Papers by Guillermo Ortiz
Avances en Ciencias e Ingeniería, 2014
Resumen es: Uti lizando elipsometria , se caracteriza l a estructura de la pelicula mediante los ... more Resumen es: Uti lizando elipsometria , se caracteriza l a estructura de la pelicula mediante los indices opticos n, k (region visible, 450 nm < < 580 nm) y el espe...
New Journal of Physics, 2018
Keller's theorem relates the components of the macroscopic dielectric response of a binary two-di... more Keller's theorem relates the components of the macroscopic dielectric response of a binary two-dimensional composite system with those of the reciprocal system obtained by interchanging its components. We present a derivation of the theorem that, unlike previous ones, does not employ the common asumption that the response function relates an irrotational to a solenoidal field and that is valid for dispersive and dissipative anisotropic systems. We show that the usual statement of Keller's theorem in terms of the conductivity is strictly valid only at zero frequency. We verify the theorem numerically in several ordered and disordered systems and discuss some of its consequences.
Optical Materials, 2020
Interfacial roughness is prevalent in 1D photonic crystals and other layered structures, but is n... more Interfacial roughness is prevalent in 1D photonic crystals and other layered structures, but is not generally accounted for in their design nor the analysis of their optical properties due to a lack of simple theoretical approaches. We present a transfer matrix formalism to incorporate the effects of interfacial roughness in the optical properties of stratified systems such as 1D photonic crystals and apply it to calculate the optical response of some nanoporous anodic alumina and porous silicon structures. We have validated our formalism by comparing our results to some experiments.
Anales AFA, 2010
En este trabajo empleamos un método recientemente reportado 1,2 para calcular la conductividad el... more En este trabajo empleamos un método recientemente reportado 1,2 para calcular la conductividad eléctrica efectiva de un sistema compuesto nanoestructurado en la aproximación de longitud de onda larga. Consideramos primero dos tipos simples de red periódica para representar la morfología de un sistema compuesto por Silicio crsitalino y Poros. Suponemos que los poros son columnares y el sistema tiene simetría de traslación en la dirección de los ejes de las columnas, por lo que pueden ser bien representados por un arreglo bidimensional de discos circulares. Encontramos que la conductividad tanto como las propiedadesópticas pueden ser controladas mediante la fracción de llenado de poros (porosidad) y el arreglo espacial de los mismos. Esto se debe a la transición de conductor a aislante del sistema cuando atraviesa por el punto de percolación de fases. El mismo resultado se verifica para el caso desordenado sin correlación entre las posiciones de los poros. Palabras clave: Silicio Poroso, Conductividad, Medios Efectivos, Percolación, Nanoestructuras. In this work we employ a recently reported method 1,2 for the effective electric conductivity calculation of nanostructured composites system assuming the long wavelength approximation. We consider first two type of simple lattice by morphologies representations of a crystalline Silicon and Porous composite system. We assume columnar porous and translation symmetry along column axes, then they are well represented by a bi-dimensional array of circular disk. We find that conductivity so as optical properties may be managed by controlling porous filling fraction (porosity) and porous spatial arrangement. This is due to transition from conductor to insulator like behavior when the system to go through phase percolation point. Same result is verified for disordered case without correlation of porous positions.
ABSTRACT We calculate numerically the self-consistent local field in a fractal aggregate illumina... more ABSTRACT We calculate numerically the self-consistent local field in a fractal aggregate illuminated by an electromagnetic wave in the long wave regime. We employ a hierarchical representation of the aggregate system that allows numerical calculations of very large clusters without truncating the long range of the dipole-dipole interactions. We employ a spectral representation of the response of the system, which allows the separation of material properties from geometrical effects. For cluster-cluster diffusion limited aggregates we find that the density of states shows singular features. The shape of the absorption band and the spatial distribution of the modes display a strong dependence on the dimension of the aggregates and of the embedding space. We discuss the presence of localization in these systems. We acknowledge the partial support from DGAPA-UNAM (grant IN110999) and CIP.
