arXiv subjects

Vassilios Yannopapas

Publications and source records attributed to Vassilios Yannopapas.

14 recordsLinked to original sources

arXiv2026

T-matrix representation of optical scattering response: Suggestion for a data format

The transition matrix, frequently abbreviated as T-matrix, contains the complete information in a linear approximation of how a spatially localized object scatters an incident field. The T-matrix is used to study the scattering response of an isolated object and describes the optical response of complex photonic materials made from ensembles of individual objects. T-matrices of certain common structures, potentially, have been repeatedly calculated all over the world again and again. This is not necessary and constitutes a major challenge for various reasons. First, the resources spent on their computation represent an unsustainable financial and ecological burden. Second, with the onset of machine learning, data is the gold of our era, and it should be freely available to everybody to address novel scientific challenges. Finally, the possibility of reproducing simulations could tremendously improve if the considered T-matrices could be shared. To address these challenges, we found it important to agree on a common data format for T-matrices and to enable their collection from different sources and distribution. This document aims to develop the specifications for storing T-matrices and associated metadata. The specifications should allow maximum freedom to accommodate as many use cases as possible without introducing any ambiguity in the stored data. The common format will assist in setting up a public database of T-matrices.

physics.optics↗

arXiv2024

Dirac plasmon polaritons and magnetic modes in topological-insulator nanoparticles

We demonstrate the existence of previously unreported magnetic modes with record-high magnetic Purcell factors in topological-insulator nanospheres. Focusing on bismuth selenide (Bi$_{2}$Se$_{3}$), and based on full electromagnetic Mie theory, we find magnetic modes arising from both displacement current loops in the bulk, and surface currents due to delocalized surface states, induced by electronic transitions between topologically protected states within the Dirac cone and discretized due to the sphere finite size. Furthermore, we discuss how Dirac plasmon polaritons, resulting from the interaction between THz photons and Dirac electrons, dramatically influence both the magnetic and the electric transitions of quantum emitters placed near Bi$_2$Se$_3$ nanospheres, significantly enhancing the corresponding Purcell factors. These findings position Bi$_{2}$Se$_{3}$ nanospheres, whose optical response is related to a richness of physical mechanisms, among the most promising candidates for enhancing light--matter interactions in nanophotonics and THz technologies.

cond-mat.mes-hall↗

arXiv2024

Multem 3: An updated and revised version of the program for transmission and band calculations of photonic crystals

We present here Multem 3, an updated and revised version of Multem 2, which syntax has been upgraded to Fortran 2018, with the source code being divided into modules. Multem 3 is equipped with LAPACK, the state-of-the art Faddeeva complex error function routine, and the Bessel function package AMOS. The amendments significantly improve both the speed, convergence, and precision of Multem 2. Increased stability allows to freely increase the cut-off value LMAX on the number of spherical vector wave functions and the cut-off value RMAX controlling the maximal length of reciprocal vectors taken into consideration. An immediate bonus is that Multem 3 can be reliably used to describe bound states in the continuum (BICs). To ensure convergence of the layer coupling scheme, it appears that appreciably larger values of convergence paramaters LMAX and RMAX are required than those reported in numerous published work in the past using Multem 2. We hope that Multem 3 will become a reliable and fast alternative to generic commercial software, such as COMSOL Multiphysics, CST Microwave Studio, or Ansys HFSS, and that it will become the code of choice for various optimization tasks for a large number of research groups. The improvements concern the core part of Multem 2, which is common to the extensions of Multem 2 for acoustic and elastic multiple scattering and to the original layer-Kohn-Korringa-Rostocker (LKKR) code. Therefore, the enhancements presented here can be readily applied to the above codes as well.

physics.comp-ph↗

arXiv2020

Light scattering by a metallic nanoparticle coated with a nematic liquid crystal

We study the optical properties of gold nanoparticles coated with a nematic liquid crystal whose director field is distributed around the nanoparticle according to the anchoring conditions at the surface of the nanoparticle. The distribution of the nematic liquid crystal is obtained by minimization of the corresponding Frank free-energy functional whilst the optical response is calculated by the discrete-dipole approximation. We find, in particular, that the anisotropy of the nematic liquid-crystal coating does not affect much the (isotropic) optical response of the nanoparticle. However, for strong anchoring of the nematic liquid-crystal molecules on the surface of nanoparticle, the inhomogeneity of the coating which is manifested by a ring-type singularity (disclination or Saturn ring), produces an enhancement of the extinction cross spectrum over the entire visible spectrum.

