arXiv subjects

Xuchen Wang

Publications and source records attributed to Xuchen Wang.

At least 19 recordsLinked to original sources

arXiv2026

Exceptional Points in Photonics: From Non-Hermitian Physics to Applications

Open photonic systems provide a versatile platform for non-Hermitian physics, enabling control over complex spectra, transport, and light-matter interactions. Exceptional points (EPs), at which eigenvalues and eigenvectors coalesce and the governing operator becomes defective, play a central role because they combine branch-point spectral topology, nonanalytic perturbative response, and controllable eigenstate conversion. This Review provides a unified framework for EP photonics by systematically distinguishing exceptional degeneracies according to the underlying operator, spectral variable, boundary conditions, and experimentally accessible observables. We discuss Hamiltonian EPs, absorbing EPs associated with scattering zeros, real-frequency scattering-matrix and Jones-matrix EPs, Bloch and Floquet EPs, and Liouvillian EPs in open quantum systems. We review their spectral topology, static and dynamical encircling, higher-order exceptional structures, and coexistence with bound states in the continuum, together with applications in sensing, lasing, coherent absorption, directional scattering, polarization and wavefront control, nonlinear optics, optical storage, nonreciprocal photonics, and quantum photonics. We also critically assess the current limitations, practical challenges, and future perspectives of EP-based photonic technologies, with particular attention to robustness, noise, scalability, and experimentally measurable performance.

physics.optics↗

arXiv2026

Wearable metasurfaces for boosting the effective area of mobile-device antennas

Wireless communications increasingly face scenarios with compact user equipments operating in propagation environments where signal blockage, absorption, and device size limitations strongly constrain link performance. Most approaches to wireless-link enhancement focus either on improving base-station antenna systems, for example, through massive MIMO architectures; on engineering the propagation environment using reconfigurable intelligent surfaces and active relays. Here, we propose an alternative user-side strategy for improving wireless links based on wearable metasurfaces that enhance the effective antenna area of compact wireless devices. The proposed passive metasurface is integrated into clothing and engineered to collect the power of electromagnetic waves incident on user's body and route the collected power toward a mobile device in the form of surface waves. This mechanism significantly increases the effective receiving and transmitting area of the device antenna and its gain without external power sources, active control electronics, or additional stand-alone hardware. We implement the concept on a hoodie textile using a scalable screen-printing process with conductive silver ink and demonstrate an approximately tenfold enhancement of the received signal at 26 GHz. This passive approach effectively brings the human body into the communication network by turning clothing into a virtual thin and flexible lens for focusing the incident power on the user equipment.

physics.optics↗

arXiv2026

Permittivity Characterization of 3D-Printed Materials at Millimeter Waves

In this study, we characterize the permittivity of various commercially available materials commonly used in 3D printing over the millimeter-wave frequency range of 70-110 GHz. The open-waveguide extraction method is employed to efficiently determine the permittivity of these 3D-printed materials, and key strategies for improving its accuracy are discussed. This methodology can be readily applied to the characterization of similar materials and extended to higher frequency ranges. The resulting permittivity data are expected to support advances in 6G and beyond wireless communication technologies, in which 3D-printed materials and millimeter-wave frequencies are anticipated to play an increasingly important role in the development of novel antennas and metasurfaces.

physics.optics↗

arXiv2026

Towards compact high-frequency nonreciprocal devices using nanoplasma-switched time-varying metasurfaces

Time-modulated systems have received growing interest in recent years. They allow us to tailor effects, such as frequency conversion, single-direction propagation, etc. For the microwave band, semiconductor elements, such as varactors, are usually used as time-modulated elements but their modulation frequency has been limited to the few-gigahertz range. Recent advances in nanoplasma switches, i.e., two-state electronic switches based on a gas discharge in a nanometer-scale gap, provide a new potential for developing time-modulated systems with high operating frequencies. Here, we develop an analytical framework based on the time-Floquet method for the design of nonreciprocal time-modulated devices based on two-state time-modulated elements, for instance, nanoplasma-based switches. A practical example of a microwave isolator operating at 100~GHz frequency is developed and studied both analytically and using full-wave simulations. A potential realization in a parallel-plate waveguide is also simulated numerically.

