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M. Nieto-Vesperinas

Publications and source records attributed to M. Nieto-Vesperinas.

29 records · Page 2Linked to original sources
arXiv2002

Theory for tailoring sonic devices: Diffraction dominates over Refraction

In the moderately long wavelength propagation regime, acoustic crystaldevices which size is of several wave lengths are studied by using thefinite-difference time domain method. It is shown by the focusing and imaging of a square shaped lens that the diffractive effects dominate over the refractive ones. Taking into account the well known Babinet principle, the major role of the device edge diffraction is shown. The first examples of imaging with a sonic plane lens and of an acoustic prism able to change the propagation direction of a plane wave are also presented here.

cond-mat↗
arXiv2002

Wedge - Shaped Absorbing Samples Look Left Handed: The Problem of Interpreting Negative Refraction, and its Solution

We report experiments of light transmissivity at wavelengths: 532 and 400 nm, through an Au film with a wedge shape. Our results mimic the negative refraction reported by others for so-called left handed materials. A mimic of negative refraction is observed, even though this medium is well known to be right handed, and thus its refractive index has a positive real part. Analogous results are obtained with a glass wedge at 320nm where absorption dominates. The experiment is explained by the wave losses that dominate over propagation, like in the observation of negative refraction, already reported in developed metamaterial wedges. We design and propose an experiment with metamaterials by using thicker wires, in correspondence with light experiments that should conclusively determine whether refraction is positive or negative.

cond-mat↗
arXiv2002

Zero Permeability Materials (ZmuM): A way out of the "Left Handed Materials" trap

Recently it has been shown that due to losses the claims on observation of negative refraction using metamaterials are questionable. In this paper we discus an interesting case that is based in almost zero permeability at a given frequency: zero permeability materials. This represents a way out of the problem of left handed materials, and constitutes a material that acts a band pass filter with a very narrow band. The metamaterials claimed to present negative refraction are just showing this effect. However, absorption can be diminished in those metamaterials by using thicker wires. Materials based in quasi metallic magnetic structures are proposed with ferromagnetic resonance at microwave frequencies under applied magnetic fields. These materials, as well as the zero permitivity in thin films, open a new ground for a new set of optical devices.

cond-mat↗
arXiv2002

Losses for microwave transmission in metamaterials for producing left-handed materials: The strip wires

This paper shows that the effective dielectric permitivity for the metamaterials used so far to obtain left-handed materials, with strip wires 0.003cm thick, is dominated by the imaginary part at 10.6- 11.5 GHz frequencies, where the band pass filter is, and therefore there is not propagation and the wave is inhomogeneous inside the medium. This is shown from finite-differences time-domain calculations using the real permitivity values for the Cu wires. For thicker wires the losses are reduced and the negative part of the permitivity dominates. As the thickness of the wires is critical for the realization of a good transparent left- handed material we propose that the strip wires should have thickness of 0.07-0.1cm and the split ring resonators 0.015-0.03cm

cond-mat↗
arXiv2001

Resonant radiation pressure on neutral particles in a waveguide

A theoretical analysis of electromagnetic forces on neutral particles in an hollow waveguide is presented. We show that the effective scattering cross section of a very small (Rayleigh) particle can be strongly modified inside a waveguide. The coupling of the scattered dipolar field with the waveguide modes induce a resonant enhanced backscattering state of the scatterer-guide system close to the onset of new modes. The particle effective cross section can then be as large as the wavelength even far from any transition resonance. As we will show, a small particle can be strongly accelerated along the guide axis while being highly confined in a narrow zone of the cross section of the guide.

physics.optics↗
arXiv1998

Statistical Distribution of Intensities Reflected from Disordered Media

A theoretical analysis of the statistical distributions of the reflected intensities from random media is presented. We use random matrix theory to analytically deduce the probability densities in the localization regime. Numerical calculations of the coupling to backward modes in surface corrugated waveguides are also put forward for comparison. Interestingly, the speckle distributions are found to be independent of the transport regime. Despite the scattering being highly non-isotropic, the predicted probability densities reproduce accurately the numerical results.

cond-mat.dis-nn↗
arXiv1998

Intensity Distribution of Modes in Surface Corrugated Waveguides

Exact calculations of transmission and reflection coefficients in surface randomly corrugated optical waveguides are presented. As the length of the corrugated part of the waveguide increases, there is a strong preference to forward coupling through the lowest mode. An oscillating behavior of the enhanced backscattering as a function of the wavelength is predicted. Although the transport is strongly non isotropic, the analysis of the probability distributions of the transmitted waves confirms in this configuration distributions predicted by Random Matrix Theory for volume disorder.

cond-mat.dis-nn↗
arXiv1998

Transition from Diffusive to Localized Regimes in Surface Corrugated Optical Waveguides

Exact calculations of the transmittance of surface corrugated optical waveguides are presented. The elastic scattering of diffuse light or other electromagnetic waves from a rough surface induces a diffusive transport along the waveguide axis. As the length of the corrugated part of the waveguide increases, a transition from the diffusive to the localized regime is observed. This involves an analogy with electron conduction in nanowires, and hence, a concept analogous to that of ``resistance'' can be introduced. We show an oscillatory behavior of both the elastic mean free path and the localization length versus the wavelength.

cond-mat.mes-hall↗