arXiv subjects

Yanan Dai

Publications and source records attributed to Yanan Dai.

12 recordsLinked to original sources

arXiv2025

Synthetic Spatiotemporal Plasmonic Vortices On Chip

Spatiotemporal vortices are polychromatic modes that intertwine orbital angular momentum (OAM) in space and time. Here we introduce a new class of such vortices, spatiotemporal plasmonic vortices (STPVs), carrying nontrivial topological spin textures. They are generated by chronotopic interference of temporally delayed plasmonic eigen-vortices, where a $\pi$-phase dislocation in the space-frequency domain maps into a 2$\pi$ spiraling phase in space-time, with the resulting focus-defocus dynamics emulate U(1) gauge transitions. Using interferometric time-resolved photoemission electron microscopy (ITR-PEEM), we directly image their nanometer-attosecond (nano-atto) evolution and control vortex number and position. Quantum-path analysis of coherent two-photon photoemission (2PP) processes reveals the nonlinear plasmonic polarization fields and angular-momentum conservation, establishing STPVs as a platform for probing spatiotemporally structured quantum matter.

cond-mat.mes-hall↗

arXiv2025

Ultrafast {\mu}eV-Precision Bandgap Engineering in Low-Dimensional Topological Insulators

Precise and ultrafast control of electronic band structures is a central challenge for advancing quantum functional materials and devices. Conventional approaches--such as chemical doping, lattice strain, or external gating--offer robust stability but remain confined to the quasi-static regime, far from the intrinsic femto- to picosecond dynamics that govern many-body interactions. Here, using cryogenic transient reflectance spectroscopy, we realize dynamic bandgap engineering in the anisotropic topological insulator $\alpha$-Bi$_4$Br$_4$ with unprecedented micro-electron-volt ($\mu$eV) precision. The exceptional sensitivity arises from the cooperative action of long-lived topological carriers, stabilized by restricted bulk-to-edge scattering phase space, together with symmetry-resolved coherent phonons that modulate inter-chain hopping. These channels jointly modify Coulomb screening and interband transitions, enabling both gradual and oscillatory control of the electronic structure. Supported by first-principles and tight-binding theory, we further demonstrate a dual-pump coherent control strategy for continuous, mode-selective tuning of electronic energies with $\mu$eV accuracy. This framework paves the way for ultrafast on-demand band-structure engineering, pointing toward new frontiers in quantum optoelectronics, precision measurement in molecular and biological systems, and attosecond control of matter.

cond-mat.mes-hall↗

arXiv2025

Geometry-Driven Lattice of Photonic Spin-Meron Tubes in Free Space

We theoretically demonstrate the first photonic spin-meron tube lattice in free space using spin-angular momentum vectors. Square-block diffraction creates $C_{4}$-symmetric beams with $\pi/2$ phase steps. Non-paraxial spin-orbit coupling then forms finite-length meron tubes ($N_{\mathrm{sk}} \approx \pm 1/2$, $> 25,\lambda$). Extending the formalism, we show that $C_{3}$ geometry of a triangular block yields a spin-skyrmion tube. Stratton-Chu theory and full-vectorial finite-difference time-domain calculations both support this material-agnostic, geometry-driven approach as a platform to explore symmetry-driven free-space topology.

physics.optics↗

arXiv2025

Magnetic order dependent photoluminescence from high energy excitons in hBN protected few-layer CrSBr

The detection and manipulation of the spin configurations in layered magnetic semiconductors hold significant interest for developing spintronic devices in two-dimensional limit. In this letter, we report a systematical study on the photoluminescence (PL) from the high energy excitons in few-layer CrSBr and its application on detecting the spin configurations. Besides the broad excitonic emission peak (Xl) at around 1.34 eV, we also observed another strong excitonic emission peak (Xh) at around 1.37 eV in hBN encapsulated 2L sample, which splits into two peaks in 3L and 4L samples. With help of the first principles calculations, we conclude that the Xh peak is associated with the transition between the top valence band and the second lowest conduction band, which is forbidden by the inversion symmetry in 1L CrSBr. Furthermore, the position and intensity of the Xh peak are strongly dependent on the interlayer magnetic order of the CrSBr samples, which provides an efficient way to probe their spin configurations. In addition, when the magnetic field is applied at the easy axis direction, we resolve an intermediate magnetic state besides the antiferromagnetic and ferromagnetic states in 3L and 4L samples. Our results reveal few-layer CrSBr as an ideal platform to study the interaction between the excitons and magnetism.

