Dissertations / Theses on the topic 'Nano-waveguides'

In this dissertation, advances in the fields of Photonics, and Plasmonics, and specifically, single cell analysis and waveguide sensing will be addressed. The first part of the dissertation is on Finite Difference Time Domain (FDTD) optimization and experimental demonstration of a nano-scale instrument that allows sensing at the cellular and subcellular levels. A new design of plasmonic coupler into a nanoscale waveguide is proposed and optimized using FDTD simulations. Following this, a subcellular nanoendoscope that can locally excite fluorescence in labelled cell organelles and collect the emitted fluorescent light for detailed spectrum analysis is fabricated and tested. The nanoendoscope has a sharp tapered tip of diameter ~ 50 nm that permits safe insertion into the cell that was confirmed by a number of viability experiments. FDTD analysis demonstrated that, with an optimized nanoendoscope taper profile, light emission and collection was very local. Thus, signal detection could be used for nano-photonic sensing of proximity of fluorophores. In further experiments, fluorescent signals were collected from individual organelles of living cells including: the nucleus of Acridine orange labelled human fibroblast cells, the nucleus of Hoechst stained live liver cells and the mitochondria of MitoTracker Red labelled MDA-MB-231 cells. In addition, this endoscope was inserted into a live organism, the nematode Caenorhabditis elegans, and in- vivo fluorescence signal was collected. Second, an innovative single step fabrication method of low loss polysilicon waveguides was developed as a potential platform for a number of photonic sensors. Optimization of a capacitively coupled plasma etching for the fabrication of a polysilicon waveguide with smooth sidewalls and low optical loss was demonstrated. A detailed experimental study on the influences of RF plasma power and chamber pressure on the roughness of the sidewalls of waveguides was conducted and waveguides were characterized using a scanning electron microscope. It was demonstrated that optimal combination of pressure (30 mTorr) and power (150 W) resulted in the smoothest sidewalls. The optical losses of the optimized waveguide were 4.1± 0.6 dB/ cm. Finally, an on-chip nanophotonic sensor for continuous blood coagulation analysis was proposed. The system was simulated using three-dimensional FDTD software. At first, the noise due to the presence of cells was calculated. Next, the design of a waveguide cladding-based filtering structure for elimination of the noise from cells was proposed and significantly decreased noise level was theoretically demonstrated.

In this dissertation, a comprehensive and rigorous analysis of BER performance in the single- and multi-channel silicon optical interconnects is presented. The illustrated computational algorithms and new results can furnish one with insight of how to engineer waveguide dimensions, optical nonlinearity and dispersion, in order to facilitate the design and construction of the ultra-fast and low-cost chip-level communications for next-generation high-performance computing systems. Two types of optical links have been intensively discussed in this dissertation, namely a strip single-mode silicon photonic waveguide and a silicon photonic crystal waveguide. Different types of optical input signals are considered here, including an ON-OFF keying modulated nonreturn-to-zero continuous-wave signal, a phase-shift keying modulated continuous-wave signal, and a Gaussian pulsed signal, all in presence of white noise. The output signal is detected and analyzed using direct-detection optical receivers. To model the signal propagation in the single- and multi-channel silicon photonic waveguides, we employ both rigorous theoretical models that incorporate all relevant linear and nonlinear optical effects and the mutual interaction between the free carriers and the optical field, as well as their linearized version valid in the low-noise power regime. Particularly, the second propagation model is designed only for optical continuous-wave signals. Equally important, the bit error rate (BER) of the transmitted signal is accurately and efficiently calculated by using the Karhunen-Loeve series expansion methods, with these approaches performed via the time-domain, frequency-domain, and Fourier-series expansion, separately. Based on the theoretical models proposed in this work, a system analysis engine has been constructed numerically. This engine can not only analyze the underlying physics of silicon waveguides, but also evaluate the system performance, which is extremely valuable for the configuration and optimization of the optical networks on chip.

