The control of all of light's degrees of freedom and its harnessing for applications is captured by the emergent field of structured light. The modern toolkit includes external modulation of light with devices such as metasurfaces and spatial light modulators, their intra-cavity insertion for structured light directly at the source, and their deployment to engineer quantum structured light at the single photon and entangled state regimes. Historically, this control has involved linear optical elements, with nonlinear optics only recently coming to the fore. This has opened unprecedented functionality while revealing new paradigms for nonlinear optics beyond plane waves. In this review we look at the recent progress in structured light with nonlinear optics, covering the fundamentals and the powerful applications they are facilitating in both the classical and quantum domains.

With increasing demand for renewable energy, perovskite solar cells (PSCs) have emerged as a promising alternative due to their high efficiency and solution-based manufacturing processes. However, the fabrication of PSCs in ambient conditions, as opposed to inert environments, remains challenging due to environmental factors such as moisture and oxygen that degrade perovskite materials. Developing air-processed PSCs is therefore critical for reducing fabrication cost, simplifying manufacturing infrastructure, and enabling scalable production compatible with industrial processes. Moreover, air processing represents a key step toward realistic deployment, bridging the gap between laboratory demonstrations and commercial applications. This perspective discusses the progress of air-processed PSCs, highlights the environmental challenges related to stability and performance, and outlines potential strategies for future research, including precursor chemistry, solvent and additive engineering, and interface optimization. In addition, emerging scalable deposition techniques, automated platforms, and machine learning-assisted control are expected to accelerate device optimization and reproducibility. Despite remaining challenges, commercializing air-processed PSCs is increasingly viable, promising a sustainable and efficient approach for solar energy technology.

Plasmonic designs for mid-infrared extraordinary optical transmission (EOT), a direct route to tailored filtering with broadband out-of-band rejection, have long been constrained by a fundamental trade-off between high transmission efficiency and narrow linewidths, a challenge rooted in the material properties of noble metals. Here, we theoretically propose and numerically demonstrate a versatile design paradigm that resolves this challenge by functionally decoupling the tasks of light coupling and resonant filtering. Our approach uses a dual-stacked noble metal-dielectric grating architecture to surpass the intrinsic limitations of single-layer structures. This paradigm provides the flexibility to engineer devices for ultra-high spectral selectivity and transmission efficiency. We demonstrate this with distinct designs: one at 10 μm with a quality factor (Q-factor) >2000 and >91% transmission; a high-Q design at 4 μm and >80% transmission; and a high-efficiency design at 4 μm with >92% transmission over a uniquely broad spectral-angular range. These generic designs produce solitary, narrow EOT peaks originating from a ``triple-coupling'' mechanism that mitigates reflection and absorption losses, with symmetry-broken configurations capable of exceeding Q-factors of 16,000 while maintaining a peak transmission efficiency > 60%. Crucially, these compact two-layer designs exhibit exceptional robustness against fabrication variations, offering a broadly applicable route to ultra-compact, low-cost infrared components, enabling advanced architectures such as angular sensing, spectro-polarimetric imaging, and isotope-resolved gas diagnostics.

Complex beams hold significant value in radar and communication systems due to their distinctive propagation characteristics. Digital metasurfaces, which can dynamically control electromagnetic (EM) waves, play an important role in realizing complex beams. Conventional analytic and optimization methods face challenges in synthesizing complex beams of low-bit digital metasurfaces due to the quantization error and the high computational complexity. Here, we propose a statistical method to realize complex beams with phase-only digital metasurfaces. To this end, we introduce tailored quantization probabilities to design the discrete random phase distributions, which approximate the continuous excitation coefficients derived from analytic methods. Based on the proposed method, we analyze the error between the realized and target patterns. These findings offer critical insights into the accuracy of random quantization. Complex patterns with cosecant, prescribed null, flat-top, and dual-beam are designed and validated in combination with a 2-bit phase coding digital metasurface. The experimental results are in good agreement with the theoretical analysis. This work pioneers the application of random phase approximation and statistical synthesis in digital metasurfaces, providing a fast and efficient route for realizing complex beams in modern radar and wireless communication technologies.

