The use of photonic crystals for light extraction from light-emitting diodes (LEDs) gives the possibility to shape the farfield emission pattern. This is of particular interest for étendue-limited LED applications that require a more directional farfield than state- of-the-art Lambertian emitters. However, the application of a photonic crystal in a LED results in directional emission only if the photonic crystal and the distribution of guided modes in the LED are tuned correctly. In this thesis, red- and blue-emitting thin-film PhC-LEDs in the AlGaInP and InGaN material systems were modelled, designed, fabricated and characterized. The first experimental results show that light extraction with photonic crystals from AlGaInP thin-film LEDs several microns thick is neither directional nor more efficient than state-of-the-art LEDs with a rough surface structure. Directional light extraction for AlGaInP PhC-LEDs is for the first time demonstrated in much thinner devices where the photonic crystal light extraction of guided modes is combined with the resonant-cavity effect. In an attempt to approach the ideal PhC-LED, strong photonic crystal farfield shaping is demonstrated in InGaN thin-film LEDs of sub-micron thickness. Analysis of their spectral farfields unexpectedly shows that high order diffraction contributes significantly to the light extraction efficiency if the mode absorption is sufficiently low. It is also demonstrated that directional photonic crystal light extraction is possible in InGaN thin-film LEDs several microns thick. The directionality stems from the modulation of the spontaneous emission caused by the proximity of the active region to the bottom mirror. Two new concepts for enhanced light extraction and high directionality are presented: Photonic crystals with two dominating lattice constants are found to outperform conventional photonic crystal LEDs. An alternative approach is the dielectric PhC-LED - FDTD simulations show that the high extraction efficiency of LEDs with surface roughness is combined with the higher directionality of photonic crystal light extraction.

<p>Microphotonics has been offering a body of ideas to prospective applicationsin optics. Among those, the concept of photonic integrated circuits (PIC’s) has recently spurred a substantial excitement into the scientific community. Relisation of the PIC’s becomes feasible as the size shrinkage of the optical elements is accomplished. The elements based on photonic crystals (PCs) represent promising candidacy for manufacture of PIC’s. This thesis is devoted to tailoring of optical properties and advanced modelling of two types of photonic crystals: (Bi₃Fe₅O₁₂/Sm₃Ga₅O₁₂)<em>^m</em> and (TiO₂/Er₂O₃)<em>^m</em> potentially applicable in the role optical isolators and optical amplifiers, respectively. Deposition conditions of titanium dioxide were first investigated to maximise refractive index and minimise absorption as well as surface roughness of titania films. It was done employing three routines: deposition at elevated substrate temperatures, regular annealing in thermodynamically equilibrium conditions and rapid thermal annealing (RTA). RTA at 500 ^oC was shown to provide the best optical performance giving a refractive index of 2.53, an absorption coefficient of 404 cm⁻¹ and a root-mean-square surface roughness of 0.6 nm. Advanced modelling of transmittance and Faraday rotation for the PCs (Bi₃Fe₅O₁₂/Sm₃Ga₅O₁₂)⁵ and (TiO₂/Er₂O₃)⁶ was done using the 4 × 4 matrix formalism of Višňovský. The simulations for the constituent materials in the forms of single films were performed using the Swanepoel and Višňovský formulae. This enabled generation of the dispersion relations for diagonal and off-diagonal elements of the permittivity tensors relating to the materials. These dispersion relations were utilised to produce dispersion relations for complex refractive indices of the materials. Integration of the complex refractive indices into the 4 × 4 matrix formalism allowed computation of transmittance and Faraday rotation of the PCs. The simulation results were found to be in a good agreement with the experimental ones proving such a simulation approach is an excellent means of engineering PCs.</p>

This thesis presents a study into structural colours that exist in natural samples, the principle aim of which is to produce experimental methods by which these colours may be examined and evaluated. In order to achieve this, previously observed structures are described, electromagnetic theory is summarised and a series of samples are examined constituting examples of the structures present in nature. The first sample discussed is the multilayer in the epicuticle of the buprestid beetle, C. raja. In order to evaluate the refractive indices of the layers contained within this structure, existing optical techniques are used to establish absolute reflection spectra for a number of angles of incidence in both linear polarisations. The approximate design for the structure is obtained by electron microscopy and modelled using Fresnel's equations. This model is then refined by a recursive least squares fitting routine to obtain the refractive indices. The second sample is the diffuse white scattering structure in the scales of two white beetles, Lepidiota stigma and Cyphochilus spp. The reflection from these scales is measured and found to be brilliantly white due to the irregular internal structure of the scales. Comparison of the Fast Fourier Transforms of TEM images of the internal structure with the diffraction pattern obtained from monochromatic laser light diffracting through a single scale demonstrate a link between this structure and photonic effects. The third sample type are found in the scales of the large true weevils, Eupholus schoenherri pettiti and E. magnificus. These scales are shown to have a domained structure in which the domains were oriented differently to each other. Single domains are shown to exhibit different colours at different orientation. The final sample is the highly regular 2-dimensional diffraction grating observed in a marine diatom, Coscinodiscus wailesii. Diffraction is demonstrated by measuring the in-plane diffraction from a single frustule for both monochromatic laser light and white light, showing an enhanced transmission for red wavelengths. Subsequent imaging of the transmitted diffraction pattern allows for the calculation of the transmitted power in each diffracted order.

