arXiv subjects

Alex M. H. Wong

Publications and source records attributed to Alex M. H. Wong.

11 recordsLinked to original sources

arXiv2026

Janus Dipoles: Fundamentals, Realizations, and Emerging Applications

The Janus dipole - featuring orthogonally oriented electric and magnetic dipoles with a 90-degree phase difference - has emerged as a powerful paradigm for wave manipulation. Unlike traditional Huygens dipoles used for directional control, this unique configuration exhibits strongly asymmetric, face-selective near-field behavior while maintaining a quasi-isotropic far-field radiation pattern. These remarkable properties make the Janus dipole an essential platform for directional wave shaping, with wide-ranging applications in on-chip photonics, quantum interactions, and wireless power transfer. This review systematically traces the rapid development of the Janus dipole from its foundational theoretical inception to its diverse implementation platforms across optical, microwave, and acoustic frequencies. In this paper, we explore the governing principles, classify realization strategies into passive Janus dipoles, active Janus dipoles, and advanced near-field coupling control, and highlight emerging frontiers. By bridging foundational electrodynamics with advanced device engineering, this paper serves as an essential reference and roadmap for researchers designing next-generation, highly integrated, and compact wave-manipulation systems.

physics.app-ph↗

arXiv2025

Channel Estimation and Passive Beamforming for Pixel-based Reconfigurable Intelligent Surfaces with Non-Separable State Response

Pixel-based reconfigurable intelligent surfaces (RISs) employ a novel design to achieve high reflection gain at a lower hardware cost by eliminating the phase shifters used in traditional RIS. However, this design presents challenges for channel estimation and passive beamforming due to its non-separable state response, rendering existing solutions ineffective. To address this, we first approximate the non-separable RIS response functions using a kernel-based method and a deep neural network, achieving high accuracy while reducing computational and memory complexity. Next, we propose a simplified cascaded channel model that focuses on dominated scattering paths with limited unknown parameters, along with customized algorithms to estimate short-term and long-term parameters separately. Finally, we introduce a low-complexity passive beamforming algorithm to configure the discrete RIS state vector, maximizing the achievable rate. Our simulation results demonstrate that the proposed solution significantly outperforms various baselines across a wide SNR range.

eess.SP↗

arXiv2023

Experiment-based deep learning approach for power allocation with a programmable metasurface

Deep learning, as a highly efficient method for metasurface inverse design, commonly use simulation data to train deep neural networks (DNNs) that can map desired functionalities to proper metasurface designs. However, the assumptions and simplifications made in the simulation model may not reflect the actual behavior of a complex system, leading to suboptimal performance of the DNNs in practical scenarios. To address this issue, we propose an experiment-based deep learning approach for metasurface inverse design and demonstrate its effectiveness for power allocation in complex environments with obstacles. Enabled by the tunability of a programmable metasurface, large sets of experimental data in various configurations can be collected for DNN training. The DNN trained by experimental data can inherently incorporate complex factors and can adapt to changed environments through its on-site data-collecting and fast-retraining capability. The proposed experiment-based DNN holds the potential for intelligent and energy-efficient wireless communication in complex indoor environments.

eess.SP↗

arXiv2023

Electromagnetic Near-Field Mutual Coupling Suppression with Active Janus Sources

Electric dipoles and magnetic dipoles are the most fundamental particles in electromagnetic theory. Huygens and Janus sources, formed by the orthogonal combination of electric and magnetic dipoles, both show good directionality in the near field. Although the Huygens source has been widely used in antennas and metasurfaces, the applications of Janus source are heretofore limited. In this paper we report the first physical construction of an active Janus source. Through full-wave simulations within the PPW environment, we show that our source achieves the directional electromagnetic near-field and quasi-isotropic far-field requisite of the Janus source. Using this fact, we demonstrate that two active Janus sources in close proximity (about 0.10 to 0.25 wavelengths) achieve a near 1000-fold reduced mutual coupling compared to electric dipole sources. The achievement of strong mutual coupling suppression and quasi-isotropic radiation make the Janus source an ideal candidate for consideration in future compact MIMO communication systems.

physics.app-ph↗

arXiv2023

Bipartite Dielectric Huygens' Metasurface for Anomalous Refraction

Huygens' metasurfaces - fundamentally based on Schelkunoff's equivalence principle, Huygens' metasurfaces consist of a two-dimensional array of Huygens' sources formed by co-located orthogonal electric and magnetic dipoles. Such metasurfaces provide electric and magnetic responses to an incoming electromagnetic (EM) wave, leading to unidirectional scattering and 2$π$ phase coverage. We herein report a near-reflectionless coarsely discretized dielectric Huygens' metasurface that performs anomalous refraction, offering a low-loss platform for wave manipulation at high frequencies as compared to their lossy metallic analogue. The coarse discretization dramatically simplifies the design, resulting in a metasurface that is highly efficient, cost-effective, and robust. In this paper, the proposed metasurface comprises two meta-atoms per period and is hence named the bipartite dielectric Huygens' metasurface. Through full-wave simulations at 28 GHz, we show that the proposed metasurface can reroute an incident EM wave from $θ_i=15^{\circ}$ to $θ_t=-44.5^{\circ}$ with very high efficiency: 87% of the scattered power is anomalously transmitted to $θ_t$. Based on our observations, a coarsely discretized dielectric Huygens' metasurface platform can be efficacious to design meta-devices with multifaceted functionalities in different frequency regimes.

