Ihar Faniayeu

We numerically investigate optical chirality generation within the interior volume of plasmonic gold helices operating in the mid‐infrared spectral range. Rather than relying on polarization or handedness switching, our approach uses geometric tuning of the helix height to achieve electromagnetic balance between the induced electric and magnetic dipole moments. By comparing single‐turn and bifilar helices, we show that electromagnetically balanced geometries generate enhanced volume‐averaged optical chiral density with comparable magnitude and sign for selected linear and circular polarization excitations. The bifilar helix provides an optical chirality enhancement approximately one order of magnitude larger than that of the single‐turn helix due to stronger electric–magnetic dipolar coupling and increased field confinement. We further demonstrate that increasing the bifilar helix height modifies the dipole balance and can reverse the sign of the optical chirality without changing the structural handedness. These results establish electromagnetically balanced plasmonic helices as a geometry‐controlled platform for enhanced chiral near‐field generation in the mid‐infrared.

Multifunctional metasurfaces are commonly realized using cascaded or few‐layer architectures, in which each layer performs a dedicated electromagnetic function. Although such designs offer increased degrees of freedom and high efficiency, they inevitably increase overall thickness and fabrication complexity. Achieving comparable multifunctional performance within a single‐layer metasurface therefore constitutes a more compact and practically attractive solution. Here, we show that a five‐function cascaded metasurface can be systematically collapsed into a single‐layer metasurface while preserving electromagnetic functionality and efficiency. The proposed design employs a helix‐inspired supercell architecture that integrates multiple frequency‐selective meta‐atoms within a single planar layer. Each meta‐atom operates independently at a designated frequency, enabling polarization‐selective reflection and transmission, circular‐polarization conversion, and absorption in an ultrathin layer. Full‐wave simulations confirm that the single‐layer metasurface reproduces the performance of its cascaded counterpart with high efficiency, polarization robustness, and angular stability across all operating bands. These results establish an approach for translating cascaded metasurface architectures into single‐layer implementations, providing a scalable pathway toward ultracompact multifunctional electromagnetic devices.

The nonreciprocal magnetoelectric effect in Tellegen materials enables exotic phenomena such as axion-modified electrodynamics and fosters the development of magnet-free nonreciprocal media. As the nonreciprocal counterpart to the well-known chiral electromagnetic response, it offers a parallel framework in which many concepts developed for chiral materials can be translated to Tellegen media, potentially unlocking new avenues for fundamental studies and applications. Although predicted over 75 years ago and observed in only a handful of natural materials with very low strength, the strong optical Tellegen effect has remained experimentally elusive. Here, we report the first experimental demonstration of a resonant optical diagonal Tellegen effect in a metasurface, showcasing a response that is 100 times greater than that of any known natural material. This optical metasurface, consisting of randomly distributed cobalt-silicon nanoscatterers with strong shape anisotropy, utilizes spontaneous magnetization to achieve a robust Tellegen effect without the need for an external magnetic field. In addition to the Tellegen response, the metasurface exhibits both gyroelectric and gyromagnetic effects, contributing to nonreciprocal cross-polarized light reflection. We introduce a technique to independently extract the amplitudes of these three effects using conventional magneto-optical single-side-illumination measurements. The observation of the resonant Tellegen effects in the optical frequency range may lead to the experimental observation of axionic electrodynamics and compact bias-free nonreciprocal optical devices.

We propose a multifunctional cascade metasurface designed for manipulating electromagnetic waves in the microwave frequency range with comprehensive theoretical analysis, design optimization, and experimental validation. A set of electrically thin metasurfaces based on planar spirals with free space impedance is used to manipulate electromagnetic waves. The metasurfaces simultaneously generate distinct, independent, and mutually compatible wave transformations at various frequencies. A multifunctional cascade metasurface integrates several functional devices, including a reflective cross‐polarizer, a transmissive cross‐polarizer, and a nonreflective absorber. Each designed metasurface has a free space impedance over a sufficiently wide frequency range that encompasses the operating frequencies of all metasurfaces. This attribute guarantees the autonomous and mutually noninterfering functioning of all metasurfaces in the cascade.