This thesis presents an investigation into some of the structural colours that are produced in nature. There are many animals and plants that produce structural colour, with a particularly high structural colour diversity in insects. Of the species that exhibit structural colours, three species are the subjects for investigation of this thesis. Those comprise a group of beetles from South-East Asia, Torynorrhina flammea, a buttery, Parides sesostris and a fruit, Margaritaria nobilis, both from South American rainforests. The structures that produce the vivid colours of these species were analysed using electron microscopy. This information aided the design and creation of three inorganic, synthetic replicas of the natural structures. The fruit of Margaritaria nobilis was structurally analysed, yielding the discovery of a novel multilayer fibre. These fibres were cylindrical in design and were found to be layered together producing the epidermis of the fruit. The multilayer structure produced a vivid blue colour appearance, which is believed to offer a selective advantage because the colour deceives birds into thinking that the fruit contains nutritious flesh. This selective advantage earns M. nobilis the label of mimetic fruit. The structure found within the M. nobilis fruit epidermis inspired the synthesis of a structure which comprises single cylindrical multilayer fibres. The synthetic fibres were manufactured from elastic materials which allow the structure to be deformed under strain and, therefore, a change in colour can be observed. As the structure was stretched, this made the layers get thinner and, therefore, the colour of the fibre blue-shifted. The fibre was able to be stretched to over twice its original length which yields a shift in peak reflected wavelength of over 200 nm. Four beetles from the Torynorrhina flammea species were investigated with the aim of replicating the nanostructures responsible for their colour appearance. The initial interest in the beetles came from their strikingly vivid colour appearances. The structure responsible for the vivid colours in all four of the subspecies is a multilayer with high structural order and over 100 laminae. Both of these attributes contribute to the saturation of the colours exhibited. The multilayer was found to be intersected by an array of rods, the long axis of which is orthogonal to the surface. The rods are believed to be the cause of an interesting diffraction phenomenon exhibited by the beetles. Using imaging scatterometry, the structure was found to diffract the colour produced by the multilayers into an annulus around a specularly reflected white spot. This inspired the synthesis of a multilayer permeated with an array of holes with the aim of replicating a system that could reproduce the annular pattern of colour reflection. The initial synthesised system comprised a quarter-wave stack with a perfectly ordered hexagonal array of holes permeating the surface orthogonally. The sample displayed the scattering characteristics of a hexagonal array, and the reflection spectra of the multilayer stack. When disordered hexagonal arrays were milled into the structure with a focussed ion beam, the scattering pattern started to show more of the green colour from the multilayer and less of the ordered scattering pattern. The highly disordered, synthesised structure displayed no hexagonal scattering pattern, but instead it showed a highly scattered bluish-green colouration. One sample was created by directly mapping out the array of holes using an image of the original array from one of the beetle samples. This sample was expected the same annular diffraction pattern as the beetles, however, the sample instead exhibited the same scattering pattern as the highly disordered array. Some structurally coloured systems in nature have more than one light scattering structure, all of which contribute to the overall colour of the system. For complicated systems such as this, it is necessary to devise a technique to characterise the individual scattering structures separately. One such species that displays a complex, multicomponent system is Parides sesostris. The male of the species displays bright green patches on the dorsal side of the forewings which are made up of thousands of green wing scales. These green scales contain a 3D gyroid poly-crystal at centre with a membrane layer surrounding the underside of each scale and a scattering structure on top. Using focussed ion beam milling techniques allowed the individual characterisation of each of these structures. The gyroid poly-crystal was found to reflect not green but blue wavelengths. This led to the discovery by