A New learning experience: voluntary preparatory course for the bachelor’s degree in engineering

This paper reports on a new experience carried out at school associated with the Technical University of Catalonia, Universitat Politècnica de Catalunya (UPC) in September 2010. The aim is to describe the organization, funding and results of a preparatory course in Physics for a bachelor's degree in Engineering. We assess the students’ opinions about the experience and whether it should be repeated. We also analyze the profile of the students who access this School, estimating their initial knowledge of physics. The results obtained provide solid evidence that 70% of new students coming from Vocational Education Schools do not meet the required minimums.Peer Reviewe

Propagació i generació de llum en nanoestructures fotòniques

Els materials nanoestructats periòdics han ofert, en les dues darreres dècades, un nou marc per a l'estudi de la interacció entre la radiació electromagnètica i la matèria. Aquestes estructures permeten modelar les propietats electromagnètiques dels materials i han esdevingut una eina idònia per confinar, guiar, suprimir, localitzar, dividir, dispersar, i filtrar la llum. L'abast del control de radiació electromagnètica va des de la propagació fins a la generació de la llum. Els cristalls fotònics han demostrat ser eficients per suprimir o afavorir mecanismes de generació de llum com l'emissió espontània o els processos no lineals.L'eix central d'aquesta tesi se centra en investigar els efectes fintis i fins a quin punt les propietats d'estructures ideals infinites o infinitament periòdiques es mantenen per a estructures que tenen un caràcter finit. Fins fa poc, els desenvolupaments tant experimentals com teòrics en el camp de cristalls fotònics es basaven, principalment, en càlculs que consideraven estructures ideals amb condicions de contorn perfectament periòdiques. Des dels inicis del camp, però, es van observar desajustos a les prediccions fetes amb aquestes condicions. Tanmateix, alguns d'ells resten, en gran part, inexplicats. En aquest el treball, tractem alguns d'aquests aspectes relacionats amb la propagació i generació del llum en cristalls fotònics finits reals, és a dir, com els que es fabriquen. Amb aquest propòsit, en realitzem un estudi tant teòric com experimental. Estudiem els efectes fintis tant en la regió de la primera banda de reflexió de Bragg com en el rang d'energies altes, on la longitud d'ona de la llum és de l'ordre o més petita que el paràmetre de xarxa.En concret, part del treball es dedica a l'estudi dels cristalls col·loïdals en el rang d'energies baixes. Desenvolupem un model vectorial 3D en l'aproximació de Rayleigh-Gans per simular estructures amb contrasts d'índexs baixos. Aquest model contempla aspectes rellevants dels cristalls reals com són ara les condicions de contorn, inclou una lleugera dispersió en el diàmetre de les esferes com també una absorció eficaç que descriu la difusió de Rayleigh i la dispersió inelàstica causada per la presència d'imperfeccions. Aquest model s'utilitza per estudiar la propagació dins de nanoestructures fotòniques reals, i per determinar-ne les propietats dispersives. Les prediccions del model es contrasten amb mesures experimentals dependents de la polarització de l'estructura de bandes parcial d'un cristall col·loïdal real. També determinem el temps de confinament del fotons, a l'extrem de la primera banda fotònica prohibida, per mitjà de la deformació d'un pols provocada per canvis en la velocitat de grup que acompanya l'atrapament dels fotons.Per explicar la propagació de llum en el rang d'energies altes, utilitzem el model vectorial KKR que ens permet determinar la velocitat de grup d'òpals artificials prims. Trobem que, per determinades freqüències, la velocitat de grup pot ser superlumínica, positiva, negativa o tendir a zero depenent del guix del cristall i la seva absorció. Aquest comportament es pot atribuir al carácter finit de l'estructura i explica observacions experimentals presents a la literatura. La mateixa propagació amb velocitats de grup anòmales pot explicar l'observació experimental de l'augment de la generació de segon harmònic en un òpal prim no lineal. Confirmant així que la disminució de velocitat de grup proporciona un mecanisme que afavoreix els processos no lineals.En darrer lloc, considerem una altra configuració en què la interacció no lineal quadràtica té lloc en una capa de menys d'una longitud d'ona de gruix. Demostrem que, en presència d'una superfície reflectora, la contribució de termes que no conserven el moment lineal de la llum, i que no tindrien cap contribució en un medi infinitament llarg, són els més determinants.Photonic periodic nanostructures have offered, in the last two decades, a new framework for the study of the interaction between electromagnetic radiation and matter. Such structures can engineer the electromagnetic properties of materials and have become a powerful tool used to confine, route, suppress, localize, split, disperse, and filter light. The scope of the electromagnetic radiation control can be extended to light propagation and generation. Photonic crystals have successfully been used as host materials to suppress or enhance light generation mechanisms such as spontaneous emission or nonlinear processes.The aim of this thesis is to investigate finite-size effects and to what extent the properties of ideal infinite or infinitely periodic structures hold for structures that are finite in size. Until recently, experimental as well as theoretical developments in the field of photonic crystals have been based, mostly, on calculations that consider ideal structures with perfectly periodic boundary conditions.Deviations from the behavior predicted form such assumptions were already observed when the field was born. However, some of them remained, for the most part, unexplained. In the present work, we tackle some of these aspects related to light propagation and generation in the real finite photonic crystals that can be fabricated. With this purpose, we perform such study from, both, an experimental as well as a theoretical perspective. We study finite-size effects in the region of the first order Bragg reflection band as well as in the high energy range where the wavelength of light is on the order or smaller than the lattice parameter.To be more specific, part of the work is devoted to the study of colloidal crystals at the range of low energy. We develop 3D full wave vector calculation in the Rayleigh-Gans approximation to simulate low index contrast structures. This model accounts for relevant real crystal's aspects such as boundary conditions, a slight dispersion in the spheres diameter and includes an effective absorption accounting for Rayleigh scattering and inelastic diffusion due to imperfections. This model is used to study light propagation within real photonic nanostructures, and to determine their dispersive properties. The predictions of the model are contrasted with experimental polarization dependent measurements of the partial band structure of an actual colloidal crystal. We also determine the experimental photon's lifetime, at the edge of the first order pseudogap, by means of the pulse reshaping induced by changes in the group velocity accompanied by the photon trapping.To explain light propagation in the high energy range, we use a vector KKR calculation that we apply to understand the group velocity of light propagating in artificial opals slabs. We show that for certain frequencies, the group velocity can either be superluminal, positive or negative or approach zero depending on the crystal size and absorption. Such behavior can be attributed to the finite character of the structure and accounts for previously reported experimental observations. The same propagation at anomalous group velocity may explain the experimental observation of second harmonic generation enhancement of light from a nonlinear opal film. Indeed, the group velocity slowing-down provides an enhancement mechanism for nonlinear processes.We finally consider another configuration such that the quadratic nonlinear interaction occurs within a sub-wavelength layer. In the presence of a nearby reflecting surface we demonstrate that the contribution of terms that do not conserve light momentum, and that would vanish in an infinitely long medium, is the most relevant one

