arXiv subjects

David R. Smith

Publications and source records attributed to David R. Smith.

At least 19 recordsLinked to original sources

arXiv2026

Semi-analytical Model of Multi-tile Rectangular Waveguide-fed Metasurfaces using Coupled Dipole Modeling Framework

We present a semi-analytical model to analyze multi-tile metasurface antennas consisting of a set of metasurface tiles and a practical power-dividing network that excites the tiles. The metasurface tiles consist of arrays of rectangular waveguides with subwavelength metamaterial radiators etched into their top walls, each of which can be accurately modeled as polarizable dipoles. The feed structure for the arrays comprises a slotted waveguide attached to their bottom wall, with coupling slots inserted into the common wall that are likewise modeled as polarizable dipoles. The proposed semi-analytical model employs a coupled-dipole framework that accurately captures dipolar interactions among constituent elements within the metasurface tiles, along with a multi-port network analysis technique that accounts for electromagnetic interactions between the tiles and the power divider, thereby forming a self-consistent formulation. The proposed model enables the prediction of key performance metrics, including overall S-parameters, radiation patterns, and gain, and is validated through full-wave numerical simulations. By significantly reducing the computational complexity associated with electrically large apertures, the proposed framework enables rapid and efficient modeling of the overall structure, thereby facilitating iterative optimization. The proposed model has potential applications as an efficient forward model for the design of wireless systems requiring large-aperture metasurface antennas, including remote sensing and next-generation wireless communication networks.

physics.optics↗

arXiv2025

Self-Organized Freeform Waveguiding

Nature offers remarkable examples of complex photonic architectures such as those responsible for the iridescent colors of butterfly wings that emerge spontaneously during growth, well before any centralized control takes place. Arising from local rules, these structures exhibit advanced optical functionalities, such as photonic band gaps, without relying on in-situ optimization or top-down design. Inspired by biological morphogenesis, we introduce an optimization-free approach for the automated generation of self-organized freeform waveguides that adapt to complex propagation paths. Our method relies on local reaction-diffusion dynamics to produce robust, spatially distributed structures. In contrast to conventional waveguides based on periodic media, which impose strong geometric constraints and require extensive fine-tuning, the proposed structures support nontrivial geometries while maintaining photonic band gap behavior. We experimentally demonstrate that these self-organized waveguides achieve superior transmission efficiency along complex paths. This optimization-free strategy enables the automated design of advanced electromagnetic components with intrinsic adaptability and resilience.

physics.app-ph↗

arXiv2025

First ionization potentials of Cr, Mo, and W calculated with SHCI

The design and performance of future fusion power plants will depend on accurate atomic data for plasma-facing material and plasma impurity species. A leading candidate for the plasma-facing material is tungsten due to its high melting point, however, the energy levels and wavefunctions of high-Z atoms with many electrons (e.g. 30 or more), including tungsten, are difficult to calculate with high accuracy. Gaps and large uncertainties in atomic data for tungsten introduce design and performance uncertainties for a fusion power plant. Specifically, improved atomic data for ionization potential, excited state energies, and collisional excitation rates are needed for the low charge states of atomic tungsten. We aim to address these shortcomings by using the semistochastic heat-bath configuration interaction (SHCI) method, which nearly exactly calculates the energies that can be determined at higher cost with the full configuration interaction. Adding well-motivated approximations to SHCI, including orbital optimization and effective core potentials, we demonstrate good agreement between our calculated first ionization potentials and the best available experimental values for chromium, molybdenum, and tungsten. The efficiency and accuracy achieved in calculating these ionization potentials demonstrates that our SHCI workflow can yield improved electron structure data for ions with many electrons, suggesting that the method could also be useful for collisional processes, such as state-selective charge exchange reactions and electron impact ionization.

