Memoria De Tesis Para Optar Al Grado De Doctor - UAM

ResumenGracias al reciente desarrollo de la nanotecnología, se ha hecho factible la fabricaciónde diferentes materiales y estructuras en la nanoescala, lo que ha permitido en lasúltimas décadas una intensa investigación de la interacción luz-materia en sistemas dereducida dimensionalidad. En este sentido la plasmónica, que estudia las resonanciaselectrónicas excitadas mediante campos electromagnéticos, atrae cada vez másatención debido a su capacidad de confinar eficientemente dichos campos en escalasnanométricas, lo que permite manipular la luz en la nanoescala y facilitar laminiaturización de numerosos dispositivos de interés.Para realizar una manipulación completa de la luz, son necesarios elementos activostales como moduladores y aisladores ópticos. Para ello, el efecto magneto-óptico (MO)es una de las mejores alternativas, con un campo magnético externo como elementode excitación. Esto permite obtener velocidades de operación ultrarrápidas ymodulaciones relativamente grandes. Además, el efecto MO es en sí mismo norecíproco, por lo que se ha aplicado ampliamente en aisladores ópticos.Por lo tanto, al combinar plasmónica y magneto-óptica en una nanoestructura, losllamados sistemas magnetoplasmónicos dispondrán de las ventajas de ambos:miniaturización de los dispositivos y manipulación activa de la luz.En esta tesis, siguiendo esta idea, se fabrica y estudia una nanoestructura típica conresonancias plasmónicas bimodales –el nanoanillo- con la funcionalidad MOadicional introducida en el seno del mismo. Tanto el protocolo de nanofabricacióncomo las propiedades ópticas / MO de diferentes tipos de nanoanillos se estudian endetalle.7

ResumenEl protocolo de fabricación se implementa basándose en la técnica de litografíacoloidal con máscara-agujero (HCL), que permite fabricar nanoestructuras conmúltiples componentes y distribuidas aleatoriamente en un área grande (más de cm2)en un solo proceso. Con este método, la ubicación espacial específica de cadaelemento consituyente del nanoanillo (Au como componente plasmónico y Co comocomponente magnético en nuestro caso) se puede controlar en las tres dimensiones(3D) con precisión en la nanoescala. En este trabajo de tesis se optimizan dosparámetros importantes del proceso HCL, a saber, la densidad de las esferas de PSutilizadas para definir los nanoagujeros en la máscara y el grosor de la máscara en sí.Controlando la morfología general de la nanoestructura y las localizaciones espacialesde los diferentes componentes dentro del nanoanillo, somos capaces de controlarfinamente las características resonantes plasmónicas de las nanoestructuras y comoconsecuencia la respuesta MO, tanto en términos de posición espectral como deintensidad.A partir de estructuras de nanodiscos de multicapas Au/Co/Au depositadas aincidencia normal, el ángulo de deposición se incrementa gradualmente hasta uncierto valor, con lo que el nanodisco evoluciona morfológicamente hacia la estructurananoanillo. Como consecuencia, la resonancia plasmónica unimodal característica delnanodisco evoluciona a la bimodal del nanoanillo. El efecto de la resonancia deplasmón en la actividad MO de estas estructuras presenta una evolución similar amedida que evoluciona la estructura - de unimodal para nanodisco a bimodal parananoanillo. Este comportamiento se explica con el modelo de hibridación plasmónicacomparando con resultados teóricos.Además, al aumentar el ángulo de deposición de la capa superior de Au, la seccióntransversal de la estructura de nanoanillo se puede sintonizar de manera detallada, conlo que las posiciones espectrales de las resonancias plasmónicas y los picos deactividad MO pueden ser controlados finamente. Por otra parte, la capa continuacentral de Co en el nanoanillo se puede substituir por sectores opuestos de Co o pordiscos opuestos de Co, lo que resulta en anisotropía óptica, con una actividad MObásicamente proporcional a la cantidad de Co.Por último, con el propósito de mejorar aún más el efecto MO en tales estructuras, segenera una brecha en el anillo superior de Au, justo encima del disco de Co, lo quepermite un mejor “enfoque” del campo electromagnético en el elemento MO activo8

Resumen(disco de Co), con un aumento adicional de la actividad MO de un factor 3. Lalocalización del campo EM en el área de separación de dicha estructura propuesta seconfirma adicionalmente tanto teóricamente mediante cálculos de distribución decampos, como experimentalmente con cartografía de señales EEL (pérdida de energíade electrones).9

