Wide-angle passive beam steering using 3D modified partial Maxwell fisheye lens.

This study presents a broadband, 3D gradient index beam-steering lens, derived from an optimized modification of the partial Maxwell fisheye (PMFE) design, achieving a boresight gain of 23 dBi, -80° to 80° beam steering, and <10 dB gain roll-off. Utilizing fused filament fabrication (FFF) to realize its intricate geometry, the design employs a novel polar space-filling curve (PSFC) to establish a 3D varying, effective permittivity distribution. Rigorous simulations and experimental validation attest to its effectiveness, marking the first 3D implementation of a PMFE-type lens to our knowledge. This research underscores the feasibility and diverse applications of a low-cost, wide-angle passive beam-steering dielectric lens.

Interface Catalysts of Ni3Fe1 Layered Double Hydroxide and Titanium Carbide for High-Performance Water Oxidation in Alkaline and Natural Conditions.

The electrocatalytic oxygen evolution reaction (OER) is important for many renewable energy technologies. Developing cost-effective electrocatalysts with high performance remains a great challenge. Here, we successfully demonstrate our novel interface catalyst comprised of Ni3Fe1-based layered double hydroxides (Ni3Fe1-LDH) vertically immobilized on a two-dimensional MXene (Ti3C2Tx) surface. The Ni3Fe1-LDH/Ti3C2Tx yielded an anodic OER current of 100 mA cm-2 at 0.28 V versus reversible hydrogen electrode (RHE), nearly 74 times lower than that of the pristine Ni3Fe1-LDH. Furthermore, the Ni3Fe1-LDH/Ti3C2Tx catalyst requires an overpotential of only 0.31 V versus RHE to deliver an industrial-level current density as high as 1000 mA cm-2. Such excellent OER activity was attributed to the synergistic interface effect between Ni3Fe1-LDH and Ti3C2Tx. Density functional theory (DFT) results further reveal that the Ti3C2Tx support can efficiently accelerate the electron extraction from Ni3Fe1-LDH and tailor the electronic structure of catalytic sites, resulting in enhanced OER performance.

Micro-dispenser-based optical packaging scheme for grating couplers.

Due to their sub-millimeter spatial resolution, ink-based additive manufacturing tools are typically considered less attractive than nanophotonics. Among these tools, precision micro-dispensers with sub-nanoliter volumetric control offer the finest spatial resolution: down to 50 µm. Within a sub-second, a flawless, surface-tension-driven spherical shape of the dielectric dot is formed as a self-assembled µlens. When combined with dispersive nanophotonic structures defined on a silicon-on-insulator substrate, we show that the dispensed dielectric µlenses [numerical aperture (NA) = 0.36] engineer the angular field distribution of vertically coupled nanostructures. The µlenses improve the angular tolerance for the input and reduces the angular spread of the output beam in the far field. The micro-dispenser is fast, scalable, and back-end-of-line compatible, allowing geometric-offset-caused efficiency reductions and center wavelength drift to be easily fixed. The design concept is experimentally verified by comparing several exemplary grating couplers with and without a µlens on top. A difference of less than 1 dB between incident angles of 7° and 14° is observed in the index-matched µlens, while the reference grating coupler shows around 5 dB contrast.

Design and fabrication of multi-material broadband electromagnetic absorbers for use in cavity-backed antennas.

We investigated the feasibility of designing and fabricating novel broadband radiofrequency (RF) absorbers for use in cavity-backed antennas. Fabricating the absorber involved a multi-material additive manufacturing (AM) approach that combined two polymer filaments: a low-loss dielectric filament and a lossy carbon-loaded filament. An iterative optimization algorithm was developed to deploy these filaments and create gradient distributions of material properties that minimize reflectance over a desired frequency band and a range of incident angles to achieve wideband electromagnetic absorption. The chosen material profiles were effectively realized using a spatially varying subwavelength lattice structure printed via fused filament fabrication. Experimentally, validation results demonstrated low reflectance over a wide frequency band, 10 to 40 GHz, and a range of incident angles, 0°-50°. Finally, this printed multi-material absorber was integrated within a cavity-backed spiral antenna and used to suppress backlobe radiation while maintaining an acceptable radiation pattern in the forward direction. While this study investigated cavity-backed antennas, these computational and experimental methods are potentially useful for a wide range of other applications.

Hollow aluminum microspheres with high mass extinction coefficients in the long wave infrared.

