arXiv subjects

M. Kafesaki

Publications and source records attributed to M. Kafesaki.

12 recordsLinked to original sources

arXiv2026

Taming the single-cylinder scattering through time-modulation -- The role of the modulation phase

Dielectric particles with time-modulated electromagnetic properties exhibit intriguing scattering phenomena, entailing unique response and possibilities in meta-structures made of such particles. In this work, we investigate the scattering properties of an infinitely-long cylinder with periodically time-modulated permittivity. We demonstrate parametric scattering amplification, depending on the strength of the modulation, as well as controllable angular scattering pattern, ranging from enhanced forward to enhanced backward, or even scattering cancellation. This angular tunability is achieved through appropriate control of the modulation phase, highlighting its critical role in multi-modal time-modulated scattering systems.

physics.optics↗

arXiv2022

XR-RF Imaging Enabled by Software-Defined Metasurfaces and Machine Learning: Foundational Vision, Technologies and Challenges

We present a new approach to Extended Reality (XR), denoted as iCOPYWAVES, which seeks to offer naturally low-latency operation and cost-effectiveness, overcoming the critical scalability issues faced by existing solutions. iCOPYWAVES is enabled by emerging PWEs, a recently proposed technology in wireless communications. Empowered by intelligent (meta)surfaces, PWEs transform the wave propagation phenomenon into a software-defined process. We leverage PWEs to i) create, and then ii) selectively copy the scattered RF wavefront of an object from one location in space to another, where a machine learning module, accelerated by FPGAs, translates it to visual input for an XR headset using PWEdriven, RF imaging principles (XR-RF). This makes for an XR system whose operation is bounded in the physical layer and, hence, has the prospects for minimal end-to-end latency. Over large distances, RF-to-fiber/fiber-to-RF is employed to provide intermediate connectivity. The paper provides a tutorial on the iCOPYWAVES system architecture and workflow. A proof-of-concept implementation via simulations is provided, demonstrating the reconstruction of challenging objects in iCOPYWAVES produced computer graphics.

eess.SP↗

arXiv2022

Topological Extraordinary Optical Transmission

The incumbent technology for bringing light to the nanoscale, the near-field scanning optical microscope, has notoriously small throughput efficiencies - of the order of 10^(-4) - 10^(-5), or less. We report on a broadband, topological, unidirectionally-guiding structure, not requiring adiabatic tapering and in principle enabling near-perfect (ideally, ~100%) optical transmission through an unstructured single (POTUS) arbitrarily-subdiffraction slit at its end. Specifically, for a slit width of just lambda_eff / 72 (lambda_0 / 138) the attained normalized transmission coefficient reaches a value of 1.52, while for a unidirectional-only (non-topological) device the normalized transmission through a lambda_eff / 21 (~lambda_0 / 107) slit reaches 1.14 - both, limited only by inherent material losses, and with zero reflection from the slit. The associated, under ideal conditions, near-perfect optical extraordinary transmission (POET) has implications, among diverse areas in wave physics and engineering, for high-efficiency, maximum-throughput nanoscopes and heat-assisted magnetic recording devices.

physics.optics↗

arXiv2022

Multi-functional metasurface architecture for amplitude, polarization and wavefront control

Metasurfaces (MSs) have been utilized to manipulate different properties of electromagnetic waves. By combining local control over the wave amplitude, phase, and polarization into a single tunable structure, a multi-functional and reconfigurable metasurface can be realized, capable of full control over incident radiation. Here, we experimentally validate a multi-functional metasurface architecture for the microwave regime, where in principle variable loads are connected behind the backplane to reconfigurably shape the complex surface impedance. As a proof-of-concept step, we fabricate several metasurface instances with static loads in different configurations (surface mount capacitors and resistors of different values in different connection topologies) to validate the approach and showcase the different achievable functionalities. Specifically, we show perfect absorption for oblique incidence (both polarizations), broadband linear polarization conversion, and beam splitting, demonstrating control over the amplitude, polarization state, and wavefront, respectively. Measurements are performed in the 4-18 GHz range inside an anechoic chamber and show good agreement with theoretically-anticipated results. Our results clearly demonstrate the practical potential of the proposed architecture for reconfigurable electromagnetic wave manipulation.

physics.app-ph↗

arXiv2019

ABSense: Sensing Electromagnetic Waves on Metasurfaces via Ambient Compilation of Full Absorption

Metasurfaces constitute effective media for manipulating and transforming impinging EM waves. Related studies have explored a series of impactful MS capabilities and applications in sectors such as wireless communications, medical imaging and energy harvesting. A key-gap in the existing body of work is that the attributes of the EM waves to-be-controlled (e.g., direction, polarity, phase) are known in advance. The present work proposes a practical solution to the EM wave sensing problem using the intelligent and networked MS counterparts-the HyperSurfaces (HSFs), without requiring dedicated field sensors. An nano-network embedded within the HSF iterates over the possible MS configurations, finding the one that fully absorbs the impinging EM wave, hence maximizing the energy distribution within the HSF. Using a distributed consensus approach, the nano-network then matches the found configuration to the most probable EM wave traits, via a static lookup table that can be created during the HSF manufacturing. Realistic simulations demonstrate the potential of the proposed scheme. Moreover, we show that the proposed workflow is the first-of-its-kind embedded EM compiler, i.e., an autonomic HSF that can translate high-level EM behavior objectives to the corresponding, low-level EM actuation commands.

