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Due to their extraordinary light confinement properties, plasmonic metasurfaces have attracted much attention in applications in the visible and IR range. Apart from the multitude of applications using the linear transmission properties, the intensity build-up in subwavelength volumes have emerged as an excellent way to observe nonlinear phenomena. The fabrication of the nanoscale structures following the traditional methods, however, still is a cumbersome process. In this work, we fabricated multiple quasi-3D plasmonic crystals via soft nanoimprint lithography, allowing us access to nanoscale variation of structural parameters with an easy scalable fabrication process. We observed the effects on surface lattice resonances as a function of varying lattice parameter, dielectric environment as well as array geometry. While the index-matching has resulted in a sharper resonance, the redshift of the resonances have resulted in a lower energy confinement. We have also experimentally reported the angular dispersion of the surface lattice resonance in both square and hexagonally arranged metasurfaces. Finally, as an initial proof of light confinement in our structures, we have detected the second harmonic signal generated from these metasurfaces, giving us 16 fold enhancement compared to the flat gold surface. To our knowledge, second harmonic generation from quasi-3D plasmonic crystals haven?t been studied, and the linear properties obtained via this study show the necessary improvements to follow in this project.

Tesi amb menció de Doctorat Internacional, (English) The use of semiconductors, metals, or ordinary dielectrics in the process of fabrication of nanodevices is at the front edge of nowadays technology. In the last decade an impressive technological progress has been made towards the miniaturization process, giving birth to the field of nanotechnology. Currently, nanostructures are routinely fabricated and integrated in different photonic devices for a variety of purposes and applications. At the nanoscale, light-matter interaction can display new phenomena, different from those occurring in homogeneous materials or even micrometer-scale optical structures and devices. This scenario makes conventional approximations to the dynamics of light-matter interactions to break down and new strategies must be sought in order to study, understand, and ultimately harness the performance of subwavelength nonlinear optical materials. This is the case of nonlinear interactions and in particular, of nonlinear frequency conversion, a fundamental physical process that lies on the basis of many modern disciplines, from bioimaging in nanomedicine to material characterization in material science and nanotechnology. Nonlinear photonics also holds great promise in laser physics with applications in information technology for optical signal processing and in the development of novel coherent light sources. Thus, a deep understanding of the specific aspects of light-matter interaction at the nanoscale is crucial if one is to properly engineer nanodevices. In this thesis we report comparative experimental and theoretical studies of nonlinear frequency conversion in different strategic materials for photonics having nanoscale dimensions. We start our study with homogeneous layers and project our results to nanostructures, where second and third harmonic conversion efficiencies drastically decrease compared to typical nonlinear optics working conditions. We have developed novel experimental set-ups capable of measuring second and third, (Català) L'ús de semiconductors, metalls o dielèctrics en el procés de fabricació de nanodispositius es troba al davant de la tecnologia actual. Durant l'última dècada s'ha fet un impressionant progrés tecnològic cap al procés de miniaturització, donant lloc al camp de la nanotecnologia. Actualment, nanoestructures es fabriquen rutinàriament i s'integren en diferents dispositius fotònics per a una varietat de propòsits i aplicacions. A l'escala nanomètrica, la interacció llum-matèria pot mostrar nous fenòmens, diferents dels que ocorren en materials homogenis o fins i tot en estructures i dispositius òptics micromètrics. En aquest escenari les aproximacions convencionals per descriure la dinàmica de les interaccions llum-matèria deixen de ser aplicables i s'han de buscar noves estratègies per estudiar, entendre i, en última instància, aprofitar el rendiment dels materials òptics no lineals de dimensions més petites que la longitud d'ona. Aquest és el cas de les interaccions no lineals i, en particular, de la conversió de freqüència no lineal, un procés físic fonamental que es troba a la base de moltes disciplines modernes, des de la bioimatge en nanomedicina fins a la caracterització de materials en ciència de materials i nanotecnologia. La fotònica no lineal també te futur potencial en la física del làser amb aplicacions en tecnologia de la informació per al processament de senyals òptics i en el desenvolupament de noves fonts de llum coherent. Per tant, una comprensió profunda dels aspectes específics de la interacció llum-materia a l'escala nanomètrica és crucial per tal de dissenyar adequadament nanodispositius. En aquesta tesi presentem estudis comparatius experimentals i teòrics de conversió de freqüència no lineal en diferents materials estratègics per a la fotònica amb dimensions nanomètriques. Comencem el nostre estudi amb capes homogènies i projectem els nostres resultats a nanoestructures, on les eficiències de conversió de segon i tercer harmònic disminu, Postprint (published version)

