arXiv subjects

Constantinos Valagiannopoulos

Publications and source records attributed to Constantinos Valagiannopoulos.

9 recordsLinked to original sources

arXiv2026

Maximum non reciprocity in metasurfaces of gyrotropic rods

Efficiently breaking time-reversal symmetry at the subwavelength scale remains a cornerstone challenge for advanced electromagnetic wave manipulation. This work presents a rigorous analytical framework, based on cylindrical wave expansion, to investigate and optimize the nonreciprocal scattering of transverse electric waves by magnetically biased plasmonic rods. An intuitive metric is introduced to quantify the breaking of time-reversal symmetry via the asymmetric lifting of degeneracy between azimuthal modes of opposite angular momentum, hosted by the gyrotropic particles. Leveraging this metric, a comprehensive mapping of the multiparametric space of operational frequency, cyclotron frequency, and cylinder optical size isolates regimes of maximum nonreciprocity. A detailed multipolar decomposition reveals that this extreme behavior stems from the phase-matched asymmetric excitation and interference of localized electric and magnetic dipole modes. Moving from individual, isolated meta-atoms to collective photonic systems, the optimized cylinders are arranged into a periodic grating. Under oblique incidence, the combination of geometric asymmetry and magnetic mode splitting, forces the metasurface to transmit light in a totally different way when excited by opposite sides. The reported findings and design principles offer a versatile blueprint for the development of dynamically tunable flat-optics isolators, directional transceivers, and advanced wavefront routers.

physics.optics↗

arXiv2025

Maximal Electromagnetic Coupling Between Arbitrary-Shaped Nanotubes

The interaction of electromagnetic waves with pairs of nanotubes having arbitrary-shaped cross sections is investigated. The study starts by thoroughly examining nanotube couples with circular boundaries to identify the structural, textural and excitation parameters that enhance the electric field concentration. Such an initial step generates a design space across which the regions of interest are determined to formulate a shape optimization problem targeting at the maximization of the signal internally to the nanotubes by modifying their boundary surface while maintaining the values for the remaining operational parameters. An IsoGeometric-Analysis-based Boundary Element Method (IGABEM) is employed for estimating the electric field around nanotube boundaries, which is subsequently used towards the overall solution of the boundary value problem. A combination of global and local optimizers along with geometric parametric models that generate valid nanotube cross sections complement the IGABEM method in the quest of optimal designs. The achieved optimal shapes attain substantial boost in the concentration of the electric field which can easily exceed, by 30 times or more, the corresponding value obtained by circular nanotube pairs. This superior performance is maintained for a large range of wave angles and nanotube areas and, accordingly, the robustness of the obtained results and their potential applicability in a wide range of applications, is demonstrated. Finally, the computational method developed in this work can be easily extended for analyzing a finite array of nanotubes, while insights gained by the reported optimal shapes pave the way for their optimization and fine tuning in collective setups like electromagnetic gratings and photonic metasurfaces.

physics.optics↗

arXiv2025

Electron Acceleration in Carbon Nanotubes

Wakefield wavelengths associated with solid-state plasmas greatly limit the accelerating length. An alternative approach employs 2D carbon-based nanomaterials, like graphene or carbon nanotubes (CNTs), configured into structured targets. These nanostructures are designed with voids or low-density regions to effectively reduce the overall plasma density. This reduction enables the use of longer-wavelength lasers and also extends the plasma wavelength and the acceleration length. In this study, we present, to our knowledge, the first numerical demonstration of electron acceleration via self-injection into a wakefield bubble driven by an infrared laser pulse in structured CNT targets, similar to the behavior observed in gaseous plasmas for LWFA in the nonlinear (or bubble) regime. Using the PIConGPU code, bundles of CNTs are modeled in a 3D geometry as 25 nm-thick carbon tubes with an initial density of $10^{22}$ cm$^{-3}$. The carbon plasma is ionized by a three-cycle, 800 nm wavelength laser pulse with a peak intensity of $10^{21}$ W cm$^{-2}$, achieving an effective plasma density of $10^{20}$ cm$^{-3}$. The same laser also drives the wakefield bubble, responsible for the electron self-injection and acceleration. Simulation results indicate that fs-long electron bunches with hundreds of pC charge can be self-injected and accelerated at gradients exceeding 1~TeV$/$m. Both charge and accelerating gradient figures are unprecedented when compared with LWFA in gaseous plasma.

physics.acc-ph↗

arXiv2022

Electromagnetic fields between moving mirrors: Singular waveforms inside Doppler cavities

