arXiv subjects

Yangjian Cai

Publications and source records attributed to Yangjian Cai.

At least 19 recordsLinked to original sources

arXiv2026

Geometric phase-space nonseparability triggers giant optical shifts

Nonseparability among multiple degrees of freedom has enabled fundamental advances in structured light and related applications. Here we unveil a previously overlooked form of nonseparability in phase space, which we term geometric phase-space nonseparability. The latter arises solely from the wavefront curvature of a conventional wave packet, such as a fundamental Gaussian beam. This phase-space structure manifests as a position-dependent transverse-momentum distribution across the beam profile leading to the giant spatial and angular beam shifts upon reflection at a planar interface that we predict analytically and observe experimentally. Remarkably, the curvature-induced phase-space correlation remains robust against spatial-coherence degradation, allowing the giant shifts to persist even in the nearly incoherent regime. Our results establish wavefront curvature as a general mechanism for engineering beam shifts across optical, acoustic, and matter-wave systems.

physics.optics↗

arXiv2026

Experimental Observation of Ghost Image Revivals via Structured Coherence

Ghost imaging retrieves an object's image from intensity correlations between two light beams, neither of which independently carries information about the object. However, conventional ghost imaging critically relies on precise object positioning and conjugate matching between the two arms, causing the image to disappear when these conditions are violated and the object is accessible from one plane only. In this Letter, we break this fundamental limitation by reporting the first experimental observation of revivals of ghost images of arbitrary objects via engineering the longitudinal intensity autocorrelation of a structured thermal light field into a coherence comb. With the reference arm fixed, translating the object produces periodic revivals of the ghost image whenever the object position matches a comb-tooth position, reminiscent of the Talbot effect. This capability enhances the flexibility of correlation imaging, enabling robust tomographic imaging of moving and non-periodic complex objects.

physics.optics↗

arXiv2026

Radially correlated partially coherent beams with a deterministic vortex structure

Partially coherent beams have attracted considerable attention due to their intrinsic resilience against complex environmental perturbations. However, the intrinsic wavefront fluctuations make it fundamentally challenging to preserve well-defined orbital angular momentum during propagation. In this work, we propose and experimentally demonstrate a class of radially correlated, partially coherent beams that carry deterministic vortex structures, generated via optical conformal mapping from Cartesian to log-polar coordinates. The resulting beams exhibit a ring-shaped coherence distribution, characterized by low coherence in the radial direction and high coherence in the azimuthal direction. This unique feature of such a beam supports a well-defined deterministic vortex phase, thereby enabling the beam to preserve its ring-shaped coherence distribution during propagation through a focusing system. Our results provide new insights into the design of new partially coherent beams and may facilitate the development of applications in optical encoding, free-space information transmission, and ultrafast light-matter interactions.

physics.optics↗

arXiv2026

Abnormal motions of optical vortex-antivortex-coupled wavepackets in the parabolic potential

The (quasi)particles or structured wavepackets in parabolic potential exhibit well-known harmonic oscillations, typically described by the Lissajous equations. However, such conventional harmonic laws rely on a fundamental assumption that the different constituent components of the (quasi)particles or wavepackets do not interact. Here we challenge this paradigm, by taking advantage of intrinsic couplings among distinct constituents-specifically by leveraging nontrivial couplings between vortices and antivortices embedded in a spatially structured wavepacket. We demonstrate theoretically and experimentally abnormal motions by considering two different optical waveforms. For a vortex-antivortexcoupled dipole mode, we reveal counterintuitive propagation regimes, including periodic annihilation and regeneration of the dipole, its non-orbital motion and realization of a critical equilibrium state without nonlinearity. For a circular chain of vortices with an antivortex set at the center, we successfully tune the oscillation frequency of the overall configuration in the potential, thus disobeying the classical Lissajous trajectories, by precisely engineering the nonlocal vortex-antivortex couplings. Since the harmonic oscillations have been proven to be fundamental physical phenomena in distinct disciplines and led to numerous important applications, our demonstrations provide different opportunities to trigger considerable investigations and potential applications, by leveraging the underlying anomalous motions of the vortex-antivortex-coupled wavepackets in the parabolic potential.

physics.optics↗

arXiv2026

Source coherence orchestrates nonlinear random wave revivals

We demonstrate that the Talbot length of periodic wave packets, long believed to be solely determined by their periodicity, is strongly affected by the source coherence in the nonlinear propagation regime. We reveal that reducing the source coherence -- and consequently the speckle size of a periodic field -- shortens the Talbot length and significantly improves the quality of the recurrent Talbot images, despite a fixed source periodicity. This effect arises from coherence-mediated nonlinear mode coupling, which alters a phase synchronization condition undergirding the wave packet revivals. Our findings expose a hidden role of coherence in governing Talbot revivals of random waves in the nonlinear regime, crucially informing the understanding and application of the Talbot effect in realistic media.