We report a combined experimental/theoretical investigation on second and third harmonic generation from a plasmonic gold nanograting, that exhibits a plasmonic resonance in the near IR. The intense field localization leads to significant enhancement in the efficiency of nonlinear optical processes from the UV to the near IR range: second harmonic generation (SHG) conversion efficiencies increase three orders of magnitude compared to a flat gold mirror, while third harmonic generation (THG) conversion efficiency increases by nearly four orders of magnitude. The spectral and angular dependence (qualitative and quantitative) of the harmonics were experimentally recorded and validated within the framework of our own theoretical model, that we employ to simulate SHG and THG efficiencies. Our approach consists of an expanded microscopic hydrodynamic model that captures the dynamics of both free and bound electrons, and faithfully describes the basic electrodynamic behaviour of linear and nonlinear optics of metals at the nanoscale., LRS, JT, and CC acknowledge Spanish Agencia Estatal de Investigación (project no. PID2019-105089GBI00/AEI/10.130397501100011033) and US Army Research Laboratory Cooperative Agreement N W911NF1920279 issued by US ARMY ACC-APG-RTP., Peer Reviewed, Objectius de Desenvolupament Sostenible::9 - Indústria, Innovació i Infraestructura::9.c - Augmentar de forma significativa l’accés a les tecnologies de la informació i la comunicació i esforçar-se a proporcionar accés universal i assequible a Internet als països menys avançats a tot tardar el 2020, Objectius de Desenvolupament Sostenible::5 - Igualtat de Gènere::5.5 - Vetllar per la participació plena i efectiva de les dones, i per la igualtat d’oportunitats de lideratge en tots els àmbits de presa de decisions en la vida política, econòmica i pública, Objectius de Desenvolupament Sostenible::9 - Indústria, Innovació i Infraestructura, Objectius de Desenvolupament Sostenible::9 - Indústria, Innovació i Infraestructura::9.5 - Augmentar la investigació científica i millorar la capacitat tecnològica dels sectors industrials de tots els països, en particular els països en desenvolupament, entre d’altres maneres fomentant la innovació i augmentant substancialment, d’aquí al 2030, el nombre de persones que treballen en el camp de la investigació i el desenvolupa­ment per cada milió d’habitants, així com la despesa en investigació i desenvolupament dels sectors públic i privat, Objectius de Desenvolupament Sostenible::5 - Igualtat de Gènere, Postprint (published version)

We report experimental observations and numerical simulations of second and third harmonic generation from a gold nanograting, which exhibits a plasmonic resonance in the near infrared. The resonance is tunable, with a spectral position that depends on the angle of incidence. All things being equal, the enhancement of nonlinear optical processes produced by the field localization in the nanograting when compared with a flat gold mirror manifests itself dramatically from the ultraviolet to the visible range: second harmonic generation conversion efficiencies increase by more than three orders of magnitude, while we report a third harmonic generation conversion efficiency enhancement factor of 3200, both in excellent agreement with our theoretical predictions. The clear inferences one may draw from our results are that our model describes the dynamics with unprecedented accuracy and that much remains to be revealed in the development of nonlinear optics of metals at the nanoscale., L.R.-S., J.T., and C.C. acknowledge the Spanish Agencia Estatal de Investigación (Project No. PID2019-105089GB-I00/AEI/10.130397501100011033) and the U.S. Army Research Laboratory Cooperative Agreement No. W911NF1920279 issued by the U.S. ARMY ACC-APG-RTP., Peer Reviewed, Postprint (published version)

© 2023 Optica Publishing Group. Users may use, reuse, and build upon the article, or use the article for text or data mining, so long as such uses are for non-commercial purposes and appropriate attribution is maintained. All other rights are reserved., Nonlinear silicon photonics offers unique abilities to generate, manipulate and detect optical signals in nano-devices, with applications based on field localization and large third order nonlinearity. However, at the nanoscale, inefficient nonlinear processes, absorption, and the lack of realistic models limit the nano-engineering of silicon. Here we report measurements of second and third harmonic generation from undoped silicon membranes. Using experimental results and simulations we identify the effective mass of valence electrons, which determines second harmonic generation efficiency, and oscillator parameters that control third order processes. We can then accurately predict the nonlinear optical properties of complex structures, without introducing and artificially separating the effective ¿(2) into surface and volume contributions, and by simultaneously including effects of linear and nonlinear dispersions. Our results suggest that judicious exploitation of the nonlinear dispersion of ordinary semiconductors can provide reasonable nonlinear efficiencies and transformational device physics well into the UV range., The authors thank Zack Coppens and Dana Dement for useful discussions and help with the sample. LRS, JT, and CC acknowledge to US Army Research Laboratory Cooperative Agreement N° W911NF1920279 issued by US ARMY ACC-APG-RTP and Spanish Agencia Estatal de Investigación (project no. PID2019-105089GBI00/AEI/10.130397501100011033). MAV was partially sponsored by the Army Research Laboratory under Cooperative Agreement N° W911NF2020078., Peer Reviewed, Postprint (published version)

