arXiv subjects

W. Luis Mochán

Publications and source records attributed to W. Luis Mochán.

At least 19 recordsLinked to original sources

arXiv2023

Efficient homogenization of multicomponent metamaterials: chiral effects

We extend an efficient homogenization procedure based on a Haydock representation of the microscopic wave operator for the calculation of the macroscopic dielectric response of a periodic composite to the case of an arbitrary number of components of arbitrary composition. As a test, we apply our numerical procedure to the calculation of the optical properties of a Bouligand structure, made of a large number of anisotropic layers stacked on top of each other and progresively rotated. This system consitutes a photonic crystal with circularly polarized electromagnetic normal modes, naturally ocurring in the cuticle of several arthropods, and which has a gap for one polarization, which corresponds to the observation of circularly polarized strong metallic like reflections. Our numerical procedure is validated through its good agreement with the analytical solution for this simple chiral system.

physics.optics↗

arXiv2023

Anisotropic optical response of arthopods's cuticle

The structure of the cuticle of arthropods consists of layers of microfilamentary chitin particles. The layers are stacked one on top of the other realizing a helical (Bouligand helix) pattern. This structure generates optical phenomena such as structural colors and birefringence that produce a metalic or iridescent appearance, polarization upon reflection and optical activity. We model the anisotropic optical responses of the cuticle of arthropods using the Photonic package to obtain the macroscopic dielectric tensor of a constitutive layer in the Bouligand helix. We find that its anisotropy depends on the shape of the chitin particles, their reticular arrangement and their filling fractions. As a main result, we obtained a cuticle model with tunable reflectance band gaps that are very sensitive to geometrical parameters such as the angle $θ$ that controls the helix pitch. We can explain the structural colors in terms of the reflectance band gap induced by the modulation of the anisotropy mediated by $θ$, instead of following the usual approach that consider multiple arrangement of layers pairs with different thickness and optical properties.

physics.optics↗

arXiv2021

Cálculo de propiedades ópticas de metamateriales

We present an introduction to metamaterials, some of their optical prop-erties, and examples of their uses. We develop an efficient theory for thecalculation of the macroscopic permittivity of binary systems and systemswith more components, in the non-retarded case and in the general case, andwe present its implementation in a computational package and illustrate itsuse. We discuss some applications regarding the design of optimized nanos-tructured optical devices and we discuss the linear and non-linear properties obtained. Presentamos una introducción a los metamateriales, algunas de sus pro-piedades ópticas y ejemplos de sus usos. Desarrollamos una teoría eficientepara el cálculo de su permitividad macroscópica en sistemas binarios o conmás componentes, en el caso no retardado y en el caso general, y presenta-mos su implementación en un paquete computacional y su uso. Finalizamosdiscutiendo algunas aplicaciones del mismo para el diseño de dispositivos ópti-cos nanoestructurados optimizados y discutimos las propiedades lineales y nolineales obtenidas.

physics.optics↗

arXiv2021

Analytical theory for three wave-mixing processes in a slightly deformed cylinder

The second order optical response of centrosymmetric materials manifests itself mostly at their surface, being strongly suppressed in their bulk. However, the overall surface response is also suppressed in nanoparticles with a centrosymmetric geometry subjected to homogeneous fields. Nevertheless, nanoparticles with a noncentrosymmetric geometry do exhibit second order optical properties. We develop an analytical theory to investigate the second order optical response of a noncentrosymmetric thin cylinder with a slightly deformed cross-section made up of a centrosymmetric material subjected to two monochromatic fields. We calculate the linear and nonlinear near fields perturbatively using the extent of the deformation away from a circular cross-section as the perturbation parameter. We obtain expressions for the quadratic hyperpolarizabilities in terms of the linear response evaluated at the three frequencies involved. We analyze the spectral features of the nonlinear response functions and explore their resonant structure for a model dielectric cylinder. Furthermore, we evaluate the second order radiated fields, the radiation patterns and efficiency of the different quadratic processes. We obtain a strong competition between electric dipolar, magnetic dipolar and electric quadrupolar contributions even for very small deformations.

physics.optics↗

arXiv2021

Entrelazados

A brief dialogue about quantum entanglement (in Spanish).

physics.pop-ph↗

arXiv2021

Ab initio theory of the Drude plasma frequency

We derive a theoretical expression to calculate the Drude plasma frequency $ω_p$ based on quantum mechanical time dependent perturbation theory. We show that in general $ω_p^2$ should be replaced by a second rank tensor, the Drude tensor $\mathcal{D}$, which we express in terms of products of the velocity expectation values integrated over the Fermi surface, and which is amiable to analytical and numerical evaluation. For the Sommerfeld's model of metals our expression yields the ubiquitous plasma frequency $ω^2_p=4πn_e e^2/m_e$. The Drude tensor takes into account the geometry of the unit cell and may be calculated from first principles for isotropic as well as anisotropic metallic systems. We present results for the noble metals, Cu, Ag, and Au without stress and subject to isotropic and uniaxial strains, and we compare the results to those available from experiment.

