Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Ce travail de thèse porte sur l'étude du couplage entre des boîtes quantiques (BQs) insérées dans des nanofils à semiconducteurs et des antennes plasmoniques. Un couplage efficace requiert une caractérisation complète des leurs propriétés optiques respectives, pour assurer un recouvrement spectral et spatial de l'émission de la boîte et du mode de l'antenne et l'alignement de la polarisation du mode plasmonique avec l'émission de la BQ.Les propriétés optiques d'antennes patchs plasmoniques circulaires ont été étudiées par cathodoluminescence (CL). Nous avons montré avec un modèle analytique de la densité locale d'états électromagnétiques (DLE) au voisinage des antennes que leurs résonances sont des superpositions de modes de Bessel d'ordre radiaux et azimutaux différents. Nous avons fabriqué et caractérisé des antennes mono et multimodes, et trouvé que la partie radiative de la DLE n'est pas la seule contribution au signal de CL. De plus, nous avons caractérisé des antennes de différentes épaisseur du plan diélectrique ou différents matériaux. L'analyse de ces résultats nous pousse à proposer une interprétation des contributions au signal de CL annexes à la partie radiative de la DLE supportée par l'antenne. Nous avons de plus démontré la fabrication d'antennes patchs en aluminium opérant dans la partie bleue du spectre électromagnétique, et appliqué la CL à d'autres géométries d'antennes.Nous avons également étudié différentes boîtes quantiques insérées dans des nanofils à semiconducteurs faits d'alliages de matériaux II-VI. Des émetteurs uniques sont étudiés par microphotoluminescence (µPL). Des mesures résolues en temps ou par microscopie de Fourier permettent une caractérisation spectrale, temporelle et la détermination de leur diagramme de rayonnement. Nous avons de plus mis en évidence les variations de propriétés optiques des émetteurs dues aux inhomogénéité de fabrication en étudiant un large ensemble de BQs. La modélisation complète des propriétés électroniques et optiques d'une boîte unique est proposée en utilisant la microscopie de Fourier résolue en polarisation, et une étape de spectroscopie magnéto-optique.Enfin, nous avons développé une méthode de lithographie électronique en deux étapes basée sur le repérage d'un émetteur unique par CL, permettant la fabrication d'antennes plasmoniques couplées de façon déterministe à des BQs insérées dans des nanofils. L'étude de ce couplage révèle un accroissement de l'absorption du faisceau d'excitation accompagné d'une accélération de l'émission de la boîte par couplage radiatif. Il en résulte une exaltation jusqu'à un facteur 2 de la µPL des boîtes. / In this work, we study the coupling between plasmonic nanoantennas and semiconducting nanowire quantum dots (NWQDs). This coupling requires spectral, spatial and polarisation matching of the antenna mode and of the NWQD emission. Hence, a full characterisation of both the antenna system and the NWQDs has to be performed to determine a relevant coupling geometry.Using cathodoluminescence (CL) we investigate the relation between the CL signal of circular patch plasmonic antennas and the electromagnetic local density of states (LDOS). The successive resonances supported by these antennas are complex superimpositions of Bessel modes of different radial and azimuthal order. Applying an analytical LDOS model, we show that we can fabricate and characterise antennas down to single mode resonances. However, the antennas CL spectrum goes beyond the radiative part of the LDOS. By changing the spacing layer thickness and the antennas materials, we propose an explanation for the origin of the additional CL signal we observe that is not related to the radiative LDOS of the patch antennas. We also demonstrate the fabrication of Al patch antennas working in the blue spectral range and apply our method to other geometries.We perform optical characterisation of different quantum dots (QDs) embedded inside semiconducting nanowires (NWs) made of II-VI materials. We use microphotoluminescence (µPL) to study the emission of single NWQDs. Time-resolved measurements and Fourier imaging allows us to extract their exciton lifetime and radiation patterns. The variability in the emission properties of the NWQDs due to inhomogeneity in the growth process are evidenced by studying a statistical set of nanowires. A complete model based on polarisation-resolved Fourier imaging and magneto-optical spectroscopy is detailed, allowing to fully determine the QD electronic and optical properties for an individual system.Finally, we develop a cathodoluminescence-based two-step electron-beam lithography technique to deterministically fabricate plasmonic antennas coupled to NWQDs, enhancing their µPL properties. The coupling results in an enhanced absorption of the pump laser inside the NW and in an increase of the radiative rate of the QD, leading to up to a two-fold intensity enhancement factor for the coupled system.

