arXiv subjects

Gennady Shvets

Publications and source records attributed to Gennady Shvets.

At least 19 recordsLinked to original sources

arXiv2026

Monoenergetic acceleration of charge-neutralized ion bunches to GeV-scale energies by the combination of a high-current electron beam and an ionization front

Compact heavy ion accelerators have numerous applications, ranging from heavy ion fusion to carbon ion radiotherapy, and testing radiation-hardened electronics. The demand could be met by developing high-gradient traveling wave plasma accelerators of high-charge ($\sim\mu\mathrm{C}$) relativistic ion beams. We will discuss a novel ion acceleration regime -- Counter-propagating ionization Front Acceleration (CFA) -- utilizing counter-propagating Ionization Front (IF) and high-current Relativistic Electron Beam (REB). Theoretical modeling and 3D PIC simulations demonstrate the possibility of using typical REBs produced by induction voltage adders propagating through a gas-filled tube undergoing laser ionization to achieve acceleration gradients in excess of $\sim 250 {\rm MeV/m}$ while accelerating micro-Coulombs of ions over meters distance. A unique energy conversion mechanism -- from the REB to electromagnetic fields to the ions is discussed, as well as the limits on the accelerated ions charge and the degree of its neutralization, acceleration gradient, and ion energy spread.

physics.plasm-ph↗

arXiv2026

Probing Azimuthal Anatomy of Hyperbolic Whispering Gallery Modes in hBN

Scattering-type scanning near-field optical microscopy (s-SNOM) is a powerful tool for investigating polaritonic modes. However, an inherent limitation of this technique is that excitation and detection occur at the same location. This constraint makes it challenging to resolve excitations with complex spatial structures, which can exhibit delicate dependence on the in-coupling conditions. Here, we present a strategy to overcome this limitation by introducing an auxiliary cavity, which serves as a stationary near-field excitation source. This configuration allows the s-SNOM tip to act solely as a detector, and decouples excitation from detection. We apply this approach to whispering gallery modes (WGMs) of hyperbolic phonon-polaritons in hexagonal boron nitride resonators. Through spatially resolved near-field maps we directly observe subwavelength polaritonic WGMs with large and discrete azimuthal momentum ($k_\phi / k_0$ up to 15). This allows us to map the frequency and angular behavior of the modes. Notably, we observe dynamic tuning of the effective refractive index by the WGMs to preserve consistent azimuthal momentum $k_\phi$ under varying excitation conditions. Numerical simulations support the experimental observations and confirm the observation of hyperbolic WGMs. This approach enables direct visualization of previously hidden mode structures in hyperbolic cavities and opens new pathways for momentum-controlled polaritonic device engineering.

cond-mat.mes-hall↗

arXiv2026

Generation and Enhancement of Persistent Nanoscale Magnetization in All-Dielectric Metasurfaces by Optically Injected and Localized Free Carriers

Time-varying dielectric metasurfaces that support sharp optical resonances with nontrivial electromagnetic field distributions constitute a unique platform for realizing temporal interfaces for metasurface-guided waves (MGWs). Rapidly changing metasurface resonance enables frequency conversion and temporal scattering of a concurrently propagating MGW. Using analytical methods and electromagnetic simulations, free carriers are generated locally to create frequency-shifted infrared MGWs. Such time interfaces can be utilized to generate large, highly localized quasistatic magnetic fields within the metasurfaces. The resulting nanoscale magnetization, supported by the residual circulating currents, persists for several optical cycles after the departure of the time-scattered MGWs. During the rectification process, the initial electromagnetic energy of the injected MGWs is partitioned between the temporally scattered MGWs, the residual motion of the free carriers, and a quasistatic magnetic field.

physics.optics↗

arXiv2025

Propagation and collisionless damping of topologically-protected surface plasma waves in non-uniformly magnetized plasma columns

Recent theoretical studies revealed the existence of topologically-protected surface plasma waves (TSPWs) in cold magnetized plasmas assumed uniform along the direction of a uniaxial magnetic field. Reflections-free propagation of the TSPWs along arbitrarily-shaped plasma boundaries oriented perpendicularly to the magnetic field was shown to be preserved even when their collisionless damping by localized upper-hybrid resonances was accounted for. Here we extend this theory to the realistic case of three-dimensional magnetic field produced by finite-sized magnetic coils. We demonstrate that when TSPWs are launched in the direction of the decreasing magnetic field, they are collisionlessly absorbed within a highly localized (evanescent) region as they propagate away from the magnetic coil. We show that the resulting wave reflection can be negligible -- in clear contrast with conventional wave reflection from the corresponding evanescence regions.