Applications of surface plasmon polaritons (SPPs) have thus far emphasized visible and near-infrared wavelengths. Extension into the long-wave infrared (LWIR) has numerous potential advantages for biosensors and waveguides, which are explored in this work. A surface plasmon resonance (SPR) biosensor that operates deep into the infrared (3-11 µm wavelengths) is potentially capable of biomolecule recognition based on both selective binding and characteristic vibrational modes. The goal is to operate such sensors at wavelengths where biological analytes are strongly differentiated by their IR absorption spectra and where the refractive index is increased by dispersion, which will provide enhanced selectivity and sensitivity. Potentially useful IR surface plasmon resonances are investigated on lamellar gratings formed from various materials with plasma frequencies in the IR wavelength range including doped semiconductors, semimetals, and conducting polymers. One outcome of this work has been the demonstration of a simple analytic formula for calculating the SPP absorption resonances in the angular reflectance spectra of gratings. It is demonstrated for Ag lamellar gratings in the 6-11 µm wavelength range. The recipe is semi-empirical, requiring knowledge of a surface-impedance modulation amplitude, which is found here by comparison to experiment as a function of the grating groove depth and the wavelength. The optimum groove depth for photon-to-SPP energy conversion was found by experiment and calculation to be ~10-15% of the wavelength. Hemicylindrical prism couplers formed from Si or Ge were investigated as IR surface plasmon couplers for the biosensor application. Strong Fabry-Perot oscillations in the angular reflectance spectra for these high index materials suggest that grating couplers will be more effective for this application in the LWIR. A variety of materials having IR plasma frequencies were investigated due to the tighter SPP mode confinement anticipated in the IR than for traditional noble metals. First doped-Si and metal silicides (Ni, Pd, Pt and Ti) were investigated due to their inherent CMOS compatibility. Rutherford backscattering spectroscopy, x-ray diffraction, scanning electron microscopy, secondary ion mass spectrometry and four point probe measurements complemented the optical characterization by ellipsometry. Calculation of propagation length and mode confinement from measured permittivities demonstrated the suitability for these materials for LWIR SPP applications. Semimetals were also investigated since their plasma frequencies are intermediate between those of doped silicon and metal silicides. The semimetal antimony, with a plasma frequency ~80 times less than that of gold was characterized. Relevant IR surface plasmon properties, including the propagation length and penetration depths for SPP fields, were determined from optical constants measured in the LWIR. Distinct resonances due to SPP generation were observed in angular reflection spectra of Sb lamellar gratings in the wavelength range of 6 to 11 µm. Though the real part of the permittivity is positive in this range, which violates the usual condition for the existence of bound SPP modes, calculations based on experimental permittivity showed that there is little to distinguish bound from unbound SPP modes for this material. The SPP mode decays exponentially away from the surface on both sides of the permittivity sign change. Water is found to broaden the IR plasmon resonances significantly at 9.25 micron wavelength where aqueous extinction is large. Much sharper resonances for water based IR SPR biosensor can be achieved in the 3.5 to 5.5 µm range. Nano-structured Au films (Au-black) were investigated as IR absorbers and possible solar cell enhancers based on surface plasmon resonance. The characteristic length scales of the structured films vary considerably as a function of deposition parameters, but the absorbance is found to be only weakly correlated with these distributions. Structured Au-black with a broad range of cluster length scales appear to be able to support multiple SPP modes with incident light coupling to the corrugated surface as seen by photoelectron emission microscopy (PEEM) and SPR experiments, supporting the hypothesis that Au-black may be a suitable material for plasmon-resonance enhancement solar-cell efficiency over the broad solar spectrum.
Ph.D.
Department of Physics
Sciences
Physics PhD

Over the past few years, surface plasmon photodetectors have been of renewed interest. This is due to their unique double functionality of combining an SPP waveguide structure with a photodetection structure. This thesis investigates the performance of a Schottky nano-photodetector integrated into a finite width metal stripe which is covered by air on top and supported by silicon at the bottom, supporting the propagation of bound SPP modes. Properties of surface plasmons, including the sub-wavelength confinement, were exploited to increase the efficiency of the detector. The detector performance was explored via applying end-fire coupling to the fundamental supported mode, then the results were used to calculate the devices responsivity, dark current, minimum detectable power, and photocurrent for various metal lengths. End fire coupling to a Schottky mode supported by a nano-structured metal was done for what is believed to be the first time.

