arXiv subjects

Kirill Koshelev

Publications and source records attributed to Kirill Koshelev.

At least 19 recordsLinked to original sources

arXiv2026

Band Engineering of Exciton Polaritons in Resonant Polaritonic Metasurfaces

Polaritonic metasurfaces provide a versatile platform for engineering hybrid light-matter states through the interplay of optical resonances and excitonic excitations. Yet, predictive models often remain phenomenological and rely on coupled-mode equations. Here, we develop an effective Hamiltonian framework for exciton polaritons in resonant polaritonic metasurfaces, derived from a semiclassical single-pole description of excitonic polarization and a Green's-function description of guided-mode resonances. The resulting non-Hermitian Hamiltonian rigorously incorporates resonant photonic harmonics, multiple excitonic degrees of freedom, and radiative losses. The model reveals selection rules governing photonic-excitonic coupling classified by orbital multipole index and polarization, and shows that the minimal number of excitonic degrees of freedom equals the number of relevant photonic modes. We apply the framework to a bulk van der Waals WS$_2$ metasurface patterned into a hexagonal lattice of triangular holes and uncover a new geometry-controlled topological transition driven by dipole-quadrupole band inversion, distinct from the conventional breathing-honeycomb-lattice transition. Full-wave simulations confirm the predicted topological phase diagram and the emergence of photonic and polaritonic edge states at a topological interface. Our results establish a theoretical multimode framework for geometry-controlled bandstructure engineering in polaritonic metasurfaces, with applications in topological, chiral, and quantum integrated photonics.

physics.optics↗

arXiv2026

Nonlinearity-induced chirality in resonant metasurfaces

Chiral metasurfaces conventionally rely on structural or extrinsic symmetry breaking, while nonlinear circular dichroism is usually treated as a resonantly enhanced consequence of an already chiral linear response. Here, we show that chirality can instead be induced by the nonlinear susceptibility of an otherwise achiral resonant metasurface. We study a membrane metasurface composed of circular holes in a square lattice made of a cubic nonlinear material such as crystalline silicon. Under normal incidence, the structure is linearly achiral and supports high-Q quasi-guided resonances exhibiting identical responses to left- and right-circularly polarized light. Using quasi-normal-mode expansion and temporal coupled-mode theory extended to the nonlinear regime, we demonstrate that a relative rotation between the principal axes of the cubic nonlinear susceptibility tensor and the metasurface axes produces unequal third-harmonic generation for opposite circular polarizations. We formulate the resulting effect of nonlinear circular dichroism without geometrical chirality in terms of helicity phase-matching criteria and discuss why such a mechanism is forbidden for second-harmonic generation in materials with second-order nonlinearity. The resulting nonlinear circular dichroism reaches 99.5% and follows a simple dependence on the relative angle, switching from fourfold to eightfold periodicity when resonances at the harmonic frequency are additionally excited. Our results establish nonlinearity-induced chirality as a fundamentally new route to chiral photonic responses without geometrical symmetry breaking, opening opportunities for nonlinear chiral optics in planar CMOS-compatible metasurfaces.

physics.optics↗

arXiv2026

Optical Resonances: From Eigenmodes to Scattering Features

Electromagnetic resonances play a central role in nanophotonics by enabling efficient confinement of electromagnetic energy and enhanced light-matter interaction. Traditionally, resonant phenomena have been described using platform-specific concepts developed within distinct research communities, including photonic crystals, plasmonics, and dielectric metasurfaces. In this Perspective, we propose a unified framework that distinguishes electromagnetic resonances as eigenmodes of open systems from their experimentally observed manifestations as scattering features. We show how resonances evolve from isolated particles to coupled oligomers and periodic structures, highlighting the roles of geometry, material response, and dimensionality. Particular attention is given to interference-driven phenomena such as bound states in the continuum, lattice resonances, anapoles, and superscattering, some of which cannot always be associated with a single eigenmode. By clarifying the relationship between eigenmodes, scattering channels, and interference effects, this Perspective provides a coherent language for interpreting resonant phenomena and identifies key challenges and opportunities for designing robust resonant photonic systems.

