Papers by Eleftherios Economou
Journal of Magnetism and Magnetic Materials, 1978
The half-filled Hubbard model can be analyzed in terms of quasi-electron and spin wave excitation... more The half-filled Hubbard model can be analyzed in terms of quasi-electron and spin wave excitations; the latter are described by HH =-1 ~nmJ,,m~nCn. The quantities J nrn were calculated and the resulting thermodynamic quantities in the one-dimensional (1D) case are in very good agreement with exact results. We outline here our approximate treatment of the Hubbard [ 1 ] hamiltonian, H= CE,nia t C VijoaiarZj, + II c-nit nil y io ijo i
Photonics and Nanostructures - Fundamentals and Applications, 2014
In this paper, we demonstrate numerically various phenomena and possibilities that can be realize... more In this paper, we demonstrate numerically various phenomena and possibilities that can be realized in THz metamaterials made of phonon-polariton materials. Such phenomena include hyperbolic dispersion relation, subwavelength imaging using backward propagation and backward radiation, total transmission and subwavelength guiding exploiting Mie-resonant scattering in permittivity near zero host, and toroidal dipolar response. The systems that we use to demonstrate most of these phenomena are two-dimensional periodic systems of m-scale rods in a host, where both rods and host are made of polaritonic alkali-halide materials.
arXiv (Cornell University), Aug 5, 2013
We present and analyze a dielectric cluster based on subwavelength LiTaO 3 polaritonic cylinders ... more We present and analyze a dielectric cluster based on subwavelength LiTaO 3 polaritonic cylinders for demonstrating toroidal response in THz regime due to mutual coupling of Mie-resonance modes of the cylinders. Based on this cluster, we demonstrate a low-loss metamaterial with the dominant toroidal response, which plays a key role in achieving resonant total transmission.
Optics Express, 2008
Using transmission and reflection measurements under normal incidence in one and three layers of ... more Using transmission and reflection measurements under normal incidence in one and three layers of a μ m-scale metamaterial consisting of pairs of shortslabs and continuous wires, fabricated by a photolithography procedure, we demonstrate the occurrence of a negative refractive index regime in the far infrared range, ~2.4-3 THz. The negative index behavior in that system at ~2.4-3 THz is further confirmed by associated simulations, which are in qualitative agreement with the experimental results.
Optics Express, 2012
In this paper we demonstrate the possibility of backward radiation from a negative permittivity p... more In this paper we demonstrate the possibility of backward radiation from a negative permittivity planar (slab) waveguide. Furthermore, we show that backward radiation can be used to achieve subwavelength imaging of a point source placed close to such a slab or to a periodic layered system of slabs. Finally, we demonstrate backwardradiation-based imaging in the case of realistic materials operating in the THz regime, such as polaritonic alkali-halide systems.
Physical Review X, 2015
Toroidal multipoles are the terms missing in the standard multipole expansion; they are usually o... more Toroidal multipoles are the terms missing in the standard multipole expansion; they are usually overlooked due to their relatively weak coupling to the electromagnetic fields. Here, we propose and theoretically study all-dielectric metamaterials of a special class that represent a simple electromagnetic system supporting toroidal dipolar excitations in the THz part of the spectrum. We show that resonant transmission and reflection of such metamaterials is dominated by toroidal dipole scattering, the neglect of which would result in a misunderstanding interpretation of the metamaterials' macroscopic response. Because of the unique field configuration of the toroidal mode, the proposed metamaterials could serve as a platform for sensing or enhancement of light absorption and optical nonlinearities.
Physical Review Letters, 2004
We analyze the transmission and reflection data obtained through transfer matrix calculations on ... more We analyze the transmission and reflection data obtained through transfer matrix calculations on metamaterials of finite lengths, to determine their effective permittivity ǫ and permeability µ. 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.
Physical Review B, 2007
We investigate, both theoretically and experimentally, a left-handed metamaterial design composed... more We investigate, both theoretically and experimentally, a left-handed metamaterial design composed of pairs of short slabs connected with continuous wires, operating in microwave frequency regime. The design was found to give left-handed behavior for a wide range of structure parameters, maintaining high impedance match with free space. We introduce a capacitor-inductor circuit description of the design and we show that this description can account for all the characteristics of its electromagnetic behavior, explaining also its superior performance.
We present and analyze the unique phenomena of enhanced THz transmission through a subwavelength ... more We present and analyze the unique phenomena of enhanced THz transmission through a subwavelength LiF dielectric rod lattice embedded in an epsilon-near-zero KCl host. Our experimental results in combination with theoretical calculations show that subwavelength waveguiding of terahertz radiation is achieved within an alkali-halide eutectic metamaterial as result of the coupling of Mie-resonance modes arising in the dielectric lattice.
We study both theoretically and experimentally the transmission properties of a lattice of split ... more 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.