physics.optics↗

arXiv2026

General framework for incoherent topological structured light and optical information encoding

Topology provides a powerful language for describing global invariants in physical systems, yet optical topology has been explored predominantly with fully coherent light. Recent studies have shown that incoherent light can host topological structures mediated by coherence singularities; however, a general framework for their construction and control has been lacking. Here, we introduce an incoherent Milnor polynomial, which establishes a theoretical framework for real-space incoherent topological structured light, in which topology and statistical coherence emerge as independent and jointly addressable degrees of freedom. This framework overcomes a fundamental limitation of coherent topological structured light, enabling arbitrary intensity engineering without altering the underlying topological configuration. Experimentally, we realize incoherent Hopf-linked and trefoil-knotted coherence singularities with programmable statistical coherence. We further demonstrate a robust optical information-encoding scheme inspired by Rubik's-cube-like rotations, where statistical coherence determines far-field intensity patterns associated with the cube's initial states, and topological structures govern controlled rotations acting as encryption keys. Our results advance incoherent topological structured light from a physical curiosity to a programmable photonic platform, opening new avenues for optical information encoding, statistical photonics, and coherence-engineered functionalities beyond coherent optical topology.

physics.optics↗

arXiv2026

Coherence toroidal vortices and statistic-veiled correlation topologies

Toroidal vortices in fluid and gas dynamics underpin a broad spectrum of scientific and technological fields, from elementary particle physics to condensed matter systems, and have recently garnered significant attention in optics because of their inherent topological stability. Here we report the experimental observation of toroidal vortices in stochastic optical wavefields with partial coherence, termed coherence toroidal vortices, which eliminates deterministic topological signatures in conventional optical degrees of freedom while unveiling statistically hidden correlation topologies. These underlying topologies-including both fundamental and higher-order hopfionic textures-emerge exclusively in second-order field correlations and are accessible only through statistical measurements. We further examine the impact of chaotic channels on the stability of these statistically veiled correlation topologies, demonstrating that their topological invariants remain robust under realistic environmental perturbations. These findings are experimentally validated and offer novel insights into the potential of toroidal light vortices serving as controllable channels for directional energy and information transfer within complex media.

physics.optics↗

arXiv2026

Partial coherence control delivers skyrmionic topological resilience and transitions

Optical skyrmions have recently unlocked topological quasiparticle textures of light, rising in prominence for next-generation ultra-robust information processing. However, to date, their study has been mainly confined to coherent laser fields. Here we extend skyrmions to more general light sources of partially coherent, stochastic optical fields. We define stochastic optical skyrmions and uncover a hidden regime where spatial coherence acts as a primary determinant of topological stability. While environmental randomness typically degrades fully coherent states, we demonstrate that engineered partial coherence provides a self-healing mechanism that preserves topology under extreme turbulence. Moreover, we show that the coherence structure can be actively tailored to trigger on-demand topological phase transitions, such as skyrmion-to-skyrmionium conversion and skyrmion lattice splitting. These findings redefine the boundaries of topological photonics, paving the way for resilient and high-fidelity information platforms that remain operational in general, non-ideal, real-world environments.

physics.optics↗

arXiv2026

Regular and irregular revivals of quasi-periodic random waves

Paraxial wave packets with discrete spatial, temporal, or spatiotemporal spectra are known to undergo periodic axial revivals on propagation in either free space or linear transparent, weakly dispersive media. Such spectacular revivals, ubiquitously encountered in physics, from optics and acoustics to condensed matter physics, are distinguished by their strict periodicity. We show theoretically and verify experimentally that ensembles of quasi-periodic random wave packets exhibit a unique revival network composed of regular (periodic) and irregular (aperiodic) revivals. Moreover, individual realizations of a statistical ensemble self-reconstruct, in general, at different propagation distances than do ensemble averages. Our results shed new light on the fundamental physics of self-reconstruction of random wave packets with structured correlations.

