arXiv subjects

C. M. Soukoulis

Publications and source records attributed to C. M. Soukoulis.

At least 37 records · Page 2Linked to original sources

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

Magnetic metamaterials at telecommunication and visible frequencies

Arrays of gold split-rings with 50-nm minimum feature size and with an LC resonance at 200-THz frequency (1500-nm wavelength) are fabricated. For normal incidence conditions, they exhibit a pronounced fundamental magnetic mode, arising from a coupling via the electric component of the incident light. For oblique incidence, a coupling via the magnetic component is demonstrated as well. Moreover, we identify a novel higher-order magnetic resonance at around 370 THz (800-nm wavelength) that evolves out of the Mie resonance for oblique incidence. Comparison with theory delivers good agreement and also shows that the structures allow for a negative magnetic permeability.

cond-mat.mtrl-sci↗

arXiv2005

Numerical Investigation of Light Scattering off Split-Ring Resonators

Recently, split ring-resonators (SRR's) have been realized experimentally in the near infrared (NIR) and optical regime. In this contribution we numerically investigate light propagation through an array of metallic SRR's in the NIR and optical regime and compare our results to experimental results. We find numerical solutions to the time-harmonic Maxwell's equations by using advanced finite-element-methods (FEM). The geometry of the problem is discretized with unstructured tetrahedral meshes. Higher order, vectorial elements (edge elements) are used as ansatz functions. Transparent boundary conditions and periodic boundary conditions are implemented, which allow to treat light scattering problems off periodic structures. This simulation tool enables us to obtain transmission and reflection spectra of plane waves which are incident onto the SRR array under arbitrary angles of incidence, with arbitrary polarization, and with arbitrary wavelength-dependencies of the permittivity tensor. We compare the computed spectra to experimental results and investigate resonances of the system.

cond-mat.mtrl-sci↗

arXiv2005

Cut-wire pairs and plate pairs as magnetic atoms for optical metamaterials

We study the optical properties of metamaterials made from cut-wire pairs or plate pairs. We obtain a more pronounced optical response for arrays of plate pairs -- a geometry which also eliminates the undesired polarization anisotropy of the cut-wire pairs. The measured optical spectra agree with simulations, revealing negative magnetic permeability in the range of telecommunications wavelengths. Thus, nanoscopic plate pairs might serve as an alternative to the established split-ring resonator design.

physics.optics↗

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↗

arXiv2005

Limits on the amplification of evanescent waves of left-handed materials

We investigate the transfer function of the discretized perfect lens in finite-difference time-domain (FDTD) and transfer matrix (TMM) simulations; the latter allow to eliminate the problems associated with the explicit time dependence in FDTD simulations. We argue that the peak observed in the FDTD transfer function near the maximum parallel momentum $k_{\|,\mathrm{max}}$ is due to finite time artifacts. We also find the finite discretization mesh acts like imaginary deviations from $μ=ε=-1$ and leads to a cross-over in the transfer function from constance to exponential decay around $k_{\|,\mathrm{max}}$ limiting the attainable super-resolution. We propose a simple qualitative model to describe the impact of the discretization. $k_{\|,\mathrm{max}}$ is found to depend logarithmically on the mesh constant in qualitative agreement with the TMM simulations.

cond-mat.mtrl-sci↗

arXiv2005

Isotropic three-dimensional left-handed meta-materials

We investigate three-dimensional left-handed and related meta-materials based on a fully symmetric multi-gap single-ring SRR design and crossing continuous wires. We demonstrate isotropic transmission properties of a SRR-only meta-material and the corresponding left-handed material which possesses a negative effective index of refraction due to simultaneously negative effective permeability and permittivity. Minor deviations from complete isotropy are due to the finite thickness of the meta-material.

cond-mat.mtrl-sci↗

arXiv2004

Intensity distribution of scalar waves propagating in random media

Transmission of the scalar field through the random medium, represented by the system of randomly distributed dielectric cylinders is calculated numerically. System is mapped to the problem of electronic transport in disordered two-dimensional systems. Universality of the statistical distribution of transmission parameters is analyzed in the metallic and in the localized regimes.In the metallic regime the universality of the transmission statistics in all transparent channels is observed. In the band gaps, we distinguish the disorder induced (Anderson) localization from the tunneling through the system due to the gap in the density of states. We show also that absorption causes rapid decrease of the mean conductance, but, contrary to the localized regime, the conductance is self-averaged with a Gaussian distribution.

cond-mat.mes-hall↗

arXiv2004

Impact of the inherent periodic structure on the effective medium description of left-handed and related meta-materials

We study the frequency dependence of the effective electromagnetic parameters of left-handed and related meta-materials of the split ring resonator and wire type. We show that the reduced translational symmetry (periodic structure) inherent to these meta-materials influences their effective electromagnetic response. To anticipate this periodicity, we formulate a periodic effective medium model which enables us to distinguish the resonant behavior of electromagnetic parameters from effects of the periodicity of the structure. We use this model for the analysis of numerical data for the transmission and reflection of periodic arrays of split ring resonators, thin metallic wires, cut wires as well as the left-handed structures. The present method enables us to identify the origin of the previously observed resonance/anti-resonance coupling as well as the occurrence of negative imaginary parts in the effective permittivities and permeabilities of those materials. Our analysis shows that the periodicity of the structure can be neglected only for the wavelength of the electromagnetic wave larger than 30 space periods of the investigated structure.

