We report the design of quasi-two-dimensional artificial structures that acoustically behave as positive, single negative, double negative or density-near-zero metamaterials. The scattering units consist of a cavity drilled in one surface of a 2D waveguide and they have an inner structure whose geometrical parameters can be selected in order to obtain the desired dynamical behavior. Finally, we report the practical realization of two samples as well as their experimental characterization showing metamaterial features.
The previous publications (Miñano et al, 2011) have shown that using a Spherical Geodesic Waveguide (SGW), it can be
achieved the super-resolution up to λ /500 close to a set of discrete frequencies. These frequencies are directly connected
with the well-known Schumann resonance frequencies of spherical symmetric systems. However, the Spherical Geodesic
Waveguide (SGW) has been presented as an ideal system, in which the technological obstacles or manufacturing
feasibility and their influence on final results were not taken into account. In order to prove the concept of superresolution
experimentally, the Spherical Geodesic Waveguide is modified according to the manufacturing requirements
and technological limitations. Each manufacturing process imposes some imperfections which can affect the
experimental results. Here, we analyze the influence of the manufacturing limitations on the super-resolution properties
of the SGW. Beside the theoretical work, herein, there has been presented the experimental results, as well.
We analyze the properties of acoustic and electromagnetic metamaterials with anisotropic constitutive parameters.
Particularly, we analyze the so-called Radial Wave Crystals, which are radially periodic structures verifying the Bloch
theorem. This type of crystals can be designed and implemented in acoustics as well as in electromagnetism by using
anisotropic metamaterials. In acoustics, we have previously predicted that they can be employed as acoustic cavities with
huge quality factors and also like dynamically driven antennas. Similar functionalities are here proven in the
electromagnetic domain with, in particular, an analysis of the functionality of practical devices operating in the
microwave regime. Starting from our recent works on anisotropic structures and their comparison in both application
fields, we present a complete discussion concerning their properties in acoustics and electromagnetics.
We review recent advances on the topic of acoustic metamaterials based on the homogenization of periodic
arrangements of sonic scatterers in a fluid or gas background. Particular emphasis is given in the application of
these structures for gradient index sonic lenses and acoustic cloaking.
A method of inverse design is applied to generate a new family of optical devices named scattering optical elemetns
(SOE). The two dimensional (2D) designs consist of a few layers of 0.4&mgr;m x 0.4&mgr;m square-shaped bars etched
in gallium arsenide. SOEs are defined as a class of computer-generated optical devices whose functionalities are
based on the multiple scattering by their individual constituents. For realization of the aforementioned devices,
two-dimensional photonic plates could be fabricated by only a single integrated circuit processing procedure
followed by micromanipulation assembling. A small library of compact SOE devices are presented: A focusing
device, a wavelength de-multiplexer, an optimized optical source, an optical MEMS switch and a cloaking device.
A method of inverse design is applied to generate a new family of optical devices. Three ultracompact devices are presented of only a few microns thick; a focusing device, a wavelength de-multiplexer and an optimized optical source. The designs consist of a few layers of 0.4μm × 0.4μm square-shaped bars etched in gallium arsenide. The proposed designs are examples of a scattering optical element, a name introduced to define a class of computer-generated optical devices whose functionalities are based on the multiple scattering by their individual constituents. For realization of the aforementioned devices, two-dimensional photonic plates could be fabricated by only a single integrated circuit processing procedure followed by micromanipulation assembling.
We use multiple scattering in conjunction with a genetic algorithm to reliably determine the optimized photonic-crystal-based structure able to perform a specific optical task. The genetic algorithm operates on a population of candidate structures to produce new candidates with better performance in an iterative process. The potential of this approach is illustrated by designing a spot size converter that has a very low F-number (F=0.47) and a conversion ratio of 11:1. Also, we have designed a coupler device that introduces the light from the optical fiber into a photonic-crystal-based wave guide with a coupling efficiency over 87% for a wavelength that can be tuned to 1.5 λ.