Previous electromagnetic computations of multilayered dielectric/metallic spheres identified the ideal dimensions and composition for achieving optimized mass extinction coefficients (m2/g). A hollow metallic sphere, with a thin metallic shell, is one such example of a spherical structure that can theoretically achieve high mass extinction coefficients in the long wave infrared (LWIR) region (8-12 µm). To this end, we endeavored to demonstrate a cost-effective and scalable manufacturing approach for synthesizing and experimentally validating the mass extinction coefficients of hollow metallic spheres. Specifically, we detail a novel approach for fabricating hollow aluminum spheres using radio frequency (RF) magnetron sputter deposition. Sacrificial high-density polyethylene polymer microspheres were used as substrates for the deposition of thin layers of aluminum. The core shell structures were subsequently thermally processed to form the hollow micron sized aluminum shells. The mass extinction coefficients of the hollow aluminum spheres were subsequently measured and compared to computational results. A strong agreement between experimental and theoretical predictions was observed. Finally, the LWIR mass extinction coefficients of the hollow spheres were compared to high aspect ratio brass flakes, a common pigment used for LWIR attenuation, and other materials and geometries that are used for LWIR filtering applications. This comparison of both performance and availability revealed that the fabricated hollow aluminum spheres exhibited competitive LWIR properties using a more scalable and cost-effective manufacturing approach.

High gain, wide-angle QCTO-enabled modified Luneburg lens antenna with broadband anti-reflective layer.

The gradient-index (GRIN) Luneburg lens antenna offers significant benefits, e.g. high aperture efficiency, low-power, minimal cost, wide beam scanning angle and broad bandwidth, over phased array antennas and reflector antennas. However, the spherical shape of the Luneburg lens geometry complicates the integration of standard planar feed sources and poses significant implementation challenge. To eliminate the feed mismatch problem, the quasi-conformal transformation optics (QCTO) method can be adopted to modify the lens' spherical feed surface into a planar one. However, Luneburg lenses designed with QCTO method are limited to poor performance due to the presence of the reflections and beam broadening arising from the quasi-conformal mapping. In this paper, we present a new method of implementing QCTO-enabled modified Luneburg lens antenna by designing a broadband anti-reflective layer along with the modified lens's planar excitation surface. The proposed anti-reflector layer is inherently broadband in nature, has a continuously tapered inhomogeneous dielectric permittivity profile along its thickness, and ensures broadband impedance matching. To show the new QCTO modified Luneburg lens antenna, an example lens antenna was designed at Ka-band (26-40 GHz) and fabricated using fused deposition modeling (FDM) based additive manufacturing technique. Electromagnetic performance of the lens antenna was experimentally demonstrated.

Iterative design of multilayered dielectric microspheres with tunable transparency windows.

Suspensions of microparticles dispersed in air or liquids are useful for designing media with desirable optical extinction properties within the visible or infrared spectrum. We describe here a numerical iterative optimization algorithm used to design multilayered concentric dielectric spheres with prescribed optical scattering properties. Our method integrates a computationally efficient rigorous electromagnetic solver, based on Mie theory, within an optimization loop to determine specific particle configurations that best meet a desired optical response. In particular, we show that this method can be used to design all-dielectric spherical particles that possess narrow tunable transparency windows while removing any angular dependency on the optical response.

Multiband absorbers for the long-wave infrared regime.

We present the development of a long-wave infrared regime multiband absorption filter with simultaneous wavelength and intensity selectivity. The approach employs a technique we call "spectral dithering" to place single-band absorbing pixels across a unit cell such that their weighted sum describes a multiband absorption spectrum. The number of absorption bands is proportional to the number of unique pixels present. Pixel patterning controls wavelength selectivity, whereas pixel distribution controls intensity selectivity. Using the rigorous coupled-wave method for modelling, a filter with three absorption bands between 6 and 14 µm is designed using the spectral dithering technique. The device is fabricated and experimentally verified using Fourier-transform infrared spectroscopy.

Cavity-based aluminum nanohole arrays with tunable infrared resonances.

