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Engheta, Nader

Publications and source records attributed to Engheta, Nader.

16 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Roadmap for Optical Metasurfaces

Metasurfaces have recently risen to prominence in optical research, providing unique functionalities that can be used for imaging, beam forming, holography, polarimetry, and many more, while keeping device dimensions small. Despite the fact that a vast range of basic metasurface designs has already been thoroughly studied in the literature, the number of metasurface-related papers is still growing at a rapid pace, as metasurface research is now spreading to adjacent fields, including computational imaging, augmented and virtual reality, automotive, display, biosensing, nonlinear, quantum and topological optics, optical computing, and more. At the same time, the ability of metasurfaces to perform optical functions in much more compact optical systems has triggered strong and constantly growing interest from various industries that greatly benefit from the availability of miniaturized, highly functional, and efficient optical components that can be integrated in optoelectronic systems at low cost. This creates a truly unique opportunity for the field of metasurfaces to make both a scientific and an industrial impact. Furthermore, the goal of this Roadmap is to mark this “golden age” of metasurface research and define future directions to encourage scientists and engineers to drive research and development in the field of metasurfaces toward both scientific excellence and broad industrial adoption.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Diffractive optical computing in free space

Abstract Structured optical materials create new computing paradigms using photons, with transformative impact on various fields, including machine learning, computer vision, imaging, telecommunications, and sensing. This Perspective sheds light on the potential of free-space optical systems based on engineered surfaces for advancing optical computing. Manipulating light in unprecedented ways, emerging structured surfaces enable all-optical implementation of various mathematical functions and machine learning tasks. Diffractive networks, in particular, bring deep-learning principles into the design and operation of free-space optical systems to create new functionalities. Metasurfaces consisting of deeply subwavelength units are achieving exotic optical responses that provide independent control over different properties of light and can bring major advances in computational throughput and data-transfer bandwidth of free-space optical processors. Unlike integrated photonics-based optoelectronic systems that demand preprocessed inputs, free-space optical processors have direct access to all the optical degrees of freedom that carry information about an input scene/object without needing digital recovery or preprocessing of information. To realize the full potential of free-space optical computing architectures, diffractive surfaces and metasurfaces need to advance symbiotically and co-evolve in their designs, 3D fabrication/integration, cascadability, and computing accuracy to serve the needs of next-generation machine vision, computational imaging, mathematical computing, and telecommunication technologies.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Open and Close-Packed, Shape-Engineered Polygonal Nanoparticle Metamolecules with Tailorable Fano Resonances

In this study, a top-down lithographic patterning and deposition process is reported for producing nanoparticles (NPs) with well-defined sizes, shapes, and compositions that are often not accessible by wet-chemical synthetic methods. These NPs are ligated and harvested from the substrate surface to prepare colloidal NP dispersions. Using a template-assisted assembly technique, fabricated NPs are driven by capillary forces to assemble into size- and shape-engineered templates and organize into open or close-packed multi-NP structures or NP metamolecules. The sizes and shapes of the NPs and of the templates control the NP number, coordination, interparticle gap size, disorder, and location of defects such as voids in the NP metamolecules. The plasmonic resonances of polygonal-shaped Au NPs are exploited to correlate the structure and optical properties of assembled NP metamolecules. Comparing open and close-packed architectures highlights that introduction of a center NP to form close-packed assemblies supports collective interactions, altering magnetic optical modes and multipolar interactions in Fano resonances. Decreasing the distance between NPs strengthens the plasmonic coupling, and the structural symmetries of the NP metamolecules determine the orientation-dependent scattering response.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Nanoimprinted Chiral Plasmonic Substrates with Three-Dimensional Nanostructures

Here, we report a large-area fabrication method to prepare chiral substrates patterned with arrays of multilayer, three-dimensional nanostructures using a combination of nanoimprint lithography and glancing angle deposition. Several structures are successfully fabricated using this method, including L-shaped, twisted arc and trilayer twisted Au nanorod structures, demonstrating its generality. As one typical example, arrays of L-shaped nanostructures, consisting of two layers of orthogonally oriented Au nanorods separated by a Ge dielectric layer in the thickness direction, exhibit giant optical chirality in the infrared region with an experimentally achieved g-factor as high as 0.38. Electromagnetic simulations show that the optical chirality results from plasmon hybridization between the two orthogonal Au segments. To demonstrate scalability, a 1 cm 2 chiral substrate is fabricated with uniform chiral optical property. This method combines both high throughput and precise geometrical control and is therefore promising for applications of chiral metamaterials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