Graphene and structurally similar 2-dimensional (2d) materials such as transition metal dichalcogenides (TMDs) and black phosphorus (BP) hold enormous potential for the next generation optoelectronics and photonics. Pairing 2d materials with printing is an emerging cost-effective large-scale device fabrication strategy. However, the current inks are far from ideal to support reproducible device fabrication. In addition, the instability of BP in ambient limits its applications. In this thesis, I present formulation of 2d material inks for inkjet printing for optoelectronic and photonic applications. To begin with, I produce mono- and few-layer 2d material flakes via ultrasonic assisted liquid phase exfoliation. This allows one-step formulation of a polymer stabilised graphene ink. For TMDs and BP, I design a binary solvent carrier for binder-free ink formulation. I show that these 2d material inks have optimal fluidic properties, drying dynamics and interaction with substrates for spatially uniform, highly controllable and print-to-print consistent large-scale printing on untreated substrates. In particular, the rapid ink drying at low temperatures leads to minimal oxidation of BP during ambient printing; the printed BP with passivation retains a stability over one month. On this basis, the printed graphene is employed as active sensing layer in CMOS integrated humidity sensors and as counter-electrodes in dye-sensitised solar cells, while the printed TMDs and BP are used to develop nonlinear photonic devices (i.e. saturable absorbers for femtosecond pulsed laser generation) and visible to near-infrared photodetectors (e.g. MoS$_2$ and BP/graphene/silicon hybrid photodetectors). Beyond inkjet printing, I present an ink formulation of commercial graphene nanoplatelets for roll-to-roll flexographic press ($\sim$100 m min$^{−1}$ printing speed). This allows hundreds of conductive electronic circuits to be printed in a minute for capacitive touchpads. Though I investigate only graphene, TMDs and BP, the ink formulation strategies can be effortlessly transferred to other 2d materials such as boron nitride, MXenes and mica. In addition to the demonstrated applications, printing of 2d materials can be potentially exploited to fabricate devices such as transistors, light emitters, energy storage conversion, and biosensors. This significantly expands the prospect of printable 2d material optoelectronics and photonics.

The fundamental limits of electronic systems in communication networks motivated scholars to think of an alternative approach to overcome problems such as demand for wider bandwidths and heat dissipation. All-optical signal processing is demonstrated as a potential solution. A major improvement in cost and speed of networking systems is expected through replacing microelectronics by photonic chips. However, the variety of operations essential to perform all-optical signal processing cannot be handled by a single material platform yet. Several III-V semiconductors, such as AlGaAs, have demonstrated potentials for photonic integration; nevertheless, there is still lack of data in literature on nonlinear optical properties of these materials. In this thesis, we extend the quest to evaluate more candidates from this class of semiconductors. Moreover, we are aiming for demonstrating the potentials of various III-V compounds for nonlinear photonics on-a-chip. In this thesis, we propose several optical waveguide designs based on quaternary III-V semiconductors AlGaAsSb and InGaAsP. We present modal analysis for waveguide designs and show that effective mode area much less than 1 $\mu m^{2}$ can be obtained. We also report specific waveguide designs that display zero-dispersion points at the specific wavelength ranges of interest. The designed waveguides are thus expected to demonstrate efficient nonlinear optical interactions. Next step is the fabrication of these devices with the goal to experimentally assess their nonlinear optical performance. The fabrication process of InGaAsP/InP strip-loaded waveguide is briefly reviewed. Following that, we report on the first, to the best of our knowledge, demonstration of third-order nonlinear optical interactions in InGaAsP/InP strip-loaded waveguides. We have performed self-phase modulation, nonlinear absorption measurements, and four-wave mixing experiments at the telecom wavelength range. The nonlinear phase shift up to 2.5 $\pi$ has been observed. Following that, we use Monte-Carlo method for design optimization and tolerance analysis of a multi-step lateral taper Spot-Size Converter in indium phosphide. An exemplary four-step lateral taper design featuring 0.35 dB coupling loss at optimal alignment of a standard single-mode fiber, $>$7 $\mu m$ 1-dB displacement tolerance in any direction of in a facet plane, and a great stability against manufacturing variances demonstrated.