En el presente trabajo se reporta la preparación y caracterización de dispositivos de junturas Sc... more En el presente trabajo se reporta la preparación y caracterización de dispositivos de junturas Schottky basados en silicio poroso nanoestructurado. Se demuestra la emisión electroluminiscente de estos dispositivos, así como en dispositivos de contacto electrolítico, preparados bajo condiciones específicas. Se presentan resultados preliminares de la medición de la intensidad de electroluminiscencia resuelta en energía de los dispositivos Schottky y la evolución temporal de la electroluminiscencia en diodos con contacto húmedo, se presenta en correlación con la caracterización eléctrica. Palabras Claves: Electroluminiscencia Silicio Poroso. The preparation and characterization of nanostructured porous silicon Schottky juncture devices is reported. Electroluminescent emission of these devices as well as that of devices with electrolytic contact, prepared under specific conditions is demonstrated. Preliminary results of measurements of energy resolved electroluminescence spectra of the devices are presented. The temporal evolution of the electroluminescence is presented in correlation with the electrical characterization of diodes with wet contact.
Journal of Optics, 2014
We investigate the capabilities of an effective non-retarded formalism (ENR) for the exploration ... more We investigate the capabilities of an effective non-retarded formalism (ENR) for the exploration and design of nanoparticle composites with specific optical properties. We consider a composite material comprising periodically distributed metallic spheres in a dielectric host matrix. The effective macroscopic dielectric function of the composite medium is obtained by means of the ENR and is used to calculate the electromagnetic response of a slab made of an inhomogeneous material. This response is compared with that obtained by using the layer Korringa-Kohn-Rostoker wave calculation method (LKKR). We analyze the optical properties for different filling fractions, especially in the vicinity of the resonance frequencies of the macroscopic dielectric function. We notice that for dense systems within the long wavelength regime, the results of some analytical theories developed by other authors do not properly describe the multipolar excitations and interactions of orders higher than the dipole, in contrast with the results obtained by using an ENR. Therefore, those methods are not suitable for the design of compound films with novel properties. We show that by appropriately choosing the parameters of the composite, it is possible to achieve a tunable absorber film, and more generally, we show that ENR is a versatile tool for the design of nanoparticle composite materials with specific properties.
physica status solidi (b), 2010
Journal de Physique IV (Proceedings), 2005
In this work we show that the thermal lens effect can be applied to highly scattering and weakly ... more In this work we show that the thermal lens effect can be applied to highly scattering and weakly absorbing materials. We apply the thermal lens effect and the z-scan technique to estimate the effective absorption coefficient of a suspension of TiO 2 particles with a mean diameter of 220 nm at two wavelengths: 488 nm and 514 nm. From the effective absorption coefficient we estimate the absorption cross section of the particles.
Novel nanofabrication techniques are capable of producing nanoparticles with controled structures... more Novel nanofabrication techniques are capable of producing nanoparticles with controled structures which include small clusters, self-assembled particles, quantum dots, vesicles, etc. The non-linear optical scattering of these structures are important for applications, and can be used for their physical characterization. The second harmonic (SH) field radiated by a single small spherical particle has surface and bulk, dipolar and quadrupolar contributions
Physica B: Condensed Matter, 2003
The bulk macroscopic dielectric response of a disordered composite is calculated employing a larg... more The bulk macroscopic dielectric response of a disordered composite is calculated employing a large finite sample instead of an infinite one obtained by replicating a unit cell, as has usually been done. We explicitly account for boundary effects through the macroscopic depolarization field, and finite size corrections are minimized by the large size of the system. The generation of the composite and the calculation of the interactions between the great number of particles become feasible using a newly developed hierarchical multi-resolution scheme. The dielectric response of a composite made up of Ag spheres embedded in glass and the composition independent spectral density agree with those from previous simulations and we discuss their differences with analytical theories based on a renormalized polarizability. Confinement effects due to inhomogeneities close to the boundary of the system are also investigated. r
We investigate the capabilities of an e ective non-retarded formalism
(ENR) for the exploration ... more We investigate the capabilities of an e ective non-retarded formalism
(ENR) for the exploration and design of nanoparticle composites with speci c optical
properties. We consider a composite material comprising periodically distributed
metallic spheres in a dielectric host matrix. The e ective macroscopic dielectric
function of the composite medium is obtained by means of the ENR and is used
to calculate the electromagnetic response of a slab made of such an inhomogeneous
material. This response is compared with that obtained using the layer Korringa-
Kohn-Rostoker wave calculation method (LKKR). We analyze the optical properties
for di erent lling fractions, especially in the vicinity of the resonance frequencies
of the macroscopic dielectric function. We notice that for dense systems within the
long wavelength regime, the results of some analytical theories developed by other
authors do not properly describe the multipolar excitations and interactions of order
higher than the dipole, in contrast to the results of ENR. Therefore, those methods
are not suitable for the design of compound lms with novel properties. We show
that appropriately choosing the parameters of the composite, it is possible to achieve
a tunable absorber lm, and more generally, we show that ENR is a versatile tool for
the design of nanoparticle composite materials with speci c properties.
