The natural frequency response of meta-atoms can be exploited to create dispersively engineered multifunctional devices, which can significantly enhance performance in radome-encapsulated antenna systems. By utilizing the transmission phase response of meta-atoms and operating in frequency bands in between their natural resonances, it is possible to create a meta-radome that collimates incoming radiation at one frequency band while appearing transparent in another. Meta-radomes provide a compelling alternative to traditional dielectric lenses, with drastically reduced size and weight. Optimization of such devices is challenging because of their complex geometry and electrically large structure. This issue can be mitigated by using various shortcut methods, which improve simulation time with minimal sacrifice to accuracy. In addition, the meta-radome design is flexible in that altering parameters such as the number of layers, layer spacing, and meta-atom geometry allows for further performance tuning. This work presents a general design of a meta-radome with emphasis on how to efficiently optimize its geometry. Comparisons to conventional dielectric lenses are made to demonstrate the viability of the proposed design technique.

We show how spin-to-orbital angular-momentum (SAM to OAM) conversion allows the generation of focused chiral fields and demonstrate numerically the utility of these fields for tweezing and separating chiral objects. The proposed setup consists of a micron-size Helmholtz hemisphere resonator fed at the pole by a circularly polarized Gaussian laser field in the visible range. The fields formed at the equator plane are shown to possess an intrinsic orbital angular momentum component with respect to the axis of the Helmholtz resonator. When brought in proximity to a metal plate, diffraction of the fields embodying OAM results in the formation of a sizable, nanoscale focused chirality density. This sub-wavelength chiral field formation is a result of an interplay between resonator cavity modes and diffraction. No plasmonic losses are involved, and the fields can be generated at tunable frequencies by varying the radius of the resonator. We demonstrate that the formed fields can move and separate radially chiral objects such as molecules on surfaces. The in-situ motion of the chiral molecules is illustrated using analytical and fully numerical simulations. The generated fields are tunable by the parameters of the input fields (such as frequency and polarization) as well as by the radius of the hemisphere. Our results demonstrate an effective tool for chiral tweezing and enantiomer separation on the nanoscale under a realistic setting and with moderate input laser intensity.

Traditional metasurface design is limited by the computational cost of full-wave simulations, preventing thorough exploration of complex configurations. Data-driven approaches have emerged as a solution to this bottleneck, replacing costly simulations with rapid neural network evaluations and enabling near-instant design for meta-atoms. Despite advances, implementing a new optical function still requires building and training a task-specific network, along with exhaustive searches for suitable architectures and hyperparameters. Pre-trained large language models (LLMs), by contrast, sidestep this laborious process with a simple fine-tuning technique. However, applying LLMs to the design of nanophotonic devices, particularly for arbitrarily shaped metasurfaces, is still in its early stages; as such tasks often require graphical networks. Here, we show that an LLM, fed with descriptive inputs of arbitrarily shaped metasurface geometries, can learn the physical relationships needed for spectral prediction and inverse design. We further benchmarked a range of open-weight LLMs and identified relationships between accuracy and model size at the billion-parameter level. We demonstrated that 1-D token-wise LLMs provide a practical tool for designing 2-D arbitrarily shaped metasurfaces. Linking natural-language interaction to electromagnetic modelling, this "chat-to-chip" workflow represents a step toward more user-friendly data-driven nanophotonics.

A Hessian-based optimization framework for accelerating the antenna design process is presented. This approach relies on leveraging the second-order derivatives of an objective function to achieve quadratic convergence, offering a significant improvement over gradient-based methods. Instead of computing the Hessian using a finite difference (FD) scheme, a custom-developed method of moments (MoM) solver was integrated with an automatic differentiation (AD) technique to evaluate the gradients at a much lower cost. This implementation requires minimal code modifications, rendering AD a highly attractive choice. Furthermore, when using the gradients in conjunction with the interior point method (IPM), the technique demonstrates superior convergence and requires fewer function evaluations compared to gradient descent (GD) and derivative-free optimization algorithms. This makes the approach very attractive compared to existing methods. Moreover, this method has the added advantage that it can be applied to arbitrary radiation and scattering problems and be readily paired with any optimization method. The accuracy and validity of the proposed method are verified through various application examples.

A transient circularly polarized excitation and its implementation in a generalized dispersive material model based discontinuous Galerkin time-domain solver are proposed for spectral analysis of chiral nanophotonic structures. The expression of a circularly polarized pulse with a certain bandwidth, which is real-valued and enables multi-physics and nonlinearity, is derived comprehensively. Numerical examples of nanophotonic structures are given in this paper. Such as reflection from metallic mirrors, transmission from S-shaped dielectric metasurfaces, and the spin response of C4 symmetrically arranged right-handed enantiomers. These examples demonstrate the accuracy and capability of the proposed method.

For electromagnetic design problems with seemingly intractable solutions, an optimization strategy best suited to the nature of the problem can be used to significantly cut the required simulation time. Traditional global optimizations require substantial simulation time for problems with large numbers of design variables, including reflectarray or gradient index (GRIN) lens structures with a large volume of unit cells. To design these complex structures at improved speeds, one can exploit the adjoint optimization (AO) approach to quickly optimize solutions with an unlimited number of parameters using physics-based gradient computation and local optimization algorithms.

