arXiv subjects

Huayu Bai

Publications and source records attributed to Huayu Bai.

3 recordsLinked to original sources

arXiv2026

Doubly resonant enhancement of second-harmonic generation with in-plane phase matching in plasmonic metasurfaces on an AlInP slab waveguide

Nonlinear metasurfaces have attracted significant interest by offering the possibility to circumvent conventional phase-matching requirements of bulk nonlinear crystals, opening the way to efficient frequency conversion over ultrashort propagation distances. Here, we experimentally demonstrate metasurfaces in which second-harmonic generation is strongly enhanced by in-plane phase matching of high-Q guided-mode resonances. To achieve this enhancement, we use hybrid metasurfaces composed of periodic arrays of gold nanodiscs on a slab waveguide made of aluminum indium phosphide (AlInP) - a low-loss epitaxial III-V semiconductor material. The metasurfaces are found to exhibit guided-mode resonances with Q factors exceeding 400 and 900 at the visible and near-infrared wavelengths, respectively, demonstrating the unique capabilities of hybrid metal-dielectric structures to support high-Q resonances despite the presence of lossy plasmonic components. Moreover, our frequency and momentum-resolved measurements demonstrate a two-orders-of-magnitude enhancement of second-harmonic generation at doubly resonant conditions. We reproduce the experimental results using numerical simulations, showing that the in-plane phase matching and spatial mode overlap are the main factors governing the enhancement. Our findings demonstrate a promising strategy to realize efficient metasurface-based frequency converters, enabling their potential applications in compact photonic systems.

physics.optics↗

arXiv2024

Enhanced $Q$ factor and robustness of photonic bound states in the continuum merging at locally bent trajectories

Bound states in the continuum (BICs) in planar photonic structures have attracted broad scientific interest owing to their exceptional capability to confine light. Topological robustness of certain BICs allows them to be moved in the momentum space by tuning the geometric parameters of the structure. In this work, we study such a BIC in a one-dimensional periodic grating, and find that its momentum-space position can be made a non-monotonic function of a geometric parameter, forming a locally bent ''V''-shaped trajectory. We show that, near the turning point of this trajectory, the robustness of the BIC and its $Q$ factor can be greatly enhanced. We tune such ''V-BICs'' to almost merge with a symmetry-protected BIC at the $Γ$-point. This creates a ''K''-shaped ultrahigh-$Q$ region containing a BIC with a much higher and more stable $Q$ factor compared to the ordinary merging BICs. The ''K-BICs'' are also found to provide a strong enhancement of the $Q$ factor in finite gratings over an extremely wide range of geometric parameters. Our findings enable further advancements in the development of ultrahigh-$Q$ BICs and their applications.

physics.optics↗

arXiv2024

Recovery of topologically robust merging bound states in the continuum in photonic structures with broken symmetry

Optical bound states in the continuum (BICs) provide a unique mechanism of light confinement that holds great potential for fundamental and applied research in optics and photonics. Of particular interest are merging BICs realized in planar periodic structures by merging accidental and symmetry-protected BICs. Topological nature of merging BICs renders their $Q$ factors exceptionally high and robust. However, the existence of accidental BICs relies on the up-down mirror symmetry of the structure. If this symmetry is broken, e.g., by a substrate, the $Q$ factor of the mode drops down. Consequently, ultrahigh-$Q$ merging BICs cannot be achieved in substrate-supported structures. Here, by studying the case of a one-dimensional periodic dielectric grating, we discover a simple method to fully compensate for the detrimental effect of breaking the up-down mirror symmetry. The method makes use of a thin layer of a high-refractive-index dielectric material on one side of the structure, allowing one to restore the diverging $Q$ factor of the accidental BIC and fully recover the merged BIC. By investigating the far-field polarization patterns of the modified gratings, we show that the integer-charge polarization vortices of the accidental BICs are restored by intersecting the momentum-space trajectories of circularly polarized half-vortices simultaneously in the upward and downward radiation directions. Our approach can enable flexible design, feasible realization, and extended applications of topologically robust BICs in various systems.

physics.optics↗