physics.app-ph↗

arXiv2023

Directional Dipole Dice Enabled by Anisotropic Chirality

Directional radiation and scattering play an essential role in light manipulation for various applications in integrated nanophotonics, antenna and metasurface designs, quantum optics, etc. The most elemental system with this property is the class of directional dipoles, including the circular dipole, Huygens dipole, and Janus dipole. A unified realization of all three dipole types and a mechanism to freely switch among them are previously unreported, yet highly desirable for developing compact and multifunctional directional sources. Here, we theoretically and experimentally demonstrate that the synergy of chirality and anisotropy can give rise to all three directional dipoles in one structure at the same frequency under linearly polarized plane wave excitations. This mechanism enables a simple helix particle to serve as a directional dipole dice (DDD), achieving selective manipulation of optical directionality via different "faces" of the particle. We employ three "faces" of the DDD to realize face-multiplexed routing of guided waves in three orthogonal directions with the directionality determined by spin, power flow, and reactive power, respectively. This construction of the complete directionality space can enable the unprecedented high-dimensional control of both near-field and far-field directionality with broad applications in photonic integrated circuits, quantum information processing, and subwavelength-resolution imaging.

physics.class-ph↗

arXiv2022

Discrete Huygens' Metasurface: Realizing Anomalous Reflection and Diffraction Mode Circulation with a Robust, Broadband and Simple Design

Metasurfaces composed of subwavelength unit cells usually require a large number of unit cells which leads to complicated design and optimization. Aggressive discretization in a metasurface can significantly reduce the number of unit cells within a period, resulting in lower requirement for phase and/or surface impedance coverage. Additionally, the enlarged unit cells will encounter negligible mutual couplings when combined together, hence making straightforward the process of metasurface design. These advantages combine to allow the design of a novel class of metasurfaces which support the high efficiency redirection of electromagnetic (EM) waves over a wide bandwidth and operation angle. Moreover, an aggressively discretized metasurface may realize functionalities such as diffraction mode circulation, which are unsupported in its continuous counterparts. In this paper we propose a simple transmissive metasurface which can realize diffraction mode circulation by refracting plane waves with incident angles of -45, 0, 45 degrees to plane waves with refraction angles of 0, 45, -45 degrees respectively. The power efficiency of each anomalous refraction is more than 80% at the design frequency of 28 GHz, and the 3-dB power efficiency bandwidth is 11%. We fabricated and measured the metasurface, the experiment results agree well with simulation results.

physics.app-ph↗

arXiv2018

Active Huygens' Box: Metasurface-Enabled Arbitrary Electromagnetic Wave Generation Inside a Cavity

This work investigates the generation of arbitrary electromagnetic waveforms inside a geometrical area enclosed by an active metasurface. We introduce the contraption of the Huygens' box, where a region of space is enclosed by an active Huygens' metasurface. We show that, upon generating the necessary electric and magnetic currents, we can create any desired electromagnetic field inside the Huygens' box. Using this method, we demonstrate through simulation and experiment the generation of travelling plane waves, a standing plane wave and a Bessel wave inside a metallic cavity. By linear superposition of these unconventional "modes", we experimentally demonstrate, for the first time, a subwavelength superoscillation focal spot formed without involving evanescent EM waves, and without an accompanying region of exorbitantly high waveform energy. The Huygens' box brings controlled waveform generation to an unprecedented level, with far-reaching implications to imaging, communication and medical therapy.

physics.app-ph↗

arXiv2018

Perfect Anomalous Reflection with a Binary Huygens' Metasurface

In this paper we propose a new metasurface that is able to reflect a known incoming electromagnetic wave into an arbitrary direction, with perfect power efficiency. This seemingly simple task, which we hereafter call perfect anomalous reflection, is actually highly non-trivial due to the differing wave impedances and complex interference between the incident and reflected waves. Heretofore, proposed metasurfaces which achieve perfect anomalous reflection require complicated, deeply subwavelength and/or multilayer element structures which allow them to couple to and from leaky and/or evanescent waves. In contrast, we demonstrate that using a Binary Huygens' Metasurface (BHM) --- a passive and lossless metasurface with only two cells per period --- perfect anomalous reflection can be achieved over a wide angular and frequency range. Through simulations and experiments at 24 GHz, we show that a properly designed BHM can anomalously reflect an incident electromagnetic wave from $θ_i = 50^\circ$ to $θ_r = -22.5^\circ$, with perfect power efficiency to within experimental precision.

physics.app-ph↗

arXiv2017

Perfect Anomalous Reflection with an Aggressively Discretized Huygens' Metasurface

This paper investigates the discretization of a periodic metasurface and demonstrates how such a surface can achieve perfect anomalous reflection. Whilst most contemporary theoretical works on metasurfaces deal with continuous current or impedance distributions, we examine how discretization affects a metasurface, and show that in some cases one can discretize a metasurface aggressively --- to the extent of having only two cells per spatial period. Such aggressive discretization can lead to great simplifications in metasurface design, and perhaps more surprisingly, a possible performance improvement from continuous metasurfaces. Using this aggressive discretization technique, we report the design of a binary Huygens' metasurface which reflects an incident plane wave at 50$^\circ$ into a reflected direction of -22.5$^\circ$. Full-wave electromagnetic simulation shows the achievement of anomalous reflection with a power efficiency of 99.1%, which dramatically surpasses the performance of a corresponding passive continuous metasurface, for which the power efficiency is fundamentally limited to 69.6%.

physics.app-ph↗

arXiv2008

A spatially shifted beam approach to subwavelength focusing

Although negative-refractive-index metamaterials have successfully achieved subwavelength focusing, image resolution is limited by the presence of losses. In this Letter, a metal transmission screen with subwavelength spaced slots is proposed that focuses the near-field beyond the diffraction limit and furthermore, is easily scaled from microwave frequencies to the optical regime. An analytical model based on the superposition of shifted beam patterns is developed that agrees very well with full-wave simulations and is corroborated by experimental results at microwave frequencies.

physics.optics↗