another group [1] that the scales contain at least one type of fluorophore. The removal of the membrane structure and some of the gyroid poly-crystal from the base of the scale resulted in the change of the overall scale structure from green to cyan. This suggests that the membrane maybe a significant source of fluorescence. Computational modelling, without fluorescence, suggests that the addition of the membrane layer to the gyroid does not shift the band-gap wavelengths; however, the overall reflection intensity does increase. The scattering structure on the top side of each scale is comprised a bi-grating which sits on top of the 3D gyroid structure. The long periodicity of the bi-grating protrudes above the surface, resulting in the very top layer of the scale to be a mono-grating. This whole structure decreases the angular-dependence of the colour by efficiently scattering the incident light into the gyroid and also scattering the reflected light from the gyroid, resulting in a double-scattering. FIB-milling was used to isolate the scattering part of the structure. Analysis of this component of the structure revealed that it was not a source of the green colour itself; however, it did show the characteristic scattering pattern of a mono-grating. The small periodicity of the bi-grating did not produce a scattering pattern since the periodicity is too small to produce optical diffraction at normal incidence. To characterise the effect of the fluorophores, the whole scale structure was photo-bleached using ultra-violet radiation for two months with the aim of destroying the fluorophores contained within the structure. The expected result occurred which was the blue-shifting of the peak reflected wavelengths. However, it could not be confirmed whether or not the photo-bleaching reduced the physical size of the light scattering structures which would, in theory, result in a blue-shift of the peak reflected wavelengths. The male P. sesostris green wing scales were also the subject for investigation for trying to make inorganic replicas of the gyroid-polycrystal. A surface sol-gel coating process was utilised to coat the green wing scales with titania. This coating process was performed using a few different methods. Half of the samples were coated with TiO2 and the other half with tin-doped TiO2. Half of each of these samples had their surfaces dendritically amplified before the coating processes and the other half were left untreated. The samples were coated with 25 surface sol-gel (SSG) cycles of each treatment at a time. After each 25 cycle treatment the samples were optically characterised. The total number of cycles applied to the samples at the end was 150. The addition of layers of titania resulted in a general red-shift that was higher for the tin-doped titania samples than for the titania samples. Another general trend found was that the samples that had their surfaces dendritically amplified, produced a lower red-shift in peak wavelength. This was contrary to the hypothesis that the amplification process was supposed to aid the SSG coating process and, therefore, increases the red-shift in peak wavelength.

Nanometer-sized semiconductor crystals, termed ‘quantum dots’, are of fundamental interest because of their size-tunable properties. Three-dimensional quantum confinement of charge carriers by the small crystal size results in discrete atomic-like electronic states. This dissertation describes the synthesis and in-depth characterization of lead chalcogenide colloidal quantum dots for forthcoming applications as near-infrared single photon emitters. An efficient single photon source that operates at telecommunication wavelengths (between 1.3 and 1.6 µm) is a basic requirement for many photonic quantum technologies, such as quantum computing and quantum cryptography. Chapters 1 and 2 of this work provide an introduction to colloidal quantum dots and their use as single photon emitters. It includes a description of photonic crystal microcavities and their ability to enhance the spontaneous emission rate of quantum dots. The synthesis and basic characterization of PbSe and PbS quantum dots is then discussed in chapter 3. In particular, a new synthetic method for the preparation of highly photoluminescent PbS quantum dots is presented. PbSe/CdSe core/shell quantum dots prepared by a cation exchange reaction are also described and a significant improvement in photo-stability is shown. Chapter 3 concludes with a description of three different surface modification techniques. PbSe core and PbSe/CdSe core/shell materials are investigated further in chapter 4 by advanced characterization techniques that include high-angle