The pursuit of compact antennas has historically sacrificed directivity. Traditionally, solutions involve adding radiators, complex lenses, or enlarging the antenna, which often introduces spurious lobes or increases system height. This study investigates a digital choke and groove aperture technology, employing recessed slots and protruded chokes, to optimize directivity and shape lobes in ultra-short horns. Through analysis and experiments, we assess the antenna's gain, volume efficiency, bandwidth, and lobe suppression. Our findings offer insights into applications and lay the groundwork for further advancements in ultra-low-profile digital aperture topologies for ultra-short horns.

Microstrip patch antennas are one of the most widely used antennas in communication systems due to their compact size, conformability, ease of implementation, and low cost. The cavity model, which assumes that a microstrip patch antenna is surrounded by four perfect magnetic conducting (PMC) walls on its periphery and perfect electric conducting (PEC) boundaries on the top (patch) and bottom (ground plane), is a well-established analytical method to evaluate its radiated fields and input impedance. In this paper, we extend the traditional cavity model approach to include the response of a microstrip patch antenna with a time-periodic substrate permittivity epsilon r(t) = epsilon r(t+ T). We show that this time-periodic permittivity modulation provides an unprecedented ability to tailor the harmonic response of the antenna. Importantly, we achieve a remarkable 40% fractional bandwidth through such a customized time-periodic permittivity modulation, which is substantially higher than the typical 1-2% fractional bandwidth of conventional microstrip patch antennas.

Data-driven approaches have revolutionized the design and optimization of photonic metadevices by harnessing advanced artificial intelligence methodologies. This review takes a model-centric perspective that synthesizes emerging design strategies and delineates how traditional trial-and-error and computationally intensive electromagnetic simulations are being supplanted by deep learning frameworks that efficiently navigate expansive design spaces. We discuss artificial intelligence implementation in several metamaterial design aspects from high-degree-of-freedom design to large language model-assisted design. By addressing challenges such as transformer model implementation, fabrication limitations, and intricate mutual coupling effects, these AI-enabled strategies not only streamline the forward modeling process but also offer robust pathways for the realization of multifunctional and fabrication-friendly nanophotonic devices. This review further highlights emerging opportunities and persistent challenges, setting the stage for next-generation strategies in nanophotonic engineering.

There has of late been a great amount of interest by the antenna design community in the synthesis of optimal phased arrays. It is no wonder, given the increased prevalence of applications such as electronically steered radar, terrestrial multiuser communications (e.g., 5G mobile communications), and space-based shaped beam antennas, among many others. A key figure of merit in the design of a phased array is the directivity pattern, meaning both the peak achievable directivity ( D-0 ) of the array as well as the peak sidelobe level (PSLL) below the main beam. Especially in the case of large element-count aperiodic arrays, these values are quite difficult to accurately determine without first performing computationally intensive numerical integrations of unpredictable radiation patterns. The literature offers several solutions to determine directivity using analytical (i.e., closed form or exact) array factor (AF) methods, but these leave much to be desired in terms of true generality. In this work, we present a new fully generalized analytical directivity solution that may be applied to planar arrays of totally arbitrary topology. The proposed element model is shown to apply to a variety of practical antennas, in addition to enabling rapid array synthesis. Moreover, the solution is shown to be in excellent agreement with prior solutions when considered as special cases (SCs).

The traditional design approaches for high-degree-of-freedom metamaterials have been computationally intensive and, in many cases, even intractable due to the vast design space. In this work, we introduce what we believe to be a novel fixed-attention mechanism into a deep learning framework to address the computational challenges of metamaterial design. We consider a 3D plasmonic structure composed of gold nanorods characterized by geometric parameters and demonstrate that a long short-term memory network with a fixed-attention mechanism can improve the prediction accuracy by 48.09% compared to networks without attention. Additionally, we successfully applied this framework for the inverse design of plasmonic metamaterials. Our approach significantly reduces computational costs, opening the door for efficient real-time optimization of complex nanostructures.

Traditionally, the asymptotic waveform evaluation (AWE) technique is combined with the method of moments (MoM) to perform fast frequency sweep calculations. In this approach, the unknown current distribution within the frequency band of interest is expanded using a Taylor or Padè approximation around a single frequency. The derivatives of the impedance matrix and excitation vector are computed based on the formulation proposed by C. R. Cockrell, et al. [C. R. Cockrell, and F. B. Beck, “Asymptotic Waveform Evaluation (AWE) Technique for Frequency Domain Electromagnetic Analysis,” NASA Tech. Memo., 110292, 1996]. Based on this initial work, several researchers have extended the technique to fast integral solver, such as the fast multipole method (FMM) [X. C. Wei, and Y. J. Zhang, “The Hybridization of Fast Multipole Method with Asymptotic Waveform Evaluation for the Fast Monostatic RCS Computation,” IEEE Trans. Antennas Propag., 52(2), 2004] and the adaptive integral method (AIM) [X. Wang, S. Gong, J. Guo, Y. Liu, and P. Zhang, “Fast and Accurate Wide-band Analysis of Antennas Mounted on Conducting Platform Using AIM and Asymptotic Waveform Evaluation Technique,” IEEE Trans. Antennas Propag., 59(12), 2011]. In all these approaches, the antenna impedance response over a band of frequencies is evaluated, eliminating the need for expensive matrix inversion. However, at the design stage, parametric analysis of antenna variables is performed to evaluate the antenna performance. This process requires a significant number of simulations, which are often computationally expensive.