The interaction of an evanescent wave and plasmonic nanostructures are simulated in Finite Element Method. Specifically, the optical absorption cross section (Cabs) of a silver nanoparticle (AgNP) and a gold nanoparticle (AuNP) in the presence of metallic (gold) and dielectric (silicon) atomic force microscope (AFM) probes are numerically calculated in COMSOL. The system was illuminated by a transverse magnetic polarized, total internally reflected (TIR) waves or propagating surface plasmon (SP) wave. Both material nanoscale probes localize and enhance the field between the apex of the tip and the particle. Based on the absorption cross section equation the author was able to demonstrate the increment of absorption cross section when the Si tip was brought closer to the AuNP, or when the Si tip apex was made larger. However, the equation was not enough to predict the absorption modification under metallic tips, especially for a AgNP's Cabs; neither it was possible to estimate the optical absorption based on the localized enhanced field caused by a gold tip. With the help of the driven damped harmonic oscillator equation, the Cabs of nanoparticles was explained. In addition, this model was applicable for both TIR and Surface Plasmon Polaritons illuminations. Fitting the numerical absorption data to a driven damped harmonic oscillator (HO) model revealed that the AFM tip modifies both the driving force (F0), consisting of the free carrier charge and the driving field, and the overall damping of the oscillator beta. An increased F0 or a decreased beta will result in an increased Cabs and vice versa. Moreover, these effects of F0 and beta can be complementary or competing, and they combine to either enhance or suppress absorption. Hence, a significantly higher beta with a small increment in F0 will result in an absorption suppression. Therefore, under a Si tip, Cabs of a AuNP is enhanced while Cabs of a AgNP is suppressed. In contrast, a Au tip suppresses the Cabs for both Au and Ag NPs. As an extension of this absorption model, further investigation of the guided mode and a close by nanostructure is proposed, where the scattered wave off the structure attenuates the guided mode with destructive interference.

Le contrôle de la microstructure et de la morphologie de surface est essentiel pour que les couches minces soient appliquées dans des dispositifs optiques et acoustiques. Des couches minces de TiO2, LaNiO3 et ZnO et leurs hétérostructures ont été obtenues par des techniques de pulvérisation cathodique à radio fréquence et de dépôt chimique en phase vapeur (CVD). L'optimisation des paramètres de dépôt, tels que la température, la pression totale de la chambre, la pression partielle d'O2 et la vitesse de croissance, a conduit à une amélioration de la qualité structurelle et fonctionnels des films minces et de leurs hétérostructures. L'orientation des couches minces épitaxiales de ZnO et TiO2 a été ajustée non seulement par le lien épitaxial avec divers substrats, mais également par les conditions de dépôt. La qualité optique des films de TiO2 a été principalement optimisée par l'élimination des défauts de microstructure et l'augmentation de la non-stoechiométrie en oxygène. Il a été démontré que les défauts ponctuels et microstructuraux dans les films polycristallins et épitaxiaux jouent un rôle clé dans les pertes de propagation optique. L'effet de la polarité du substrat sur les propriétés structurelles, optiques et acoustiques des films minces à base de ZnO a également été étudié. Les couches sacrificielles et / ou d'initiation de croissance ont été identifiées pour l'intégration hétérogène de films acoustiques et optiques fonctionnels sur substrats semi-conducteurs
The control of microstructure and surface morphology is essential for the thin films to be applied in optical and acoustic devices. Thin films of TiO2, LaNiO3 and ZnO and their heterostructures in this work were obtained by radio frequency (RF) magnetron sputtering and metalorganic chemical vapor deposition (MOCVD) techniques. The optimization of deposition parameters, such as temperature, total chamber pressure, O2 partial pressure and growth rate, led to high structural quality of functional thin films and their heterostructures. The orientation of epitaxial ZnO and TiO2 thin films was tuned not only through lattice matching with various substrates, but as well through deposition conditions. The optical quality of TiO2 films was mostly optimized through elimination of microstructural defects and increasing oxygen non-stoichiometry. It was shown that microstructural and lattice defects in polycrystalline and epitaxial films played a key role in optical propagation losses. Effect of substrate polarity on the structural, optical and acoustic properties of ZnO-based thin films was studied, as well. The sacrificial and/or seed layers were identified for heterogeneous intégration of functional acoustical and optical films with semiconductor substrates