physics.plasm-ph↗

arXiv2025

Simultaneous amplification and shaping of excimer lasers using Stimulated Brillouin Scattering in the strongly damped limit

Attaining practical Inertial Fusion Energy (IFE) depends on how efficiently one can couple the driver energy to the nuclear fusion fuel for compression and ignition. While the excimer lasers provide an efficient alternative compared to existing laser technology, it is unclear how the lasers can be harnessed to form a pulse with desired pulse shape and intensity. Stimulated Brillouin Scattering (SBS) provides a path to compressing long, energetic pulses to short intense ones. We consider the equations governing SBS in the Strongly Damped Limit (SDL) and find that it is possible to almost completely specify the final pulse shape by providing an appropriate initial seed pulse. We provide analytic expressions for reverse-engineering the initial seed shape and delineate physical limits concerning the prepulse level.

physics.optics↗

arXiv2025

Nontrivial low-frequency topological waves at the boundary of a magnetized plasma

The topological properties of a magnetized cold gaseous plasma have recently been explored and the existence of topologically protected edge states has been established. These studies are limited to a magnetized plasma, where ions are infinitely massive and provide a neutralizing background. When ion motion is included, a new class of low-frequency unidirectional topological waves (TSPWs) emerges in the dispersion relation. The group velocity of these waves is in the opposite direction of high-frequency topological electron waves for a given magnetic field direction. The Berry curvature and Chern numbers are calculated to establish nontrivial topological phase. Additionally, we demonstrate a unique characteristic of ion dominated TSPW propagating above the ion cyclotron frequency: their collisionless damping via coupling to the continuum of lower-hybrid resonant modes localized inside a smooth plasma-vacuum interface. These finding broadens the possible applications and observations of these exotic excitations in space and laboratory plasmas.

physics.plasm-ph↗

arXiv2025

Co-existing topological and Volkov-Pankratov plasmonic edge states in magnetized graphene

Graphene placed in a perpendicular magnetic field supports optical modes known as magnetoplasmons which are transversally confined to the graphene layer. Unlike ordinary graphene plasmons, these magnetoplasmonic surface waves are characterized by a band gap corresponding to the cyclotron frequency. In addition, these magnetoplasmon bands are topological, characterized by a non-zero Chern number. This leads to the existence of topologically protected edge states at domain edges where the Chern number changes. Since the Chern number is dependent on the direction of the magnetic field, edge states exist at domain edges across which the magnetic field flips direction. Physically, the magnetic field can only flip direction at gradual domain edges with finite width creating topological heterojunctions. These topological heterojunctions support extra edge states known as Volkov-Pankratov edge states which can enter the band gap and support propagation in both directions. The number of Volkov-Pankratov states at a heterojunction varies as a function of the width of the gradual domain edge.

cond-mat.mes-hall↗

arXiv2025

Broadband localization of light at the termination of a topological photonic waveguide

Localized optical field enhancement enables strong light-matter interactions necessary for efficient manipulation and sensing of light. Specifically, tunable broadband energy localization in nanoscale hotspots offers a wide range of applications in nanophotonics and quantum optics. We experimentally demonstrate a novel principle for the local enhancement of electromagnetic fields, based on strong suppression of backscattering. This is achieved at a designed termination of a topologically non-trivial waveguide that nearly preserves the valley degree of freedom. The symmetry origin of the valley degree of freedom prevents edge states to undergo intervalley scattering at waveguide discontinuities that obey the symmetry of the crystal. Using near-field microscopy, we reveal that this can lead to strong confinement of light at the termination of a topological photonic waveguide, even without breaking time-reversal symmetry. We emphasize the importance of symmetry conservation by comparing different waveguide termination geometries, confirming that the origin of suppressed backscattering lies with the near-conservation of the valley degree of freedom, and show the broad bandwidth of the effect.

physics.optics↗

arXiv2024

Analytic model of stable shock-like structures in laser interaction with underdense plasma for identifying of phase and polarization dependent regime of laser wakefield accelerators

We present an analytical model describing a stable shock-like structure that is formed when an ultra-intense laser propagates through an underdense plasma. It is shown that such structures exist in a wide range of laser-plasma parameters, with a unique sub-luminal shock front velocity for each parameter. Numerical methods to accurately describe such shock-front is developed. The formalism is applied to describe the parameter space in which the Carrier-Envelope-Phase (CEP) effect under which phase and polarization dependent super-ponderomotive effects becomes significant. The developed formalism will enable quick identification of regimes in which CEP effects become significant, expediting designing of Laser Wakefield Accelerators operating in the superponderomotive regime.

