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.

Phased array metamaterial systems are enabling new classes of refractive and diffractive optical elements through spatial-phase engineering. In this letter, we develop design principles for reconfigurable optical antennas and metasurfaces. We theoretically demonstrate 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 2ߨ with less than 3dB loss in intensity. Such tunable metasurfaces may form the basis for reconfigurable metadevices that enable unprecedented control over the electromagnetic wave front.Antenna and metamaterial systems that can dynamically steer optical frequency beams is a pivotal area of research with applications ranging from Lidar technologies 1 , sensors 2,3 , to interand intra-chip communication 4 in nano-photonic circuits. Steering beams with metamaterials requires the ability to continuously tune the phase of scattered, reflected, or transmitted waves between 0 and 2π. Thus, there has been extensive research to construct devices which control the phase of scattered waves in a low loss manner. 5 Metamaterial devices that exploit phase engineering include flat optical elements, 6 polarization converters, 7-9 holographic projectors, 10 and optical vortex beam generators. 11 Arbitrary control of the wavelength dependent phase enables the miniaturization of bulk refractive (linear) and diffractive (interference based) optical elements to metamaterial based devices. [12][13][14] Phased array metamaterials and metasurfaces comprise assemblies of engineered optical antennas. Optical antenna systems that control the phase of transmitted, reflected, or scattered fields have mainly been built from metallic elements. Researchers have demonstrated numerous metallic optical antennas based on scaled-down bow-tie, 15 patch, 16 and split-ring 17 RF design principles for extreme concentration of light. 18 These single-element antennas typically emit dipolar radiation patterns. By exploiting phased-array concepts, multi-element optical antenna systems can support directional radiation patterns. transimitting 12 or reflecting 21-24 infrared metasurfaces, wherein the optical phase is engineered through antenna shape and size. 25 A broader challenge is to achieve such metamaterial phased arrays in a reconfigurable manner. To achieve reconfigurable properties, researchers have considered electromechanically tuning the dimensions of the resonators, 26 phase change media based antennas, 27 graphene based mod...

The interdependences of different phase transitions in Mott materials are fundamental to the understanding of the mechanisms behind them. One of the most important relations is between the ubiquitous structural and electronic transitions. Using IR spectroscopy, optical reflectivity and x-ray diffraction we show that the metal-insulator transition (MIT) is coupled to the structural phase transition in V2O3 films. This coupling persists even in films with widely varying transition temperatures and strains. Our findings are in contrast to recent experimental findings and theoretical predictions. Using V2O3 as a model system, we discuss the pitfalls in measurements of the electronic and structural states of Mott materials in general, calling for a critical examination of previous work in this field. Our findings also have important implications for the performance of Mott materials in next-generation neuromorphic computing technology.

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 ΔT ∼ 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.

III-Nitride light emitting diodes (LEDs) are the backbone of ubiquitous lighting and display applications. Imparting directional emission is an essential requirement for many LED implementations. Although optical packaging 1 , nano-patterning 2,3 and surface roughening 4 techniques can enhance LED extraction, directing the emitted light requires bulky optical components. Optical metasurfaces provide precise control over transmitted and reflected waveforms, suggesting a new route for directing light emission. However, it is difficult to adapt metasurface concepts for incoherent light emission, due to the lack of a phase-locking incident wave. In this Letter, we demonstrate metasurface-based design of InGaN/GaN quantum-well structures that generate narrow, unidirectional transmission and emission lobes at arbitrary engineered angles. We show that the directions and polarization of emission differ significantly from transmission, in agreement with an analytical Local Density of Optical States (LDOS) model. The results presented in this Letter open a new paradigm for exploiting metasurface functionality in light emitting devices. Light emitting diodes (LED) are rapidly enabling solid-state solutions to commercial lighting applications. Imparting unidirectionality to LEDs is a challenging, problem with a host of applications 5-7 awaiting a scalable solution. Directional emission is naturally observed in lasing systems, 13,14 where all of the resonators are emitting coherently. By modifying the local density