In the design and fabrication of ceramic filters, the quality of metallization is crucial to minimize resistive losses and ensure optimal resonator performance. This work presents the design and fabrication of a monoblock dual-mode filter with two distinct types of couplings, based on barium titanate (BaTiO₃) ceramics, operating at S-band frequencies. Sputtering deposition was used to create a 5 nm gold seed layer, on which a 30 μm copper metallization was grown through electroplating. This method guarantees high conductivity in the resonator coating, and test results demonstrated that the fabricated device offers very good filtering performance with a minimal insertion loss of 0.57 dB.

Low-emissivity glass, commonly employed in building curtain walls strongly reflects and weakly transmits millimeter-wave signals, thereby hindering signal propagation. To address this issue, this paper introduces a novel method that leverages the low-emissivity film itself to design a metasurface for enhanced signal transmission. Two specific metasurface designs are presented. The simulation results validate the proposed method. For the design targeting linearly polarized waves, a 23 dB enhancement in the transmitted electric field is achieved compared to that of uncoated glass. The design for circularly polarized waves achieves a 22 dB enhancement. Both metasurfaces exhibit excellent wide-angle performance, maintaining single-point focusing up to a 30° incidence angle with an electric field enhancement exceeding 15 dB. The proposed millimeter-wave transparent metasurface features a simple structure, supports wide-angle incidence, and can be deployed over large areas with adjustable focal points to meet communication requirements. This work provides a reliable solution for mitigating millimeter-wave transmission loss through low-emissivity glass.

Conventional full-wave methods face prohibitive computational costs for far-field scattering optimization of metasurface-integrated cavity structures. To address this limitation, a lightweight residual neural network is introduced within a two-stage scattering prediction framework. This framework effectively mitigates model degradation. The first stage employs shallow convolutional networks to extract local phase-coupling features. The second stage integrates residual layers with fully connected layers to refine cross-scale scattering responses. A compact CNN-ResNet surrogate model is developed for rapid cavity scattering prediction. With only 2.5×10⁴ parameters and training on 500 full-wave samples spanning 6.0-16.0 GHz, the model achieves high computational efficiency. The proposed approach directly maps binary phase-coded matrices to far-field electromagnetic characteristics. Extensive validation on a cavity structure across 6.0-16.0 GHz demonstrates excellent accuracy. The per-sample runtime is reduced from hours to milliseconds while maintaining prediction errors below 3 dB. These results confirm the effectiveness of the approach in enabling fast and accurate electromagnetic scattering prediction for complex cavity environments. The approach provides a practical solution for metasurface-integrated cavity optimization.

In response to the demand for broadband antennas in satellite communications, this paper sets out the proposal of a broadband circularly polarised metasurface antenna. Based on the theory of characteristic mode analysis of super surface, a pair of characteristic modes with the potential to realize circular polarization broadband are obtained and used as the modes to be excited. At the same time, the metasurface current is analyzed; the position of the floor gap is determined according to the results; and the shape of the floor gap is designed to better stimulate the characteristic mode. Subsequently, the power is transmitted through the microstrip line gap coupling feeding structure to excite the selected mode. Finally, an MTS antenna with dimensions of 0.9λ₀ × 0.9λ₀ × 0.076λ₀ at a centre frequency of 5 GHz was determined. The antenna was modeled using CST, a 3D electromagnetic simulation software, and then physically tested for verification. The experimental findings indicate that the impedance bandwidth of the antenna in question is 4.20-5.83 GHz (relative bandwidth of 32.6%). Furthermore, the 3 dB axial ratio bandwidth is 4.38-5.97 GHz (relative bandwidth of 30.7%).

