arXiv subjects

Andriy Shevchenko

Publications and source records attributed to Andriy Shevchenko.

10 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↗

arXiv2025

Near-field enhancement by a metasurface at octupole plasmon resonance in periodic disc dimers

Local intensity enhancement by plasmonic nanoparticles is widely used in optics and photonics. However, the effect is usually based on dipole resonances in the particles. Recently, it has been shown that quadrupole and octupole resonances can exhibit comparable, or even higher near-field enhancement. In this work, we focus on the near-field enhancement by a metasurface composed of gold-disc dimers arranged in a rectangular array. We find that, owing to an octupole plasmon resonance coupled to a surface lattice resonance, exceptionally high near-field enhancement in the dimer gaps can be achieved in the visible spectral range. To gain insight into the effect, we develop an analytical model for the effective dipole and octupole polarizabilities of the particles in an array, and discover, that at decreasing array periods, the dipole polarizability tends to vanish, while the octupole polarizability rapidly increases. Hence, octupole resonances can find applications in high-density arrays of plasmonic resonators. We propose a method to numerically evaluate multipole polarizabilities of a single particle, applying it to the gold dimer that we consider. The influence of the array on the effective polarizabilities is then verified by numerical calculations and a good agreement is obtained. Our results may open new avenues for investigating the properties of periodic plasmonic structures based on higher-order multipole resonances and their applications.

physics.optics↗

arXiv2025

Exact electromagnetic multipole expansion using elementary current multipoles

Multipole expansion plays an important role in the description of electromagnetic scatterers, allowing them to be accurately characterized by a small set of expansion coefficients. However, to describe electromagnetic excitations inside a scatterer, the current density in it should be decomposed into current multipoles, which include nonradiating current configurations (anapoles) that are absent in the classical field-based expansion. Unfortunately, the use of current multipoles has so far been limited by the absence of an exact and general expression for the current multipole moments beyond their point-multipole approximation. Here, we derive such an expression and present the exact mapping relations between the classical and current multipole moments. We use our theory to calculate the scattering and extinction cross sections for large, wavelength-scale, optical scatterers supporting multipole excitations up to the sixth order, showing perfect agreement with the Mie theory. We also demonstrate the ability of current multipole expansion to describe anapole excitations beyond the small-scatterer approximation, which allows us to derive the exact anapole condition and reveal the actual current configurations and their contributions to scattering. Our theoretical framework is valid for electromagnetic scatterers of arbitrary sizes and shapes without restrictions on the multipole orders, complementing the existing theory of electromagnetic multipole expansion. The minimalistic and universal character of current multipoles makes them a convenient tool for characterizing and designing diverse electromagnetic scattering systems of arbitrary complexity.

physics.optics↗

arXiv2025

Towards ultracompact photonic chips using higher-order modes in closely spaced waveguides

Photonic integrated circuits are gaining traction in the field of telecommunications and information processing for their low-loss and high-throughput data transmission in comparison to electronic integrated circuits. However, they are still not used as widely as their electronic counterparts due to a relatively large footprint of photonic chips. One limiting factor to their size is the need to separate optical components by distances on the order of the working wavelength or larger to minimize optical crosstalk between them. In this work, we consider the fundamental and higher-order modes in closely spaced straight and bent waveguides with relatively small cross sections and find that higher-order modes allow one to substantially reduce the crosstalk in both cases. This can be used to considerably reduce the dimensions of photonic chips. We also propose on-chip components that allow selective excitation of higher-order modes. In addition, we design a directional coupler, a 3-dB splitter, and a Mach-Zehnder interferometer capable of operating on higher-order modes. Other ultracompact photonic-chip components, such as optical interconnects, switches, transceivers, and phased waveguide arrays for on-chip LiDAR scanners, can be designed as well based on similar principles.

physics.optics↗

arXiv2025

Spatial Filtering with Nonlocal Non-Hermitian Metasurfaces

Spatial filtering of optical fields has widespread applications ranging from beam shaping to optical information processing. However, conventional spatial filters are bulky and alignment-sensitive. Here, we present nonlocal non-Hermitian metasurfaces that can act as exceptionally effective optical spatial filters while being highly compact and insensitive to both lateral and longitudinal displacements. The metasurface design is based on a resonant waveguide grating in which radiative losses of the modes are tailored to realize a symmetry-protected bound state in the continuum in the middle of a non-Hermitian flat band. Using this design, we propose a compact spatial filtering device operating over an angular range of approximately 1 degree around normal incidence. In addition to being ultrathin and robust against translational misalignment, the proposed metasurfaces are easy to manufacture, which makes them an attractive alternative to conventional spatial filters, holding a potential to become a widely used optical component.

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 $\Gamma$-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↗

arXiv2023

Near-field enhancement by waveguide-plasmon polaritons in a nonlocal metasurface

Localized surface plasmons in metal nanoparticles are widely used in nano-optics to confine and enhance optical fields. It has been previously shown that, if the nanoparticles are distributed periodically, an additional enhancement can be achieved by coupling the localized surface plasmons to the diffraction orders of the lattice, forming surface lattice resonances. In this work, we study an even further improvement of the near-field enhancement by placing a metal-dielectric slab waveguide beneath the lattice of the particles to excite coupled waveguide-plasmon polaritons. These excitations can extend over many periods of the lattice, making the metasurface highly nonlocal. We numerically demonstrate that the approach can provide a significant extra increase in the near-field intensity -- by a factor of 80 over that produced by a single-particle plasmon resonance and by 7 over the lattice-resonance enhancement. The described enhancement mechanism can be used to design extraordinarily efficient nonlocal optical metasurfaces for many applications, including surface-enhanced Raman spectroscopy, fluorescence spectroscopy, nonlinear optics, and solar energy harvesting.

physics.optics↗

arXiv2023

Enabling infinite $Q$ factors in absorbing optical systems

Resonant optical systems have widespread applications in science and technology. However, their quality ($Q$) factors can be significantly deteriorated, if some of their parts exhibit optical absorption. Here, we show that by coupling a lossy mode of such a structure to two independent lossless modes, one can create a nonradiating and absorption-free bound state in the continuum (BIC). The $Q$ factor of such a BIC is theoretically unlimited despite interaction with an absorbing structure. We use this mechanism to design a plasmonic metasurface with $Q$ factors that are close to $10^7$ in the visible spectral range. The proposed mechanism is general and can be used to engineer ultrahigh-$Q$ resonances in various absorbing structures.

physics.optics↗

arXiv2010

Electromagnetic force density in dissipative isotropic media

We derive an expression for the macroscopic force density that a narrow-band electromagnetic field imposes on a dissipative isotropic medium. The result is obtained by averaging the microscopic form for Lorentz force density. The derived expression allows us to calculate realistic electromagnetic forces in a wide range of materials that are described by complex-valued electric permittivity and magnetic permeability. The three-dimensional energy-momentum tensor in our expression reduces for lossless media to the so-called Helmholtz tensor that has not been contradicted in any experiment so far. The momentum density of the field does not coincide with any well-known expression, but for non-magnetic materials it matches the Abraham expression.

physics.optics↗