cond-mat.other↗

arXiv2021

Optimization of wide-band quasi-omnidirectional 1-D photonic structures

We have designed, optimized, fabricated and characterized highly reflective quasi-omnidirectional (angular range of $0-60^\circ$) multilayered structures with a wide spectral range. Two techniques, chirping (a continuous change in thicknesses) and stacking of Bragg-type sub-structures, have been used to enhance the reflectance with minimum thickness for a given pair of refractive indices. Numerical calculations were carried out employing the transfer matrix method and we optimized the design parameters to obtain maximal reflectance averaged over different spectral ranges for all angles. We fabricated some of the optimized structures with porous silicon dielectric multilayers with low refractive contrast and compared their measured optical properties with the calculations. Two chirped structures with thicknesses 21.6 $μ$m and 60.4 $μ$m, resulting in quasi omnidirectional mirrors with bandwidths of 360 nm and 1800 nm, centered at 1160 nm and 1925 nm respectively have been shown. In addition, we fabricated a stacked sub-Bragg mirror structure with a quasi omnidirectional bandwidth of 1800 nm (centered at 1850 nm) and a thickness of 41.5 $μ$m, which is almost two third (in thickness) of the chirped structure. Thus, our techniques allowed us to obtain relatively thin quasi-omnidirectional mirrors with wide bands over different wavelength ranges. Our analysis techniques can be used for the optimization of the reflectance not only in multilayered PS systems with different refractive index contrasts but also for systems with other types of materials with low refractive index contrast. The present study could also be useful for obtaining omnidirectional dielectric mirrors in large spectral regions using different materials, flat focusing reflectors, thermal regulators, or, if defects are included, as filters or remote chemical/biosensors with a wide angular independent response.

physics.optics↗

arXiv2020

Stable Calculation of Optical Properties of Large Non-Periodic Dissipative Multilayered Systems

The calculation of the transfer matrix for a large non-periodic multilayered system may become unstable in the presence of absorption. We discuss the origin of this instability and we explore two methods to overcome it: the use of a total matrix to solve for all the fields at all the interfaces simultaneously and an expansion in the Bloch-like modes of a periodic artificially repeated system. We apply both methods to obtain the reflectance spectra of multilayered chirped structures composed of nanostructured porous silicon (PS). Both methods yield reliable and numerically stable results. The former allows an analysis of the field within all layers while the latter is much more efficient computationally, allowing the design of novel structures and the optimization of their parameters. We compare numerical and experimental results across a wide spectral range from the infrared to the ultraviolet.

physics.optics↗

arXiv2019

Mie Scattering in the Macroscopic Response and the Photonic Bands of Metamaterials

We present a general approach for the numerical calculation of the effective dielectric tensor of metamaterials and show that our formalism can be used to study metamaterials beyond the long wavelength limit. We consider a system composed of high refractive index cylindric inclusions and show that our method reproduces the Mie resonant features and photonic band structure obtained from a multiple scattering approach, hence opening the possibility to study arbitrarily complex geometries for the design of resonance-based negative refractive metamaterials at optical wavelengths.

physics.optics↗

arXiv2019

Recursive Calculation of the Optical Response of Multicomponent Metamaterials

We develop a recursive computational procedure to efficiently calculate the macroscopic dielectric function of multi-component metamaterials of arbitrary geometry and composition within the long wavelength approximation. Although the microscopic response of the system might correspond to non-Hermitian operators, we develop a representation of the microscopic fields and of the response, and we introduce an appropriate metric that makes all operators symmetric. This allows us to use a modified Haydock recursion, introducing complex Haydock coefficients that allow an efficient computation of the macroscopic response and the microscopic fields. We test our procedure comparing our results to analytical ones in simple systems, and verifying they obey a generalized multicomponent Keller's theorem and the Mortola and Stefé's theorem for four component metalic and dielectric systems.

physics.optics↗

arXiv2019

Analytical Theory of Second Harmonic Generation from a Nanoparticle with a Non-Centrosymmetric Geometry

We analytically investigate the effect of a non-centrosymmetric geometry in the optical second harmonic (SH) generation from a particle made of a centrosymmetric material, in the interior of which quadratic optical processes are suppressed. We consider a cylindrical particle with a cross-section that is slightly deformed away from a circle and with a radius much smaller than the wavelength. We calculate the induced linear and nonlinear fields perturbatively in terms of the deformation parameter and obtain the nonlinear dipolar and quadrupolar hyperpolarizabilities, whose spectra we evaluate for metallic and dielectric materials. We show that for very small deformations the dipolar contribution to the response competes with the quadrupolar term, and may even be dominant. We explore the spectra of the hyperpolarizability and identify the contributions to its structure for metallic and dielectric particles. We also discuss the nature of SH radiation at various frequencies and find that it may be dominated by the dipolar or the quadrupolar term, or that both may compete yielding non-symmetric radiation patterns. Our calculation may be employed to assess, calibrate and test numerical SH calculations.