This paper systematically analyzes the electromagnetic coupling characteristics between microstrip resonators and proposes a novel structure that enables mutual cancellation of electromagnetic coupling, effectively reducing the spacing between resonators. Based on this approach, a 14th-order compact high-temperature superconducting (HTS) microstrip bandpass filter is designed and implemented. By constructing a folded symmetric resonator structure to minimize the total electromagnetic coupling energy, and by optimizing the non-uniform coupling gaps in conjunction with the coupling characteristics, precise control of the coupling paths is achieved, leading to a significantly enhanced compactness. The filter is fabricated using double-sided YBCO HTS thin films and tested at liquid nitrogen temperature (77 K). Both simulation and measurement results show that the filter operates within the 0.96~1.06 GHz frequency band, exhibits an insertion loss below 0.4 dB, an out-of-band rejection better than 78 dB, and a passband edge roll-off rate exceeding 60 dB/MHz, demonstrating excellent performance in terms of low loss, wide bandwidth, and high suppression.

Generalized sidelobe canceller (GSC) based adaptive beamformer possesses a main advantage of superior interference rejection due to its capability in tracking the interference characteristics. However, its performance is very sensitive to even a small mismatch in array scenarios. For example, the mismatch due to mutual coupling between array sensors is a common phenomenon in practical environments. Two common problems considered are as follows. (1) The existing adaptive array beamformers are very sensitive to MCE. (2) The existing robust methods inevitably suffer from the problems, including additional computational complexity and estimate accuracy. In this paper, we present an efficient method to deal with the performance degradation induced by the MCE to achieve robust beamforming. The proposed method simply utilizes a well-known scheme, namely the zero-forcing (ZF) equalizer. The ZF equalizer simply preprocesses the data vector received by the antenna array and then inputs the processed data vector into a GSC based adaptive array processor. The combination of a ZF equalizer and a GSC based adaptive array processor results in an adaptive array beamformer providing satisfactory beamforming performance in the presence of the MCE. The performance analysis regarding the proposed method is analyzed. Simulation results are also presented for confirmation and comparison. The simulation results show that the ZF equalizer alleviates the MCE and the GSC based adaptive beamformer can subdue the background noise enhanced by ZF equalizer.

This paper investigates the circuit theory of elementary passive topology exhibiting reconfigurable positive/negative delay (RPND) effect. This novel evaluated framework enables identification of the first-order L-topology constituted by RC-network operating under RPND effect. The investigated passive L-cell can operate in both negative and positive group delay (NGD or PGD) mode depending on the RC-network parameter. After establishing the NGD existence condition, the design equations versus the RPND effect including the target parameter values are formulated. To validate the theory, an RC-circuit representing a RPND Proof-of-Concept (PoC) was designed, implemented and tested especially in the time-domain by verifying the time-advance signature corresponding to the NGD operation mode. By tuning a PoC resistor, experimentation of pulse and arbitrary waveform signals confirm the feasibility to observe RPND reconfigurability. In the NGD mode, it is observed that outputs in time-advance of their own inputs about -3 ms. The RPND circuit is particularly useful for adjusting delay effect and signal synchronization in the communication system.

The history of science is filled with legendary ``happy accidents", from penicillin to the microwave oven. These breakthroughs are often attributed to luck. But Prof. Keisuke Goda (University of Tokyo) and his team argues in a recent paper in Progress In Electromagnetics Research (Vol. 184, 14--23, 2025: doi:10.2528/PIER25100702) [1], entitled ``Serendipity Engineering with Photonics: Harnessing the Unexpected in Biology and Medicine," that such accidental discoveries can be systematically cultivated. It introduces ``serendipity engineering" --- the intentional design of tools and research cultures to make unexpected, yet meaningful findings more likely.