Optical properties of photonic crystals have emerged as being pivotal in the field of nanophotonics. Their artificial periodicity can alter and create new effective dispersion relations compared to bulk materials, allowing us to control the flow of light and light-matter interactions. Specifically, the enhancement of the electromagnetic field obtained either at the surface or inside the nanostructure is very important for applications in optical sensing or light emission. In particular, it is known that by gradually varying the period of the PhC in the direction of propagation of light, a process known as chirping the samples, different wavelengths are localised at different positions in the crystal. This wavelength dependant light localisation has been demonstrated numerically in one and two dimensions in the literature, but there has been little work done to study the phenomenon in three dimensions and experimentally. This project has studied the wavelength dependent localisation of light in chirped woodpile PhC samples in the visible and infrared, both from a numerical and experimental point of view. In the end, a qualitative match between simulations and experiments has been obtained. These structures, in combination with different types of absorptive materials placed at the surface or at particular positions inside the structure target application in sensing., Las propiedades ópticas de cristales fotónicos han resultado ser clave en el campo de la nano-fotónica. Su periodicidad, creadas artificialmente, nos permite diseñar nuevas relaciones de dispersión efectivas, imposibles de obtener en materiales convencionales. Esto nos permite controlar el flujo de luz y las interacciones luz-materia. Concretamente, la amplificación del campo electromagnético en la superficie o dentro de la estructura manométrica es crucial en aplicaciones relacionadas con sensores ópticos y emisión de luz. En particular, se sabe que variando gradualmente el periodo de los cristales en la dirección de propagación de la luz, un proceso conocido como añadir "chirp" en los cristales, diferentes longitudes de onda se localizan en diferentes posiciones dentro de los cristales. Esta localización en función de la longitud de onda se ha estudiado sobre todo numéricamente en cristales de una o dos dimensiones, sin embargo, pocos estudios han estudiado el fenómeno de manera experimental o en cristales tridimensionales. Específicamente este proyecto se ha centrado en la caracterización de cristales "woodpile" en el visible y en el infrarrojo, tanto experimental como numéricamente. Finalmente, se ha llegado a una relación cualitativa entre simulaciones y experimentos. Creemos que estas estructuras, combinadas con diferentes tipos de materiales absorbentes situados en la superficie o en posiciones concretas dentro de los cristales, pueden tener aplicaciones en sensores., Les propietats òptiques dels cristalls fotònics han resultat ser clau en el camp de la nano-fotònica. La seva periodicitat, creada artificialment, ens permet dissenyar noves relacions de dispersió efectives, impossibles d'obtenir en materials convencionals. Això ens permet controlar el flux de llum i les interaccions llum-matèria. Concretament, l'amplificació del camp electromagnètic a la superfície o dins l'estructura nanomètrica és crucial en aplicacions relacionades amb sensors òptics i emissió de llum. En particular, se sap que fent variar gradualment el període dels cristalls en la direcció de propagació de la llum, procés conegut com a afegir "chirp" als cristalls, diferents longituds d'ona es localitzen a diferents posicions dins els cristalls. Aquesta localització en funció de la longitud d'ona ha estat comprovada numèricament en cristalls d'una o dues dimensions, però pocs estudis han estat fets de cara a estudiar el fenomen experimentalment o cristalls tridimensionals. Aquest projecte s'ha centrat en la caracterització de cristalls woodpile en el visible i l'infraroig, tan experimental com numèricament. Finalment, s'ha aconseguit arribar a una relació qualitativa entre simulacions i experiments. Creiem que aquestes estructures, combinades amb diferents tipus de materials absorbents situats a la superfície o en posicions concretes dins els cristalls tenen potencials aplicacions com a sensors.

We report contrasted experimental and theoretical results on second and third harmonic generation from an amorphous silicon nanolayer. We perform this study by analysing the harmonic optical signals generated when fs and ps pulses tuned at 800nm and 1064nm, respectively, interact with the material. We measure second and third harmonic efficiencies as functions of angle of incidence and compare with numerical simulations based on a microscopic hydrodynamic model that accounts for different possible contributions to the nonlinear polarization. We measure and estimate the efficiencies of the nonlinear signals using no assumptions about effective surface or volume nonlinearities, enabling proper explanations about the microscopic origins of SH and TH signals arising from the amorphous silicon nanolayer., Peer Reviewed, Postprint (published version)

This project is devoted to the study and implementation of single-shot characterization method of ultrashort laser pulses using second- harmonic generation in nonlinear ferroelectric crystals. The main objective is the development of a program that uses the experimental measurement of the auto- correlation trace and measures the duration and other parameters of the Gaussian pulse by image processing., Este proyecto se dedica al estudio e implementación del método de "single-shot characterization" de pulsos láser ultracortos utilizando generación de segundo harmónico en cristales ferroeléctricos no lineales. El objetivo principal es el desarrollo de un programa que utiliza la medida experimental de la traza de autocorrelación y mide la duración y otros parámetros del pulso gaussiano por procesamiento de imagen., Aquest projecte es dedica a l'estudi i implementació del mètode de "single-shot characterization" de polsos ultracurts utilitzant generació de segon harmonic en cristalls ferroelèctrics no lineals. L'objectiu principal és el desenvolupament d'un programa que faci servir la mesura experimental de la traça d'autocorrelació i calculi la durada i altres paràmetres del pols gaussià mitjançant processament d'imatges.