cond-mat.soft↗

arXiv2020

Manipulating the photonic Hall effect with hybrid Mie-exciton resonances

We examine the far-field optical response, under-plane wave excitation in the presence of a static magnetic field, of core-shell nanoparticles involving a gyroelectric component, either as the inner or the outer layer, through analytic calculations based on appropriately extended Mie theory. We focus on absorption and scattering of light by bismuth-substituted yttrium iron garnet (Bi:YIG) nanospheres and nanoshells, combined with excitonic materials such as organic-molecule aggregates or two-dimensional transition-metal dichalcogenides, and discuss the hybrid character of the modes emerging from the coupling of the two constituents. We observe the excitation of strong magneto-optic phenomena and explore, in particular, the response and tunability of a magneto-transverse light current, indicative of the photonic Hall effect. We show how interaction between the Bi:YIG and excitonic layers leads to a pair of narrow bands of highly directional scattering, emerging from the aforementioned hybridization, which can be tuned at will by adjusting the geometrical or optical parameters of the system. Our theoretical study introduces optically anisotropic media as promising templates for strong coupling in nanophotonics, offering a means to combine tunable magnetic and optical properties, with potential implications both in the design of all-dielectric photonic devices but also in novel clinical applications.

cond-mat.mes-hall↗

arXiv2020

Control of nonlinear optical phenomena and spatially structured optical effects in a four-level quantum system near a plasmonic nanostructure

We investigate the nonlinear optical response of a four-level double-V-type quantum system interacting with a pair of weak probe fields while located near a two-dimensional array of metal-coated dielectric nanospheres. Such a quantum system contains a V-type subsystem interacting with surface plasmons, and another V-type subsystem interacting with the free-space vacuum. A distinctive feature of the proposed setup is its sensitivity to the relative phase of the applied fields when placed near the plasmonic nanostructure. We demonstrate that due to the presence of the plasmonic nanostructure, the third-order (Kerr-type) susceptibility for one of the laser fields can be significantly modified while another probe field is acting. Moreover, the Kerr nonlinearity of the system can be controlled and even enhanced by varying the distance of the quantum system from the plasmonic nanostructure.We also show that the Kerr nonlinearity of such a system can be controlled by adjusting the relative phase of the applied fields. The results obtained may find potential applications in on-chip nanoscale photonic devices. We also study the light-matter interaction in the case where one probe field carries an optical vortex, and another probe field has no vortex. We demonstrate that due to the phase sensitivity of the closed-loop double V-type quantum system, the linear and nonlinear susceptibility of the nonvortex probe beam depends on the azimuthal angle and orbital angular momentum (OAM) of the vortex probe beam. This feature is missing in open four-level double V-type quantum system interacting with free-space vacuum, as no quantum interference occurs in this case. We use the azimuthal dependence of optical susceptibility of the quantum system to determine the regions of spatially-structured transmittance.

quant-ph↗

arXiv2015

Dynamic Effective Medium Theory for Two-Dimensional Non-Magnetic Metamaterial Lattices using Multipole Expansion

We present a formulation for deriving effective medium properties of infinitely periodic two-dimensional metamaterial lattice structures beyond the static and quasi-static limits. We utilize the multipole expansions, where the polarization currents associated with the supported Bloch modes are expressed via the electric dipole, magnetic dipole, and electric quadrupole moments per unit length. We then propose a method to calculate the Bloch modes based on the lattice geometry and individual unit element structure. The results revert to well-known formulas in the quasistatic limit and are useful for the homogenization of nanorod-type metamaterials which are frequently used in optical applications.

physics.optics↗

arXiv2013

Arrays of carbon nanoscrolls as deep-subwavelength magnetic metamaterials

We demonstrate theoretically that an array of carbon nanoscrolls acts as a hyperbolic magnetic metamaterial in the THz regime with genuine subwavelength operation corresponding to wavelength-to-structure ratio of about 200. Due to the low sheet resistance of graphene, the electromagnetic losses in an array of carbon nanoscrolls are almost negligible offering a very sharp magnetic resonance of extreme positive and negative values of the effective magnetic permeability. The latter property leads to superior imaging properties for arrays of carbon nanoscrolls which can operate as magnetic endoscopes in the THz where magnetic materials are scarce. Our optical modelling is supplemented with ab initio density-functional calculations of the self-winding of a single layer of graphene onto a carbon nanotube so as to form a carbon nanoscroll. The latter process is viewed as a means to realize ordered arrays of carbon nanoscrolls in the laboratory based on arrays of aligned carbon nanotubes which are nowadays routinely fabricated.