Optical nonreciprocity is manifested as a difference in the transmission of light for the opposite directions of excitation. Nonreciprocal optics is traditionally realized with relatively bulky components such as optical isolators based on the Faraday rotation, hindering the miniaturization and integration of optical systems. Here we demonstrate free-space nonreciprocal transmission through a metasurface comprised of a two-dimensional array of nanoresonators made of silicon hybridized with vanadium dioxide (VO₂). This effect arises from the magneto-electric coupling between Mie modes supported by the resonator. Nonreciprocal response of the nanoresonators occurs without the need for external bias; instead, reciprocity is broken by the incident light triggering the VO₂ phase transition for only one direction of incidence. Nonreciprocal transmission is broadband covering over 100 nm in the telecommunication range in the vicinity of λ = 1.5 µm. Each nanoresonator unit cell occupies only ~0.1 λ³ in volume, with the metasurface thickness measuring about half-a-micron. Our self-biased nanoresonators exhibit nonreciprocity down to very low levels of intensity on the order of 150 W/cm² or a µW per nanoresonator. We estimate picosecond-scale transmission fall times and sub-microsecond scale transmission rise. Our demonstration brings low-power, broadband and bias-free optical nonreciprocity to the nanoscale.

The nonreciprocal magnetoelectric effect, also known as the Tellegen effect, promises a number of groundbreaking phenomena connected to fundamental (e.g., electrodynamics of axion and relativistic matter) and applied physics (e.g., magnetless isolators). We propose a three-dimensional metamaterial with an isotropic and resonant Tellegen response in the visible frequency range. The metamaterial is formed by randomly oriented bi-material nanocylinders in a host medium. Each nanocylinder consists of a ferromagnet in a single-domain magnetic state and a high-permittivity dielectric operating near the magnetic Mie-type resonance. The proposed metamaterial requires no external magnetic bias and operates on the spontaneous magnetization of the nanocylinders. By leveraging the emerging magnetic Weyl semimetals, we further show how a giant bulk effective magnetoelectric effect can be achieved in a proposed metamaterial, exceeding that of natural materials by almost four orders of magnitude.

Nonreciprocal devices, including optical isolators, phase shifters, and amplifiers, are pivotal for advanced optical systems. However, exploiting natural materials is challenging due to their weak magneto‐optical (MO) effects, requiring substantial thickness to construct effective optical devices. In this study, it is demonstrated that subwavelength metasurfaces supporting bound states in the continuum (BICs) and made of conventional ferrimagnetic material can exhibit strong nonreciprocity in the Faraday configuration and near‐unity magnetic circular dichroism (MCD). These metasurfaces enhance the MO effect by 3–4 orders of magnitude compared to a continuous film of the same material. This significant enhancement is achieved by leveraging Huygens' condition in the metasurface whose structural units support paired electric and magnetic dipole resonances. The multi‐mode temporal coupled mode theory (CMT) is developed for the observed enhancement of the MO effect, and the findings with the full‐wave simulations are confirmed.

Tunable metal–insulator–metal (MIM) Fabry–Pérot (FP) cavities that can dynamically control light enable novel sensing, imaging and display applications. However, the realization of dynamic cavities incorporating stimuli‐responsive materials poses a significant engineering challenge. Current approaches rely on refractive index modulation and suffer from low dynamic tunability, high losses, and limited spectral ranges, and require liquid and hazardous materials for operation. To overcome these challenges, a new tuning mechanism employing reversible mechanical adaptations of a polymer network is proposed, and dynamic tuning of optical resonances is demonstrated. Solid‐state temperature‐responsive optical coatings are developed by preparing a monodomain nematic liquid crystalline network (LCN) and are incorporated between metallic mirrors to form active optical microcavities. LCN microcavities offer large, reversible and highly linear spectral tuning of FP resonances reaching wavelength‐shifts up to 40 nm via thermomechanical actuation while featuring outstanding repeatability and precision over more than 100 heating–cooling cycles. This degree of tunability allows for reversible switching between the reflective and the absorbing states of the device over the entire visible and near‐infrared spectral regions, reaching large changes in reflectance with modulation efficiency ΔR = 79%.