Mechanical uniaxial tensile strain affects the performance of high power laser diode bars (LDBs) inducing changes in the properties of the laser emission. Properties such as temperature tunning coefficient, far field divergence, polarization, threshold current and slope efficiency are altered. In this work we study how those variations happen and if the behaviour correlates with studies carried out to date. The experimental results seem to coincide with the stipulations regarding the variations depending on the position of emitters in the bars. However, when we variate the uniaxial tensile stress applied there are not correlations in our results, and therefore this question needs to be analysed in more detail.

Chiral metasurfaces can transmit one circularly polarized light, while almost completely refuse the opposite handedness. In this project, a comparison of the optical properties of metallic vs high dielectric 2D chiral metasurfaces is made. Firstly, we discuss the general method to fabricate a chiral metasurface using the scalable technique of nanoimprinting soft lithography. Subsequently, numerical finite-difference time-domain (FDTD) simulations have been performed and compared with the experimentally measured chiroptical response of the nanoimprinted samples. These calculations show that each kind of metasurface presents a particular circular dichroism (CD), as plasmonic metasurfaces present better CD in transmittance due to their higher absorption, whereas the high dielectric materials can potentially generate chiral photoluminescence.

With huge amounts of data being generated every second, the demand for parallelized, high speed, and efficient computing power is rising rapidly, pushing the limits of existing computing paradigms. In this circumstance, photonic computing hardware is a promising alternative to conventional electronics with prospects of speed and remarkably power efficient at accelerating multiply-accumulate (MAC) operations. Moreover, optical computing enables massive parallelism over their electronic counter parts through wavelength division multiplexing. This work involves the design and fabrication of an integrated photonic tensor core (PTC) capable of performing 60 millon MAC operations per second. Optical computing hardware makes use of multiple electro-optic and digital-analog converters. This work also involves the design and characterisation of a dedicated electronic interface to feed data to the PTC. In order to demonstrate the application potential, we perform convolution processing on 2D images in the optical domain with the newly developed hardware.

The use of semiconductors, metals, or ordinary dielectrics in the process of fabrication of nanodevices is at the front edge of nowadays technology, exploiting the properties of light propagation and localization at nanometric scale in new and surprising ways. Understanding accurately how light interacts with these materials at the nanoscale is crucial if one is to properly engineer nano-devices. When the nanoscale is reached, light-matter interactions display new phenomena where conventional approximations may not always be applicable and they should be either revised or voided. In this work, we measure the efficiency of second and third harmonic generation from gold nanolayers. The experimental results are compared with numerical simulations based on a detailed microscopic hydrodynamic model that considers different effects playing a role in the nonlinear response, not usually considered by more generic models. The agreement between experimental and theoretical results proves the importance of all these contributions., Peer Reviewed, Postprint (published version)

Nanolasers are a fundamental piece of all-optical integrated circuits and their several applications. In this work, the nanolaser is a system of two coupled nanocavities in a semiconductor photonic crystal, characterized by a high Qfactor, tunable coupling, a nonlinear quantum well gain medium, and improved beaming. A theoretical introduction including the physical basis of the system and the sample description is provided. Also, the experimental set-up and the measurement methodology are described. ASThe system is firstly characterized under the optical pump regime, in which laser regime phenomena like before-threshold wavelength blueshift are observed. The coupled system exhibits a hybridization between bonding and antibonding modes, altered by system properties like coupling or detuning between cavities. Mode competition and mode reversing are also observed for several situations. ASFurthermore, the system is taken as two coupled resonators and coherently injected. The transmitted linear resonances are recorded for different parameters, observing mode hybridization, mode extinction, and mode reversing around zero couplings. Moreover, the phenomenon of avoided mode crossing is firstly observed in a system of photonic crystal nanocavities., Los nanoláseres son una pieza fundamental de los circuitos integrados totalmente ópticos y su varias aplicaciones. En este trabajo, el nanolaser es un sistema de dos acoplados nanocavidades en un cristal fotónico semiconductor, caracterizado por un factor Q alto, un acoplamiento sintonizable, un medio de ganancia de pozo cuántico no lineal y una mejora radiante. Una introducción teórica que incluye la base física del sistema. y se proporciona la descripción de muestra. Además, de la configuración experimental se describe la metodología de medición. AS El sistema se caracteriza en primer lugar bajo el régimen de bombeo óptico, en el que se observan fenómenos del régimen del láser como el desplazamiento al azul de la longitud de onda antes del umbral. El sistema acoplado exhibe una hibridación entre unión y antienlazamiento. Modos, alterados por propiedades del sistema como acoplamiento o desafinación entre cavidades. La competencia de modo y la inversión de modo, también se observan en varias situaciones. AS Además, el sistema se toma como dos resonadores acoplados y se inyecta coherentemente. Las resonancias lineales transmitidas se registran para diferentes parámetros: modo de observación, de hibridación, modo de extinción,e inversión de modo alrededor de acoplamientos cero. Además, el fenómeno del cruce de modos evitado se observa en primer lugar en un sistema de nanocavidades de cristales fotónicos., Els nanolasers són una peça fonamental de circuits òptics integrats i les seves diverses aplicacions. En aquest treball, el nanolaser és un sistema de dos nanocavitats d'un cristall fotònic semiconductor, caracteritzat per un elevat factor de qualitat, un acoblament modificable, un medi actiu no lineal amb pous quàntic mitjà i un "beaming" millorat. Es proporciona una introducció teòrica que inclou la base física del sistema i també la descripció de la mostra. A més, es descriu la configuració experimental i la metodologia de mesura. El sistema es caracteritza primer pel règim de pompatge òptic, en el qual s'observen fenòmens de règim làser com el desplaçament blau de longitud d'ona abans del llindar làser. El sistema acoblat presenta una hibridació entre modes, alterats per propietats del sistema com l'acoblament o desafinació entre cavitats. La competència i inversió entre modes també es caracteritzen per diverses situacions. A més, el sistema es pren com a dos ressonadors acoblats i s'injecta de manera coherent. Les ressonàncies lineals transmeses es registren per diferents paràmetres, observant la hibridació dels modes, l'extinció dels modes i la inversió dels modes al voltant de zero acoblaments. A més, el fenomen del creuament de modes evitat s'observa en primer lloc en un sistema de nanocavitats de cristalls fotònics.