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AbstractLight-matter interaction is an old but still active and fascinating field, with newphenomena and theories emerging consistently. Thanks to the development of thenanotechnology in the last decades, fabrication of different materials and structures inthe nanoscale becomes feasible, therefore the light-matter interaction in nanoscaleobtains a vast and intensive research. Among this field, plasmonics, which studies theresonantly oscillating electrons (plasmons) driven by the light field in the metallicnanostructures, draws more and more attention due to its ability to confine the lightfield efficiently in nanometric scale. This ability, which makes it possible tomanipulate light in nanoscale and actually can miniaturize greatly the device volume,has already found potential application in the integrated optical circuits for opticalcomputation. Furthermore, the confined light field in such a small space results in thegreatly enhanced localized electromagnetic (EM) field which can be utilized toenhance the other light-matter interaction, e.g. the famous application in surfaceenhanced Raman scattering (SERS). On the other hand, this enhanced localized fieldis sensitive to the environmental refractive indices, which makes it useful for theconcept of “lab-on-chip” for biological and chemical sensing.For a full light circuit, light manipulators such as light modulator and optical isolator,are necessary, which allow modulating light wavevector and permit the light to passthrough only along one direction but the opposite direction is forbidden(non-reciprocal effect), respectively. For this purpose, magneto-optical (MO) effect isa good option to manipulate the light actively (rotate the light polarization or controllight reflection intensity) by the active magnetic field since it can realize an ultrafastoperation speed (femtosecond level) and relatively large modulation depth. Moreover,the MO effect itself is non-reciprocal. Therefore, by combining the plasmonics and11

Abstractmagneto-optics in one nanostructure, the so-called magnetoplasmonics will give usboth of their advantages: miniaturization of the devices and active light (or plasmon)manipulation. As a light-matter interaction process, the MO effect can also benefitfrom the localized field at the plasmonic resonance, which gives rise to theplasmon-enhanced MO effect. On the other hand, instead of measuring the lightintensity and peak shift in the plasmonics-based sensing platform, MO signal containsthe phase information of the light (e.g. Kerr ellipticity), which actually has beenproved to give a much better signal-to-noise ratio as the sensing platform.In this thesis, following these trends, a typical plasmonic nanostructure-nanoring- isstudied with the further introduced MO functionality. Both the fabrication protocoland the optical/MO properties are studied.The protocol is implemented based on the hole-mask colloidal lithography (HCL, Fig.1) technique that allows fabricating multicomponent nanostructures randomlydistributed on a large area (over cm2).Fig. 1 Sketch for hole-mask colloidal lithography (HCL) method. The material (Au in thefigure) deposits through the hole in the HCL template. With tilted angle deposition androtation in the substrate, a nanoring structure can be formed.With the HCL method, the specific position of plasmonic and magnetic components(Au and Co in this case, respectively) can be controlled in three dimensions with theprecision in nanoscale. Two important parameters in the HCL template, the density of12

Abstractthe PS spheres used to define the holes in the template and the thickness of the Aumask, are optimized to adapt to the nanoring structure fabrication.By controlling the overall morphology and the spatial location of the differentcomponents (Fig. 2), we are able to finely control the plasmonic resonantcharacteristics of the nanostructures and as a consequence the MO response, both interms of spectral position and intensity. Starting from the Au/Co/Au trilayer nanodiskstructures which are normally deposited, the unimodal plasmonic resonance evolvesto the bimodal resonance of the nanoring as the deposition angle increases to a certainvalue. The enhanced MO effect of these structures exhibits similar evolution as thestructures evolve. This behavior is explained with the plasmonic hybridization model.Furthermore, by increasing the deposition angle during the deposition process, thecross-section of the nanoring structure can be tuned continuously, therefore thespectral positions of the plasmonic resonance and the enhanced MO peaks can becontrolled in a fine way. The Co continuous layer in the nanoring can be furthervaried into opposite 2 Co sectors and 2 Co dots, which results in optical anisotropy,and the MO effect is found to be almost proportional to the Co amount.At last, with the purpose to further improve the MO effect in such structures, a gap isgenerated in the top Au ring above the Co dot, which can further focus the EM field inthe MO-active Co dot region and result in a further enhancement of the MO effect bya factor of 3 compared with the structures without such a gap.CoAuFig. 2 Structures studied in this thesis. From left to right: Au/Co/Au nanodisk, Au/Co/Aunanoring, Co sectors nanoring, Co dots nanoring, and ring/split ring with a gap on top ofCo dot.13