Physical Review B, 2003
Due to the scale invariance of fractal aggregates, light scattered from them often decays as a po... more Due to the scale invariance of fractal aggregates, light scattered from them often decays as a power of the scattering wave vector. The exponent in this power law has been usually interpreted as the geometrical fractal dimension. However, the validity of this interpretation is questionable for frequencies close to the resonances of the system, for which multiple scattering becomes important.
Ion Mobility Spectrometry (IMS) is a well-known, sensitive and rapid technique to detect dangerou... more Ion Mobility Spectrometry (IMS) is a well-known, sensitive and rapid technique to detect dangerous organic compounds. We propose a system in which a crown type discharge generates a ionic flux that is swept towards an array of collectors by a transverse electric field. The ions are separated as they enter the cell according to their mobility. Thus, the distribution of the charge collected at the detector assembly constitutes a fingerprint for each organic compound. Simulations of our cell and experiments were performed for small amount of Acetone, Ethanol and Toluene. The dependence on the cell parameters of the current and charge versus time of flight was analyzed. Our simulation reproduces only qualitatively the experimental results. However, a PCA statistical analysis of the simulated results shows that such a fingerprint can be used to identify those compounds with certainty.
Journal of Micromechanics and Microengineering
Ion-mobility spectrometry (IMS) is an analytical technique used to separate and identify ionized ... more Ion-mobility spectrometry (IMS) is an analytical technique used to separate and identify ionized gas molecules based on their mobility in a carrier buffer gas. Such methods come in a large variety of versions that currently allow ion identification at and above the millimeter scale. Here, we present a design for a cross-flow-IMS method able to generate and detect ions at the sub-millimeter scale. We propose a novel ion focusing strategy and tested it in a prototype device using Nitrogen as a sample gas, and also with simulations using four different sample gases. By introducing an original lobular ion generation localized to a few ten of microns and substantially simplifying the design, our device is able to keep constant laminar flow conditions for high flow rates. In this way, it avoids the turbulences in the gas flow, which would occur in other ion-focusing cross-flow methods limiting their performance at the sub-millimeter scale. Scalability of the proposed design can contribute to improve resolving power and resolution of currently available cross-flow methods.
4th Iberoamerican Meeting on Optics and 7th Latin American Meeting on Optics, Lasers, and Their Applications, 2001
Optical properties and scaling in fractal aggregates. [Proceedings of SPIE 4419, 752 (2001)]. Gui... more Optical properties and scaling in fractal aggregates. [Proceedings of SPIE 4419, 752 (2001)]. Guillermo P. Ortiz, W. Luis Mochan. Abstract. Within the first born approximation, the calculated angular distribution of the light scattered ...
Journal of the Optical Society of America. A, Optics, image science, and vision, 2005
The energy flux within an opaque medium near an interface is not the sum of an incident plus a re... more The energy flux within an opaque medium near an interface is not the sum of an incident plus a reflected term, as there is a synergistic contribution to the time averaged Poynting vector which involves simultaneously both the incident and reflected fields. Therefore, the well known formula R + T = 1, where R is the reflectance and T the transmittance, does not hold, and furthermore, R and T loose their accepted meaning. We illustrate the perils of assuming energy flux additivity by calculating the transmission and reflection spectrum of a film over a substrate normally illuminated by incoherent light at frequencies in the neighborhood of an optical resonance. We also show that the usual relation between the scattering, absorption, and extinction cross sections for particles immersed within a dissipative host have to be modified to account for the non additivity.