cond-mat.mes-hall↗

arXiv2024

Plasmonic Vortices Host Magnetoelectric Interactions

The vector cross product and pseudoscalar dot products of electric (E) and magnetic (H) fields are separately finite in vacuum transverse electric and magnetic (TEM) plane waves, and angular momentum structured light. Current theories of interactions beyond the standard model of particle physics invoke non-zero dot(E,H) as the source term in the axion law that describes interactions with the cosmological dark matter axion particles outside of the quartet of Maxwells equations. The non-zero dot(E,H) also drives relativistic spin-charge magnetoelectric excitations of axion quasiparticles at a distinctively higher condensed matter scale in magnetic and topological materials. Yet, how to drive coherent dot(E,H) responses is unknown, and provides motivation to examine the field polarizations in structured light on a deep sub-diffraction limited spatial scale and sub-optical cycle temporal scale by ultrafast nonlinear photoemission electron microscopy. By analytical theory and ultrafast coherent photoemission electron microscopy, we image dot(E,H) fields in surface plasmon polariton vortex cores at subwavelength scales, where we find that the magnetoelectric relative to the dipole density is intensified on a ~10 nm diameter scale as a universal property of plasmonic vortex fields. The generation and nanoscale localization of dot(E,H) fields introduces the magnetoelectric symmetry class, having the parity and time reversal broken, but the joint parity-time reversal symmetry preserved. The ability to image the optical fields of plasmonic vortex cores opens the research of ultrafast microscopy of magnetoelectric responses and interactions with axion quasiparticles in solid state materials.

cond-mat.mes-hall↗

arXiv2024

Transient Magnetoelastic Coupling in CrSBr

Recent research has revealed remarkable properties of the two-dimensional (2D) van der Waals layered crystal CrSBr, which is both a semiconductor and an A-type antiferromagnet. Here we show the role of strong magnetoelastic coupling in the generation and propagation of coherent magnons in CrSBr. Time and spatially resolved magneto-optical Kerr effect (tr-MOKE) microscopy reveals two time-varying transient strain fields induced by out-of-plane transverse and in-plane longitudinal lattice displacements. These transient strain fields launch coherent wavepackets of magnons, optical and acoustic at 24.6 GHz and 33.4 GHz, respectively. These findings suggest mechanisms for controlling and manipulating coherent magnons from distinct magnetoelastic couplings in this 2D van der Waals magnetic semiconductor.

cond-mat.mtrl-sci↗

arXiv2021

Spin-Orbit Coupled Exciton-Polariton Condensates in Lead Halide Perovskites

Spin-orbit coupling (SOC) is responsible for a range of spintronic and topological processes in condensed matter. Here we show photonic analogs of SOCs in exciton-polaritons and their condensates in microcavities composed of birefringent lead halide perovskite single crystals. The presence of crystalline anisotropy coupled with splitting in the optical cavity of the transverse electric (TE) and transverse magnetic (TM) modes gives rise to a non-Abelian gauge field, which can be described by the Rashba-Dresselhaus Hamiltonian near the degenerate points of the two polarization modes. With increasing density, the exciton polaritons with pseudospin textures undergo phase transitions to competing condensates with orthogonal polarizations. Unlike their pure photonic counterparts, these exciton polaritons and condensates inherit nonlinearity from their excitonic components and may serve as quantum simulators of many-body SOC processes.

cond-mat.quant-gas↗

arXiv2021

Disarranged Zone Learning (DZL): An unsupervised and dynamic automatic stenosis recognition methodology based on coronary angiography

We proposed a novel unsupervised methodology named Disarranged Zone Learning (DZL) to automatically recognize stenosis in coronary angiography. The methodology firstly disarranges the frames in a video, secondly it generates an effective zone and lastly trains an encoder-decoder GRU model to learn the capability to recover disarranged frames. The breakthrough of our study is to discover and validate the Sequence Intensity (Recover Difficulty) is a measure of Coronary Artery Stenosis Status. Hence, the prediction accuracy of DZL is used as an approximator of coronary stenosis indicator. DZL is an unsupervised methodology and no label engineering effort is needed, the sub GRU model in DZL works as a self-supervised approach. So DZL could theoretically utilize infinitely huge amounts of coronary angiographies to learn and improve performance without laborious data labeling. There is no data preprocessing precondition to run DZL as it dynamically utilizes the whole video, hence it is easy to be implemented and generalized to overcome the data heterogeneity of coronary angiography. The overall average precision score achieves 0.93, AUC achieves 0.8 for this pure methodology. The highest segmented average precision score is 0.98 and the highest segmented AUC is 0.87 for coronary occlusion indicator. Finally, we developed a software demo to implement DZL methodology.