In this contribution, a method to fabricate a diamond structure with a complete PBG in the near infrared is proposed. The procedure starts by building an opal composed of two types of microspheres (organic and inorganic) in a body-centered-cubic symmetry by means of a micro-robotic technique. Then, the organic particles may be selectively removed to obtain a diamond structure of inorganic particles. Once this structure is assembled its filling fraction may be controlled by sintering. Subsequently this template can be infiltrated with an adequate high refractive index material. In this way, the method can be extended to make diamond inverse opals of, for instance, silicon with gap to mid gap ratios as large as 13% for moderate filling fractions. An overview of micromanipulation as well as previous experimental results will be offered to show the feasibility of this method.
Transmission of light through linear defects in two-dimensional (2D) photonic crystals has been already successfully demonstrated in two ways: numerical simulations and experimental measurements. Recently, novel waveguides have been proposed in which the propagation of photons is performed via hopping due to overlapping between nearest-neighbors defect cavities. These waveguides are commonly referred to as coupled-cavity waveguides (CCW). In this work, we present a comprehensive analysis of the light transmission (TM modes) in CCW's created in hexagonal 2D photonic crystals made of high-index dielectric rods. Numerical simulations of the transmission are performed using a 2D Finite-Difference Time-Domain method. A plane wave algorithm and a simple one-dimensional (1D) tight-binding model are employed to describe the miniband which allows the light transport. It is shown that modifying the individual cavities along the CCW one can control the average frequency and the dispersion relation of the miniband. The results also show that this novel guiding method can be used to develop 1310nm/1550nm Coarse-WDM optical demultiplexers employing bended waveguides.
This paper reports a new method for faceting artificial opals based on micromanipulation techniques. By this means it was possible to fabricate an opal prism in a single domain with different faces: (111), (110) and (100), which were characterized by Scanning Electron Microscopy and Optical Reflectance Spectroscopy. Their spectra exhibit different characteristics depending on the orientation of the facet. While (111)-oriented face gives rise to a high Bragg reflection peak at about a/(lambda) equals 0.66 (where a is the lattice parameter), (110) and (100) faces show much less intense peaks corresponding to features in the band structure at a/(lambda) equals 1.12 and a/(lambda) equals 1.07 respectively. Peaks at higher energies have less obvious explanation.
When rows of cylinders are periodically removed from a hexagonal array of dielectric cylinders, a new two-dimensional (2D) photonic crystal (PC) arises. The new structure consists of a lattice of vacancies embedded in the initial hexagonal lattice. We called it Suzuki Phase because it remains similar structures discovered in the 60's by K. Suzuki studying alkali halides. A plane-wave algorithm as well as a 2D finite difference-time-domain method has been employed to study the photonic properties of this PC as a function of the filling fraction (f) in the case of high dielectric cylinders ((epsilon) equals 13.6) in air. For TM- modes, it is shown that in a certain range of f an isolated miniband appears in the gap of the initial hexagonal lattice. The miniband, which is created by the coupling of defect states, is described by a tight-binding formalism with two parameters. Also, the frequencies of the two possible vacancy defects in the SP have been obtained and their symmetry analyzed.
We have studied different strained InGaAs/GaAs ultrathin quantum wells grown on vicinal surfaces for various terrace lengths and In contents. From photoluminescence experiments we observe an enhancement of the continuum density of states of quantum wells wit large In content (x equals 0.35). We associate this behavior to the localization of carriers in regions of quasi one-dimensional confinement which are induced by fluctuations in the lateral periodicity of the strained layers. This assumption is supported by time resolved measurements and explained through theoretical calculations.
Time resolved luminescence experiments performed on GaA1As-GaAs-A1As nonsymmetric modulation n-doped thick Quantum Wells show an atypical behaviour regarding similar but symmetric ( GaA1As-GaAs-GaA1As ) Quantum Wells. In the latter system the luminescence has the standard radiative lifetimes in the range reported in the literature. Much shorter lifetimes for nonsymmetric Quantum Wells are a clear indication that a non radiative mechanism is associated with AlAs- GaAs interface. The temperature dependence ofthe lifetime and magneto-optical experiments suggest the existence of a non-radiative level 5 meV above the third electric subband of the Quantum Wells. Transfer ofcharge from GaAs ( F point ) to AlAs ( X point ) is proposed as a mechanism to shorten the lifetime ofthe luminescence process in agreement with theoretical calculations. 1_