Nous étudions les propriétés photoniques dans le plan de monocouches de nanostructures auto-organisées. L'objectif de cette l'étude est d'appliquer les nanostructures auto-organisées pour la réalisation de cavités à cristaux photoniques (CP), et de nouvelles sources de lumière organiques. Le premier chapitre présente les opales et les opales-inverse réalisées à partir de nanoparticules auto-organisées et leurs propriétés optiques. Dans une deuxième partie, sont introduits les cristaux photoniques, leur physique et les outils numériques pour les quantifier. Le deuxième chapitre se concentre sur l'étude des propriétés photoniques de réseaux périodiques bidimensionnels de monocouches de nanoparticules diélectriques auto-organisées. La transmission optique dans le plan du cristal et l'existence de bandes interdites photoniques sont systématiquement étudiées en utilisant la méthode numérique des différences finies dans le domaine temporel en trois dimensions (3D FDTD). Les structures étudiées sont des monocouches de sphères diélectriques entourés d'air («opales») ou des sphères d'air entourées par un matériau diélectrique («opales inverses») en treillis triangulaire, avec et sans substrat de verre. Les bandes interdites photoniques (BIP) sont étudiées en fonction du contraste d'indice et de la compacité des sphères. Pour les structures sans substrat, la BIP est observée pour les faibles indices de réfraction des matériaux. Toutefois, la présence d'un substrat de verre réduit les BIP. Défaut microcavité conçu en opales et opales inverses sont alors pris en compte. Le meilleur facteur de qualité sont obtenus avec des inverses-opales lorsque la compacité (r/a ) est d'environ 0,32. Une expérience pour mesurer la propagation dans le plan dans des monocouches de opales est présentée. Dans le troisième chapitre de cette étude, nous présentons une nouvelle approche de nanostructuration bidimensionnelle qui utilise la photolithographie et des nanoparticules auto-organisée pour nanostructurer de diodes électroluminescentes organiques. Cette technique utilise la photolithographie classique, un photo masque réutilisable faits de micro nanoparticules auto-organisées, et une résine photosensible. Le masque est constitué de micro-sphères de SiO2 et de polystyrène monodisperses de taille sub-micronique déposées d'une manière auto-organisée sur un substrat de quartz. Le principe de fonctionnement est similaire à celui de la photolithographie classique, sauf que deux configurations peuvent être distingués : le mode contact-dure et le mode contact-doux. Dans la première configuration, chaque microsphère agit comme une lentille micro-boule qui focalise la lumière et expose la partie de la résine photosensible au-dessous d'elle. Le motif résultant reproduit l'agencement du réseau triangulaire des sphères avec la même période. Dans le mode de contact-doux un comportement de masque de phase est obtenu qui se traduit par des périodes de réseau égales à la moitié du diamètre de la sphère. La période de réseaux et le diamètre des trous les plus petits obtenus avec une source de lumière de 405 nm sont respectivement 750 nm et 420 nm. Finalement, cette nouvelle technique de structuration de motifs bidimensionelle est appliquée à la nanostructuration d'OLEDs. Comme exemple, des OLEDs nano-structurées avec des couleurs d'émission vertes et rouges sont réalisées et présentées. / We investigate the in-plane photonic properties of monolayer of self-organized nanostructures. We aim at investigating the contribution of photonic self-organized nanostructures to organic