annular dark field (HAADF) imaging, energy-filtered transmission electron microscopy (EF-TEM) imaging, energy-dependent X-ray photo-electron spectroscopy (XPS), small angle X-ray scattering (SAXS), and small angle neutron scattering (SANS). The information obtained from these techniques is combined to form a structural model of the PbSe core and PbSe/CdSe core/shell quantum dots with greater complexity than previously reported. In chapter 5, the temperature-dependent photoluminescence from PbSe and PbSe/CdSe core/shell quantum dots is discussed and a thermal model is presented that accounts for the large (non-trivial) temperature dependence of the Stokes shift and photoluminescence lineshape over the entire temperature range (4.5 to 295 K). Chapter 6 examines two scalable methods to integrate the colloidal quantum dots into silicon two-dimensional photonic crystal slab microcavities (a requirement for efficient single photon emission). Finally, conclusions and possible future work are discussed in chapter 7. / Graduate

A modelagem numérica de cristais fotônicos tridimensionais é o objeto de estudo deste trabalho. Especificamente, o método das diferenças finitas no domínio do tempo (FDTD) é utilizado para a modelagem de um cristal FCC (face-centered-cubic) formado por opalas de látex imersas em ar. Por meio de uma análise comparativa com o cristal formado por opalas inversas (opalas de ar incrustadas em uma matriz dielétrica com alto índice de refração), é mostrado que o baixo contraste de índice de refração do cristal de látex é característica preponderante para a inexistência de uma banda proibida completa. No entanto, podem ser observadas bandas fotônicas proibidas ao longo de algumas direções de propagação, como é evidenciado através da investigação da difração de Bragg relativa à família de planos cristalinos (111). Sempre que possível os resultados numéricos são comparados com os dados experimentais disponíveis. / The numerical modeling of tridimensional photonic crystals is the object of study in this work. Especifically, the finite difference time domain method (FDTD) is used for the modeling of a FCC (face-centered-cubic) crystal composed by latex opals immersed in air. Through a comparative analysis to a crystal composed by inverse opals (close-packed air opals in a dielectric matrix with high refractive index), it is shown that the low contrast of the latex crystal is the crucial characteristic to prevent the rising of a complete photonic band gap. However, photonic band gaps can be observed for certain directions of propagation, as it is demonstrated by the investigation of Bragg diffraction related to the (111) crystalline planes. Wherever possible, the numerical results are compared to available experimental data.

Graphene has emerged as an appealing material for a variety of optoelectronic applications due to its unique electrical and optical characteristics. In this thesis, I will present recent advances in integrating graphene and graphene-boron nitride (BN) heterostructures with confined optical architectures, e.g. planar photonic crystal (PPC) nanocavities and silicon channel waveguides, to make this otherwise weakly absorbing material optically opaque. Based on these integrations, I will further demonstrate the resulting chip-integrated optoelectronic devices for optical interconnects. After transferring a layer of graphene onto PPC nanocavities, spectral selectivity at the resonance frequency and orders-of-magnitude enhancement of optical coupling with graphene have been observed in infrared spectrum. By applying electrostatic potential to graphene, electro-optic modulation of the cavity reflection is possible with contrast in excess of 10 dB. And furthermore, a novel and complex modulator device structure based on the cavity-coupled and BN-encapsulated dual-layer graphene capacitor is demonstrated to operate at a speed of 1.2 GHz. On the other hand, an enhanced broad-spectrum light-graphene interaction coupled with silicon channel waveguides is also demonstrated with ∼0.1 dB/μm transmission attenuation due to graphene absorption. A waveguide-integrated graphene photodetector is fabricated and shown 0.1 A/W photoresponsivity and 20 GHz operation speed. An improved version of a similar photodetector using graphene-BN heterostructure exhibits 0.36 A/W photoresponsivity and 42 GHz response speed. The integration of graphene and graphene-BN heterostructures with nanophotonic architectures promises a new generation of compact, energy-efficient, high-speed optoelectronic device concepts for on-chip optical communications that are not yet feasible or very difficult to realize using traditional bulk semiconductors.