physics.optics↗

arXiv2025

Light matter interaction in van der Waals heterostructures with Mie voids

Recently introduced concept of Mie voids allows to enhance the field localization inside air cavities embedded in high-index materials. Mie voids provide an alternative approach to conventional dielectric resonators that confine optical fields within bulk high-index materials. Building on this concept, here we present a hybrid photonic platform that integrates monolayer WS2 with Mie void resonators patterned in a high-index Bi2Te3 substrate. By carefully aligning the dipolar void resonance with the excitonic transition of WS2, we achieve substantially enhanced photoluminescence and second-harmonic generation. Far-field imaging of the harmonic fields reveals spatially resolved hotspots that directly map localized resonant modes, with their positions tunable by cavity geometry and pump wavelength. This approach enables real-space control of nonlinear emission at the single-resonator level, offering a robust and reconfigurable platform for next-generation nonlinear photonics and surface-enhanced optical sensing.

physics.optics↗

arXiv2025

Third-Order Spontaneous Parametric Down Conversion in Dielectric Nonlinear Resonant Metasurfaces

We propose a general scheme to investigate photon triplet generation (PTG) via third-order spontaneous parametric downconversion (TOSPDC) in $χ^{(3)}$ nonlinear structures. Our approach leverages the quantum-classical correspondence between TOSPDC and its reverse classical process, three-wave sum-frequency generation (TSFG), to efficiently estimate the PTG rate. We apply this framework to nonlinear metasurfaces supporting quasi-bound states in the continuum (qBICs) in the optical range. From numerical analysis of non-collinear TSFG with degenerate input waves at qBIC wavelengths, we predict wavelength-tunable three-photon emission with spatio-angular correlations. These findings establish a novel method for modelling TOSPDC and also highlight the potential of nonlinear resonant metasurfaces as compact free-space photon triplet sources with quantum state control.

physics.optics↗

arXiv2025

Light-cone-proximal quasi-BICs for chiral lasing at grazing angles

Chiral quasi-bound states in the continuum (q-BICs) have recently emerged in metaphotonics as resonances that combine ultrahigh quality factors with near-unity circular polarization. However, these states are typically confined to the Gamma-point (normal incidence) due to their symmetry-protected origins. We propose a new mechanism for realizing light-cone-proximal chiral q-BICs at large oblique angles, enabled by the divergence of the radiative local density of states near the light cone. Using dielectric metasurfaces with a monoclinic lattice and broken in-plane mirror symmetry, we demonstrate that tuning the lattice angle allows for robust control of these resonances. The resulting chiral q-BICs exhibit near-unity circular dichroism in transmission and fully circularly polarized emission at angles exceeding 50 degrees from normal. This approach paves the way for directional chiral lasing at grazing angles and for photonic devices operating efficiently in off-normal geometries.

physics.optics↗

arXiv2025

Excitonic bound states in the continuum in van der Waals heterostructure metasurfaces

We investigate the formation of excitonic bound states in the continuum in van der Waals (vdW) heterostructures composed of two-dimensional excitonic vdW layers and an optically resonant patterned vdW thin film. We show that the radiative losses of the exciton can be completely suppressed - not through conventional methods such as total internal reflection, Bragg mirrors, or metallic layers - but instead via destructive interference of exciton emission rates to distinct optical modes of the metasurface. We formulate the general conditions of excitonic BICs as a vanishing Purcell factor with non-vanishing vacuum local density of states at the exciton frequency. We propose a mechanism to achieve excitonic quasi-BICs with almost complete suppression of radiation via exciton coupling with a guided-mode resonance and multiple Fabry-Pérot modes. We show that in unpatterned vdW slabs, the Purcell factor suppression is defined exclusively by the slab's permittivity achieved via positioning the 2D exciton layer in the minimum of the mode electric field. We confirm through numerical simulations that, in periodically patterned heterostructure metasurfaces, the Purcell factor can be suppressed by more than five orders of magnitude, and this effect is not due to vanishing local electric fields. Our results demonstrate the formation of excitonic quasi-BICs and their potential for advancing quantum optics and information processing.