The use of semiconductors, metals and conductive oxides in the process of fabrication of actual nano devices is at the front edge of nowadays technology, exploiting the properties of light propagation and localization at nanometric scale in new and surprising ways. At these scales the usual theory describing the nonlinear (NL) effects of electromagnetic fields should be revisited and analyzed. We report a collection of experimental results of nonlinear harmonic generation in different nanolayers: semiconductors, conductive oxides and metals. The comparison of these experimental results with numerical predictions of our theoretical model identifies, distinguishes and explains the different nonlinear contributions to the harmonics generated by these materials at nanoscale. Our model accounts for surface, magnetic and bulk nonlinearities arising from free and bound charges, preserving linear and nonlinear dispersion, nonlocal effects due to pressure and viscosity., Postprint (published version)

Currently, nanostructures are routinely fabricated and integrated in different photonic devices for a variety of purposes and applications. For instance, in order to engineer properly nano-antennas or filters, it is important to understand accurately how light interacts with metals, semiconductors, or ordinary dielectrics at the nanoscale. When the nanoscale is reached, light-matter interactions displays new phenomena and conventional approximations may not always be applicable. Thus, new strategies must be sought in order to study and understand light-matter interactions at the nanoscale. In this work, we present experimental results of second and third harmonic generation from gold nanolayers, raveling novel behavior at nanoscale. These measurements are compared with numerical simulations based on a microscopic hydrodynamic model which accounts for surface, magnetic and bulk nonlinearities arising from both free and bound charges, preserving linear and nonlinear dispersion, nonlocal effects due to pressure and viscosity, and an intensity dependent free electron density, to which we refer as hot electrons contribution., Postprint (published version)

Understanding how light interacts with matter at the nanoscale is pivotal if one is to properly engineer nano-antennas, filters and other devices whose geometrical features approach atomic size. We report experimental results on second and third harmonic generation from 20 nm- and 70 nm-thick gold layers, for TE- and TM-polarized incident light pulses. We discuss the relative roles that bound electrons and an intensity dependent free electron density (hot electrons) play in third harmonic generation. While planar structures are generally the simplest to fabricate, metal layers that are only a few nanometers thick and partially transparent are almost never studied. Yet, transmission offers an additional reference point to compare experimental measurements with theoretical models. Our experimental results are explained well within the context of the microscopic hydrodynamic model that we employ to simulate second and third harmonic conversion efficiencies. Using our experimental observations we estimate |¿(3)1064¿¿|˜10-18 (m/V)2, triggered mostly by hot electrons., Postprint (published version)

We present a theoretical and experimental analysis of the plasmon surface latticeresonances arising from gold nanoparticle assemblies. The metamolecules of the array arecomposed of colloidal gold nanoparticles and as the system is heated up, creating a singlepolycrystalline metamolecule. They were characterized by several microscopy methods forretrieving the shape. Subsequently, the optical resonances from the annealed plasmonic crystalswere obtained by measuring the transmission at normal incidence and at several angles, and thencompared to FDTD simulations for adjudicating field components to each resonance. Finally,photoluminiscence experiments were carried out in order to analyze the photoluminiscenceenhancement and the potential applications for lasing.