physics.plasm-ph↗

arXiv2024

Acceleration and focusing of multispecies ion beam using a converging laser-driven shock

We demonstrate an ion acceleration scheme capable of simultaneously focusing and accelerating a multispecies ion beam with monoenergetic spectra to a few micron radius. The focal length and ion mean energy can be independently controlled: the former by using a different front-surface shape and the latter by tuning the laser-plasma parameters. We interpret the results using simple models and validate the results using first-principles simulations. The scheme is applicable to different laser transverse profiles and multi-ion species target, and limiting factors for the ion focusing are delineated. The generated ion beam exhibits high charge, low emittance, and high energy flux and is of interest to various applications including Inertial Confinement Fusion (ICF), high flux neutron generation, and biomedical applications.

physics.plasm-ph↗

arXiv2024

Topological Directional Coupler

Interferometers and beam splitters are fundamental building blocks for photonic neuromorphic and quantum computing machinery. In waveguide-based photonic integrated circuits, beam-splitting is achieved with directional couplers that rely on transition regions where the waveguides are adiabatically bent to suppress back-reflection. We present a novel, compact approach to introducing guided mode coupling. By leveraging multimodal domain walls between microwave topological photonic crystals, we use the photonic-spin-conservation to suppress back-reflection while relaxing the topological protection of the valley degree of freedom to implement tunable beam splitting. Rapid advancements in chip-scale topological photonics suggest that the proposed simultaneous utilization of multiple topological degrees of freedom could benefit the development of novel photonic computing platforms.

cond-mat.mes-hall↗

arXiv2024

Arbitrary control over multimode wave propagation for machine learning

Controlled multimode wave propagation can enable more space-efficient photonic processors than architectures based on discrete components connected by single-mode waveguides. Instead of defining discrete elements, one can sculpt the continuous substrate of a photonic processor to perform computations through multimode interference in two dimensions. Here we designed and demonstrated a device with a refractive index that can be rapidly reprogrammed across space, allowing arbitrary control of wave propagation. The device, a two-dimensional programmable waveguide, uses parallel electro-optic modulation of the refractive index of a slab waveguide with about $10^4$ programmable spatial degrees of freedom. We implemented neural network inference on benchmark tasks with up to $49$-dimensional vectors in a single pass, without digital pre-processing or post-processing. Theoretical and numerical analyses further indicated that two-dimensional programmable waveguides may offer not only a constant-factor reduction in device area but also a scaling benefit, with the area required growing as $N^{1.5}$ rather than $N^2$.

physics.optics↗

arXiv2023

High quality nanocavities through multimodal confinement of hyperbolic polaritons in hexagonal boron nitride

A conventional optical cavity supports modes which are confined because they are unable to leak out of the cavity. Bound state in continuum (BIC) cavities are an unconventional alternative, where light can leak out, but is confined by multimodal destructive interference. BICs are a general wave phenomenon, of particular interest to optics, but BICs and multimodal interference have never been demonstrated at the nanoscale. Here, we demonstrate the first nanophotonic cavities based on BIC-like multimodal interference. This novel confinement mechanism for deep sub-wavelength light shows orders of magnitude improvement in several confinement metrics. Specifically, we obtain cavity volumes below 100x100x3nm^3 with quality factors about 100, with extreme cases having 23x23x3nm^3 volumes or quality factors above 400. Key to our approach, is the use of pristine crystalline hyperbolic dispersion media (HyM) which can support large momentum excitations with relatively low losses. Making a HyM cavity is complicated by the additional modes that appear in a HyM. Ordinarily, these serve as additional channels for leakage, reducing cavity performance. But, in our experiments, we find a BIC-like cavity confinement enhancement effect, which is intimately related to the ray-like nature of HyM excitations. In fact, the quality factors of our cavities exceed the maximum that is possible in the absence of higher order modes. The alliance of HyM with BICs in our work yields a radically novel way to confine light and is expected to have far reaching consequences wherever strong optical confinement is utilized, from ultra-strong light-matter interactions, to mid-IR nonlinear optics and a range of sensing applications.

physics.optics↗

arXiv2023

Deep Subwavelength Topological Edge State in a Hyperbolic Medium

Topological nanophotonics presents the potential for cutting-edge photonic systems, with a core aim revolving around the emergence of topological edge states. These states are primed to propagate robustly while embracing deep subwavelength confinement that defies diffraction limits. Such attributes make them particularly appealing for nanoscale applications, where achieving these elusive states has remained challenging. We unveil the first experimental proof of deep subwavelength topological edge states by implementing periodic modulation of hyperbolic phonon polaritons within a Van der Waals heterostructure. This finding represents a significant milestone in the field of nanophotonics, and it can be directly extended to and hybridized with other Van der Waals materials in various applications. The extensive scope for material substitution facilitates broadened operational frequency ranges, streamlined integration of diverse polaritonic materials, and compatibility with electronic and excitonic systems.