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Chapter 1State of the Art and Motivation1.1 IntroductionOptics is the topic studying light generation, control and detection [1]. Plenty ofapplications in different areas and daily life have been realized in this fascinatingtopic utilizing different properties of the light: communication, computing, medicine,sensing, and so on.As an example, optical signal can be used instead of the electronic signal in amicroprocessor used in computing, which is the heart of the current informationtechnology and semiconductor industry, for the digital information exchange betweentransistors and integrated circuits [2,3]. This is proposed to solve the actual problemexisting in the current technology: according to Moore’s law, the number of thesemiconductor transistors in the dense integrated circuit is doubled approximatelyevery two years and now the characteristic dimensions are on the order of tens ofnanometers or even smaller, therefore the metallic nanowire interconnects currentlyused are not sufficient to provide the necessary capacity required by this exponentiallygrowing transistor number, due to the limited bandwidth of the electronic signal. Theoptical interconnects such as fiber optic cables, on the other hand, can provide abandwidth much higher ( 1000 times) than the electronic signal. However, due to thediffraction limit, the traditional optical or photonic components mainly based on thedielectrics, such as the optical fiber, lens, etc., are too bulky to be integrated with theelectronic integrated circuits. Even the photonic crystal, which is a partial solution forthis miniaturization and integration problem, is still too large in dimension comparedwith the electronic circuit component because its typical period is on the order of halfof a wavelength (around hundreds of nanometers).15

Chapter 1PlasmonicsIn this sense, surface plasmon-based photonics, or “plasmonics”, may offer a bettersolution to this problem [2-4]. Surface plasmons (SPs) are light waves propagatingalong, typically, a metal/dielectric interface, and coupled with the collectiveoscillation of electrons [ 5 ]. These waves are confined to the surface withexponentially decaying fields in both metal and dielectrics—on the metallic side, thedecaying field relies on the skin depth which is on the order of 10nm. In this way,both of the advantageous large bandwidth of the optical signal and the compatibilitywith the current semiconductor technology can be kept, e.g. Cu, Al metals, etc. [6].Therefore, the miniaturization and integration problems are both solved. Furthermore,the ultrafast dynamics of the surface plasmons, being relaxation times around 10femtoseconds and coherent evolution times even shorter, makes them even moresuited for the fast processing of the digital information [7].The potential applications of surface plasmons have become realizable in the last twoor three decades, mainly thanks to the advances in nanofabrication techniques, whichmake it possible the production and integration of nanoscale surface plasmonicdevices. Besides this, plasmonic devices have also found applications in many otherfields. For example, the spectral response of the plasmonic structures is very sensitiveto the variation in the refractive index of the surrounding medium, allowing thedetection of, for example, molecules in the vicinity of the plasmonic surface. This hasbeen used extensively in biological and chemical sensing applications and has showna high sensitivity [7, 8 ]. On the other hand, at plasmonic resonance, theelectromagnetic (EM) field can be localized and enhanced greatly in the near field ofthe nanostructure, which can be applied to enhance the light-matter interactionprocess sensitive to the EM field intensity, such as the surface enhanced Ramanscattering (SERS) [9], surface enhanced infrared absorption (SEIRA) [10], secondand higher harmonics generation [11], and so on.Active Plasmonics: MagnetoplasmonicsTo realize purely plasmonic circuits, the surface plasmons carrying the digital signalshould be manipulated and processed actively. Active plasmonic devices such asswitches and modulators, are necessary in this sense. It is known that light can bemanipulated with different mechanisms, for example, the thermo-, electro-, and16

1.1 Introductionmagneto-optical effects, etc. Correspondingly, all these effects can in principle beapplied to manipulate surface plasmons and generate active plasmonic devices [3].Among them, plasmonics mediated by a magnetic field [ 12 ] and termed as“magnetoplasmonics” [13], is a competitive candidate due to the intrinsically ultrafastproperty of the magnetic reversal [14] and the relatively large modulation depth [15].Magneto-optical (MO) effect was first discovered by M. Faraday in 1845, as arotation in the light polarization after light transmitted through a heavy glass (silicateborate of lead) in magnetic field (Faraday effect) [16] and later by J. Kerr for thereflected light from a pole of an electromagnet of iron (Kerr effect) [17]. In the lastyears, the MO effect has been applied to modulate the surface plasmons, and indeed,hybrid structures based on noble metal-ferromagnetic metal (e.g. Au/Co) have been

Memoria de tesis para optar al grado de Doctor Magnetoplasmonic Nanorings: Novel Architectures with Tunable Magneto-optical Activity in Wide Wavelength Ranges Hua Yu Feng Tesis codirigida por Dr. Feng Luo y Prof. Alfonso Cebollada Tutor Prof. Rodolfo Miranda Marzo 2017 . for my parents and my love.