physics.atom-ph↗

arXiv2024

Equivalent Circuit Models for Waveguide-Fed, Resonant, Metamaterial Elements

We propose an approach to extracting equivalent circuit models for waveguide-fed, resonant metamaterial elements, such as the complementary, electric inductive-capacitive element (cELC). From the scattering parameters of a single waveguide-fed cELC, effective electric and magnetic polarizabilities can be determined that can be expressed in terms of equivalent lumped element circuit components. The circuit model provides considerable insight into the electromagnetic scattering properties of cELCs as a function of their geometric parameters and imparts intuition useful for element optimization. We find that planar, inherently resonant, waveguide-fed elements exhibit a set of common properties that place constraints on their coupling, maximum radiation, and other key scattering parameters. In addition, unlike simple slots and other non-resonant irises, resonant elements introduce an effective transformer to the equivalent circuit that accounts for the field enhancement occurring in such elements at resonance. We introduce a general and robust method to determine the effective circuit parameters by fitting to the extracted polarizability, extending the approach to resonant metamaterial elements integrated with physical lumped circuit components, such as packaged capacitors or varactors. We find excellent agreement between the analytical predictions and full-wave simulations, such that with one or two full-wave simulations the properties of the cELC can be determined for any externally added lumped elements. This approach can be leveraged to dramatically increase the efficiency of metasurface aperture design, especially when libraries of element responses are required.

physics.app-ph↗

arXiv2024

Entangled two-photon absorption for the continuous generation of excited state populations in plasma

Entangled two-photon absorption (ETPA) may be a viable technique to continuously drive an excited state population in plasma for high-bandwidth spectroscopy measurements of localized plasma turbulence or impurity density. Classical two-photon absorption commonly requires a high-intensity, pulsed laser, but entangled photons with short entanglement time and high time correlation may allow for ETPA using a lower intensity, continuous-wave laser. Notably, ETPA with non-collinear entangled photon generation allows for cross-beam spatial localization of the absorption or fluorescence signal using a single laser source. Entangled photon generation, the ETPA cross-section, candidate transitions for an Ar-II species, and plans for a proof-of-principle measurement in a helicon plasma are discussed.

quant-ph↗

arXiv2024

Equivalence of Polarizability and Circuit Models for Waveguide-Fed Metamaterial Elements

A common variant of a metasurface antenna consists of an array of metamaterial elements coupled to a waveguide feed. The guided wave excites the metamaterial elements, coupling energy from the waveguide mode to radiation. Under appropriate conditions, each sub-wavelength metamaterial element can be modeled as a polarizable dipole, with the polarizability determined by an extraction procedure from the computed or measured waveguide scattering parameters. Here we establish the equivalence of this polarizability description of a metamaterial element with an equivalent circuit model, providing an additional tool for metasurface design that offers significant insight and a path towards efficiently modeling very large apertures. With this equivalence established, more complicated external circuits that include lumped elements and devices such as diodes and transistors can be integrated into the metamaterial element, which can then be transformed into an equivalent polarizability for modeling in the coupled dipole framework. We derive appropriate circuit models for several basic metamaterial elements, which provide direct relationships between the equivalent circuit parameters of an element and its effective polarizability. These expressions are confirmed using scattering parameters for several example structures obtained via full-wave simulations.

physics.app-ph↗

arXiv2024

Low frequency coherent Raman imaging robust to optical scattering

We demonstrate low-frequency interferometric impulsive stimulated Raman scattering (ISRS) imaging with high robustness to distortions by optical scattering. ISRS is a pump-probe coherent Raman spectroscopy that can capture Raman vibrational spectra. Recording of ISRS spectra requires isolation of a probe pulse from the pump pulse. While this separation is simple in non-scattering specimens, such as liquids, scattering leads to significant pump pulse contamination and prevent the extraction of a Raman spectrum. We introduce a robust method for ISRS microscopy that works in complex scattering samples. High signal-to-noise ISRS spectra are obtained even when the pump and probe pulses pass through many scattering layers.

physics.optics↗

arXiv2023

Simultaneous Transmitting and Reflecting (STAR)-RIS for Harmonious Millimeter Wave Spectrum Sharing