In the manuscript a novel design of microstrip patch antenna with moderate degree of complexity is proposed in terms of metamaterial based unit cells as radiating patch on the top as well as metamaterial based periodic structure as defected ground structure at the bottom (MRPMGS) for Intelligent Transportation System (ITS) applications. The novel design of patch antenna exhibited multibands with broad-band transmission patterns, improved high gain and compact structure. The MRPMGS has a three layered structure with overall dimensions of 32 mm × 28 mm × 1.6 mm. The top layer with radiating patch has unit cell(s) with dimensions of 3.6 mm × 3.6 mm, and at the bottom the defective ground structure (DGS) has unit cell(s) with dimensions of 4 mm × 4 mm. The middle layer is of FR4 substrate with 1.6 mm thickness. The MRPMGS has experimental (simulated) transmission frequencies at 11.54 GHz (11.24 GHz), 12.91 GHz (12. 98 GHz), and 13.20 GHz (13.48 GHz) with reflection coefficients of -20.91 dB (-25.16 dB), -26.19 dB (-29.36 dB), and -18.94 dB (-26.02 dB) respectively. The VSWR varies between 1 and 3. The radiation efficiency reaches 80%, and high gain varying between 2.35 and 5.5 is achieved at the desired frequencies.

This paper presents a numerical study of a novel method for auto-calibration of scatteringparameter measurements in a near-field microwave sensor system. The here proposed method is applied to estimation of the average complex permittivity in a measurement domain from the scattering parameters, corrupted by gain uncertainties in the measurement instruments. Simultaneously with the average complex permittivity, the gain uncertainties are also estimated. The characteristic property of the proposed method is that no simplified mathematical model of the measurement domain is assumed, and instead a set of a-priori calibrated measurements is used. Numerical studies demonstrate the performance of the method in noiseless and noisy settings with and without nuisance stochastic perturbations in the measurement domain. An approach to compensate for the stochastic perturbations in the measurement domain permittivity is proposed, and it demonstrates an improved performance of the method in numerical examinations.

Time reversal is an established wave imaging and focusing method that has proved to be robust and compatible with super-resolution imaging and focusing, i.e., to provide images and foci with subwavelength features beyond the diffraction limit. The method has been applied to numerous wave systems. We propose a systematic review of super-resolution time reversal applied to electromagnetic waves in the radio-frequency regime. We examine the theoretical foundation, the methods and the applications of radio-frequency super-resolution time-reversal. We explain a seeming contradiction between a widely used model of resolution due to the time-reversal cavity, and a common approach of Fourier optics highlighting the significance of evanescent waves for super-resolution. We also present an application of one of the algorithms of time-reversal imaging (known as TR-MUSIC) to measurements in a highly reflective environment, such as a resonant cavity. Finally, we outline open questions and applications.

This paper proposes a single-fed antenna with large frequency ratio. The antenna integrates a metasurface antenna and a dielectric resonator antenna (DRA), capable of simultaneous operation in both microwave and millimeter-wave bands with single-port feeding. The microwave band achieves resonance through the metasurface antenna, while the millimeter-wave band resonates by exciting the HEM_12δ higher-order mode of the DRA. The proposed metasurface antenna achieves 24% (5.6 GHz-7.13 GHz) impedance bandwidth, 10% (6.2 GHz-6.9 GHz) axial ratio bandwidth, and 6.07 dBi peak gain in the microwave band; the DRA provides 13.4% (27.47 GHz-31.43 GHz) impedance bandwidth and 6.4 dBi peak gain in the millimeter-wave band. With simple structure and excellent performance, this antenna achieves a frequency ratio of 5.2, making it suitable for 5G communication scenarios requiring concurrent Sub-8 GHz and FR2 operation.