photonic crystal (PhC) cavities, and novel organic light sources.The first chapter presents bulk opals and inverse-opals made of self-organized nanoparticles and their optical properties. In a second part, photonic crystals are introduced as well as the physics and the numerical tools to quantify them.The second chapter deals with the study of photonic properties of two-dimensional periodic array of monolayer of self-organized dielectric nanoparticles. The in-plane optical transmission and the existence of photonic band gap are systematically studied by using the 3D finite-difference time domain (3D FDTD) method. The structures studied are monolayer of dielectric spheres surroundedby air ('opals') and air spheres infiltrated with dielectric material ('inverse opals') in triangularlattice, with and without glass substrate. The dependence of photonic band gaps (PBGs) on therefractive index and on the compactness of spheres is studied. For self-sustained structures, PBG isobserved for relatively low refractive indices of materials. However, the presence of a glasssubstrate reduces the PBGs. Defect microcavity designed in opals and in inverse opals are then considered. The best quality factor are obtained with inverse-opals when the compactness (r/a ratio) is around 0.32. An experiment to measure the in-plane propagation in monolayers of opals is presented. In the third chapter of this study, we present a new approach of two-dimensional patterning based on self-organized nanoparticle photolithography for nanostructuration of organic light emitting diodes. This technique uses conventional photolithography, a reusable photomask made of self organized micro nanoparticles, and a conventional photoresist. The mask consists of micro and submicronsized SiO2 or Polystyrene mono-dispersed spheres deposited in a self-organized manner on aquartz substrate. The principle of operation is similar to the one of conventional photolithography except that two configurations can be distinguished : The hard-contact mode and the soft-contact mode. In the first configuration, each microsphere acts as a micro ball-lens that focuses the light and expose the part of the photoresist underneath the spheres. The resulting pattern reproduce the triangular lattice arrangement of the spheres with the same period. In the soft contact mode a phase mask behavior is obtained which results in lattice periods being the half of the sphere diameters. Lattice periods and hole diameter as small as 750 nm and 420 nm respectively are demonstrated with a 405 nm light source. Eventually, this new two-dimensional patterning technique is applied to the nanostructuration of OLEDs. As an example, green and red patterned OLEDs are demonstrated.

Most of the time, in a nano-fabrication facility, the efforts of a researcher are devoted to optimising the fabrication process in order to avoid defects and obtain the best result in terms of precision and quality of the fabricated device. However, it is inevitable that during the sample fabrication, a variable intrinsic amount of disorder is introduced. This feature can be exploited to develop novel applications spanning different areas of optics. A perfect unclonable cryptographic system based on new integrated optical fingerprints chip is presented and a proof of concept is provided. The role of disorder at the nanoscale is further studied in the fabrication processes such as electron beam lithography and dry-etching. In this scenario, the randomness is the starting point to develop new technologies for structural coloration and holograms.

