arXiv subjects

Ken Qin

Publications and source records attributed to Ken Qin.

6 recordsLinked to original sources

arXiv2026

Spatiotemporal Optical Vortices From All-Dielectric Bilayer Metagratings

Spatiotemporal optical vortices (STOVs) carry transverse orbital angular momentum within the space-time domain, rendering them powerful tools for constructing high-dimensional and quantum optical fields. However, most existing approaches rely on highly lossy metallic structures or complex pulse-shaping systems. Here, we propose an all-dielectric route to STOV generation based on symmetry-protected bound states in the continuum (BICs) in a bilayer metagrating and provide proof-of-concept validation of its key momentum-frequency signatures. By simply introducing a lateral shift between the upper and lower layers of the vertical slots on the dielectric metagrating, the {\Gamma}-point BIC transforms into a quasi-BIC (qBIC) with momentum-dependent directional radiation and asymmetric coupling. This qBIC further leads to an isolated zero-transmission dip associated with a clear phase singularity and branch cut in the momentum-frequency response, enabling stable STOV generation under excitation by a spatiotemporal Gaussian pulse. The multipole analysis of the qBIC reveals the key role of the Kerker-type interference between its electric and magnetic dipoles and associated asymmetric coupling in the STOV generation. Experimentally, free-space transmission measurements reveal a transmission zero and a branch cut that agree excellently with theoretical analysis. Therefore, our work provides an experimentally validated, scalable route for manipulating spatiotemporal optical fields on low-loss all-dielectric metasurfaces via only gliding offsets, with potential applications in directional coupling of quantum light sources and spatiotemporal shaping of single-photon wave packets.

physics.optics↗

arXiv2025

Kirigami-based Flexible Metasurface with Reconfigurable Intrinsic Chirality from Zero to Near-unity

Chiral responses in electromagnetic metasurfaces are typically categorized as extrinsic, resulting from asymmetric interactions between the structure and incident waves, and intrinsic, arising from three-dimensional symmetry breaking of the unit cell. However, most existing metasurface designs target only one type of chirality and lack a unified, continuously tunable platform for broader chiroptical control. To address this limitation, the designed kirigami-based flexible metasurface is proposed for dynamic, continuous modulation of chirality, which expands the control scope to both extrinsic and intrinsic chiral responses within a single, reconfigurable platform. Initially, the unfolded metasurface exhibits extrinsic chirality under oblique incidence. By introducing well-designed kirigami-based cuts and folds, the metasurface transitions from a planar and achiral configuration to a three-dimensional chiral geometry that breaks the mirror symmetry, thereby exhibiting tunable intrinsic chirality and asymmetric extrinsic chirality. As the folding angle increases, the resulting deformation enables continuous tuning of the chiral response, with circular dichroism and its asymmetry under oblique incidences progressively increasing and reaching pronounced levels across the X-band. Our work provides a lightweight, easy-fabricated, and mechanically reconfigurable metasurface, which offers strong potential for future development in adaptive photonic systems and advanced chiroptical technologies.

physics.optics↗

arXiv2025

Hybrid Cavity from Tunable Coupling between Anapole and Fabry-Perot Resonance or Anti-resonance

Enhancing light-matter interactions depends critically on the ability to tailor photonic modes at subwavelength scales, and combining distinct resonant modes has shown remarkable potential unattainable by individual resonances alone. Despite recent advances in anapole metasurfaces for energy confinement and Fabry-Perot (FP) cavities for spectral control, their synergistic coupling and resulting opportunities remain largely unexplored due to challenges such as precise nanoscale assembly. Here, we demonstrate that embedding a terahertz (THz) anapole metasurface within a tunable FP cavity results in a hybrid cavity that demonstrates exotic properties as the anapole transitions between coupling to FP resonances and anti-resonances via cavity-length tuning. At room temperature, we observe ultrastrong coupling (> 30% of the anapole frequency) between anapoles and FP resonances, generating tunable-dispersion polaritons that blend favorable properties of both modes. Meanwhile, anapole spectrally aligns with FP anti-resonances, leading to weak coupling that narrows the linewidth of the anapole's transmission peak by two orders of magnitude and enhances its local density of states (LDOS) near the metasurface correspondingly. With exceptional capabilities including formation of polaritons and significant enhancement of LDOS, the hybrid cavity enables strong interaction with functional materials, paving the way for exploration of quantum optics, molecular sensing, and ultrafast nonlinear photonics.

physics.optics↗

arXiv2025

Giant and Rapidly Switching Intrinsic Chirality Enabled by Toroidal Quasi-Bound States in the Continuum