Touch interaction is an important function in various electronic systems. In this paper, a touch sensing method based on an electromagnetically induced transparency (EIT) microstrip structure is proposed. The design consists of a U-shaped two-port microstrip transmission line with four open stubs oriented in four directions. Two transmission narrow bands are generated by the proposed structure at around 1.8 GHz and 3.5 GHz, corresponding to the EIT effect. When finger-like objects approach the terminals of these open stubs, their transmission characteristics change significantly, as indicated by variations in the S-parameter response. To precisely determine the touch position, a shifting vector method is introduced based on the variations of S-parameters at different touch positions on the board plane. Both simulation and experimental results demonstrate a touch localization accuracy of 94.4% with a spatial resolution of 3 mm. The proposed design offers a low-cost and compact platform that integrates touch interaction and RF communication, showing strong potential for future interactive electronic and communication systems.

The modulation of topological polarization singularities in momentum space in photonics has attracted much attention due to their relations with bound states in the continuum (BICs), unidirectional guided resonances, and chirality. Current modulation strategies that rely on structural symmetry breaking or phase-change materials are challenging to achieve dynamic and flexible modulation of polarization singularities. Recently, magneto-optical (MO) modulation of light provides a promising theoretical strategy for the dynamic modulation of polarization singularities. However, the dynamics of transverse electric (TE)/transverse magnetic (TM)-mode singularities under varying magnetic fields remain elusive in the MO photonic crystal (PhC) slab. Herein, we systematically investigate the dynamic modulation of topological polarization singularities in the PhC slab based on the MO effect. In-plane (x/y) magnetic fields have no effect on the TE mode of the MO PhC slab. However, the fields induce splitting and separation of vortex polarization singularity (V point) of the TM mode into a pair of circular polarization points (C points), enabling extrinsic chirality without breaking the structural symmetry. A magnetic field along the z direction enables near-unity circular dichroisms (CDs) over a broad angular range when circular polarizations are formed at off-Γ points for the TE and TM modes. Furthermore, by introducing single symmetry breaking (in-plane symmetry breaking for TE, out-of-plane symmetry breaking for TM) with magnetic field tuning, one of the C points can be shifted to the Γ point, resulting in intrinsic chiral quasi-BICs (QBICs) with ultra-high Q-factors and near-unity CDs. This study provides a dynamic and flexible modulation approach for polarization singularities, which enhances light-matter interactions for applications in advanced chiral photonic devices and tunable optoelectronic devices.

In this work, we present a single-layer, wideband circularly polarized (CP) metasurface antenna fed by a coplanar waveguide (CPW). The proposed antenna employs a rotated CPW feed structure, which achieves circular polarization without adding dielectric layers, significantly simplifying the antenna structure and improving manufacturability, while incorporating two stubs to adjust impedance matching and broaden the bandwidth. Measurement results indicate a -10 dB impedance bandwidth covering 4.23-6.2 GHz (a fractional bandwidth of 37.7%), and a 3-dB axial ratio bandwidth of 5.26-6.32 GHz (a fractional bandwidth of 18.3%), with a peak measured gain of 9 dBi. The antenna targets Sub-6 GHz with strong 5G integration potential.

Temporal metasurfaces offer a promising platform for new-architecture wireless communications by enabling fast modulation of both electromagnetic waves and digital information. Optical control of these metasurfaces is particularly attractive as it establishes a direct physical bridge between optical and microwave signals, forming the foundation for optoelectronic hybrid communication systems. However, existing schemes are confined to static pre-alignment of the laser beam with the metasurface, lacking real-time spatial alignment capability essential for real-world mobile applications. Here, we propose and realize a mobile hybrid wireless communication system based on the designed laser-tracking-modulated microwave temporal metasurface. This communication system is constructed by integrating a photodiode-based microwave temporal metasurface, a vision-assisted laser-tracking transmitter, and a microwave receiver, enabling direct laser-to-microwave signal conversion sustained by dynamic alignment. Experimental results demonstrate that the system maintains successfully a stable hybrid communication link while the laser transmitter is in motion. This work provides a viable strategy for establishing stable hybrid wireless links for moving platforms and drones in high-mobility scenarios.