This work details the successful computational design, fabrication, and characterization of a cavity-based aluminum nanohole array. The designs incorporate arrays of aluminum nanoholes that are patterned on a dielectric-coated (SiO2 or ZnSe) aluminum base mirror plane. This architecture provided a means of exploring the coupling of the localized resonances, exhibited by the aluminum nanohole array, with the cavity resonance that is generated within the dielectric spacer layer, which resides between the base plane mirror and the nanohole array. Rigorous coupled wave analysis (RCWA) was first used to computationally design the structures. Next, a range of lithographic techniques, including photolithography, E-beam lithography, and nanosphere lithography, were used to fabricate the structures. Finally, infrared spectroscopy and scanning electron microscopy (SEM) were used to characterize the spectral and structural properties of the multilayered devices, respectively. The overall goal of this study was to demonstrate our ability to design and fabricate aluminum-based structures with tunable resonances throughout the infrared region, i.e. from the short-wave through longwave infrared regions of the electromagnetic spectrum (1.5 -12 µm).

General optimization of tapered anti-reflective coatings.

An efficient, general optimized method is outlined that achieves antireflective tapers using lossless, non-dispersive dielectrics. The method modifies the derivative of a perfect antireflective wave amplitude distribution rather than the index of refraction distribution. Modifying the derivative of the wave amplitude distribution minimizes the potential index of refraction distributions and ensures perfect antireflection at one frequency, incidence angle, and linear polarization combination. Additional combinations of frequency, incident angle, and linear polarization can be targeted at a particular reflection coefficient within the optimization. After the method is outlined, three examples are shown with one being fabricated and validated at radiofrequencies.

Fiber optic micro sensor for the measurement of tendon forces.

A fiber optic sensor developed for the measurement of tendon forces was designed, numerically modeled, fabricated, and experimentally evaluated. The sensor incorporated fiber Bragg gratings and micro-fabricated stainless steel housings. A fiber Bragg grating is an optical device that is spectrally sensitive to axial strain. Stainless steel housings were designed to convert radial forces applied to the housing into axial forces that could be sensed by the fiber Bragg grating. The metal housings were fabricated by several methods including laser micromachining, swaging, and hydroforming. Designs are presented that allow for simultaneous temperature and force measurements as well as for simultaneous resolution of multi-axis forces.The sensor was experimentally evaluated by hydrostatic loading and in vitro testing. A commercial hydraulic burst tester was used to provide uniform pressures on the sensor in order to establish the linearity, repeatability, and accuracy characteristics of the sensor. The in vitro experiments were performed in excised tendon and in a dynamic gait simulator to simulate biological conditions. In both experimental conditions, the sensor was found to be a sensitive and reliable method for acquiring minimally invasive measurements of soft tissue forces. Our results suggest that this sensor will prove useful in a variety of biomechanical measurements.

Design of cubic-phase optical elements using subwavelength microstructures.

We describe a design methodology for synthesizing cubic-phase optical elements using two-dimensional subwavelength microstructures. We combined a numerical and experimental approach to demonstrate that by spatially varying the geometric properties of binary subwavelength gratings it is possible to produce a diffractive element with a cubic-phase profile. A test element was designed and fabricated for operation in the LWIR, approximately lambda=10.6 microm. Experimental results verify the cubic-phase nature of the element.

Design of two-dimensional polarization-selective diffractive optical elements with form-birefringent microstructures.

We describe a design methodology for synthesizing polarization-sensitive diffractive optical elements based on two-dimensional form-birefringent microstructures. Our technique yields a single binary element capable of producing independent phase transformations for horizontally and vertically polarized illumination. We designed two elements for operation at 10.6 microm and fabricated them in silicon. Qualitative experimental results agree with design predictions.

Numerical evaluation of heating of the human head due to magnetic resonance imaging.

In this paper, we present a numerical model for evaluating tissue heating during magnetic resonance imaging (MRI). Our method, which included a detailed anatomical model of a human head, calculated both the electromagnetic power deposition and the associated temperature elevations during an MRI head examination. Numerical studies were conducted using a realistic birdcage coil excited at frequencies ranging from 63 to 500 MHz. The model was validated both experimentally and analytically. The experimental validation was performed at the MR test facility located at the Food and Drug Administration's Center for Devices and Radiological Health.

Applying a mapped pseudospectral time-domain method in simulating diffractive optical elements.

A new technique for the analysis of two-dimensional diffractive optical elements, by use of the pseudospectral time-domain (PSTD) method, is presented. In particular, the method uses a nonuniform (NU) grid and a mapping technique to obtain very accurate spatial derivatives in an efficient manner. To this end, we present the formulation of the PSTD method by using a NU grid and compare its application to the analysis with that of the finite-difference time-domain (FDTD) method. Using only a fraction of the memory and a fraction of the computation time used by FDTD, the mapped PSTD was able to obtain very close results to FDTD.