physics.optics↗

arXiv2026

Investigating Bystander Privacy in Chinese Smart Home Apps

Bystander privacy in smart homes has been widely studied in Western contexts, yet it remains underexplored in non-Western countries such as China. In this study, we analyze 49 Chinese smart home apps using a mixed-methods approach, including privacy policy review, UX/UI evaluation, and assessment of Apple App Store privacy labels. While most apps nominally comply with national regulations, we identify significant gaps between written policies and actual implementation. Our traceability analysis highlights inconsistencies in data controls and a lack of transparency in data-sharing practices. Crucially, bystander privacy -- particularly for visitors and non-user individuals -- is largely absent from both policy documents and interface design. Additionally, discrepancies between privacy labels and actual data practices threaten user trust and undermine informed consent. We provide design recommendations to strengthen bystander protections, improve privacy-oriented UI transparency, and enhance the credibility of privacy labels, supporting the development of inclusive smart home ecosystems in non-Western contexts.

cs.HC↗

arXiv2025

From Nucleobases to DNA: Clustering-Triggered Emission and Pressure-Induced Emission Enhancement

The photophysical properties of deoxyribonucleic acid (DNA) are fundamental to life sciences and biophotonics. While previous studies have generally been restricted to fluorescence, attributing it to pi-pi* transitions and charge transfer within nucleobases in dilute solution, these understandings fail to explain the pronounced visible emission in physiological and aggregated states, and moreover, ignore the possible phosphorescence. Addressing this critical gap, we systematically investigate native DNA across its structural hierarchy, from nucleobases to single-stranded chains, under varying states. We demonstrate that DNA exhibits excitation-dependent emission in aggregates and moreover room-temperature phosphorescence (RTP) in the solid state. These behaviors are rationalized by the clustering-triggered emission (CTE) mechanism, where nucleobases and electron-rich nonaromatic moieties like sugar and phosphate synergistically contribute to DNA photophysics. High-pressure experiments reveal a 207-fold luminescence enhancement for nucleotides at 26 GPa, largely retained after decompression, underscoring the precise control of emission by intermolecular interactions. This study not only elucidates the intrinsic luminescence mechanism of DNA and but also establishes pressure modulation as a versatile approach for developing new nucleic acid-inspired luminescent materials.

physics.chem-ph↗

arXiv2025

Non-Foster Photonic Time Crystals

Photonic time crystals (PhTCs) are spatially uniform media whose material parameters vary periodically in time, opening momentum bandgaps within which the fields of electromagnetic modes can grow exponentially in time. To date, PhTCs have utilized only passive, lossless materials with "positive" dispersion (Foster materials), and a theoretical framework addressing active materials with "negative" dispersion (non-Foster materials) in PhTCs and their associated physical properties remains undeveloped. Here, we explore the two classes of isotropic PhTCs with embedded non-Foster inclusions: a bulk medium with periodically modulated negative permittivity, and a metasurface whose surface capacitance alternates between positive and negative values. Employing an analytical transfer-matrix formulation, we demonstrate that non-Foster permittivity modulation not only broadens momentum bandgaps without bounds but also provides a gain rate that increases linearly with momentum. Remarkably, the proposed isotropic PhTCs support exponential amplification down to zero frequency-a regime inaccessible in conventional isotropic PhTCs. These results open new avenues for ultra-broadband wave control, high-gain signal processing, and energy-harvesting devices that leverage the unique dispersion of active, time-modulated circuitry.