Strongly localised plasmons in metallic nano-structures offer exciting characteristics for guiding and focusing light on the nano-scale, opening the way for the development of new types of sensors, circuitry and improved resolution of optical microscopy. The work presented in this thesis focuses on two major areas of plasmonics research - nano-focusing structures and nano-sized waveguides. Nano-focusing structures focus light to an area smaller than the wavelength and will find applications in sensing, efficiently coupling light to nano-scale devices, as well as improving the resolution of near field microscopy. In the past the majority of nano-focusing structures have been nano-scale cones or tips, which are capable of focusing light to a spot of nano-scale area whilst enhancing the light field. The alternatives are triangular nano-focusing structures which have received far less attention, and only one type of triangular nano-focusing structure is known – a sharp V-groove in a metal substrate. This structure focuses light to a strip of nano-scale width, which may lead to new applications in microscopy and sensing. The difficulty with implementing the V-groove is that the structure is not robust and is quite difficult to fabricate. This thesis aims to develop new triangular nano-focusing devices which will overcome these difficulties, whilst still producing an intense light source on the nano-scale. The two proposed structures presented in this thesis are a metallic wedge submerged in uniform dielectric and a tapered metal film lying on a dielectric substrate, the latter being the easier to fabricate and the more structurally sound and robust. The investigation is performed using the approximation of continuous electrodynamics, the geometrical optics approximation and the zero-plane method. The second aim of this thesis is to investigate plasmonic waveguides and couplers for the development of nano-optical circuitry, more compact photonic devices and sensors. The research will attempt to fill the gaps in the current knowledge of the V-groove waveguide, which consists of a sharp triangular groove in a metal substrate, and the gap plasmon waveguide, which consists of a rectangular slot in a thin metal film. The majority of this work will be performed using the author’s in house finite-difference time-domain algorithm and FEMLAB as well as the effective medium method and geometric optics approximation. The V-groove may be used as either a nano-focusing or waveguiding device. As a waveguide the V-groove is one of the most promising plasmonic waveguides in the optical regime. However, there exist quite a number of gaps in the current knowledge of V-groove waveguides which this thesis will attempt to fill. In particular, the effect of rounded groove tip on plasmon propagation has been assessed for the V-groove. The investigation of rounded groove tip is important, as due to modern fabrication processes it’s not possibly to produce an infinitely sharp groove, and the existing literature has not considered the impact of this problem. The thesis will also investigate the impacts of the inclusion of dielectric filling in the groove on plasmon propagation parameters. This research will be important for optimising the propagation characteristics of the mode for certain applications, but it may also lead to easier methods of fabricating the V-groove device and prevent oxidation of the metal film. The gap plasmon waveguide is easier to fabricate than the V-groove, and is a new type of sub-wavelength waveguide which displays many advantages over other types of plasmon waveguides, including ease of fabrication, almost 100% transmission around sharp bends, sub-wavelength localisation and long propagation distances of the guided mode, etc. This waveguide may prove invaluable in the development of compact photonic devices. In the past the modes supported by this structure were not thoroughly analysed and the possibility of using this structure to develop sub-wavelength couplers for sensing and nano-optical circuits was not considered in detail. This thesis aims to resolve these issues. In conclusion, the results of this thesis will lead to a better understanding of Vgroove and gap plasmon waveguide devices for the development of nano-optical circuits, compact photonic devices and sensors. This thesis also proposes two new nano-focusing structures which are easier to fabricate than the V-groove structure and will lead to applications in sensing, coupling light efficiently into nano-scale devices and improving the resolution of near-field microscopy.

Strongly localised plasmons in metallic nano-structures offer exciting characteristics for guiding and focusing light on the nano-scale, opening the way for the development of new types of sensors, circuitry and improved resolution of optical microscopy. The work presented in this thesis focuses on two major areas of plasmonics research - nano-focusing structures and nano-sized waveguides. Nano-focusing structures focus light to an area smaller than the wavelength and will find applications in sensing, efficiently coupling light to nano-scale devices, as well as improving the resolution of near field microscopy. In the past the majority of nano-focusing structures have been nano-scale cones or tips, which are capable of focusing light to a spot of nano-scale area whilst enhancing the light field. The alternatives are triangular nano-focusing structures which have received far less attention, and only one type of triangular nano-focusing structure is known – a sharp V-groove in a metal substrate. This structure focuses light to a strip of nano-scale width, which may lead to new applications in microscopy and sensing. The difficulty with implementing the V-groove is that the structure is not robust and is quite difficult to fabricate. This thesis aims to develop new triangular nano-focusing devices which will overcome these difficulties, whilst still producing an intense light source on the nano-scale. The two proposed structures presented in this thesis are a metallic wedge submerged in uniform dielectric and a tapered metal film lying on a dielectric substrate, the latter being the easier to fabricate and the more structurally sound and robust. The investigation is performed using the approximation of continuous electrodynamics, the geometrical optics approximation and the zero-plane method. The second aim of this thesis is to investigate plasmonic waveguides and couplers for the development of nano-optical circuitry, more compact photonic devices and sensors. The research will attempt to fill the gaps in the current knowledge of the V-groove waveguide, which consists of a sharp triangular groove in a metal substrate, and the gap plasmon waveguide, which consists of a rectangular slot in a thin metal film. The majority of this work will be performed using the author’s in house finite-difference time-domain algorithm and FEMLAB as well as the effective medium method and geometric optics approximation. The V-groove may be used as either a nano-focusing or waveguiding device. As a waveguide the V-groove is one of the most promising plasmonic waveguides in the optical regime. However, there exist quite a number of gaps in the current knowledge of V-groove waveguides which this thesis will attempt to fill. In particular, the effect of rounded groove tip on plasmon propagation has been assessed for the V-groove. The investigation of rounded groove tip is important, as due to modern fabrication processes it’s not possibly to produce an infinitely sharp groove, and the existing literature has not considered the impact of this problem. The thesis will also investigate the impacts of the inclusion of dielectric filling in the groove on plasmon propagation parameters. This research will be important for optimising the propagation characteristics of the mode for certain applications, but it may also lead to easier methods of fabricating the V-groove device and prevent oxidation of the metal film. The gap plasmon waveguide is easier to fabricate than the V-groove, and is a new type of sub-wavelength waveguide which displays many advantages over other types of plasmon waveguides, including ease of fabrication, almost 100% transmission around sharp bends, sub-wavelength localisation and long propagation distances of the guided mode, etc. This waveguide may prove invaluable in the development of compact photonic devices. In the past the modes supported by this structure were not thoroughly analysed and the possibility of using this structure to develop sub-wavelength couplers for sensing and nano-optical circuits was not considered in detail. This thesis aims to resolve these issues. In conclusion, the results of this thesis will lead to a better understanding of Vgroove and gap plasmon waveguide devices for the development of nano-optical circuits, compact photonic devices and sensors. This thesis also proposes two new nano-focusing structures which are easier to fabricate than the V-groove structure and will lead to applications in sensing, coupling light efficiently into nano-scale devices and improving the resolution of near-field microscopy.