eess.SP↗

arXiv2010

Magnetic response of nanoscale left-handed metamaterials

Using detailed simulations we investigate the magnetic response of metamaterials consisting of pairs of parallel slabs or combinations of slabs with wires (including the fishnet design) as the length-scale of the structures is reduced from mm to nm. We observe the expected saturation of the magnetic resonance frequency when the structure length-scale goes to the sub-micron regime, as well as weakening of the effective permeability resonance and reduction of the spectral width of the negative permeability region. All these results are explained by using an equivalent resistor-inductor-capacitor (RLC) circuit model, taking into account the current-connected kinetic energy of the electrons inside the metallic parts through an equivalent inductance, added to the magnetic field inductance in the unit-cell. Using this model we derive simple optimization rules for achieving optical negative permeability metamaterials of improved performance. Finally, we analyze the magnetic response of the fishnet design and we explain its superior performance regarding the high attainable magnetic resonance frequency, as well as its inferior performance regarding the width of the negative permeability region.

physics.optics↗

arXiv2005

Spontaneous emission rates of dipoles in photonic crystal membranes

We show theoretically that finite two-dimensional (2D) photonic crystals in thin semiconductor membranes strongly modify the spontaneous emission rate of embedded dipole emitters. Three-dimensional Finite-Difference Time-Domain calculations show over 7 times inhibition and 15 times enhancement of the emission rate compared to the vacuum emission rate for judiciously oriented and positioned dipoles. The vertical index confinement in membranes strongly enhances modifications of the emission rate as compared to vertically unconfined 2D photonic crystals. The emission rate modifications inside the membrane mimic the local electric field mode density in a simple 2D model. The inhibition of emission saturates exponentially as the crystal size around the source is increased, with a $1/e$ length that is inversely proportional to the bandwidth of the emission gap. We obtain inhibition of emission only close to the slab center. However, enhancement of emission persists even outside the membrane, with a distance dependence which dependence can be understood by analyzing the contributions to the spontaneous emission rate of the different vertically guided modes of the membrane. Finally we show that the emission changes can even be observed in experiments with ensembles of randomly oriented dipoles, despite the contribution of dipoles for which no gap exists.

cond-mat.other↗

arXiv2005

Spontaneous emission in the near-field of 2D photonic crystals

We show theoretically that photonic crystal membranes cause large variations in the spontaneous emission rate of dipole emitters, not only inside but also in the near-field above the membranes. Our three-dimensional finite difference time-domain calculations reveal an inhibition of more than five times and an enhancement of more than ten times for the spontaneous emission rate of emitters with select dipole orientations and frequencies. Furthermore we demonstrate theoretically, the potential of a nanoscopic emitter attached to the end of a glass fiber tip as a local probe for mapping the large spatial variations of the photonic crystal local radiative density of states. This arrangement is promising for on-command modification of the coupling between an emitter and the photonic crystal in quantum optical experiments.

cond-mat.other↗

arXiv2004

Tuning of High-Q Photonic Crystal Microcavities by a Subwavelength Near-field Probe

We demonstrate that the resonance frequencies of high-Q microcavities in two-dimensional photonic crystal membranes can be tuned over a wide range by introducing a subwavelength dielectric tip into the cavity mode. Three-dimensional finite-difference time-domain (FDTD) simulations show that by varying the lateral and vertical positions of the tip, it is possible to tune the resonator frequency without lowering the quality factor. Excellent agreement with a perturbative theory is obtained, showing that the tuning range is limited by the ratio of the cavity mode volume to the effective polarizability of the nano-perturber.

cond-mat.other↗

arXiv2004

Electric coupling to the magnetic resonance of split ring resonators

We study both theoretically and experimentally the transmission properties of a lattice of split ring resonators (SRRs) for different electromagnetic (EM) field polarizations and propagation directions. We find unexpectedly that the incident electric field E couples to the magnetic resonance of the SRR when the EM waves propagate perpendicular to the SRR plane and the incident E is parallel to the gap-bearing sides of the SRR. This is manifested by a dip in the transmission spectrum. A simple analytic model is introduced to explain this interesting behavior.

cond-mat.mtrl-sci↗

arXiv2004

Effective medium theory of left-handed materials

We analyze the transmission and reflection data obtained through transfer matrix calculations on metamaterials of finite lengths, to determine their effective permittivity and permeability. Our study concerns metamaterial structures composed of periodic arrangements of wires, cut-wires, split ring resonators (SRRs), closed-SRRs, and both wires and SRRs. We find that the SRRs have a strong electric response, equivalent to that of cut-wires, which dominates the behavior of left-handed materials (LHM). Analytical expressions for the effective parameters of the different structures are given, which can be used to explain the transmission characteristics of LHMs. Of particular relevance is the criterion introduced by our studies to identify if an experimental transmission peak is left- or right-handed.

cond-mat.mtrl-sci↗

arXiv1998

Comment on "Energy Velocity of Diffusion Waves in Strongly Scattering Media"

A comment on the recently published article (PRL 79, 3166 (1997)) is given. The issue is if there is a unique self-consistent way in determining the effective medium in a random medium. The recently introduced effective medium theory (PRL 75, 3442 (1995)) based on the principle of energy density homogenization is also discussed.

cond-mat.dis-nn↗