Optics Express, 2010
We present a very efficient recursive method to calculate the effective optical response of metam... more We present a very efficient recursive method to calculate the effective optical response of metamaterials made up of arbitrarily shaped inclusions arranged in periodic 3D arrays. We apply it to dielectric particles embedded in a metal matrix with a lattice constant much smaller than the wavelength of the incident field, so that we may neglect retardation and factor the geometrical properties from the properties of the materials. If the conducting phase is continuous the low frequency behavior is metallic, and if the conducting paths are thin, the high frequency behavior is dielectric. Thus, extraordinary-transparency bands may develop at intermediate frequencies, whose properties may be tuned by geometrical manipulation.
Physica B: Condensed Matter, 2003
It is well known that under the first Born approximation, the differential cross-section ds=dO fo... more It is well known that under the first Born approximation, the differential cross-section ds=dO for light scattered by a fractal of dimension d f scales with wave vector Q as Q Àd f : The use of this formula to interpret experiments performed at frequencies close to the resonances of the system for which multiple scattering is not negligible is questionable. We study the scaling properties of the light scattered by colloidal aggregates employing a novel multi-resolution hierarchical algorithm which allows the study of large systems taking fully into account the long range of the interactions in multiple scattering calculations. We obtain the conditions under which scaling may be present and find that even under resonant conditions, the scattering cross-section may scale with the fractal dimension d f ; but only if the aggregate is larger than a dissipation and frequency-dependent length-scale L h ; confirming within a full vectorial calculation a result previously found for scalar interactions. For smaller aggregates, ds=dO might scale with a different exponent or it might not scale at all, depending on the frequency of light employed. This could explain the discrepancies between experiments performed on similar systems within the strong scattering regime. r
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Papers by Guillermo Ortiz
(ENR) for the exploration and design of nanoparticle composites with speci c optical
properties. We consider a composite material comprising periodically distributed
metallic spheres in a dielectric host matrix. The e ective macroscopic dielectric
function of the composite medium is obtained by means of the ENR and is used
to calculate the electromagnetic response of a slab made of such an inhomogeneous
material. This response is compared with that obtained using the layer Korringa-
Kohn-Rostoker wave calculation method (LKKR). We analyze the optical properties
for di erent lling fractions, especially in the vicinity of the resonance frequencies
of the macroscopic dielectric function. We notice that for dense systems within the
long wavelength regime, the results of some analytical theories developed by other
authors do not properly describe the multipolar excitations and interactions of order
higher than the dipole, in contrast to the results of ENR. Therefore, those methods
are not suitable for the design of compound lms with novel properties. We show
that appropriately choosing the parameters of the composite, it is possible to achieve
a tunable absorber lm, and more generally, we show that ENR is a versatile tool for
the design of nanoparticle composite materials with speci c properties.
(ENR) for the exploration and design of nanoparticle composites with speci c optical
properties. We consider a composite material comprising periodically distributed
metallic spheres in a dielectric host matrix. The e ective macroscopic dielectric
function of the composite medium is obtained by means of the ENR and is used
to calculate the electromagnetic response of a slab made of such an inhomogeneous
material. This response is compared with that obtained using the layer Korringa-
Kohn-Rostoker wave calculation method (LKKR). We analyze the optical properties
for di erent lling fractions, especially in the vicinity of the resonance frequencies
of the macroscopic dielectric function. We notice that for dense systems within the
long wavelength regime, the results of some analytical theories developed by other
authors do not properly describe the multipolar excitations and interactions of order
higher than the dipole, in contrast to the results of ENR. Therefore, those methods
are not suitable for the design of compound lms with novel properties. We show
that appropriately choosing the parameters of the composite, it is possible to achieve
a tunable absorber lm, and more generally, we show that ENR is a versatile tool for
the design of nanoparticle composite materials with speci c properties.