physics.optics↗

arXiv2025

Stacked Time-Varying Metasurfaces

Spatiotemporal metasurfaces offer unique opportunities for wave manipulation, however, their practical realization is often constrained by the requirement for in-plane spatial modulation, which necessitates a large number of time-varying elements. In this work, we introduce an alternative architecture based on a cascade of spatially uniform metasurfaces subjected to periodic temporal modulation. Although all metasurfaces share the same modulation frequency, their individual modulation functions are independently engineered to achieve a desired complex electromagnetic response. We develop a general theoretical framework for the design and optimization of such stacked metasurface systems, composed of dense arrays of cylindrical meta-atoms with time-varying plasma and/or collision frequencies. The effectiveness of the approach is demonstrated through the optimization of metasurface designs that enable magnet-free isolation at the fundamental frequency and a temporal analogue of circulators. Furthermore, we show that a metasurface stack can be implemented using only a few time-modulated elements embedded within a parallel-plate waveguide, opening new avenues for extremely compact, versatile, and scalable spatiotemporal platforms for next-generation photonic and microwave systems.

physics.optics↗

arXiv2025

Terahertz frequency conversion at plasma-induced time boundary

We report on the frequency conversions of terahertz (THz) waves at ultrafast time boundaries created via femtosecond laser-induced air-to-plasma phase transitions. Our combined experimental and theoretical approach reveals that the abrupt change in refractive index at the ultrafast time boundaries drives both the red and blue shifts over the broadband THz spectrum due to the dispersive plasma, with distinctive amplitude variations. The present study contrasts these effects with those from spatial boundaries, highlighting the superior efficacy of temporal manipulations for spectral engineering. These findings not only deepen the understanding of light-matter interactions in time-varying media but also pave the way for innovative applications in THz technology and lay the groundwork for the observation of temporal reflection effects, photonic time crystals, and spatio-temporally modulated matter.

physics.optics↗

arXiv2025

A strictly predefined-time convergent and anti-noise fractional-order zeroing neural network for solving time-variant quadratic programming in kinematic robot control

This paper proposes a strictly predefined-time convergent and anti-noise fractional-order zeroing neural network (SPTC-AN-FOZNN) model, meticulously designed for addressing time-variant quadratic programming (TVQP) problems. This model marks the first variable-gain ZNN to collectively manifest strictly predefined-time convergence and noise resilience, specifically tailored for kinematic motion control of robots. The SPTC-AN-FOZNN advances traditional ZNNs by incorporating a conformable fractional derivative in accordance with the Leibniz rule, a compliance not commonly achieved by other fractional derivative definitions. It also features a novel activation function designed to ensure favorable convergence independent of the model's order. When compared to five recently published recurrent neural networks (RNNs), the SPTC-AN-FOZNN, configured with $0<\alpha\leq 1$, exhibits superior positional accuracy and robustness against additive noises for TVQP applications. Extensive empirical evaluations, including simulations with two types of robotic manipulators and experiments with a Flexiv Rizon robot, have validated the SPTC-AN-FOZNN's effectiveness in precise tracking and computational efficiency, establishing its utility for robust kinematic control.

eess.SY↗

arXiv2024

A Predefined-Time Convergent and Noise-Tolerant Zeroing Neural Network Model for Time Variant Quadratic Programming With Application to Robot Motion Planning

This paper develops a predefined-time convergent and noise-tolerant fractional-order zeroing neural network (PTC-NT-FOZNN) model, innovatively engineered to tackle time-variant quadratic programming (TVQP) challenges. The PTC-NT-FOZNN, stemming from a novel iteration within the variable-gain ZNN spectrum, known as FOZNNs, features diminishing gains over time and marries noise resistance with predefined-time convergence, making it ideal for energy-efficient robotic motion planning tasks. The PTC-NT-FOZNN enhances traditional ZNN models by incorporating a newly developed activation function that promotes optimal convergence irrespective of the model's order. When evaluated against six established ZNNs, the PTC-NT-FOZNN, with parameters $0 < \alpha \leq 1$, demonstrates enhanced positional precision and resilience to additive noises, making it exceptionally suitable for TVQP tasks. Thorough practical assessments, including simulations and experiments using a Flexiv Rizon robotic arm, confirm the PTC-NT-FOZNN's capabilities in achieving precise tracking and high computational efficiency, thereby proving its effectiveness for robust kinematic control applications.

