Abdoulaye Ndao

Nonreciprocal lasing in topological cavities of arbitrary geometries

Topological lasing Resonant cavities that confine light are crucial components of lasers. Typically, these cavities are designed to high specification to get the best possible output. That, however, can limit their integration into photonic devices and optical circuits. Bahari et al. fabricated resonant cavities of arbitrary shape within a hybrid photonic crystal structure. The confinement of light to topologically protected edge states resulted in lasing at communication wavelengths. Relaxing the resonant cavity design criteria should be useful in designing photonic devices. Science, this issue p. 636 Resonant cavities of arbitrary shape can be designed to provide lasing into topically protected edge states. Resonant cavities are essential building blocks governing many wave-based phenomena, but their geometry and reciprocity fundamentally limit the integration of optical devices. We report, at telecommunication wavelengths, geometry-independent and integrated nonreciprocal topological cavities that couple stimulated emission from one-way photonic edge states to a selected waveguide output with an isolation ratio in excess of 10 decibels. Nonreciprocity originates from unidirectional edge states at the boundary between photonic structures with distinct topological invariants. Our experimental demonstration of lasing from topological cavities provides the opportunity to develop complex topological circuitry of arbitrary geometries for the integrated and robust generation and transport of photons in classical and quantum regimes.

Low-loss LiNbO 3 tapered-ridge waveguides made by optical-grade dicing

We report on low-loss vertical tapers for efficient coupling between confined LiNbO3 optical ridge waveguides and Single Mode Fibers. 3D-Pseudo-Spectral-Time-Domain calculations and Optical-Coherence-Tomography-based methods are advantageously used for the numerical and experimental study of the tapers. The tapered-section is done simultaneously with the ridge waveguide by means of a circular precision dicing saw, so that the fabrication procedure is achieved in only two steps. The total insertion losses through a 1.6 cm long ridge waveguide are measured to be improved by 3 dB in presence of the taper. These tapered-ridge waveguides open the way to the low-cost production of low-loss phase modulators or resonators.

From parabolic-trough to metasurface-concentrator: assessing focusing in the wave-optics limit

Metasurfaces are promising tools toward novel designs for flat optics applications. As such, their quality and tolerance to fabrication imperfections need to be evaluated with specific tools. However, most such tools rely on the geometrical optics approximation and are not straightforwardly applicable to metasurfaces. In this Letter, we introduce and evaluate for metasurfaces parameters such as intercept factor and slope error usually defined for solar concentrators in the realm of ray-optics. After proposing definitions valid in physical optics, we put forward an approach to calculate them. As examples, we design three different concentrators based on three specific unit cells and assess them numerically. The concept allows for comparison of the efficiency of the metasurfaces and their sensitivities to fabrication imperfections and will be critical for practical systems implementation.

Hybridized metamaterial platform for nano-scale sensing

Plasmonic/metamaterial sensors are being investigated for their high sensitivity, fast response time, and high accuracy. We propose, characterize and experimentally realize subwavelength bilayer metamaterial sensors operating in the near-infrared domain. We measure the figure-of-merit (FOM) and the bulk sensitivity (S) of the two fundamental hybridized modes and demonstrate both numerically and experimentally that the magnetic dipolar mode, degenerate with the electric quadrupolar mode, has higher sensitivity to a variation of the refractive index compared to the electric dipolar mode. In addition, the hybridized system exhibits a four fold increase in the FOM compared to a standard dipolar plasmonic system.

Local phase method for designing and optimizing metasurface devices

Metasurfaces have attracted significant attention due to their novel designs for flat optics. However, the approach usually used to engineer metasurface devices assumes that neighboring elements are identical, by extracting the phase information from simulations with periodic boundaries, or that near-field coupling between particles is negligible, by extracting the phase from single particle simulations. This is not the case most of the time and the approach thus prevents the optimization of devices that operate away from their optimum. Here, we propose a versatile numerical method to obtain the phase of each element within the metasurface (meta-atoms) while accounting for near-field coupling. Quantifying the phase error of each element of the metasurfaces with the proposed local phase method paves the way to the design of highly efficient metasurface devices including, but not limited to, deflectors, high numerical aperture metasurface concentrators, lenses, cloaks, and modulators.

Integration of Nanomaterials into Three-Dimensional Vertical Architectures

A novel layer-by-layer three-dimensional (3D) architecture allowing one to expand device fabrication in the vertical direction and integrating functional nanomaterials is presented by emulating civil engineering. The architecture uses SU-8 pillars as structural columns, which support multiple horizontal suspended thin films. The films then serve as platforms for the integration of nanomaterials and nanodevices. Multiple graphene layers suspended across SU-8 pillars with precise control on their vertical spacing are demonstrated. In addition to graphene, silicon nitride films that offer high strength yield and thickness control are also presented. Metallic microstructures, plasmonic nanostructures, semiconducting quantum dots, and monolayer graphene on the suspended films are achieved to prove the capability of integrating functional nanomaterials. This work provides the potential to integrate highly compact micro/nanoscale devices at different vertical levels with high surface density, which allows for more capabilities and functionalities in a single device.

A Monte Carlo approach for investigating the fabrications imperfections for high-efficiency metasurface solar concentrators

We introduce and evaluate, for metasurfaces, parameters such as the intercept factor usually defined for solar concentrators in the realm of ray-optics. The concept provides a path to design highly efficiency metasurface solar concentrators.