cond-mat.mes-hall↗

arXiv2025

Resonant helical dichroism in twisted dielectric metastructures

Circular dichroism, arising from interactions with light fields of opposite spin angular momentum, has become a fundamental tool for molecular characterization. Meanwhile, helical dichroism (HD) - the dichroic response to vortex beams carrying opposite orbital angular momentum (OAM) - offers an alternative approach for probing chiral molecules and photonic structures. Previous demonstrations of HD have been limited to non-resonant light-matter interactions with chiral micro- and nanostructures, leaving the realization of resonance helical dichroism largely unexplored. Here, we present the design and implementation of twisted dielectric metastructures, composed of an array of rotated silicon trimer nanostructures harnessing nonlocal photonic modes with a high quality factor of several dozen that enable strong resonant HD for OAM values up to $10$. We experimentally demonstrate resonantly enhanced HD for strongly focused OAM beams with the magnitude of topological charges from $1$ to $3$. Our findings pave the way for resonant nanophotonics involving OAM beams, unlocking the full potential of structured light for applications in molecular sensing, optical imaging, nonlinear optics, and optical data storage.

physics.optics↗

arXiv2024

Chirality encoding in resonant metasurfaces governed by lattice symmetries

Chiral metasurfaces provide invaluable tools capable of controlling structured light required for biosensing, photochemistry, holography, and quantum photonics. Here we suggest and realize a universal strategy for controlling the chiral response of resonant metasurfaces via the interplay of meta-atom geometry and lattice arrangements within all five possible planar Bravais symmetries. By introducing chiral gradient metasurfaces, we illustrate how our approach allows producing a predictable chiral response tunable by simple parameter variations. We highlight that symmetry-controlled chiral response provides an additional degree of freedom in optical signal processing, and showcase this with simultaneous mid-IR image encoding in two fundamental quantities, transmission and circular dichroism. Our proposed concept represents a universal toolkit for on-demand design and control of chiral metastructures that has potential for numerous applications in life sciences, quantum optics and more.

physics.optics↗

arXiv2024

Chiral Dichroism in Resonant Metasurfaces with Monoclinic Lattices

We demonstrate that chiral response can be achieved in resonant metasurfaces with a monoclinic lattice symmetry (the so-called Bravais oblique lattices) where the mirror symmetry is broken by the lattice asymmetry and also by a substrate, whereas each individual meta-atom remains fully achiral. We describe the underlying physics by introducing a mode chirality parameter as a quantitative measure of the lattice chiral eigenmodes. We confirm experimentally selective linear and nonlinear chiral interaction of resonant silicon metasurfaces with circularly polarized light.

physics.optics↗

arXiv2024

Nonlinear chiral metasurfaces based on structured van der Waals materials

Nonlinear chiral photonics explores nonlinear response of chiral structures, and it offers a pathway to novel optical functionalities not accessible through linear or achiral systems. Here we present the first application of nanostructured van der Waals materials to nonlinear chiral photonics. We demonstrate the three orders of magnitude enhancement of the third-harmonic generation from hBN metasurfaces driven by quasi-bound states in the continuum and accompanied by strong nonlinear circular dichroism at the resonances. This novel platform for chiral metaphotonics can be employed for achieving large circular dichroism combined with high-efficiency harmonic generation in a broad frequency range.

physics.optics↗

arXiv2024

Even-order optical harmonics generated from centrosymmetric-material metasurfaces

Generation of even-order optical harmonics requires noncentrosymmetric structures being conventionally observed in crystals lacking the center of inversion. In centrosymmetric systems, even-order harmonics may arise, e.g., at surfaces but such effects are usually very weak. Here we observe optical harmonics up to 4-th order generated under the normal incidence from centrosymmetric dielectric metasurfaces empowered by resonances. We design silicon metasurfaces supporting optical quasibound states in the continuum and guided-mode resonances, and demonstrate the enhancement of second-harmonic signals by over three orders of magnitude compared to nonresonant thin films. Under the optimal conditions, the brightness of the second harmonic approaches that of the third harmonic, and the 4th-order harmonic becomes detectable.

physics.optics↗

arXiv2023

Quasi-Babinet principle in dielectric resonators and Mie voids

Advancing resonant nanophotonics requires novel building blocks. Recently, cavities in high-index dielectrics have been shown to resonantly confine light inside a lower-index region. These so-called Mie voids represent a counterpart to solid high-index dielectric Mie resonators, offering novel functionality such as resonant behavior in the ultraviolet spectral region. However, the well-known and highly useful Babinet's principle, which relates the scattering of solid and inverse structures, is not strictly applicable for this dielectric case as it is only valid for infinitesimally thin perfect electric conductors. Here, we show that Babinet's principle can be generalized to dielectric systems within certain boundaries, which we refer to as the quasi-Babinet principle and demonstrate for spherical and more generically shaped Mie resonators. Limitations arise due to geometry-dependent terms as well as material frequency dispersion and losses. Thus, our work not only offers deeper physical insight into the working mechanism of these systems but also establishes simple design rules for constructing dielectric resonators with complex functionalities from their complementary counterparts.