Circular dichroism (CD), arising from spin-selective light-matter interactions controlled by chirality, is critical for advanced applications such as chiral imaging and ultrasensitive biosensing. However, CD of chiral natural materials is inherently constrained owing to molecular symmetry and thermodynamic stability. Recently, artificially engineered metasurfaces incorporating chiral quasi-bound states in the continuum (Q-BICs) have emerged as a promising solution, which enables near-unity CD responses. However, their current designs heavily rely on complex three-dimensional geometries, posing significant challenges for integration with planar on-chip platforms. To address the stringent challenges, we demonstrate a truly planar metasurface that achieves giant intrinsic chiral responses by utilizing a chiral Q-BIC dominated by out-of-plane toroidal dipoles (Tz). With deep-subwavelength ({\lambda}/20) thickness, our metasurface exhibits outstanding intrinsic CD values in both simulations (>0.90) and experiments (~0.80). Moreover, in contrast to previous electric or magnetic chiral Q-BICs, the toroidal Q-BIC produces a rapidly switching CD response - transitioning sharply between positive and negative giant CD values within ~0.2 GHz, and the switching is highly sensitive to small oblique incidence of opposite angles. Therefore, our scheme provides a planar platform for studying chiral light-matter interactions involving toroidal dipoles, important for future development of polarization- and angle-sensitive photonic and optoelectronic devices.

physics.optics↗

arXiv2025

Nonlocal Generation of Fano Resonance with No Symmetry Breaking in THz Hybrid Metasurfaces

Fano resonance, arising from the interference between a discrete resonance and a continuum of states, results in sharp and asymmetric line shapes and has significant applications in advanced photonic devices, particularly in sensing, filtering, and nonlinear optics. Nowadays, metasurfaces comprised of engineering microstructures play a crucial role in generation and manipulation of Fano resonance in photonics. However, current metasurfaces dominantly rely on local symmetry breaking of the microstructures to induce Fano resonances, which significant limits their tunability and scalable fabrication for practical applications. To address the challenge, a metal-dielectric hybrid metasurface is demonstrated to achieve nonlocal generation of Fano resonance with no symmetry breaking in the terahertz (THz) band. The Fano resonance, including its existence and peak frequency, is sensitively controlled by the thickness and dielectric constant of the dielectric layer, which is experimentally observed. Our analysis elucidates that the metallic layer with a pair of dumbbell holes leads to the band folding and coupling of guided modes within the dielectric layer. When the thickness or dielectric constant surpasses a critical value, the guided mode resonance falls below the diffraction limit, resulting in a unique nonlocal Fano resonance due to the interaction between the resonance and background transmission facilitated by dumbbell holes. Furthermore, the Fano transmission peak corresponds to an anapole excitation, revealed by multipole calculations. Benefiting from the ability to control the Fano resonance with no symmetry breaking, the proposed hybrid THz metasurface will advance broad applications in the fields of sensors, optical switches, and tunable filters.

physics.optics↗

arXiv2025

Direct Observation of Strongly Tilted Dirac Points at General Positions in the Reciprocal Space

Type-II Dirac points (DPs), which occur at the intersection of strongly tilted and touching energy bands, exhibit many intriguing physical phenomena fundamentally different from the non-tilted type-I counterparts. Over the past decade, their discovery has spurred extensive research into electronic systems and other Bloch-wave systems, such as photonic and phononic crystals. However, current studies typically focus on type-II DPs along high-symmetry directions in the first Brillouin zone (FBZ) under mirror symmetry conditions, which are highly restrictive and limit further investigations and applications. To overcome the stringent constraint, here we identify and demonstrate the emergence of type-II DPs at general positions inside the FBZ without requiring the mirror symmetry. The type-II DPs, being accidental degeneracies, are experimentally realized on a metacrystal slab with H-shaped metallic patterns. Our findings indicate that even in the absence of mirror symmetry, type-II DPs can emerge at designated locations inside the FBZ by simply rotating the H-shaped patterns and adjusting geometrical and physical parameters. Furthermore, based on the rotated type-II DPs, off-axis conical diffractions have been both realized and experimentally observed. Meanwhile, we discovered that during the rotation process, the type-II DPs transform into off-axis type-I DPs, but still strongly tilted, resulting in the emergence of negative refractions. Hence, the generic method we propose for inducing type-II or strongly tilted type-I DPs without the high-symmetry limitations opens potential avenues for related research. For example, the observed off-axis conical diffraction and negative refraction could inspire future development and applications in photonics and other Bloch-wave systems.

physics.optics↗