We present free-space microwave measurements on composite metamaterials (CMMs) consisting of spli... more We present free-space microwave measurements on composite metamaterials (CMMs) consisting of split ring resonators (SRRs) and wires either on the same dielectric board or on alternating boards. Our experimental results disprove the widely held belief that the occurrence of a CMM transmission peak within the stop bands of the SRRs alone and wires alone constitutes a clear demonstration of left-handed (LH) behavior. This belief is based on the assumption that the stop bands of SRRs alone and wires alone are not affected by the simultaneous presence of both. We show here that this assumption is wrong: The effective plasma frequency, p Ј, of the CMM is actually substantially lower than the wires-only plasma frequency, p ; furthermore, the in-plane wires, as opposed to the off-plane case, push the magnetic resonance frequency of the SRRs, m , to a higher value, m Ј , for the CMM. We conclude that the criterion for deciding whether a peak in the transmission spectrum through a CMM is really left-handed is for the peak to be located above m Ј and below p Ј. Our results provide a definite way for experimentally identifying p Ј.
+Abstract. Within the binary, self-consistent static approximation to the Hubbard model, local mo... more +Abstract. Within the binary, self-consistent static approximation to the Hubbard model, local moments are created out of itinerant electrons. The magnetic Ising-type interactions of these local moments are calculated and explicit results are obtained. Relaxing the static
character of the approximation transforms the interactions to Heisenberg type and renormalises their size. Renormalisation is necessary in order to satisfy certain sum rules. Our results
are in good agreement with exact many-body results.
Physical Review B, 1980
A Slater-Koster Hamiltonian for Si is constructed using four (one s and three p) orthogonal orbit... more A Slater-Koster Hamiltonian for Si is constructed using four (one s and three p) orthogonal orbitals per site. This Hamiltonian reproduces reasonably accurately the empirical pseudopotential band structure not only for the valence but for the conduction band as well. It also determines correctly the position of the bound level of an unreconstructed vacancy demonstrating thus that the effects of electronic self-consistency are minor.
Physical review. B, Condensed matter, 1994
An extension of the well-known coherent-potential approximation is developed for the study of var... more An extension of the well-known coherent-potential approximation is developed for the study of various properties of random arrangements of spherical dielectric scatterers. Some of the short-range order is taken into account by considering a coated sphere as the basic scattering unit. A generalization of the energy-transport velocity is obtained. The validity of our approach is checked by comparison with experimental results, as well as with numerical calculations. Results for the long-wavelength effective dielectric constant, phase velocity, energy-transport velocity, mean free path, and diffusion coefficient are presented and compared with experiments on scattering from dielectric spheres. In addition, our findings suggest that the positions of the band gaps in periodic dielectric structures are closely related with the range of localized states in random dielectric media.
Physical review. B, Condensed matter, 1994
We study the time dependence of a combined system of an electron described by a tight-binding mod... more We study the time dependence of a combined system of an electron described by a tight-binding model interacting with vibrational degrees of freedom in a one-dimensional lattice. The time evolution of our coupled model is described by a nonlinear system of differential-difference equations. Apart from localized solutions (polarons) and extended (Bloch-like) solutions, we found that for a small parameter range, there exists a periodic exchange of energy between the electronic and vibrational degrees of freedom: a small part of the electronic energy is transferred to the lattice and then completely back to the electron with an impressive regularity.
Physical review. B, Condensed matter, Jan 15, 1993
Using the transfer-matrix technique for the propagation of electromagnetic waves in dielectric st... more Using the transfer-matrix technique for the propagation of electromagnetic waves in dielectric structures, we calculate the transmission coefficient versus the frequency of the incident wave for different polarizations in two-dimensional periodic and/or random arrangements of dielectric cylinders. This technique has been applied to cases where the plane-wave method fails or becomes too time consuming, such as when the dielectric constant
Physical review. B, Condensed matter, 1991
... E.N. Economou Research Center of Crete, Foundation for Research and Technology (FO.R.TH.), In... more ... E.N. Economou Research Center of Crete, Foundation for Research and Technology (FO.R.TH.), Institute for Electronic Structure and Lasers, Heraklion, P ... The results demonstrate the fracton excitations at the percolation concentration p, as expected from the Alexander-Orbach ...
Physical review. B, Condensed matter, Jan 15, 1995
We study the transport properties of a two-dimensional randomly disordered dielectric medium. The... more We study the transport properties of a two-dimensional randomly disordered dielectric medium. The medium consists of infinitely long dielectric cylinders with a real dielectric constant ε{sub {ital a}}, embedded in a different dielectric medium with ε{sub {ital b}}=1. The transport velocity is calclated within the low-density approximation of the Bethe-Salpeter equation and within the coated extension of the well-known coherent-potential
Optics Express, 2009
We present a planar design of a metamaterial exhibiting electromagnetically induced transparency ... more We present a planar design of a metamaterial exhibiting electromagnetically induced transparency that is amenable to experimental verification in the microwave frequency band. The design is based on the coupling of a split-ring resonator with a cut-wire in the same plane. We investigate the sensitivity of the parameters of the transmission window on the coupling strength and on the circuit elements of the individual resonators, and we interpret the results in terms of two linearly coupled Lorentzian resonators. Our metamaterial designs combine low losses with the extremely small group velocity associated with the resonant response in the transmission window, rendering them suitable for slow light applications at room temperature.
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Papers by Eleftherios Economou
character of the approximation transforms the interactions to Heisenberg type and renormalises their size. Renormalisation is necessary in order to satisfy certain sum rules. Our results
are in good agreement with exact many-body results.
character of the approximation transforms the interactions to Heisenberg type and renormalises their size. Renormalisation is necessary in order to satisfy certain sum rules. Our results
are in good agreement with exact many-body results.