eess.IV↗

arXiv2021

Anomalous reflection at the interface of binary synthetic photonic lattices

We construct a binary synthetic photonic lattice theoretically with an effective magnetic field by projecting two fiber loops' light intensity and adjusting the phase distribution precisely. By tuning the phase modulator, wave vector, and propagation constant of an effective waveguide, the interface's transmittance could be manipulated. Further light dynamics show that the light pulse can achieve total reflection without diffraction and exchanges the light energy in two optical fiber loops completely when phase distribution and wave vector meet certain conditions. Our study may provide a new way to realize optical switches in optical interconnection and optical communication.

physics.optics↗

arXiv2020

PT-symmetric topological near-zero interface state

Photonic systems with parity-time (PT) symmetry and topology are attracting considerable attentions. In this work, topological near-zero edge states are studied in PT-symmetric photonic lattice and the results indicate that the near-zero edge states can be broken spontaneously in spite of the unbroken PT symmetry. To achieve the stable topological near-zero mode, a binary lattice with carefully designed PT-symmetric is proposed. Further study shows such a structure supports a stable topological interface state experiences phase transition similar to the bulk states in infinite lattice and thus possess real-eigenvalues even with unbroken PT phase. Our study enriches the content of non-Hermitian topological physics and might have potential applications in the fields of topological lasing and quantum computation.

physics.optics↗

arXiv2019

The Birth of a Plasmonic Topological Quasiparticle on the Nanofemto Scale

At interface of the classical and quantum physics Maxwell and Schrödinger equations describe how optical fields drive and control electronic phenomena at THz or PHz frequencies and on ultra-small scales to enable lightwave electronics. Light striking a metal surface triggers electric field-electron particle/wave interactions to coherently imprint and transfer its attributes on the attosecond time scale. Here we create and image by ultrafast photoemission electron microscopy a new quasiparticle of optical field-collective electron interaction where the design of geometrical phase creates a plasmonic topological spin angular momentum texture. The spin texture resembles that of magnetic meron quasiparticle, is localized within 1/2 wavelength of light, and exists on ~20 fs (2^10-14 s) time scale of the plasmonic field. The quasiparticle is created in a nanostructured silver film, which converts coherent linearly polarized light pulse into an evanescent surface plasmon polariton light-electron wave with a tailored geometric phase to form a plasmonic vortex. Ultrafast coherent microscopy imaging of electromagnetic waves propagating at the local speed of light of 255 nm/fs, electromagnetic simulations, and analytic theory find a new quasiparticle within the vortex core, with topological spin properties of a meron that are defined by the optical field and sample geometry. The new quasiparticle is an ultrafast topological defect whose chiral field breaks the time-inversion symmetry on the nanoscale; its creation, symmetry breaking topology, and dynamics pertain to contexts ranging from the cosmological structure creation to topological phase transitions in quantum liquids and gases, and may act as a transducer for quantum information on the nanofemto scale.

cond-mat.mes-hall↗

arXiv2019

Ultrafast Microscopy of a Plasmonic Spin Skyrmion

We present an ultrafast microscopy imaging experiment and a general analytical description of a new quasiparticle composed of plasmonic Skyrmion-like spin texture at the core of a surface plasmon polariton (SPP) vortex. The illumination of a circular coupling structure milled in an Ag film by circularly polarized light (CPL) couples its spin angular momentum (SAM) into orbital angular momentum (OAM) of SPPs launching them to form a plasmonic vortex. The coupling of the cycloidal motion of the SPP polarization at the 2D interface, with their orbital swirl at the vortex core causes the plasmonic field to generate 3D SAM pseudovectors, whose topological texture has integer Skyrmion number and is homotopic to a twisted magnetic Skyrmion quasiparticle with the boundary defined by an optical L-line singularity contour. An analytical description finds that the dielectric discontinuity at the Ag/vacuum interface supports on each side entwined twisted Skyrmion pairs that are characterized by stable topological textures with opposite Skyrmion numbers. The SAM texture of the Skyrmion pair within the primary vortex ring corresponds to a monopole-hedgehog type SAM texture, with a SAM singularity at the vortex core. Interferometric time-resolved two-photon photoemission electron microscopy (ITR-2P-PEEM) imaging of the nanofemto spatiotemporal evolution of the SPP fields and simulation by an analytical model, establish the twisted topological plasmonic SAM Skyrmion quasiparticle at the vortex core. The SAM textures can probe and simulate topological responses in trivial and topological materials that can be coupled in the near-field of the SPP vortex. The theory anticipates different field structures and the accompanying topological spin textures that construct single Skyrmion and meron-like quasiparticles as well as their arrays.

cond-mat.mes-hall↗