A fotônica em silício é um campo de pesquisas emergente com grande potencial para contribuir com a resolução de alguns dos problemas tecnológicos da atualidade. O gargalo imposto por interconexões metálicas na expansão da taxa de transmissão de dados em sistemas de comunicação como os de computadores de alto desempenho talvez seja um dos maiores desafios a serem superados. A propagação de luz em baixas velocidades de grupo e com controle de dispersão é uma das linhas de pesquisa atuais nas quais se busca explorar de forma mais eficiente as propriedades ópticas do silício, e assim, aumentar a compatibilidade entre componentes fotônicos e a tecnologia CMOS (Complementary Metal- Oxide-Semiconductor) por meio da diminuição das dimensões e do consumo de energia de componentes ópticos ativos. Dessa forma, espera-se diminuir os custos de fabricação e viabilizar a produção em larga escala de dispositivos integrados optoeletrônicos, que poderiam ser utilizados em sistemas de comunicação de curtas distâncias e assim ampliar a largura de banda disponível. Investigações recentes têm demonstrado que a fabricação de dispositivos baseados em cristais fotônicos planares possui grande potencial para controlar simultaneamente a velocidade de grupo e a dispersão, além de permitir a redução do tamanho de elementos como curvas, divisores de potência e cavidades ressonantes devido ao efeito do confinamento dos campos através do bandgap fotônico. Dessa forma, esse trabalho aborda um estudo sobre a integração monolítica entre guias de onda, curvas de 60º e junções em Y que operam em baixas velocidades de grupo e com reduzida dispersão, construídos em cristais fotônicos planares formados por uma matriz periódica de furos em uma membrana de silício suspensa em ar. Essa investigação englobou atividades bastante intensivas, tanto de simulações por métodos numéricos, como de processos de fabricação dedicados à nanofotônica, assim como de caracterizações ópticas. Ao longo das discussões são identificados e analisados os mecanismos que afetaram de forma mais crítica a eficiência dos dispositivos propostos. Também foram avaliados os maiores problemas enfrentados nos processos de fabricação, e suas possíveis soluções foram apontadas. Os resultados demonstraram a possibilidade teórica de realizar tal integração de forma eficiente. O melhor entendimento sobre a relação entre a dispersão e os parâmetros geométricos dos guias de onda permitiram modelar curvas e divisores de potência que exibiram, respectivamente, larguras de banda em torno de 56 e 40 nm, cobrindo regiões do espectro com elevados índices de grupo. Foi possível fabricar cristais fotônicos com uma qualidade próxima das já reportadas na literatura sobre o tema e assim foram estabelecidas bases bastante sólidas para a fabricação de tais dispositivos localmente, sem a necessidade expressa de acessar centros de fabricação no exterior. / Silicon photonics is an emerging research field that has great potential to contribute to solving some of the technological problems nowadays. Maybe, one of the greatest challenges to be overcome is the bottleneck imposed by electrical interconnections in the expansion of the bandwidth of communication systems such as those of high performance computers. Slow light propagation in dispersionless media is a hot topic in the current research fields that seek to more efficiently explore the silicon optical properties, and thus, increase the compatibility between photonic components and CMOS technology by decreasing the footprint and power consumption of active optical components. This way, the manufacturing costs it is expected to be reduced by making the large-scale production of integrated optoelectronic devices feasible, and so, they could be used in short distance communication systems to expand the available bandwidth. Recent researches has also shown that photonic crystal slab waveguides are very promising to simultaneously control group velocity and devices dispersion, as well as in the reduction of the size of elements such as bends, power splitters and nanocavities due to the fields confinement through the photonic bandgap effect. Thus, this work addresses a study of the monolithic integration of slow light and dispersionless waveguides, 60º bends, and Y-junctions fabricated in air-bridge photonic crystal slabs formed by the drilling of a periodic array of air holes in a silicon membrane. The research was accomplished with intensive activities in numerical simulations, as well as in nanophotonic manufacturing processes, and optical characterizations. Throughout the discussions were identified and analyzed the mechanisms that more critically affected the devices efficiency. The major problems faced in the manufacturing processes were also evaluated, and their possible solutions were pointed out. The results demonstrated a theoretical possibility of performing such integration more efficiently. Having a better understandment about the relation between the photonic crystal waveguides geometrical parameters and their dispersion allowed the modeling of bends and power splitters which exhibited 3 dB bandwidths that covered, respectively, ranges around 56 and 40 nm, along spectral regions with very high group indices. It was possible to fabricate photonic crystals with a quality close to those already reported in the literature on this subject and thus, very solid bases were established for the manufacture of such devices locally, without the necessity of accessing manufacturing centers abroad.