Ultralight Angstrom-Scale Optimal Optical Reflectors

High reflectance in many state-of-the-art optical devices is achieved with noble metals. However, metals are limited by losses and, for certain applications, by their high mass density. Using a combination of ab initio and optical transfer matrix calculations, we evaluate here the behavior of graphene-based angstrom-scale metamaterials and find that they could act as nearly perfect reflectors in the mid–long-wave infrared (IR) range. The low density of states for electron–phonon scattering and interband excitations leads to unprecedented optical properties for graphene heterostructures, especially alternating atomic layers of graphene and hexagonal boron nitride, at wavelengths greater than 10 μm. At these wavelengths, these materials exhibit reflectivities exceeding 99.7% at a fraction of the weight of noble metals, as well as plasmonic mode confinement and quality factors that are greater by an order of magnitude compared to noble metals. These findings hold promise for ultracompact optical components and waveguides for mid-IR applications. Moreover, unlike metals, the photonic properties of these heterostructures could be actively tuned via chemical and/or electrostatic doping, providing exciting possibilities for tunable devices.

2D heterotructures↗
DOE OSTI2017

Bi-continuous Multi-component Nanocrystal Superlattices for Solar Energy Conversion

Our SISGR program studied an emerging class of nanomaterials wherein different combinations of semiconductor or semiconductor and plasmonic nanocrystals (NCs) are self-assembled into three-dimensional multi-component superlattices. The NC assemblies were designed to form bicontinuous semiconductor NC sublattices with type-II energy offsets to drive charge separation onto electron and hole transporting sublattices for collection and introduce plasmonic NCs to increase solar absorption and charge separation. Our group is expert in synthesizing and assembling an extraordinary variety of artificial systems by tailoring the NC building blocks and the superlattice unit cell geometry. Under this DOE BES Materials Chemistry program, we introduced chemical methods to control inter-particle distance and to dope NC assemblies, which enabled our demonstration of strong electronic communication between NCs and the use of NC thin films as electronic materials. We synthesized, assembled and structurally, spectroscopically, and electrically probed NC superlattices to understand and manipulate the flow of energy and charge toward discovering the design rules and optimizing these complex architectures to create materials that efficiently convert solar radiation into electricity.

14 SOLAR ENERGY↗
DOE OSTI2014

Optical isolation via unidirectional resonant photon tunneling

We show that tri-layer structures combining epsilon-negative and magneto-optical material layers can exhibit unidirectional resonant photon tunneling phenomena that can discriminate between circularly polarized (CP) waves of given handedness impinging from opposite directions, or between CP waves with different handedness impinging from the same direction. This physical principle, which can also be interpreted in terms of a Fabry-Perot-type resonance, may be utilized to design compact optical isolators for CP waves. Within this framework, we derive simple analytical conditions and design formulae, and quantitatively assess the isolation performance, also taking into account the unavoidable imperfections and nonidealities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1989

Periodic chiral structures

The electromagnetic properties of a structure that is both chiral and periodic are investigated using coupled-mode equations. The periodicity is described by a sinusoidal perturbation of the permittivity, permeability, and chiral admittance. The coupled-mode equations are derived from physical considerations and used to examine bandgap structure and reflected and transmitted fields. Chirality is observed predominantly in transmission, whereas periodicity is present in both reflection and transmission.

Jaggard, Dwight L.↗
NASA NTRS1989

One- and two-dimensional dyadic Green's functions in chiral media

The one-dimensional and two-dimensional dyadic Green functions are determined for an unbounded lossless reciprocal chiral medium which is electromagnetically described by a set of symmetric constitutive relations. It is shown that, as in the three-dimensional case, the medium supports two eigenmodes of propagation with two different wavenumbers, corresponding to the right- and left-circularly polarized waves, respectively. The eigenmode amplitudes a and b are similar to those of the three-dimensional case.

Engheta, Nader↗
NASA NTRS1988

Radiation characteristics of a source in a thin substrate mounted over a dielectric medium

The radiation pattern of a line source is calculated for the case in which the source is lying on the top or the bottom surface of a lossless dielectric substrate that is mounted on the top of semiinfinite dielectric medium. It is found that in both cases the pattern along the interfaces has a null; that the pattern in the upper semiinfinite medium has a single lobe; and that the pattern in the lower semiinfinite medium has many lobes, the number of which varies with the substrate thickness. In both cases, the power radiated into the lower medium is more than that radiated into the upper medium. Applications of this calculation to remote sensing, microstrip antenna technology, and antenna arrays are discussed.

Engheta, Nader↗