physics.optics↗

arXiv2023

Scattering matrix for chiral harmonic generation and frequency mixing in nonlinear metasurfaces

We generalize the concept of optical scattering matrix ($S$-matrix) to characterize harmonic generation and frequency mixing in planar metasurfaces in the limit of undepleted pump approximation. We show that the symmetry properties of such nonlinear $S$-matrix are determined by the microscopic and macroscopic symmetries of the metasurface. We demonstrate that for description of degenerate frequency mixing processes such as optical harmonic generation, the multidimensional $S$-matrix can be replaced with a reduced two-dimensional $S$-matrix. We show that for metasurfaces possessing specific point group symmetries, the selection rules determining the transformation of the reduced nonlinear $S$-matrix are simplified substantially and can be expressed in a compact form. We apply the developed approach to analyse chiral harmonic generation in nonlinear metasurfaces with various symmetries including rotational, in-plane mirror, and out-of-plane mirror symmetries. For each of those symmetries, we confirm the results of the developed analysis by full-wave numerical calculations. We believe our results provide a new paradigm for engineering nonlinear optical properties of metasurfaces which may find applications in active and nonlinear optics, biosensing, and quantum information processing.

physics.optics↗

arXiv2023

Nonlinear chiral metaphotonics

We review the physics and some applications of photonic structures designed for the realisation of strong $\textit{nonlinear chiroptical response}$. We pay much attention to the recent strategy of utilizing different types of $\textit{optical resonances}$ in metallic and dielectric subwavelength structures and metasurfaces, including surface plasmon resonances, Mie resonances, lattice guided modes, and bound states in the continuum. We summarize earlier results and discuss more recent developments for achieving large circular dichroism combined with the high efficiency of nonlinear harmonic generation.

physics.optics↗

arXiv2022

Resonant chiral effects in nonlinear dielectric metasurfaces

We study the resonant enhancement of linear and nonlinear chiroptical effects in asymmetric silicon metasurfaces supporting multipolar Mie resonances and quasi-bound states in the continuum (quasi-BICs). We demonstrate theoretically and observe in experiment the pronounced linear circular dichroism at the quasi-BIC resonances. We further find that both local field enhancement and third-harmonic signal are large for Mie resonances and some quasi-BIC modes. We explain the selectivity of the nonlinear enhancement by employing the concept of critical coupling being more favorable for the modes with moderately large radiative quality factors ($Q$ factors). We demonstrate experimentally strong nonlinear chiroptical response associated with high efficiency of the third-harmonic generation and large nonlinear circular dichroism varying from $+0.918\pm0.049$ to $-0.771\pm0.004$ for the samples with different asymmetries. We believe our results suggest a general strategy for engineering nonlinear chiroptical response in dielectric resonant metasurfaces.

physics.optics↗

arXiv2022

Nonlinearity-induced optical torque

Optically-induced mechanical torque leading to the rotation of small objects requires the presence of absorption or breaking cylindrical symmetry of a scatterer. A spherical non-absorbing particle cannot rotate due to the conservation of the angular momentum of light upon scattering. Here, we suggest a novel physical mechanism for the angular momentum transfer to non-absorbing particles via nonlinear light scattering. The breaking of symmetry occurs at the microscopic level manifested in nonlinear negative optical torque due to the excitation of resonant states at the harmonic frequency with higher projection of angular momentum. The proposed physical mechanism can be verified with resonant dielectric nanostructures, and we suggest some specific realizations.

physics.optics↗

arXiv2022

Bound states in the continuum in photonic structures

Bound states in the continuum provide a remarkable example of how a simple problem solved about a century ago in quantum mechanics can drive the research on a whole spectrum of resonant phenomena in wave physics. Due to their huge radiative lifetime, bound states in the continuum have found multiple applications in various areas of physics devoted to wave processes, including hydrodynamics, atomic physics, and acoustics. In this review paper, we present a comprehensive description of bound states in the continuum and related effects, focusing mainly on photonic dielectric structures. We review the history of this area, basic physical mechanisms in the formation of bound states in the continuum, and specific examples of structures supporting such states. We also discuss their possible applications in optics, photonics, and radiophysics.

physics.optics↗