The opening of the millimeter wave (mmWave) spectrum bands for 5G communications has motivated the need for novel spectrum sharing solutions at these high frequencies. In fact, reconfigurable intelligent surfaces (RISs) have recently emerged to enable spectrum sharing while enhancing the incumbents' quality-of-service (QoS). Nonetheless, co-existence over mmWave bands remains persistently challenging due to their unfavorable propagation characteristics. Hence, initiating mmWave spectrum sharing requires the RIS to further assist in improving the QoS over mmWave bands without jeopardizing spectrum sharing demands. In this paper, a novel simultaneous transmitting and reflecting RIS (STAR-RIS)-aided solution to enable mmWave spectrum sharing is proposed. In particular, the transmitting and reflecting abilities of the STAR-RIS are leveraged to tackle the mmWave spectrum sharing and QoS requirements separately. The STAR-RIS-enabled spectrum sharing problem between a primary network (e.g. a radar transmit-receive pair) and a secondary network is formulated as an optimization problem whose goal is to maximize the downlink sum-rate over a secondary multiple-input-single-output (MISO) network, while limiting interference over a primary network. Moreover, the STAR-RIS response coefficients and beamforming matrix in the secondary network are jointly optimized. To solve this non-convex problem, an alternating iterative algorithm is employed, where the STAR-RIS response coefficients and beamforming matrix are obtained using the successive convex approximation method. Simulation results show that the proposed solution outperforms conventional RIS schemes for mmWave spectrum sharing by achieving a 14.57% spectral efficiency gain.

cs.IT↗

arXiv2022

Automatic Identification of Edge Localized Modes in the DIII-D Tokamak

Fusion power production in tokamaks uses discharge configurations that risk producing strong Type I Edge Localized Modes. The largest of these modes will likely increase impurities in the plasma and potentially damage plasma facing components such as the protective heat and waste divertor. Machine learning-based prediction and control may provide for online mitigation of these damaging modes before they grow too large to suppress. To that end, large labeled datasets are required for supervised training of machine learning models. We present an algorithm that achieves 97.7% precision when automatically labeling Edge Localized Modes in the large DIII-D tokamak discharge database. The algorithm has no user controlled parameters and is largely robust to tokamak and plasma configuration changes. This automatically-labeled database of events can subsequently feed future training of machine learning models aimed at autonomous Edge Localized Mode control and suppression.

physics.plasm-ph↗

arXiv2022

Reflectarray Design Using a Discrete Dipole Framework

We propose and numerically validate a patch reflectarray modeling approach suitable for small patches that describes each patch as a pair of polarizable magnetic dipoles. We introduce an extraction technique to obtain the effective polarizability of the patch dipoles via full-wave simulations on individual patches, followed by a beamforming design routine valid under weakly scattering configurations. This dipole framework serves as an alternative to the ray tracing model often used in reflectarray designs, in which rays are drawn from the feed point and scattered off of the patch elements. Whereas the ray tracing method solves the design problem in terms of phase delays, the dipole framework presented here has the potential to accurately design and predict beam patterns using a fully dipolar treatment of empirically characterized patches. We illustrate this technique by applying it to two modulation strategies: a variable patch size reflectarray in which the phase can be continuously tuned (grayscale patch response), and a fixed patch size (binary patch response) in which on/off modulation is achieved through selective patch electrical shorting. Methods for incorporating these cases into the dipole design framework are discussed and the results compared to those from full wave simulation.

physics.app-ph↗

arXiv2022

Hollow Rectangular Waveguide-fed Holographic Beamforming Antenna Additively Manufactured (3D Printed) with Conductive Polymer

We present the design and fabrication of 3D printed holographic beamforming antennas. The antennas utilize additively manufactured hollow rectangular waveguides that feed radiating rectilinear slots inserted into the upper conducting wall. The lengths of the individual slots are altered to implement a holographic beamforming solution designed using a coupled dipole formalism. For rapid verification, the designed antennas are fabricated using a desktop dual-extrusion fused filament 3D printer. The body of each antenna and its inner conducting surface are respectively printed using polylactic acid and biodegradable conductive polyester composite material (i.e., Electrifi), which is later deposited with a layer of copper on its surface to improve surface conductivity and reduce surface roughness. The beamforming performance of the fabricated antennas is confirmed via experiments. The 3D printed metasurface antennas using the proposed fabrication technique illustrate emerging capabilities in the rapid prototyping of complex electromagnetic structures.