cs.RO↗

arXiv2024

Radiation Pattern Synthesis with Uniform Nonlocal Metasurfaces

One of the main applications of electromagnetic metasurfaces (MSs) is to tailor spatial field distributions. The radiation pattern of a given source can be desirably modified upon reflection on an MS having proper spatial modulation of its local macroscopic parameters. At the microscopic level, spatial modulation requires individually engineered meta-atoms at different points. In contrast, the present research demonstrates the opportunity for radiation pattern engineering in the reflection regime without using any spatial modulation. The principle consists in the deliberate tailoring of the surface impedance of an unmodulated but spatially dispersive (nonlocal) MS. A 2D synthesis problem with a magnetic line current source is solved analytically by finding a required form of the surface impedance as a function of the tangential wave vector in both visible and evanescent parts of the spatial spectrum. To prove the principle, three different pattern shapes are implemented via full-wave numerical simulations by tuning the spatial dispersion in a realistic mushroom-type high-impedance electromagnetic surface with loaded vias. This work extends the synthesis methods and the application area of spatially dispersive MSs, showing the latter as a promising platform for new types of antennas.

physics.optics↗

arXiv2024

A CT-guided Control Framework of a Robotic Flexible Endoscope for the Diagnosis of the Maxillary Sinusitis

Flexible endoscopes are commonly adopted in narrow and confined anatomical cavities due to their higher reachability and dexterity. However, prolonged and unintuitive manipulation of these endoscopes leads to an increased workload on surgeons and risks of collision. To address these challenges, this paper proposes a CT-guided control framework for the diagnosis of maxillary sinusitis by using a robotic flexible endoscope. In the CT-guided control framework, a feasible path to the target position in the maxillary sinus cavity for the robotic flexible endoscope is designed. Besides, an optimal control scheme is proposed to autonomously control the robotic flexible endoscope to follow the feasible path. This greatly improves the efficiency and reduces the workload for surgeons. Several experiments were conducted based on a widely utilized sinus phantom, and the results showed that the robotic flexible endoscope can accurately and autonomously follow the feasible path and reach the target position in the maxillary sinus cavity. The results also verified the feasibility of the CT-guided control framework, which contributes an effective approach to early diagnosis of sinusitis in the future.

cs.RO↗

arXiv2024

Inverse-designed dispersive time-varying nanostructures

Time-varying nanostructures allow us to control the spatial and temporal properties of light. The temporal modulation of the nanostructures constitutes an additional degree of freedom to control their scattering properties on demand and in a reconfigurable manner. However, these additional parameters create a vast design space, raising challenges in identifying optimal designs. Therefore, tools from the field of photonic inverse design must be used to optimize the degrees of freedom of the system to facilitate predefined optical responses. To further develop this field, here we introduce a differentiable transition (T-) matrix-based inverse design framework for dispersive time-varying nanostructures. The electron density of the material of the nanostructures is modulated non-adiabatically as a generic periodic function of time. Using the inverse design framework, the temporal shape of the electron density can be manipulated to reach the target functionality. Our computational framework is exploited, exemplarily, in two instances. First, the decay rate enhancement of oscillating dipoles near time-varying spheres is controlled on demand. Second, using spatiotemporal metasurfaces, a system supporting asymmetric transmission of light at visible frequencies is designed. Our work paves the way toward programmable spatiotemporal metasurfaces and space-time crystals for a future generation of reconfigurable functional photonic devices.

physics.optics↗

arXiv2024

A Minimal Set of Parameters Based Depth-Dependent Distortion Model and Its Calibration Method for Stereo Vision Systems