In the framework of the quasi-static approximation (QSA), some theoretical studies were conducted within the local response approximation (LRA). In these studies, certain plasmonic and plexcitonic systems were proposed, and their spectroscopic properties investigated. The QSA allows us to study metal nanoparticles (MNPs) and inter-particle distances that are small compared to the wavelength of light in the medium surrounding the MNPs, while the LRA enables us to utilize the bulk dielectric response of the metal in consideration. We have studied the following properties in detail: localized surface plasmon resonances (LSPRs), plasmon-induced transparency (PIT), and plasmon-enhanced fluorescence (PEF), while exciton-induced transparency (EIT) has only been partly studied. LSPR and PIT are properties of plasmonic systems while PEF and EIT are properties of plexcitonic systems. Both PIT and EIT are forms of electromagnetically-induced transparency. We started by constructing a geometry-based theoretical model that predicts the LSPR formula of any member of a certain group of single MNPs, using the LSPR for the most complex MNP geometry in the group. The model shows that from the LSPR of a nanorice, one could predict the LSPRs of concentric nanoshells, solid and cavity nanorods and nanodisks, respectively, and solid and cavity nanospheres. These formulae serve as quick references for predicting LSPRs since they can easily be compared to LSPRs obtained from spectral analysis. Likewise, we studied LSPR in addition to PIT in a nanoegg-nanorod dimer. We proposed this dimer in order to investigate how the interplay between plasmon coupling and MNP sizes affects PIT in complex geometries such as nanoeggs. Our result shows that the formation of PIT dips — regions in the dimer spectra where little or no incident radiation is absorbed by the dimer — are strongly-dependent on the nanorod size, due to the dependence of the plasmon coupling strength on the half-length of the nanorod. We investigated the phenomenon of PEF using a nanoegg-emitter system and a nanorod-emitter system, respectively. Emitters are organic or inorganic materials whose radiative decay rates increase dramatically when placed near a MNP subjected to plasmon excitation. Our theoretical results show that the choice of the MNP-emitter system to use depends on both the intrinsic quantum yield of the emitter and the antenna efficiency of the MNP. Theory shows that PEF is more substantial when the former is very low, and it will always occur if the latter is greater than the former. A nanorod-emitter system should serve as the preferred choice, due to the relatively easier synthesis of nanorods compared to nanoeggs, and the large longitudinal polarizability of nanorods as a result of the lightning rod effect. However, our theoretical model also shows that a nanoegg-emitter system can rival the PEF parameters obtained in a nanorod-emitter system, due to an increase in the Purcell factor of the emitter with increasing core-offset of the nanoegg, resulting from the presence of dipole-active modes in the nanoegg. / Thesis (PhD (Physics))--University of Pretoria, 2020. / University of Pretoria / National Research Foundation (NRF) / Physics / PhD (Physics) / Unrestricted

This thesis presents optical characterizations of interfaces in ferromagnetic heterostructures and thin films used for spin polarized electronic devices. In these experiments, femtosecond laser spectroscopies are exploited to investigate the interface magnetization reversal, spin precession, and band offset, which are crucial in determining the performances of spintronic devices.;First, magnetization-induced second-harmonic-generation (MSHG) is applied to study interface magnetism in a hybrid structure containing a noncentrosymmetric semiconductor---Fe/AlGaAs. The reversal process of Fe interface layer magnetization is compared with the bulk magnetization reversal. In Fe/AlGaAs (001), the interface magnetization is found to be decoupled from the bulk magnetization based on the different switching characteristics---single step switching occurs at the interface layer, whereas two-jump switching occurs in the bulk. In contrast, the interface layer in Fe/AlGaAs (110) is rigidly coupled with the bulk Fe, indicating a strong impact of electronic structure on the magnetic interaction despite the same chemical composition. Furthermore, a time-resolved MSHG study demonstrates a coherent interface magnetization precession in Fe/AlGaAs (001), implying the feasibility of fast precessional control of interfacial spin. The interface magnetization precession exhibits a higher frequency and opposite phase for a given applied field compared to the bulk magnetization precession.;Second, uniform magnetization precession in the Lac0.67Ca 0.33MnO3 (LCMO) and La0.67Sr0.33MnO 3 (LSMO) films grown on different substrates are investigated by time-resolved magneto-optic Kerr effect. The parameters of magnetic anisotropy are determined from the field dependence of the precession frequency. The strain-free LCMO films grown on NdGaO3 exhibit a uniaxial in-plane anisotropy induced by the tilting of the oxygen octahedra in NdGaO3 An easy-plane magnetic anisotropy is found in the tensile-strained films grown on SrTiO 3, whereas the compressive-strained film grown on LaAlO3 exhibits an easy normal-to-plane axis.;Third, a table-top internal photoemission system is developed to measure the band offsets across semiconductor heterointerfaces by utilizing an optical parametric amplifier as the bright light source. The conduction band offsets DeltaE c = 660 meV and 530 meV at the CdCr2Se4-GaAs and CdCrZSe4-ZnSe interfaces are determined from the threshold energies of the photocurrent spectrum. The band offset is shown to be reduced by engineering the interface bonding and stoichiometry.

