk arrays. We show that wire resonators support dielectric resonances at low temperatures, a damped scattering response at intermediate temperatures, and plasmonic resonances at high temperatures. In disk resonators, however, upon heating, there is a pronounced enhancement of scattering at intermediate temperatures and a substantial narrowing of the phase transition. These findings may lead to the design of novel nanophotonic devices that incorporate thermally switchable plasmonic–dielectric behavior

Enhancing Organic Semiconductor–Surface Plasmon Polariton Coupling with Molecular Orientation

Due to strong electric field enhancements, surface plasmon polaritons (SPPs) are capable of drastically increasing light-molecule coupling in organic optoelectronic devices. The electric field enhancement, however, is anisotropic, offering maximal functional benefits if molecules are oriented perpendicular to the interface. To provide a clear demonstration of this orientation dependence, we study SPP dispersion and SPP-mediated photoluminescence at a model Au/small-molecule interface where identical molecules can be deposited with two very different molecular backbone orientations depending on processing conditions. First, we demonstrate that thin films of <i>p</i>-SIDT­(FBTTh₂)₂ can be deposited with either all “in-plane” (parallel to substrate) or a 50/50 mix of in-plane/“out-of-plane” (perpendicular to substrate) optical transition dipoles by the absence or presence, respectively, of diiodooctane during spin-coating. In contrast to typical orientation control observed in organic thin films, for this particular molecule, this corresponds to films with conjugated backbones purely in-plane, or with a 50/50 mix of in-plane/out-of-plane backbones. Then, using momentum-resolved reflectometry and momentum-resolved photoluminescence, we study and quantify changes in SPP dispersion and photoluminescence intensity arising solely from changes in molecular orientation. We demonstrate increased SPP momentum and a 2-fold enhancement in photoluminescence for systems with out-of-plane oriented transition dipoles. These results agree well with theory and have direct implications for the design and analysis of organic optoelectronic devices