physics.app-ph↗

arXiv2022

An Integrated, Phase-Controlled Power Divider for Metasurface Array Antennas

We present the design of a phased, modulated, power distribution network for metasurface array antennas. The specific metasurface array design comprises a series of waveguides, each feeding a sub-array of dynamically tunable metamaterial elements that radiate at microwave (X-band) frequencies. To remain a one-port device, the composite array requires a power divider that can excite each branch waveguide with equal power. Further, the power divider must also apply a prescribed phase shift to each branch waveguide to mitigate metasurface-specific grating lobes. The presented design features hollow metal waveguides, with a conducting sheet patterned with metamaterial irises serving as the upper wall. A single hollow metal main waveguide feed is used to couple power to each of the branch waveguide. Carefully oriented slots control both the magnitude and phase of the field distributed from the main feed to each branch. All waveguides can be machined from a single block of metal, with a metamaterial-patterned printed circuit board laminated to the top of the assembly. The resulting design is mechanically robust and capable of high power operation while retaining the low-cost, low-power consumption, lightweight, and low-profile features common in metasurface antenna designs.

physics.app-ph↗

arXiv2022

Optimizing Polarizability Distributions for Metasurface Apertures with Lorentzian-Constrained Radiators

We present a design strategy for selecting the effective polarizability distribution for a metasurface aperture needed to form a desired radiation pattern. A metasurface aperture consists of an array of subwavelength metamaterial elements, each of which can be conceptualized as a radiating, polarizable dipole. An ideal polarizability distribution can be determined by using a holographic approach to first obtain the necessary aperture fields, which can then be converted to a polarizability distribution using equivalence principles. To achieve this ideal distribution, the polarizability of each element would need to have unconstrained magnitude and phase; however, for a single, passive, metamaterial resonator the magnitude and phase of the effective polarizability are inextricably linked through the properties of the Lorentzian resonance, with the range of phase values restricted to a span of at most 180 degrees. Here, we introduce a family of mappings from the ideal to the available polarizability distributions, easily visualized by plotting both polarizabilities in the complex plane. Using one of these mappings it is possible to achieve highly optimized beam patterns from a metasurface antenna, despite the inherent resonator limitations. We introduce the mapping technique and provide several specific examples, with numerical simulations used to confirm the design approach.

physics.app-ph↗

arXiv2022

Nanogap-Enhanced Infrared Spectroscopy with Template-Stripped Wafer-Scale Arrays of Buried Plasmonic Cavities

We have combined atomic layer lithography and template stripping to produce a new class of substrates for surface-enhanced infrared absorption (SEIRA) spectroscopy. Our structure consists of a buried and U-shaped metal-insulator-metal waveguide whose folded vertical arms efficiently couple normally incident light. The insulator is formed by atomic layer deposition (ALD) of Al2O3 and precisely defines the gap size. The buried nanocavities are protected from contamination by a silicon template until ready for use and exposed by template stripping on demand. The exposed nanocavity generates strong infrared resonances, tightly confines infrared radiation into a gap that is as small as 3 nm (λ/3300), and creates a dense array of millimeter-long hotspots. After partial removal of the insulators, the gaps are backfilled with benzenethiol molecules, generating distinct Fano resonances due to strong coupling with gap plasmons, and a SEIRA enhancement factor of 10^5 is observed for a 3 nm gap. Because of the wafer-scale manufacturability, single-digit-nanometer control of the gap size via ALD, and long-term storage enabled by template stripping, our buried plasmonic nanocavity substrates will benefit broad applications in sensing and spectroscopy.