Nonlinear optics is the study of interactions of materials with intense light beams made possible by the invention of laser. Arguably the most trivial but technologically most important nonlinear optical effect is the intensity-dependent nonlinear refraction: an intense light beam can temporarily and reversibly change the refractive index of a material. However, the changes to the refractive index of a material due to the presence of a strong laser beam are very weak---maximum on the order of $10^{-3}$---and tend to be a small fraction of the linear refractive index. It must be noted that at optical frequencies vacuum has a refractive index of 1 and glass has a refractive index of 1.5. Thus, one of the foundational assumptions of nonlinear optics is that the nonlinear optical changes to material properties are always a small perturbation to the linear response. In the 58-year history of nonlinear optics, one of the overarching themes of research has been to find ways to increase the efficiency of nonlinear interactions. This thesis is a collection of six manuscripts motivated by our experimental finding that at least in a certain class of materials the above long-standing view of nonlinear optics does not necessarily hold true. We have found that in a material with low refractive index, known as an epsilon-near-zero material or ENZ material, the nonlinear changes to the refractive index can be a few times larger than the linear refractive index, i.e. the nonlinear response becomes the dominant response of the material in the presence of an intense optical beam. We believe that the results presented in this thesis collectively make a convincing case that ENZ materials are a promising platform for nonlinear nano-optics.

At the same time as Moore’s law is reaching it’s limits, there has been exponential growth in required computation power, most notably driven by the widespread deployment of artificial intelligence (AI) and deep learning (DL) models, such as ChatGPT. The unprecedented modern, and projected, bandwidth density requirements between compute needs for high performance (HPC) and data center (DC) applications leads directly to an equally unprecedented need to supply and dissipate extreme amounts of power in ever smaller volumetric units. While at smaller scales this becomes a question of power dissipation limits for discrete components, in aggregate the power consumed across the full system quickly adds up to becoming an environmentally significant energy drain. Traditional electronic interconnects have failed to keep pace, both in terms of supporting bandwidth density and achieving sufficient energy per-bit efficiency, leading to optical interconnects becoming the dominant form of high-bandwidth communication between nodes at shorter and shorter reaches. Co-packaged silicon photonics (SiPh)s have been proposed as a promising solution for driving these optical interconnects. In fact, SiPh engines have already become widely accepted in the commercial ecosystem, specifically for network switches and plugable optical modules for mid- (10 m - 500 m) and long-haul (≥2 km) applications. The work in this thesis proposes novel integrated SiPh interconnect architectures, as well asnovel devices that enable them, in order to push SiPh driven interconnects down into the inter-chip scale, inside the compute and memory nodes (< cm), as well as all the way out to the low-earth orbit (LEO) inter-satellite scale (> 1000 km). In the case of the former, recent advances in chip-scale Kerr frequency comb sources have allowed for fully integrated ultra-broadband dense wavelength division multiplexing (DWDM). To take full advantage of these integrated DWDM sources, similar advances must be made at both the architecture and device levels. In the latter case, interest in inter-constellation connectivity is growing as LEO becomes saturated with varying satellites owned by private and public entities. For these constellations to communicate directly, a new class of satellite must join the sky, with adaptive communication capabilities to translate Baud rate and modulation format between otherwise incompatible constellations. To support each of these applications with integrated photonics solution, advances in both SiPh architectures and the devices that comprise them. This work first presents an overview of the system-level challenges associated with such links, including novel proposed integrated interconnect architectures, and then explores novel photonic devices that are designed to enable critical functionality and overcome system-level limitations. The advances demonstrated in this thesis provide a clear direction towards realizing a future fully permeated by ultra-efficient optical connectivity, supporting resource disaggregation and all-to-all connectivity from green hyper-scale data centers all the way to LEO.