At a time when computing resources, telecommunication requirements and data processing are exploding worldwide, nanophotonics has emerged in the last decade as an important tool that, together with microelectronics, provides answers to the new technological needs of our society. In particular, nanophotonics requires the development of robust and energy-efficient nanoscale light sources with advanced optical functionalities. In this project, a laser source based on a system with an active optical nanocavity made of a photonic crystal is investigated. It is evanescently coupled to silicon on insulator waveguide circuitry, forming what is called a hybrid Si/III-V photonic crystal nanolaser, which makes the most of both materials. The theoretical model for this specific nanolaser is simulated, and the system fabrication process described, followed by the experimental characterization of its emission properties, i.e. output power and emission wavelength, around the laser threshold value. The influence of geometrical parameters of the structure on the emission wavelength is proven, and the most strongly coupled configuration is measured for 350 nm of waveguide width. The fabrication is shown to be reproducible. Finally, a novel hybrid system consisting of two identical nanocavities coupled to the waveguide is presented. The preliminary work on this coupled cavity system deals with the construction of a novel nano-heating system. It exploits the thermorefractive effect to arbitrarily shift and match the emission wavelength of both cavities to take advantage of the rich physics of such a system., En un momento en que los recursos informáticos, las necesidades de las telecomunicaciones y el procesamiento de datos están explotando en todo el mundo, la nanofotónica ha surgido en la última década como una herramienta importante que, junto con la microelectrónica, proporciona soluciones a las nuevas necesidades tecnológicas de nuestra sociedad. En particular, la nanofotónica requiere el desarrollo de fuentes de luz a nanoescala robustas y energéticamente eficientes con funcionalidades ópticas avanzadas. En este proyecto se investiga una fuente láser, basada en un sistema con una nanocavidad óptica activa hecha de un cristal fotónico. Se acopla evanescentemente a circuitos de guía de onda de silicio sobre aislador, formando lo que se llama un nanoláser híbrido de cristal fotónico III-V/Si, que aprovecha al máximo ambos materiales. Se simula el modelo teórico de este nanoláser específico y se describe el proceso de fabricación del sistema, seguido de la caracterización experimental de sus propiedades de emisión, es decir, de la potencia de salida y de la longitud de onda de emisión, alrededor del valor umbral del láser. Se confirma la influencia de los parámetros geométricos de la estructura en la longitud de onda de emisión, y la configuración más fuertemente acoplada se mide para 350 nm de ancho de guía de onda. Se demuestra que la fabricación es reproducible. Finalmente, se presenta un novedoso sistema híbrido que consta de dos nanocavidades idénticas acopladas a la guía de onda. El trabajo preliminar en este sistema de cavidad acoplada se ocupa de la construcción de un nuevo sistema, llamado nanocalefactor. Explota el efecto termorrefractivo para cambiar arbitrariamente y hacer coincidir la longitud de onda de emisión de ambas cavidades para aprovechar las ricas propiedades físicas de dicho sistema., En un moment en què els recursos informàtics, les necessitats de les telecomunicacions i el processament de dades estan explotant a tot el món, la nanofotònica ha sorgit en l'última dècada com una eina important que, juntament amb la microelectrònica, proporciona solucions a les noves necessitats tecnològiques de la nostra societat. En particular, la nanofotònica requereix el desenvolupament de fonts de llum a nanoescala robustes i energèticament eficients amb funcionalitats òptiques avançades. En aquest projecte s'investiga una font làser, basada en un sistema amb una nanocavitat òptica activa feta de cristall fotònic. S'acobla evanescentement a circuits de guia d'ona de silici sobre aïllador, formant el que es diu un nanoláser híbrid de cristall fotònic III-V/Si, que aprofita al màxim tots dos materials. Se simula el model teòric d'aquest nanoláser específic i es descriu el procés de fabricació del sistema, seguit de la caracterització experimental de les seves propietats d'emissió, és a dir, de la potència de sortida i de la longitud d'ona d'emissió, al voltant del valor llindar del làser. Es confirma la influència dels paràmetres geomètrics de l'estructura en la longitud d'ona d'emissió, i la configuració més fortament acoblada es mesura per a 350 nm d'ample de guia d'ona. Es demostra que la fabricació és reproduïble. Finalment, es presenta un nou sistema híbrid que consta de dues nanocavitats idèntiques acoblades a la guia d'ona. El treball preliminar en aquest sistema de cavitat acoblada es tracta de la construcció d'un nou sistema, anomenat nanocalefactor. Aquest explota l'efecte termorrefractiu per a canviar arbitràriament i fer coincidir la longitud d'ona d'emissió de totes dues cavitats per tal d'aprofitar les riques propietats físiques d'aquest sistema.

We have presented a combined experimental and numerical method that can be used to predict complex nonlinear dispersion curves in almost any material, based exclusively on the experimental determination of harmonic generation conversion efficiencies. We used a hydrodynamic approach that faithfully duplicates linear dispersion, and predicted the wavelength dependent nonlinear response. The method is particularly useful when analytical solutions are not available, as is the case for conductive oxides like ITO, which displays nonlocal effects that trigger an effective anisotropy, and extendable to second order bulk nonlinearities. Our experimental results for THG in silicon suggest that it is possible to exploit its large nonlinear response in the visible and UV ranges, thus opening up new prospects for silicon photonics (Scalora et al., 2019), Postprint (published version)