physics.optics↗

arXiv2021

Meta-Programmable Analog Differentiator

We show experimentally that the fundamental ingredient of wave-based signal differentiation, namely zeros of the scattering matrix that lie exactly on the real axis, can be imposed at will and in situ by purposefully perturbing an overmoded random scattering system. The resulting unprecedented flexibility overcomes current limitations of wave-based differentiators, both regarding their extreme vulnerability due to fabrication inaccuracies or environmental perturbations as well as their lack of in situ adaptability. Moreover, in addition to current miniaturization efforts, we suggest that integrability of wave processors can also be achieved by endowing existing bulky everyday-life systems that naturally scatter waves with a second signal-processing functionality. We demonstrate our technique by placing a programmable metasurface inside a 3D disordered metallic box: the hundreds of available degrees of freedom allow us to impose at will reflection zeros on a connected port, such that the reflected signal envelope is the temporal derivative of the incident one. We demonstrate our ability to toggle between differentiation of envelopes modulated onto distinct carriers. We also parallelize multiple differentiation operations on the same device and implement higher-order differentiators. Our "over-the-air" temporal differentiator for microwave carriers may find civilian and military applications in processing wireless communication or radar signals, for data segmentation and compression, as well as machine vision and hearing. Our generic concept is also applicable to optical, acoustic, and elastic scattering.

physics.app-ph↗

arXiv2021

Holographic Metasurface Antennas for Uplink Massive MIMO Systems

We propose an uplink massive MIMO system using an array of holographic metasurfaces as a sector antenna. The antenna consists of a set of rectangular waveguide-fed metasurfaces combined along the elevation direction into a planar aperture, each with subwavelength-sized metamaterial elements as radiators. The metamaterial radiators are designed such that the waveguide-fed metasurface implements a holographic solution for the guided (or reference) mode, generating a fan beam towards a prescribed direction, thereby forming a multibeam antenna system. We demonstrate that a narrowband uplink massive MIMO system using the metasurfaces can achieve the sum capacity close to that offered by the Rayleigh channel at 3.5 GHz. We show that metasurfaces supporting multiple fan beams can achieve high spatial resolution in the azimuth directions in sub-6 GHz channels, and thereby form uncorrelated MIMO channels between the base station and users. Also, the proposed metasurface antenna is structurally simple, low-cost, and efficient, and thus is suitable to alleviate RF hardware issues common to massive MIMO systems equipped with a large antenna system.

eess.SP↗

arXiv2021

Design of Conformal Array of Rectangular Waveguide-fed Metasurfaces

We present a systematic design method for a cylindrical conformal array of rectangular waveguide-fed metasurfaces. The conformal metasurface consists of multiple curved rectangular waveguides loaded with metamaterial elements\textemdash electrically small irises\textemdash inserted into the upper conducting walls of the waveguides. Each element radiates energy into free space to contribute to an overall radiation pattern. Thus, the geometry or electrical configuration of each of the individual metamaterial elements needs to be tailored to generate a desired pattern. In general, due to difficulties in modeling the effect of curvature, the design of conformal metasurface arrays has relied on full-wave simulations or experiments. In this study, we propose a design method utilizing the analytic model of a planar metasurface accounting for metamaterial elements' locations and orientations over a surface with curvature. Although approximate, we demonstrate that such an alteration along with the framework of dipolar modeling of planar elements can be used for the analysis of conformal arrays with small curvature. We then design a conformal array metasurface using the method combined with CMA-ES optimizer. Through numerical simulations, we confirm the validity of the proposed design method. Applications include the design of metasurfaces for radar, communications, and imaging systems for automobiles and airplanes.

physics.app-ph↗

arXiv2020

Electronically Steered Nyquist Metasurface Antenna

Mobile devices, climate science, and autonomous vehicles all require advanced microwave antennas for imaging, radar, and wireless communications. The cost, size, and power consumption of existing technology, however, has hindered the ubiquity of electronically steered systems. Here, we propose a metasurface antenna design paradigm that enables electronic beamsteering from a passive lightweight circuit board with varactor-tuned elements. Distinct from previous metasurfaces (which require dense element spacing), the proposed design uses Nyquist spatial sampling of half a wavelength. We detail the design of this Nyquist metasurface antenna and experimentally validate its ability to electronically steer in two directions. Nyquist metasurface antennas can realize high performance without costly and power hungry phase shifters, making them a compelling technology for future antenna hardware.

physics.app-ph↗