Axisymmetric photonic structures with PT-symmetry

Copyright 2016 Society of Photo-Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.PT-symmetric structures in photonic crystals, combining refractive index and gain-loss modulations is becoming a research field with increasing interest due to the light directionality induced by these particular potentials. Here, we consider PT-symmetric potentials with axial symmetry to direct light to the crystal central point obtaining a localization effect. The axial and PT-symmetric potential intrinsically generates an exceptional central point in the photonic crystal by the merge of both symmetries. This particular point in the crystal lattice causes field amplitude gradients with exponential slopes around the crystal center. The field localization strongly depends on the phase of the central point and on the complex amplitude of the PT-potential. The presented work analyzes in a first stage 1D linear PT-axisymmetric crystals and the role of the central point phase that determines the defect character, i.e. refractive index defect, gain-loss defect or a combination of both. The interplay of the directional light effect induced by the PT-symmetry and the light localization around the central point through the axial symmetry enhances localization and allows higher field concentration for certain phases. The linearity of the studied crystals introduces an exponential growth of the field that mainly depends on the complex amplitude of the potential. The work is completed by the analysis of 2D PT-axisymmetric potentials showing different spatial slopes and growth rates caused by symmetry reasons.Peer ReviewedPostprint (published version

Polarization-independent broadband bidirectional optical cloaking using a new type of inverse scattering approach

(c) 2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other works.Since the advent of transformation optics a decade ago [1], the ability to achieve optical cloaking has become a matter of practical realization. However, so far extreme material requirements and large device areas have significantly posed an obstacle to realize compact cloaking schemes that are fully functional. Here, by taking a different approach and by following our recently developed general theorem to control the scattering behaviour of an arbitrary object on a specific demand [2], we show that nearly perfect bidirectional optical cloaking effect can be generated for any type of object with a given shape and size. Contrary to previous approaches, we reveal that such a method is always able to produce local refractive indices larger than one and that neither gain nor lossy materials are required. Furthermore, by means of numerical calculations, we demonstrate a highly tunable broad operational bandwidth of 550 nm (covering 650-1200 nm interval) and an angular aperture of 36° for both directions and polarizations. With these unprecedented features, we expect that the present work will hold a great potential to enable a new class of optical cloaking structures that will find applications particularly in communication systems, defence industry and in other related fields.Peer ReviewedPostprint (author's final draft

Light control by scattering cancellation in ordered and disordered non-Hermitian media, direct and inverse design