Depth position highly affects lens distortion, especially in close-range photography, which limits the measurement accuracy of existing stereo vision systems. Moreover, traditional depth-dependent distortion models and their calibration methods have remained complicated. In this work, we propose a minimal set of parameters based depth-dependent distortion model (MDM), which considers the radial and decentering distortions of the lens to improve the accuracy of stereo vision systems and simplify their calibration process. In addition, we present an easy and flexible calibration method for the MDM of stereo vision systems with a commonly used planar pattern, which requires cameras to observe the planar pattern in different orientations. The proposed technique is easy to use and flexible compared with classical calibration techniques for depth-dependent distortion models in which the lens must be perpendicular to the planar pattern. The experimental validation of the MDM and its calibration method showed that the MDM improved the calibration accuracy by 56.55% and 74.15% compared with the Li's distortion model and traditional Brown's distortion model. Besides, an iteration-based reconstruction method is proposed to iteratively estimate the depth information in the MDM during three-dimensional reconstruction. The results showed that the accuracy of the iteration-based reconstruction method was improved by 9.08% compared with that of the non-iteration reconstruction method.

cs.CV↗

arXiv2024

Reconfigurable Superdirective and Superabsorptive Aperiodic Metasurfaces

In this paper, we present a general theory of aperiodic subwavelength arrays for controlling electromagnetic waves. The considered platform is formed by an array of electrically small loaded scatterers above a ground plane. While the array is geometrically periodic, all the loads can be in general different, so that the distributions of currents induced by plane waves are not periodic. To allow analytical solutions, we study arrays of thin wires or strips loaded by bulk loads. We demonstrate a practical way of creating tunable and reconfigurable multifunctional devices, on examples of superdirective beam splitters, focusing lenses establishing subdiffraction focusing, and absorbers going beyond perfect absorption. Contrary to the constraints imposed by the Floquet theorem in periodic counterparts like periodic metasurfaces or metagratings, where a fixed angle of incidence and period dictate the propagating directions of reflected waves, the proposed aperiodic designs allow controlling all propagating modes in any direction, which provides more freedom in manipulating electromagnetic waves. We hope that these results can be useful in multiple applications, such as telecommunications, radar techniques, signal processing, and energy harnessing.

physics.app-ph↗

arXiv2024

Going Beyond Perfect Absorption: Reconfigurable Super-directive Absorbers

In the context of electromagnetic absorption, it is obvious that for an infinite planar periodic structure illuminated by a plane wave, the maximum attainable absorptance, i.e., perfect absorption, is theoretically limited to 100% of the incident power. Here we show that an intriguing possibility of overcoming this limit arises in finite-size resonant absorbing arrays. We present a comprehensive analysis of a simple two-dimensional strip array over an infinite perfectly conducting plane, where the strips are loaded by reconfigurable impedance loads. The absorptance is defined as the ratio of the dissipated power per unit length of the strips to the incident power on the unit length of the array width. The results show that even regular arrays of impedance strips can slightly overcome the limit of 100% absorptance, while using aperiodic arrays with optimized loads, absorptance can be significantly increased as compared with the scenario where the strips are identical. In principle, by tuning the reconfigurable loads, high super-unity absorptance can be realized for all angles of illumination.

physics.app-ph↗

arXiv2023

Smith-Purcell radiation from time grating

Smith-Purcell radiation (SPR) occurs when an electron skims above a spatial grating, but the fixed momentum compensation from the static grating imposes limitations on the emission wavelength. It has been discovered that a temporally periodic system can provide energy compensation to generate light emissions in free space. Here, we introduce temporal SPR (t-SPR) emerging from a time grating and propose a generalized t-SPR dispersion equation to predict the relationship between radiation frequency, direction, electron velocity, modulation period, and harmonic orders. Compared to conventional SPR, t-SPR can: 1) Provide a versatile platform for manipulating SPR emission through temporal modulation (e.g., period, amplitude, wave shape). 2) Exhibit strong robustness to the electron-grating separation, alleviating the constraints associated with extreme electron near-field excitation. 3) Introduce additional energy channels through temporal modulation, enhancing and amplifying emission.

physics.optics↗