This study examines the effects of time-modulated permittivity in a lossy cavity enclosed by perfect magnetic conductor (PMC) walls, where the modulation frequency is set to twice the input frequency to observe parametric amplification. A parametric study of three key parameters is conducted to identify the conditions under which parametric amplification occurs. The findings show that parametric amplification is achievable even in the presence of material loss.

An arbitrary high order spectral-element time-domain method with local time-stepping (LTS-ADER-SETD) is proposed for transient electromagnetics simulation. An expression for the second-order wave-equation-based LTS-ADER-SETD method is comprehensively derived by replacing the time derivatives with space derivatives. Innovatively, this technology achieves subdomain coupling without temporal synchronization constraints, enabled by a unified treatment of local time-stepping (LTS) scheme. The proposed method achieves exponential error reduction with increasing spatiotemporal discretization order while significantly enhancing computational efficiency in multiscale electromagnetic analysis. Numerical convergence results demonstrate the high accuracy in both the space domain and time domain, even with an extremely large time step ratio. Further validation is provided, indicating the proposed method is computationally efficient for electromagnetic problems with strongly varying element sizes.

The capability to simultaneously transmit and receive signals in the same frequency band, known as in-band full-duplex or simultaneous transmit and receive, increases the spectral efficiency and decreases the latency of communication systems. Moreover, sensing systems can also benefit from this capability. However, the primary challenge for successful implementation remains the susceptibility to self-interference. This article reviews various approaches to overcome the challenges presented by self-interference, focusing primarily on those that can be addressed by taking advantage of knowledge and understanding of electromagnetics, antennas, and propagation.

As the requirements for antenna performance continue to grow, multi-band and multi-functional apertures are increasingly sought after to meet system needs in a variety of application areas. However, it can often be cost prohibitive to replace legacy systems, and thus solutions for enhancing their existing behavior are desirable. For example, enhancing scan range or gain at certain bands with an augmentation of the system may be sufficient to extend the life of legacy systems. Moreover, realizing performance enhancement through a low-cost solution makes the proposition even more enticing. To this end, gradient index (GRIN) lenses offer designers the potential to enhance aperture gain and scan range. GRIN lenses may also be considered for potential radome solutions, where environmental protection and/or visual obfuscation is needed.

Chiral metamaterials based on mirror-symmetry broken resonators are engineered to achieve a strong interaction with circularly polarized (CP) waves. However, in contrast to the widely-observed enhanced circular dichroism (CD), moderate/narrow-band optical activity (OA) responses are usually seen in chiral metadevices. Here, by engineering the chirality parameter that determines the effective refractive indices associated with the two CP eigenstates, a three-dimensional (3D) metamaterial consisting of 3D-printed ceramic meta-atoms exhibiting broadband strong OA with nondispersive near-zero ellipticity in the millimeter-wave regime is demonstrated. With four-fold rotational symmetry (C-4), the 3D dielectric metamaterial shows near-identical co-polarized transmission magnitude but distinct phase retardations under left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) illumination over a broad frequency range. Field analysis indicates that the observed OA behavior originates from the handedness-dependent light-matter interaction between the chiral meta-atoms and CP waves. Furthermore, large transmission CD and CP polarization conversion are observed in a metamaterial with C-2-symmetry, demonstrating the proposed systems' versatility in CP wave control. As revealed by the present study, coherently exploring the complex parametric space offered by 3D meta-atoms based on state-of-the-art 3D fabrication techniques can become a promising paradigm for engineering metamaterials with sophisticated functionalities.

In physics, geometrical symmetry is a fundamental property of importance since it is associated with physical conservation. Here, we reveal an exceptional form of symmetry for a family of knots that are both chiral and three-dimensional (3D) rotationally symmetric about every axis of a standard Cartesian coordinate system. We call these unique knotted structures chiral balls. Moreover, chirality can bring about polarization transformation in electromagnetic waves. As a consequence of their 3D rotational symmetry, we further expect the polarization transformation performance of chiral balls to exhibit ultra-wide angle-independent behavior. Such a remarkable property has not been previously reported on in the literature. As a case study, we investigate the intrinsic electromagnetic scattering properties of a representative conductive chiral ball using characteristic mode analysis and then further verify them by measuring its radiation performance. The result shows that the chiral ball can exhibit an unprecedented extraordinary omnidirectional circularly polarized electromagnetic scattering property. Because of their unique properties, chiral balls are expected to not only have a profound impact on the fields of electromagnetics and optics but also potentially far beyond.