Unveiling Local Optical Properties Using Nanoimaging Phase Mapping in High-Index Topological Insulator Bi2Se3 Resonant Nanostructures.

Topological insulators are materials characterized by an insulating bulk and high mobility topologically protected surface states, making them promising candidates for future optoelectronic and quantum devices. Although their electronic properties have been extensively studied, their mid-infrared (MIR) properties and prospective photonic capabilities have not been fully uncovered. Here, we use a combination of far-field and near-field nanoscale imaging and spectroscopy to study chemical vapor deposition-grown Bi2Se3 nanobeams (NBs). We extract the MIR optical constants of Bi2Se3, revealing refractive index values as high as n ∼ 6.4, and demonstrate that the NBs support Mie resonances across the MIR. Local near-field reflection phase mapping reveals domains of various phase shifts, providing information on the local optical properties of the NBs. We experimentally measure up to 2π phase-shift across the resonance, in excellent agreement with finite-difference time-domain simulations. This work highlights the potential of Bi2Se3 for quantum circuitry, nonlinear generation, high-Q metaphotonics, and photodetection.

Salt-induced stability and modified interfacial energetics in self-faceting emulsion droplets.

HYPOTHESIS: The counterintuitive temperature-controlled self-faceting of water-suspended, surfactant-stabilized, liquid oil droplets provides new opportunities in engineering of smart liquids, the properties of which are controllable by external stimuli. However, many emulsions exhibiting self-faceting phenomena have limited stability due to surfactant precipitation. The emulsions' stability may be enhanced, and their inter-droplet electrostatic repulsion tuned, through controlled charge screening driven by varying-concentration added salts. Moreover, in many technologically-relevant situations, salts may already exist in the emulsion's aqueous phase. Yet, salts' impact on self-faceting effects has never been explored. We hypothesize that the self-faceting transitions' temperatures, and stability against surfactant precipitation, of ionic-surfactants-stabilized emulsions are significantly modified by salt introduction. EXPERIMENTS: We explore the temperature-dependent impact of NaCl and CsCl salt concentration on the emulsions' phase diagrams, employing optical microscopy of emulsion droplet shapes and interfacial tension measurements, both sensitive to interfacial phase transitions. FINDINGS: A salt concentration dependent increase in the self-faceting transition temperatures is found, and its mechanism elucidated. Our findings allow for a significant enhancement of the emulsions' stability, and provide the physical understanding necessary for future progress in research and applications of self-faceting phenomena in salt-containing emulsions.

Chemical Vapor Deposition of Spherical Amorphous Selenium Mie Resonators for Infrared Meta-Optics.

Applying direct growth and deposition of optical surfaces holds great promise for the advancement of future nanophotonic technologies. Here, we report on a chemical vapor deposition (CVD) technique for depositing amorphous selenium (a-Se) spheres by desorption of selenium from Bi2Se3 and re-adsorption on the substrate. We utilize this process to grow scalable, large area Se spheres on several substrates and characterize their Mie-resonant response in the mid-infrared (MIR) spectral range. We demonstrate size-tunable Mie resonances spanning the 2-16 µm spectral range for single isolated resonators and large area ensembles. We further demonstrate strong absorption dips of up to 90% in ensembles of particles in a broad MIR range. Finally, we show that ultra-high-Q resonances arise in the case where Se Mie-resonators are coupled to low-loss epsilon-near-zero (ENZ) substrates. These findings demonstrate the enabling potential of amorphous Selenium as a versatile and tunable nanophotonic material that may open up avenues for on-chip MIR spectroscopy, chemical sensing, spectral imaging, and large area metasurface fabrication.

Denser fluids of charge-stabilized colloids form denser sediments.

Granular matter, where solid-like elasticity emerges in the absence of crystalline order, has been actively studied over the last few decades, targeting fundamental physical understanding of granular packings and glasses, abundant in everyday life and technology. We employ charge-stabilized sub-micron particles in a solvent, known as colloids, to form granular packings through a well-controlled process, where initially homogeneous and thermodynamically equilibrated colloidal fluids form solid sediments, when subjected to an effective gravity in a centrifuge. We demonstrate that particles' volume fraction φj in these sediments increases linearly with that in the initial fluid φ0, setting an upper limit φRCP≈ 0.64 on both φj and φ0, where φRCP coincides with the well-known, yet highly controversial, 'random close packing' density of spheres, providing new insight into the physics of granular packings. The observed φj(φ0) dependence is similar to the one recently reported for colloidal hard spheres, sterically stabilized by surface-linked polymer combs (S. R. Liber, et al., Proc. Natl. Acad. Sci. U. S. A., 2013, 110, 5769-5773). However, the lower limit on sediment densities drops to φj≈ 0.49 in the present work, suggesting that sedimented charge-stabilized silica are able to overcome mutual electrostatic repulsions, forming gel-like structures stabilized by occasional van der Waals contacts. Finally, by introducing particle size polydispersity, which significantly modifies fluid structure and sedimentation dynamics, we almost completely diminish the φj(φ0) dependence, bringing φj(0) close to its value in frictionless systems.