This project aims to contribute to two important research areas in Photonics: nonlinear optics and sub-wavelength nano-structured optical materials (nanomaterials). These inter-connected fields have major applications in the development of photonic devices at the nanoscale(NanoPhotonics). Nowadays nanomaterials are produced and integrated into an important number of devices and applications of Photonics. At nanometric scale, new phenomena arise and new strategies must be sought in order to understand and develop methods to harness the performance of deeply sub-wavelength optical materials having geometrical features that are a few atomic diameters in size. Nonlinear Optics (NLO) covers different processes, such as parametric frequency conversion, nonlinear scattering, ultrashort pulse generation, and characterization and sensing, to name a few. Traditionally, most efforts have been focused on the study of efficient harmonic generation in phase-matching (momentum conservation) conditions in thick materials having large nonlinearities (NL), when the harmonic generation is dominated by the electric dipole (?bulk?) contribution to the polarization density of the materials. However, when the material size is reduced to the nanoscale, the efficiency of the bulk contribution is drastically reduced or even vanish and other nonlinear contributions, arising from electric quadrupole or magnetic dipole interactions, become important and must be considered. For instance, in centrosymmetric materials like metals or Si, the bulk quadratic nonlinearity vanishes so that other nonlinear sources become essential to explain NL effects, such as surface harmonic generation. The presence of these terms can account for NL conversion in different situations traditionally considered as hostile environments for a harmonic generation. The experimental detection of very weak harmonic signal generated at the nanoscale is essential to obtain unique and unprecedented information in surface charact

[EN] In this work, we numerically investigate the diffraction management of longitudinal elastic waves propagating in a two-dimensional metallic phononic crystal. We demonstrate that this structure acts as an "ultrasonic lens", providing self-collimation or focusing effect at a certain distance from the crystal output. We implement this directional propagation in the design of a coupling device capable to control the directivity or focusing of ultrasonic waves propagation inside a target object. These effects are robust over a broad frequency band and are preserved in the propagation through a coupling gel between the "ultrasonic lens" and the solid target. These results may find interesting industrial and medical applications, where the localization of the ultrasonic waves may be required at certain positions embedded in the object under study. An application example for non-destructive testing with improved results, after using the ultrasonic lens, is discussed as a proof of concept for the novelty and applicability of our numerical simulation study.

[EN] This work envisages a detailed study of two-dimensional defect localization and reconstruction, using laser generated ultrasound and its application as a remotely controlled non-destructive testing method. As an alternative to full ultrasonic or full optical approaches, we propose a hybrid configuration where ultrasound is generated by impact of laser pulses, while the detection is done with conventional transducers. We implement this approach for defect reconstruction in metallic elements and show that it combines advantages of both photonic and ultrasonic devices, reducing the drawbacks of both methods. We combine our experimental results with a high-resolution signal processing procedure based on the synthetic aperture focusing technique for the benefit of the final two-dimensional visualization of the defects.

We demonstrate a novel dispersion-scan (d-scan) scheme for single-shot temporal characterization of ultrashort laser pulses. The novelty of this method relies on the use of a highly dispersive crystal featuring antiparallel nonlinear domains with a random distribution and size. This crystal, capable of generating a transverse second-harmonic signal, acts simultaneously as the dispersive element and the nonlinear medium of the d-scan device. The resulting in-line architecture makes the technique very simple and robust, allowing the acquisition of single-shot d-scan traces in real time. The retrieved pulses are in very good agreement with independent frequency-resolved optical grating measurements. We also apply the new single-shot d-scan to a terawatt-class laser equipped with a programmable pulse shaper, obtaining an excellent agreement between the applied and the d-scan retrieved dispersions., Postprint (author's final draft)

In this work, we numerically investigate the diffraction management of longitudinal elastic waves propagating in a two-dimensional metallic phononic crystal. We demonstrate that this structure acts as an “ultrasonic lens”, providing self-collimation or focusing effect at a certain distance from the crystal output. We implement this directional propagation in the design of a coupling device capable to control the directivity or focusing of ultrasonic waves propagation inside a target object. These effects are robust over a broad frequency band and are preserved in the propagation through a coupling gel between the “ultrasonic lens” and the solid target. These results may find interesting industrial and medical applications, where the localization of the ultrasonic waves may be required at certain positions embedded in the object under study. An application example for non-destructive testing with improved results, after using the ultrasonic lens, is discussed as a proof of concept for the novelty and applicability of our numerical simulation study., Postprint (published version)