Phenomena of wave propagation in dynamically varying structures have reemerged as the temporal variations of the medium's properties can extend the possibilities for electromagnetic wave manipulation. While the dynamical change of the electromagnetic medium's properties is a difficult task, the movement of scatterers is not. In this paper, we analyze the electromagnetic fields trapped inside two smoothly moving mirrors. We employ the method of characteristics and take into account the relativistic phenomena to show that the temporally and spatially local Doppler effects can filter and amplify the electromagnetic signal, tailoring the $k-$ and $ω-$content of the transients. It is shown using the Doppler factor and the change of the distance between neighbor characteristics that the dynamical movement of the boundaries can lead to condensated characteristics resulting in field amplification or dilution of the characteristics resulting in the attenuation of the signal. In the case of periodically moving mirrors the field distribution is shown that asymptotically leads to exponentially growing delta-like wave packets at discrete points of space with a limiting number of peaks due to the fact that the velocity of the mechanical vibrations can not exceed that of light. The theoretical analysis is also verified by FDTD simulations and is connected with the theory of mode locking.

physics.optics↗

arXiv2021

Nanotubes as Sinks for Quantum Particles

Nanotubes with proper thickness, size and texture make ultra-efficient sinks for the quantum particles traveling into specific background media. Several optimal semiconducting cylindrical layers are reported to achieve enhancement in the trapping of matter waves by 2-3 orders of magnitude. The identified shells can be used as pieces in quantum devices that involve the focusing of incident beams from charge pumps and superconducting capacitors to radiation pattern controllers and matter-wave lenses.

physics.app-ph↗

arXiv2021

Metasurface-Coated Devices: A New Paradigm for Energy-Efficient and Secure 6G Communications

The sixth-generation (6G) era comes with the challenge of offering highly energy-efficient and autonomous communications securely. In this direction, we report energy efficiency (EE), energy harvesting (EH), and secure performance by employing power-collecting metasurface-coated devices capable of supporting ultra-low-power (ULP) transmissions. Contrary to reconfigurable intelligent surfaces (RIS), where the reflected signal can be combined at the receiver by being treated as transmitted from a relay, the proposed metasurface claddings can be deployed at either or both the transmitter and receiver. The passive metasurface-coated devices can achieve ultra-high EE and EH besides the signal detection, combined with an enhanced secrecy rate at the legitimate user and/or improved spying capabilities of the eavesdroppers under ULP transmission. To quantify their efficiency, we provide a holistic model for the utilization of the metasurface shells. Building upon the aforementioned model, preliminary results are presented that reveal the unprecedented superiority of the proposed concept compared to the RIS paradigm. Additionally, we enumerate the main advantages of the new concept and define its role in the 6G era. Finally, possible research directions are discussed.

eess.SP↗

arXiv2020

Maximal Interaction of Electromagnetic Radiation with Corona-Virions

Absorption and scattering of the impinging electromagnetic waves are the two fundamental operations describing the energy exchange of any, organic or inorganic, particle with its environment. In the case of virion cells, substantial extinction power, counting both absorbing and scattering effects, is a prerequisite for performing a variety of coupling actions against the viral particles and, thus, a highly sought-after feature. By considering realistic dispersion for the dielectric permittivity of proteins and a core-shell modeling allowing for rigorous formulation via Mie theory, we report optical extinction resonances for corona-virions at mid-infrared range that are not significantly perturbed by changes in the objects size or the background host. Our findings indicate the optimal regime for interaction of photonic radiation with viral particles and may assist towards the development of equipment for thermal damage, disintegration or neutralization of coronavirus cells.

physics.app-ph↗

arXiv2019

Enhanced stability, bistability, and exceptional points in saturable active photonic couplers

A generic photonic coupler with active and lossy parts, gain saturation and asymmetric characteristics is examined. Saturable activity is shown to be able to enhance the overall stability of the steady states, prevent evolution to undesirable unbounded modes and allow for bistable operation in specific regions of parametric space. Both stability and bistability are studied in the phase space of the system, where the basins of attraction of each state are identified, providing an accurate description of the dependence of the electric fields on the initial conditions. Continuous families of exceptional points are detected via suitable regulation of the coupling and asymmetry features of the configuration. In this way, a complete description of the nonlinear dynamics landscape is provided, which should be crucial for multiple application-driven designs incorporating such a ubiquitous optical component.

physics.optics↗

arXiv2019

Metasurface-Enabled Interference Suppression at Visible-Light Communications

Light can be used for wireless information transmission apart from illumination; that is the key idea behind visible-light communication (VLC), one of the disruptive technologies of our days. It combines remarkably high data rates due to ultrashort wavelengths with huge reliability and security due to small distances; nevertheless, it substantially suffers from interference of neighboring light sources in multiple-link configurations. In this manuscript, we investigate a pair of light emitting diodes (LEDs) interfering each other and propose a simple nanoslit metasurface that radically increases the directivity of the transmitting beams. As a result, enhancement of the signal-to-interference ratio by several orders of magnitude is reported. The considered generic setup retains its beneficial features in the presence of realistic design defects and, accordingly, may inspire standardization efforts towards the adoption of VLC in next-generation heterogeneous communication networks.

eess.SP↗