cond-mat.mtrl-sci↗

arXiv2004

Negative refraction and superlensing in a 2D photonic crystal structure

We experimentally and theoretically studied a new left-handed (LH) structure based on a photonic crystal (PC) with a negative refractive index. The structure consists of triangular array of rectangular dielectric bars with dielectric constant 9.61. Experimental and theoretical results demonstrate the negative refraction and the superlensing phenomena in the microwave regime. The results show high transmission for our structure for a wide range of incident angles. Furthermore, surface termination within a specific cut of the structure excite surface waves at the interface between air and PC and allow the reconstruction of evanescent waves for a better focus and better transmission. The normalized average field intensity calculated in both the source and image planes shows almost the same full width at half maximum for the source and the focused beam.

cond-mat.mtrl-sci↗

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↗

arXiv2004

EM wave propagation in two-dimensional photonic crystals: a study of anomalous refractive effects

We systematically study a collection of refractive phenomena that can possibly occur at the interface of a two-dimensional photonic crystal, with the use of the wave vector diagram formalism. Cases with a single propagating beam (in the positive or the negative direction) as well as cases with birefringence were observed. We examine carefully the conditions to obtain a single propagating beam inside the photonic crystal lattice. Our results indicate, that the presence of multiple reflected beams in the medium of incidence is neither a prerequisite nor does it imply multiple refracted beams. We characterize our results in respect to the origin of the propagating beam and the nature of propagation (left-handed or not). We identified four distinct cases that lead to a negatively refracted beam. Under these findings, the definition of phase velocity in a periodic medium is revisited and its physical interpretation discussed. To determine the ``rightness'' of propagation, we propose a wedge-type experiment. We discuss the intricate details for an appropriate wedge design for different types of cases in triangular and square structures. We extend our theoretical analysis, and examine our conclusions as one moves from the limit of photonic crystals with high index contrast between the constituent dielectrics to photonic crystals with low modulation of the refractive index. Finally, we examine the ``rightness'' of propagation in the one-dimensional multilayer medium, and obtain conditions that are different from those of two-dimensional systems.

cond-mat.other↗

arXiv2003

Theoretical investigation of one-dimensional cavities in two-dimensional photonic crystals

We study numerically the features of the resonant peak of one-dimensional (1-D) dielectric cavities in a two-dimensional (2-D) hexagonal lattice. We use both the transfer matrix method and the finite difference time-domain (FDTD) method to calculate the transmission coefficient. We compare the two methods and discuss their results for the transmission and quality factor Q of the resonant peak. We also examine the dependence of Q on absorption and losses, the thickness of the sample and the lateral width of the cavity. The Q- factor dependence on the width of the source in the FDTD calculations is also given.

cond-mat↗

arXiv2003

Resonant and anti-resonant frequency dependence of the effective parameters of metamaterials

We present a numerical study of the electromagnetic response of the metamaterial elements that are usedto construct materials with negative refractive index. For an array of split ring resonators (SRR) we find that the resonant behavior of the effective magnetic permeability is accompanied by an anti-resonant behavior of the effective permittivity. In addition, the imaginary parts of the effective permittivity and permeability are opposite in sign. We also observe an identical resonant versus anti-resonant frequency dependence of the effective materials parameters for a periodic array of thin metallic wires with cuts placed periodically along the length of the wire, with roles of the permittivity and permeability reversed from the SRR case. We show in a simple manner that the finite unit cell size is responsible for the anti-resonant behavior.

cond-mat.mtrl-sci↗

arXiv2003

Absorption losses in periodic arrays of thin metallic wires

We analyze the transmission and reflection of the electromagnetic wave calculated from transfer matrix simulations of periodic arrangements of thin metallic wires. The effective permittivity and the absorption is determined. Their dependence on the wire thickness and the conductance of the metallic wires is studied. The cutoff frequency or effective plasma frequency is obtained and is compared with analytical predictions. It is shown that the periodic arrangement of wires exhibits a frequency region in which the real part of the permittivity is negative while its imaginary part is very small. This behavior is seen for wires with thickness as small as 17 $μ$m with a lattice constant of 3.33 mm.

cond-mat.mtrl-sci↗

arXiv2003

Mode Repulsion and Mode Coupling in Random Lasers

We studied experimentally and theoretically the interaction of lasing modes in random media. In a homogeneously broadened gain medium, cross gain saturation leads to spatial repulsion of lasing modes. In an inhomogeneously broadened gain medium, mode repulsion occurs in the spectral domain. Some lasing modes are coupled through photon hopping or electron absorption and reemission. Under pulsed pumping, weak coupling of two modes leads to synchronization of their lasing action. Strong coupling of two lasing modes results in anti-phased oscillations of their intensities.

cond-mat.dis-nn↗

arXiv2002

Negative refraction and left-handed behavior in two-dimensional photonic crystals

We systematically examine the conditions of obtaining left-handed (LH) behavior in photonic crystals. Detailed studies of the phase and group velocities as well as the phase np and group ng refractive index are given. The existence of negative refraction does not guarantee the existence of negative index of refraction and so LH behavior. A wedge type of experiment is suggested that can unambiguously distiguinsh between cases of negative refraction that occur when left-handed behavior is present, from cases that show negative refraction without LH behavior.

cond-mat↗