This PhD thesis is devoted to the design, development and implementation of a non-contact hybrid non-destructive testing (NDT) method applied to the analysis of metallic objects that contain embedded defects or fractures. We propose a hybrid opto-acoustic technique that combines laser generated ultrasound as exciter and ultrasound transducers as receivers. This work envisages a detailed study of the detection and one, two or three-dimensional reconstruction of defects, using the proposed hybrid technique and its application as a remotely controlled non-contact NDT. Our device combines several advantages of both photonic and ultrasonic techniques, while reduces some of the drawbacks of both individual methods. Our method relay on the combination of experimental results with high-resolution signal processing procedures based on different mathematical algorithms. Our basic experimental setup uses a nanosecond pulsed laser at 532nm wavelength that impacts onto the surface of the object under study. The laser pulse is rapidly absorbed into a shallow volume of material and creates a localized thermo-elastic expansion inducing a broadband ultrasound pulse that propagate inside the material. The laser beam scans a selected area of the object surface, being remotely controlled by means of a programmable XY scanner. For each excitation point, the ultrasound waves propagate through the object are reflected or scattered by material 3D defects. They are detected by ultrasound transducers and recorded with a PC data-acquisition system for a further process and analysis. As a first step, the time of flight analysis provides enough data for the location and size of the defect in 1D view. The detection capabilities of internal defects in a metallic sample are studied by means of wavelet transform, chosen due to its multi-resolution time-frequency characteristics. A novel algorithm using a density-based spatial clustering is applied to the resulting time frequency maps to estimate th, Esta tesis doctoral versa sobre el diseño, estudio e implementación de un método híbrido, sin contacto, de ensayos no destructivos (NDT, non-destructive testing) para el análisis de objetos metálicos que contienen defectos o fracturas internas. Proponemos una técnica híbrida opto-acústica que combina ultrasonidos generados por impacto láser como excitador y transductores de ultrasonidos como receptores. El trabajo plantea un estudio detallado de la detección y reconstrucción en 1D, 2D y 3D de defectos presentes en un objeto metálico, usando la técnica híbrida de NDT sin contacto y controlado remotamente. Nuestro dispositivo presenta varias ventajas de las técnicas fotónicas y de ultrasonidos, reduciendo al mismo tiempo algunos inconvenientes de dichos métodos tomados por separado. Nuestro método combina resultados experimentales con simulaciones numéricas basadas en el procesado de señal de alta resolución. El montaje experimental consiste en un láser pulsado de ns a una longitud de onda de 532 nm, que impacta sobre la superficie del objeto. El pulso láser se absorbe, creando una expansión termoelástica localizada que induce un pulso de ultrasonidos de banda ancha que se propaga en el material. El láser, controlado remotamente, realiza un barrido sobre un área seleccionada de la superficie del objeto. Por cada punto de excitación, el ultrasonido se propaga a través del objeto y se refleja o dispersa en los defectos del material. Dichas ondas se detectan mediante transductores y se registran en un sistema de adquisición de datos para su ulterior procesado. En un primer paso, mediante el análisis del tiempo de vuelo, podemos localizar y determinar el tamaño del defecto en una vista 1D. Las capacidades de detección de defectos internos en una muestra metálica se estudian también mediante transformación wavelet debido a sus características de multi-resolución en tiempo y frecuencia. Se aplica un algoritmo novedoso de agrupamiento (clustering) espacial y se usan los mapa, Postprint (published version)

We report comparative experimental and theoretical studies of the second and third harmonic generation from a 20 nm-thick indium tin oxide layer in proximity of the epsilon-near-zero condition. Using a tunable optical parametric amplifier, we record both spectral and angular dependence of the generated harmonic signals close to this particular point. In addition to the enhancement of the second harmonic efficiency close to the epsilon-near-zero wavelength, at oblique incidence, third harmonic generation displays an unusual behavior, predicted but not observed before. We implement a comprehensive, first-principles hydrodynamic approach able to simulate our experimental conditions. The model is unique, flexible, and able to capture all major physical mechanisms that drive the electrodynamic behavior of conductive oxide layers: nonlocal effects, which blueshift the epsilon-near-zero resonance by tens of nanometers; plasma frequency redshift due to variations of the effective mass of hot carriers; charge density distribution inside the layer, which determines the nonlinear surface and magnetic interactions; and the nonlinearity of the background medium triggered by bound electrons. We show that, by taking these contributions into account, our theoretical predictions are in very good qualitative and quantitative agreement with our experimental results. We expect that our results can be extended to other geometries where epsilon-near-zero nonlinearity plays an important role., Peer Reviewed, Postprint (published version)

This work envisages a detailed study of two-dimensional defect localization and reconstruction, using laser generated ultrasound and its application as a remotely controlled non-destructive testing method. As an alternative to full ultrasonic or full optical approaches, we propose a hybrid configuration where ultrasound is generated by impact of laser pulses, while the detection is done with conventional transducers. We implement this approach for defect reconstruction in metallic elements and show that it combines advantages of both photonic and ultrasonic devices, reducing the drawbacks of both methods. We combine our experimental results with a high-resolution signal processing procedure based on the synthetic aperture focusing technique for the benefit of the final two-dimensional visualization of the defects., Postprint (author's final draft)

En col·laboració amb la Universitat Autònoma de Barcelona (UAB) i la Universitat de Barcelona (UB), Quantum annealers are ideal platforms for the efficient resolution of optimization problems, such as those that arise in areas as chemical computing when studying the properties of the ground state of molecules. In this paper, we propose an algorithm for calculating the ground state of molecules relying on Gaussian basis sets, a technique used by classical algorithms due to its great computational simplicity. The algorithm provides accurate results for small molecules such as H2 or LiH and, although using a direct product wave function, it scales polynomially with the dimension of the system and linearly with the number of qubits per coefficient