physics.optics↗

arXiv2022

Transverse hypercrystals formed by periodically modulated phonon-polaritons

Photonic crystals and metamaterials are two overarching paradigms for manipulating light. Combining the two approaches leads to hypercrystals: hyperbolic dispersion metamaterials that undergo periodic modulation and mix photonic-crystal-like aspects with hyperbolic dispersion physics. So far, there has been limited experimental realization of hypercrystals due to various technical and design constraints. Here, we create nanoscale hypercrystals with lattice constants ranging from 25 nm to 160 nm and measure their collective Bloch modes and dispersion with scattering nearfield microscopy. We demonstrate for the first time dispersion features such as negative group velocity, indicative of bandfolding, and signatures of sharp density of states peaks, expected for hypercrystals (and not for ordinary polaritonic crystals). These density peaks connect our findings to the theoretical prediction of an extremely rich hypercrystal bandstructure emerging even in geometrically simple lattices. These features make hypercrystals both fundamentally interesting, as well as of potential use to engineering nanoscale light-matter interactions.

cond-mat.mes-hall↗

arXiv2022

Polarization control of electron injection and acceleration in the plasma by a self steepening laser pulse

We describe an interplay between two injection mechanism of background electrons into an evolving plasma bubble behind an intense laser pulse: one due to the overall bubble expansion, and another due to its periodic undulation. The two mechanisms occur simultaneously when an intense laser pulse propagating inside a plasma forms a shock like steepened front. Periodic undulations of the plasma bubble along the laser propagation path can either inhibit or conspire with electron injection due to bubble expansion. We show that Carrier Envelope Phase (CEP) controlled plasma bubble undulation induced by the self steepening laser pulse produces a unique electron injector: Expanding Phase controlled Undulating Bubble (EPUB). The longitudinal structure of the electron bunch injected by the EPUB can be controlled by laser polarization and power, resulting in high-charge (multiple nano Coulombs) high-current (tens of kilo Amperes) electron beams with ultra short (femtosecond scale) temporal structure. Generation of high energy betatron radiation with polarization and CEP controlled energy spectrum and angular distribution is analyzed as a promising application of EPUB produced beams.

physics.acc-ph↗

arXiv2022

WAND-PIC: an accelerated three-dimensional quasi-static particle-in-cell code

We introduce a quasi-static particle-in-cell (PIC) code -- WAND-PIC -- which does not suffer from some of the common limitations of many quasi-static PICs, such as the need for a predictor-corrector method in solving electromagnetic fields. We derive the field equations under quasi-static (QS) approximation and find the explicit form of the "time" derivative of the transverse plasma current. After that, equations for the magnetic fields can be solved exactly without using the predictor-corrector method. Algorithm design and code structure are thus greatly simplified. With the help of explicit quasi-static equations and our adaptive step size, plasma bubbles driven by the large beam charges can be simulated efficiently without suffering from the numerical instabilities associated with the predictor-corrector method. In addition, WAND-PIC is able to simulate the sophisticated interactions between high-frequency laser fields and beam particles through the method of sub-cycling. Comparisons between the WAND-PIC and a first-principle full PIC code (VLPL) are presented. WAND-PIC is open-source, fully three-dimensional, and parallelized with the in-house multigrid solver. Scalability, time complexity, and parallelization efficiency up to thousands of cores are also discussed in this work.

physics.plasm-ph↗

arXiv2022

Mode-selective Single-dipole Excitation and Controlled Routing of Guided Waves in a Multi-mode Topological Waveguide

Topology-linked binary degrees of freedom of guided waves have been used to expand the channel capacity of and to ensure robust transmission through photonic waveguides. However, selectively exciting optical modes associated with the desired degree of freedom is challenging and typically requires spatially extended sources or filters. Both approaches are incompatible with the ultimate objective of developing compact mode-selective sources powered by single emitters. In addition, the implementation of highly desirable functionalities, such as controllable distribution of guided modes between multiple detectors, becomes challenging in highly-compact devices due to photon loss to reflections. Here, we demonstrate that a linearly-polarized dipole-like source can selectively excite a topologically robust edge mode with the desired valley degree of freedom. Reflection-free routing of valley-polarized edge modes into two spatially-separated detectors with reconfigurable splitting ratios is also presented. An optical implementation of such a source will have the potential to broaden the applications of topological photonic devices.

physics.optics↗