physics.optics↗

arXiv2026

Discord-Enabled Teleportation-Inspired Optical Imaging at a Distance

In quantum teleportation, a pair of entangled photons are prerequisite to serve as the quantum channel for quantum state transfer distantly. Here, we report a new strategy of quantum-teleportation-inspired classical optical imaging, which also works non-locally at a distance; however, only a classically correlated light source is used instead of entanglement. In our experiment, we explore the pseudo-thermal light source to offer the teleportation-like channel and employ the sum-frequency generation to perform the Bell-like state measurement. We successfully demonstrate the teleportation-inspired optical imaging of simple characters, Taiji diagram, and the superposition of orbital angular momentum modes. Moreover, we experimentally observe that a better coherence of pseudo-thermal light will result in a lower contrast of the formed images, and thus revealing that non-zero quantum discord offered by pseudo-thermal light, regardless of zero entanglement, plays the pivotal role in sustaining the teleportation-like channel for imaging at a distance.

physics.optics↗

arXiv2026

Observation of Janus Chirality for Coherent Thermal Emission from Metasurfaces

Metasurfaces emerged as a powerful tool for controlling thermal radiation, yet achieving coherent emission with opposite circular handednesses remains a highly challenging problem. Here, we demonstrate experimentally the Janus chiral thermal emission from metasurfaces with opposite circular handednesses on either side of a single device. We employ anisotropic metasurfaces supporting high-Q resonances with photonic flatbands enabling near-unity circular dichroism through in-plane symmetry control. Our experiments confirm the Janus coherent emission, and they are validated by the results of the coupled-mode theory. The flatband resonant metasurfaces enabling a control of chiral thermal emission provide an efficient platform for spin-controlled light-matter interaction.

physics.optics↗

arXiv2025

Spatiotemporally Localized Optical Links and Knots

Optical links and knots have attracted growing attention owing to their exotic topologic features and promising applications in next-generation information transfer and storage. However, current protocols for optical topology realization rely on paraxial propagation of spatial modes, which inherently limits their three-dimensional topological structures to longitudinal space-filling. In this work we propose and experimentally demonstrate a scheme for creating optical knots and links that are localized in space within a transverse plane of a paraxial field, as well as in time. These spatiotemporal topological structures arise from polychromatic wave fields with tightly coupled spatial and temporal degrees of freedom that can be realized in the form of superpositions of toroidal light vortices of opposite topological charges. The (2+1)-dimensional nature of a toroidal light vortex imparts spatiotemporally localized wave fields with nontrivial topological textures, encompassing both individual and nested links or knots configurations. Moreover, the resulting topological textures are localized on an ultrashort timescale propagate at the group velocity of the wave packets and exhibit remarkable topological robustness during propagation as optical carriers. The nascent connection between spatiotemporally localized fields and topology offers exciting prospects for advancing space-time photonic topologies and exploring their potential applications in high-capacity informatics and communications.

physics.optics↗

arXiv2025

Propagation Dynamics of Photonic Toroidal Vortices Mediated by Orbital Angular Momenta

The dynamics of vortex rings in fluids have long captivated researchers due to the intriguing complexity of their behavior, despite the apparent simplicity of their structure. In optics, photonic toroidal vortices constitute a novel class of three-dimensional, space-time nonseparable structured light fields that carry transverse orbital angular momentum. However, as solutions to the dispersive form of Maxwell's equations, these wavepackets do not survive upon nondispersive propagation, and their dynamics remain elusive. In this article, the dynamics of photonic toroidal vortices under various dispersion regimes, mediated by both transverse and longitudinal orbital angular momentum, are investigated through simulations and experiments. The results reveal that the motion of a toroidal vortex is strongly affected by the presence of longitudinal orbital angular momentum. The swirling flow destabilizes the toroidal structure under dispersion conditions and induces topological transformations in the vortex line characterized by its annihilation and subsequent reformation in vacuum. Remarkably, the renascent toroidal vortex exhibits robust propagation in vacuum while maintaining its toroidal structure. These findings are supported by experimental validation and highlight the potential of photonic toroidal vortices as controllable channels for directional energy and information transfer.

physics.optics↗

arXiv2025

Topological links and knots of speckled light mediated by coherence singularities

Links and knots are exotic topological structures that have garnered significant interest across multiple branches of natural sciences. Coherent links and knots, such as those constructed by phase or polarization singularities of coherent light, have been observed in various three-dimensional optical settings. However, incoherent links and knots - knotted or connected lines of coherence singularities - arise from a fundamentally different concept. They are hidden in the statistic properties of a randomly fluctuating field, making their presence often elusive or undetectable. Here, we theoretically construct and experimentally demonstrate such topological entities of incoherent light. By leveraging a state-of-the-art incoherent modal-decomposition scheme, we unveil incoherent topological structures from fluctuating light speckles, including Hopf links and Trefoil knots of coherence singularities that are robust against coherence and intensity fluctuations. Our work is applicable to diverse wave systems where incoherence or practical coherence is prevalent, and may pave the way for design and implementation of statistically-shaped topological structures for various applications such as high-dimensional optical information encoding and optical communications.