Dans cette thèse, une étude des microstructures périodiques et de leurs applications à la modulation optique par ondes acoustiques est présentée. Plus spécifiquement, le sujet traite du couplage opto-mécanique dans les cavités des cristaux phoXoniques. Cette étude montre comment la théorie des perturbations fournit un outil efficace d’analyse et de prédiction du comportement de la modulation dans de telles structures. Cette méthode permet également d’économiser du temps de calcul en comparaison aux calculs numériques purs. L'étude théorique de la propagation des ondes dans les milieux périodiques est d'abord introduite, puis les paramètres de l'existence simultanée des bandes interdites photoniques et phononiques sont déduites. Le développement d’une méthode semi-analytique ayant pour but d’analyser l'efficacité du couplage acousto-optique dans les structures périodiques artificielles est ensuite réalisé. La théorie des perturbations est développée jusqu'au 2ème ordre. Celle-ci, associée à des considérations de symétrie, est utilisée pour l'interprétation des résultats. Une illustration de la versatilité de la méthode, basée d'une cavité ponctuelle L1 sur substrat silicium, est présentée. Les résultats obtenus sont en accord avec ceux donnés par une méthode purement numérique. / In this thesis, a study of periodic microstructures and their applications to optical modulation by acoustical waves is presented. More specifically, it deals with opto-mechanical coupling in phoXonic crystal cavities. This study shows how the perturbation theory provides an efficient tool to analyse and predict the behaviour of modulation in such structures. Moreover, when compared to pure numerical ones, this method leads to calculation time saving. The theory of periodic media is first introduced and then we derive the parameters for the simultaneous existence of photonic and phononic bandgaps. We end up by the development of a semi-analytical method to analyze acousto-optical coupling efficiency in artificial periodic structures. The perturbation theory is developed up to 2nd order and is used together with symmetry considerations for interpretations. An illustration of the versatility of the developed method is presented using an L1 point defect cavity on silicon substrate and validated with classical numerical results.

Dans une cellule solaire, il existe un compromis entre l’efficacité d’absorption des photons et le rendement quantique de collection des charges électriques. Dans les semi-conducteurs organiques, la longueur de diffusion des porteurs est limitée à une centaine de nanomètres, si bien qu’il est nécessaire de travailler avec des couches photo-actives ultraminces (< 100 nm). Pour limiter l’épaisseur physique des matériaux utilisés tout en maintenant une absorption élevée, il est possible d’utiliser les propriétés des cristaux photoniques (CP), pour allonger la durée d’interaction des photons avec le milieu absorbant. Cela consiste à former un CP dans la couche active ou à son voisinage et d’exciter des modes résonants de la structure. Ce travail de thèse est divisé en plusieurs parties. Dans un premier temps, à l’aide d’outils numériques, nous nous intéressons aux phénomènes qui régissent le gain d’absorption lors du couplage d’une onde plane avec un mode résonant d’une membrane à CP. Ensuite, nous étudions une cellule à CP, où l’électrode d’ITO est nano-structurée, et nous optimisons le gain d’absorption d’une couche photo-active ultramince (50 nm). Enfin, dans un travail expérimental, nous fabriquons des cristaux colloïdaux bidimensionnels à base de microsphères diélectriques par différentes méthodes d’auto assemblage. / In a solar cell, there is a trade-off between light absorption capacity and internal quantum efficiency. In organic semi conductors, charge carrier diffusion is limited to a few hundred nanometers, which implies to work with very thin active layers (< 100 nm). In order to limit the thickness of the material while keeping high light absorption, it is possible to use the properties of photonic crystals (PC) to enhance light matter interaction duration. It consists in forming a PC in or around the active layer, and to excite a resonant mode of the formed photonic structure. The work of this thesis is divided into several parts. In a first approach, using numerical tools, we investigate the phenomena that give rise to absorption enhancement when a plane wave is coupled to a resonant mode of a PC membrane. We then study a nano-structured cell architecture, in which the ITO electrode is periodically patterned, and we optimize absorption enhancement in the thin active layer (50 nm). Finally, in an experimental work, we fabricate two dimensional colloidal crystals formed by dielectric microsphere self assembly.