cond-mat.mes-hall↗

arXiv2012

Topological bands in two-dimensional networks of metamaterial elements

We show that topological frequency band structures emerge in two-dimensional electromagnetic lattices of metamaterial components without the application of an external magnetic field. The topological nature of the band structure manifests itself by the occurrence of exceptional points in the band structure or by the emergence of one-way guided modes. Based on an EM network with nearly flat frequency bands of nontrivial topology, we propose a coupled-cavity lattice made of superconducting transmission lines and cavity QED components which is described by the Janes-Cummings-Hubbard model and can serve as simulator of the fractional quantum Hall effect.

cond-mat.str-el↗

arXiv2011

Gapless surface states in a lattice of coupled cavities: a photonic analog of topological crystalline insulators

We show that a tetragonal lattice of weakly interacting cavities with uniaxial electromagnetic response is the photonic counterpart of topological crystalline insulators, a new topological phase of atomic band insulators. Namely, the frequency band structure stemming from the interaction of resonant modes of the individual cavities exhibits an omnidirectional band gap within which gapless surface states emerge for finite slabs of the lattice. Due to the equivalence of a topological crystalline insulator with its photonic-crystal analog, the frequency band structure of the latter can be characterized by a $Z_{2}$ topological invariant. Such a topological photonic crystal can be realized in the microwave regime as a three-dimensional lattice of dielectric particles embedded within a continuous network of thin metallic wires.

cond-mat.str-el↗

arXiv2009

Circular dichroism in planar nonchiral plasmonic metamaterials

It is shown theoretically that a nonchiral, two-dimensional array of metallic spheres exhibits optical activity as manifested in calculations of circular dichroism. The metallic spheres occupy the sites of a rectangular lattice and for off-normal incidence they show a strong circular-dichroism effect around the surface plasmon frequencies. The optical activity is a result of the rectangular symmetry of the lattice which gives rise to different polarizations modes of the crystal along the two orthogonal primitive lattice vectors. These two polarization modes result in a net polar vector, which forms a chiral triad with the wavevector and the vector normal to the plane of spheres. The formation of this chiral triad is responsible for the observed circular dichroism, although the structure itself is intrinsically nonchiral.

physics.optics↗

arXiv2008

Subwavelength imaging of light by arrays of metal-coated semiconductor nanoparticles: a theoretical study

Rigourous calculations of the imaging properties of metamaterials consisting of metal-coated semiconductor nanoparticles are presented. In particular, it is shown that under proper choice of geometric and materials parameters, arrays of such particles exhibit negative refractive index within the region of the excitonic resonance of the semiconductor. The occurrence of negative refractive index is predicted by the extended Maxwell-Garnett theory and confirmed by a layer-multiple scattering method for electromagnetic waves. By using the same method it is shown that within the negative refractive-index band, arrays of such nanoparticles amplify the transmitted near-field emitted while simultaneously narrow down its spatial profile leading to subwavelength resolution. The effect of material losses to the imaging properties of the arrays is also addressed.

cond-mat.mtrl-sci↗

arXiv2007

A general theory for the van der Waals interactions in colloidal systems based on fluctuational electrodynamics

A rigourous theory for the determination of the van der Waals interactions in colloidal systems is presented. The method is based on fluctuational electrodynamics and a multiple-scattering method which provides the electromagnetic Green's tensor. In particular, expressions for the Green's tensor are presented for arbitrary, finite, collections of colloidal particles, for infinitely periodic or defected crystals as well as for finite slabs of crystals. The presented formalism allows for {\it ab initio} calculations of the vdW interactions is colloidal systems since it takes fully into account retardation, many-body, multipolar and near-fields effects.

cond-mat.other↗

arXiv2006

Thermal emission from three-dimensional arrays of gold nanoparticles

We study the blackbody spectrum from slabs of three-dimensional metallodielectric photonic crystals consisting of gold nanoparticles using an ab initio multiple-scattering method. The spectra are calculated for different photonic-crystal slab thicknesses, particle radii and hosting materials. We find in particular that such crystals exhibit a broadband emission spectrum above a specific cutoff frequency with emissivity of about 90%. The studied photonic crystals can be used as efficient selective emitters and can therefore find application in thermophotovoltaics and sensing.

cond-mat.other↗