physics.optics↗

arXiv2018

Second-Harmonic Generation in Nano-Structured Metamaterials

We conduct a theoretical and numerical study on the second-harmonic (SH) optical response of a nano-structured metamaterial composed of a periodic array of inclusions. Both the inclusions and their surrounding matrix are made of centrosymmetrical materials, for which SH is strongly suppressed, but by appropriately choosing the shape of the inclusions, we may produce a geometrically non-centrosymmetric system which does allow efficient SH generation. Variations in the geometrical configuration allows tuning the linear and quadratic spectra of the optical response of the system. We develop an efficient scheme for calculating the nonlinear polarization, extending a formalism for the calculation of the macroscopic dielectric function using Haydock's recursion method. We apply the formalism developed here to an array of holes within an Ag matrix, but it can be readily applied to any metamaterial made of arbitrary materials and for inclusions of any geometry within the long-wavelength regime.

cond-mat.mes-hall↗

arXiv2017

Keller's Theorem Revisited

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.

physics.optics↗

arXiv2016

Optical and Electrical Properties of Nanostructured Metallic Electrical Contacts

We study the optical and electrical properties of silver films with a graded thickness obtained through metallic evaporation in vacuum on a tilted substrate to evaluate their use as semitransparent electrical contacts. We measure their ellipsometric coefficients, optical transmissions and electrical conductivity for different widths, and we employ an efficient recursive method to calculate their macroscopic dielectric function, their optical properties and their microscopic electric fields. The topology of very thin films corresponds to disconnected islands, while very wide films are simply connected. For intermediate widths the film becomes semicontinuous, multiply connected, and its microscopic electric field develops hotspots at optical resonances which appear near the percolation threshold of the conducting phase, yielding large ohmic losses that increase the absorptance above that of a corresponding homogeneous film. Optimizing the thickness of the film to maximize its transmittance above the percolation threshold of the conductive phase we obtained a film with transmittance T = 0.41 and a sheet resistance $R_{\square}^{\mathrm{max}}\approx2.7Ω$. We also analyze the observed emission frequency shift of porous silicon electroluminescent devices when Ag films are used as solid electrical contacts in replacement of electrolytic ones.

physics.optics↗

arXiv2016

Tailored Optical Polarization in Nano-Structured Metamaterials

A very efficient method for the calculation of the effective optical response of nano-structured composite systems allows the design of metamaterials tailored for specific optical polarization properties. We use our method to design 2D periodic arrays of sub-wavelength dielectric inclusions within nanometric supported metallic thin films which behave as either an almost perfect linearly dichroic system, as a controllable source of circular polarized light, as a system with a large circular dichroism, or as a circular polarizer. All of these systems may be tuned over a wide energy range.

physics.optics↗

arXiv2013

The Casimir Effect for Arbitrary Optically Anisotropic Materials

We extend a fictitious-cavity approach to calculate the Casimir effect for cavities bounded by flat anisotropic materials. We calculate the energy, force and torque in terms only of the optical coefficients of the walls of the cavity. We calculate the Casimir effect at zero and finite temperature for some simple systems. As a non trivial application, we calculate the torque between a semi-infinite anisotropic plate and an anisotropic film. We study the effect of the film thickness in the torque and find an optimal width that maximizes the torque.

quant-ph↗

arXiv2013

Effectiveness of the GT200 Molecular Detector: A Double-Blind Test

The GT200 is a device that has been extensively used by the Mexican armed forces to remotely detect and identify substances such as drugs and explosives. A double blind experiment has been performed to test its effectivity. In seventeen out of twenty attempts, the GT200 failed in the hands of certified operators to find more than 1600 amphetamine pills and four bullets hidden in a randomly chosen cardboard box out of eight identical boxes distributed within a 90m$\times$20m ballroom. This result is compatible with the 1/8 probability expected for a completely ineffectual device, and is incompatible with even a moderately effective working one.

physics.soc-ph↗

arXiv2009

Effective Dielectric Response of Metamaterials

We use a homogenization procedure for Maxwell's equations in order to obtain in the local limit the frequency ($ω$) dependent macroscopic dielectric response $ε^M(ω)$ of metamaterials made of natural constituents with any geometrical shape repeated periodically with any structure. We illustrate the formalism calculating $ε^M(ω)$ for several structures. For dielectric rectangular inclusions within a conducting material we obtained a very anisotropic response which changes along one direction from conductor-like at low $ω$ to a resonant dielectric-like at large $ω$, attaining a very small reflectance at intermediate frequencies unrelated to surface plasmon excitation and which can be tuned through geometrycal tayloring. A similar behavior is obtained for other shapes close to the percolation threshold.

cond-mat.mtrl-sci↗