physics.optics↗

arXiv2025

On-Demand Pulse Shaping with Partially Coherent Pulses in Nonlinear Dispersive Media

In this Letter, we employ the complex screen method to investigate the dynamic evolution of partially coherent pulses with specified properties as they propagate through a nonlinear Kerr medium. Our results reveal that partially coherent pulses can retain stable pulse characteristics and exhibit enhanced robustness when the source coherence is reduced. Importantly, by adjusting the source pulse properties, the far-zone pulse properties can be customized on demand, even in highly nonlinear environments. These findings are of significant importance for applications such as pulse shaping, free-space optical communication, information encryption etc. in nonlinear media. Notably, the results offer valuable insights for mitigating nonlinear effects in light beams within the spatial domain.

physics.optics↗

arXiv2024

Bound states in the continuum of infinite quality factor in finite unit cells

A theory based on the superposition principle is developed to uncover the basic physics of the wave behavior in a finite grating of N unit cells. The theory reveals that bound states in the continuum (BICs) of infinite quality factor (Q-factor) can be supported by such grating when the perfect reflection is introduced at its boundaries. If geometrical perturbations are introduced in the structure, the dark BICs transit to bright quasi-BICs of finite Q-factor, whose spectral behaviors are nearly the same as that of quasi-BICs supported by infinite gratings. When the boundaries are replaced with metallic mirrors of high reflectivity, the Q-factor of the resonant mode is reduced to be finite; however, it can be much larger than that in the corresponding nanostructure of open boundaries and can be tuned in a large range by varying the number of unit cells or boundary conditions.

physics.optics↗

arXiv2024

Ultrafast bursts of tailored spatiotemporal vortex pulses

Orbital angular momentums (OAMs) of light can be categorized into longitudinal OAM (L-OAM) and transverse OAM (T-OAM). Light carrying time-varying L-OAM, known as self-torqued light, was recently discovered during harmonic generation and has been extensively developed within the context of optical frequency combs (OFCs). Meanwhile, ultrafast bursts of optical pulses, analogous to OFCs, are sought for various light-matter interaction, spectroscopic and nonlinear applications. However, achieving transiently switchable T-OAM of light on request, namely spatiotemporal vortex pulse bursts, with independently controlled spatiotemporal profile of each comb tooth, remain unrealized thus far. In this work, the experimental generation of spatiotemporal vortex bursts featured with controllable time-dependent characteristics is reported. The resultant bursts comprised of spatiotemporal optical vortex comb teeth have picosecond timescale switchable T-OAMs with defined arrangement, manifesting as spatiotemporal torquing of light. We also show ultrafast control of T-OAM chirality, yielding pulse bursts with staggered azimuthal local momentum density, resembling Kármán vortex streets. This approach enables the tailoring of more intricate spatiotemporal wavepacket bursts, such as high-purity modes variation in both radial and azimuthal quantum numbers of spatiotemporal Laguerre-Gaussian wavepackets over time, which may facilitate a host of novel applications in ultrafast light-mater interactions, high-dimensional quantum entanglements, space-time photonic topologies as well as spatiotemporal metrology and photography.

physics.optics↗

arXiv2024

Spatiotemporal optical vortices with controllable radial and azimuthal quantum numbers

Optical spatiotemporal vortices with transverse photon orbital angular momentum (OAM) have recently become a focal point of research. In this work we theoretically and experimentally investigate optical spatiotemporal vortices with radial and azimuthal quantum numbers, known as spatiotemporal Laguerre-Gaussian (STLG) wavepackets. These 3D wavepackets exhibit phase singularities and cylinder-shaped edge dislocations, resulting in a multi-ring topology in its spatiotemporal profile. Unlike conventional ST optical vortices, STLG wavepackets with non-zero p and l values carry a composite transverse OAM consisting of two directionally opposite components. We further demonstrate mode conversion between an STLG wavepacket and an ST Hermite-Gaussian wavepacket through the application of strong spatiotemporal astigmatism. The converted STHG wavepacket is de-coupled in intensity in space-time domain that can be utilized to implement the efficient and accurate recognition of ultrafast STLG wavepackets carried various p and l. This study may offer new insights into high-dimensional quantum information, photonic topology, and nonlinear optics, while promising potential applications in other wave phenomena such as acoustics and electron waves.

physics.optics↗