Non-Hermitian Physics has emerged as a fertile ground for a smart control of waves. Here, we present direct and inverse-design strategies to achieve ‘on demand’ dynamical manipulation of light by non-Hermitian potentials. The direct approach is based on our recently proposed generalized Hilbert Transform relating the real and imaginary distributions of the complex permittivity to induce spatial symmetry breaking to control scattering, widening the concept Kramers Kronig relations in space. A recipe to design complex potentials to tailor the propagation of light following any vector field, or to generate invisible potentials where light propagates as in free space. The procedure may be applied on any given arbitrary background permittivity distribution being regular or random, extended or localized. Moreover, it is possible to keep the design parameters within realistic limits, even avoiding gain. Beyond this fundamental approach, we also we also present supervised and unsupervised learning techniques for knowledge acquisition in non-Hermitian systems which accelerate the inverse the “on demand” design process. The different proposals may have direct applications to control the wave dynamics in semiconductor lasers or other linear and nonlinear physical systems including cloaking sensors and arbitrary shaped objects.Objectius de Desenvolupament Sostenible::9 - Indústria, Innovació i InfraestructuraPostprint (author's final draft

Beam shaping mechanism in spatially modulated edge emitting broad area semiconductor amplifiers

We investigate beam shaping in broad area semiconductor amplifiers induced by a periodic modulation of the pump on a scale of several microns. The study is performed by solving numerically a (2+1)-dimensional model for the semiconductor amplifier. We show that, under realistic conditions, the anisotropic gain induced by the pump periodicity can show narrow angular profile of enhanced gain of less than one degree, providing an intrinsic filtering mechanism and eventually improving the spatial beam quality

Structured Meta-Mirrors for Beam Spatial Filtering

The work presents optical spatial filtering in reflection based on translationally invariant meta-mirrors. The meta-structure is generated by a thin grating presenting a transverse modulation of the refraction index on the sub-micron scale located in front of a mirror. We analyze the angular spectrum of the reflected waves for different types of structured meta-mirrors as well as the filtering effects of these meta-structures in reflected beams. The comparison between FDTD simulations of full Maxwell equations and different approximated models allows to determine the filtering contribution from the structured cavity and from Mie resonances associated to elements generating the grating.Peer ReviewedPostprint (author's final draft

Slow light enabled wavelength demultiplexing

Photonic crystal waveguides supporting band gap guided modes hold great potential to tailor the group velocity of propagating light. We propose and explore different wavelength demultiplexer design approaches that utilize slow light concept. By altering the dielectric filling factors of each waveguide segment, one can show that different frequencies can be separated and extracted at different locations along the cascaded waveguide. Furthermore, to eliminate the inherent reflection loss of such a design, a composite structure involving a tapered waveguide with a side-coupled resonator is also presented. Such a structure features not only a forward propagating wave but also a backward propagating wave acting as a feedback mechanism for the drop channels. We show that by careful design of the waveguide and the resonator, the destructive and instructive interference of these waves can effectively eliminate the reflection loss and increase the coupling efficiency, respectively. Numerical and experimental verification of the proposed structures show that the targeted frequencies can be coupled out with low cross-talks and moderate quality factors, while maintaining a compact size. © 2016 IEEE.Peer ReviewedPostprint (published version

Non-Hermitian broad aperture semiconductor lasers based on PT-symmetry

In this paper we propose a novel configuration to regularize the complex spatiotemporal dynamics of broad area lasers into bright light beam. It has recently been shown that arbitrary non-Hermitian optical potentials based on local Parity-Time (PT-) symmetry may tailor and control the flow of light, due to the asymmetric mode coupling. We now provide a comprehensive analysis on how this can be applied to stabilize the emission from broad aperture semiconductor lasers. The mechanism relies on a non-Hermitian configuration of the laser potential achieved by simultaneous spatial modulation of the refractive index and gain-loss profiles. This allows concentrating the light into a bright and narrow output beam. We provide a numerical analysis on Vertical Cavity Surface Emitting lasers and Broad Area Semiconductor Lasers. The results indicate a significant intensity enhancement and concentration of the emitted stabilized beam. The proposed mechanism may be technologically achievable, and it is expected to be applicable to regularize the radiation of other broad aperture and microlasers, which typically emit quite random and irregular light patterns. Besides, the reported concentration effect is universal, and could be extended to random and quasi-periodic background potentials.Peer ReviewedPostprint (author's final draft

Narrowing of the far field in spatially modulated edge-emitting broad area semiconductor amplifiers

We perform a detailed theoretical analysis of the far field narrowing in broad-area edgeemitting semiconductor amplifiers that are electrically injected through the contacts periodically modulated in both, longitudinal and transverse, directions. The beam propagation properties within the semiconductor amplifier are explored by a (1+2)-dimensional traveling wave model and its coupled mode approximation. Assuming a weak field regime, we analyze the impact of different parameters and modulation geometry on the narrowing of the principal far field component