Light-emitting metalenses and meta-axicons for focusing and beaming of spontaneous emission.

Phased-array metasurfaces have been extensively used for wavefront shaping of coherent incident light. Due to the incoherent nature of spontaneous emission, the ability to similarly tailor photoluminescence remains largely unexplored. Recently, unidirectional photoluminescence from InGaN/GaN quantum-well metasurfaces incorporating one-dimensional phase profiles has been shown. However, the possibility of generating arbitrary two-dimensional waveforms-such as focused beams-is not yet realized. Here, we demonstrate two-dimensional metasurface axicons and lenses that emit collimated and focused beams, respectively. First, we develop off-axis meta-axicon/metalens equations designed to redirect surface-guided waves that dominate the natural emission pattern of quantum wells. Next, we show that photoluminescence properties are well predicted by passive transmission results using suitably engineered incident light sources. Finally, we compare collimating and focusing performances across a variety of different light-emitting metasurface axicons and lenses. These generated two-dimensional phased-array photoluminescence waveforms facilitate future development of light sources with arbitrary functionalities.

Robust Coupling between Structural and Electronic Transitions in a Mott Material.

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 is coupled to the structural phase transition in V_{2}O_{3} 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 V_{2}O_{3} 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.

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 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.

Designing Multipolar Resonances in Dielectric Metamaterials.

Dielectric resonators form the building blocks of nano-scale optical antennas and metamaterials. Due to their multipolar resonant response and low intrinsic losses they offer design flexibility and high-efficiency performance. These resonators are typically described in terms of a spherical harmonic decomposition with Mie theory. In experimental realizations however, a departure from spherical symmetry and the use of high-index substrates leads to new features appearing in the multipolar response. To clarify this behavior, we present a systematic experimental and numerical characterization of Silicon disk resonators. We demonstrate that for disk resonators on low-index quartz substrates, the electric and magnetic dipole modes are easily identifiable across a wide range of aspect-ratios, but that higher order peaks cannot be unambiguously associated with any specific multipolar mode. On high-index Silicon substrates, even the fundamental dipole modes do not have a clear association. When arranged into arrays, resonances are shifted and pronounced preferential forward and backward scattering conditions appear, which are not as apparent in individual resonators and may be associated with interference between multipolar modes. These findings present new opportunities for engineering the multipolar scattering response of dielectric optical antennas and metamaterials, and provide a strategy for designing nano-optical components with unique functionalities.

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.