A major challenge in modern photonics and nano-optics is the diffraction limit of light which does not allow field localisation into regions with dimensions smaller than half the wavelength. Localisation of light into nanoscale regions (beyond its diffraction limit) has applications ranging from the design of optical sensors and measurement techniques with resolutions as high as a few nanometres, to the effective delivery of optical energy into targeted nanoscale regions such as quantum dots, nano-electronic and nano-optical devices. This field has become a major research direction over the last decade. The use of strongly localised surface plasmons in metallic nanostructures is one of the most promising approaches to overcome this problem. Therefore, the aim of this thesis is to investigate the linear and non-linear propagation of surface plasmons in metallic nanostructures. This thesis will focus on two main areas of plasmonic research –– plasmon nanofocusing and plasmon nanoguiding. Plasmon nanofocusing – The main aim of plasmon nanofocusing research is to focus plasmon energy into nanoscale regions using metallic nanostructures and at the same time achieve strong local field enhancement. Various structures for nanofocusing purposes have been proposed and analysed such as sharp metal wedges, tapered metal films on dielectric substrates, tapered metal rods, and dielectric V-grooves in metals. However, a number of important practical issues related to nanofocusing in these structures still remain unclear. Therefore, one of the main aims of this thesis is to address two of the most important of issues which are the coupling efficiency and heating effects of surface plasmons in metallic nanostructures. The method of analysis developed throughout this thesis is a general treatment that can be applied to a diversity of nanofocusing structures, with results shown here for the specific case of sharp metal wedges. Based on the geometrical optics approximation, it is demonstrated that the coupling efficiency from plasmons generated with a metal grating into the nanofocused symmetric or quasi-symmetric modes may vary between ~50% to ~100% depending on the structural parameters. Optimal conditions for nanofocusing with the view to minimise coupling and dissipative losses are also determined and discussed. It is shown that the temperature near the tip of a metal wedge heated by nanosecond plasmonic pulses can increase by several hundred degrees Celsius. This temperature increase is expected to lead to nonlinear effects, self-influence of the focused plasmon, and ultimately self-destruction of the metal tip. This thesis also investigates a different type of nanofocusing structure which consists of a tapered high-index dielectric layer resting on a metal surface. It is shown that the nanofocusing mechanism that occurs in this structure is somewhat different from other structures that have been considered thus far. For example, the surface plasmon experiences significant backreflection and mode transformation at a cut-off thickness. In addition, the reflected plasmon shows negative refraction properties that have not been observed in other nanofocusing structures considered to date. Plasmon nanoguiding – Guiding surface plasmons using metallic nanostructures is important for the development of highly integrated optical components and circuits which are expected to have a superior performance compared to their electronicbased counterparts. A number of different plasmonic waveguides have been considered over the last decade including the recently considered gap and trench plasmon waveguides. The gap and trench plasmon waveguides have proven to be difficult to fabricate. Therefore, this thesis will propose and analyse four different modified gap and trench plasmon waveguides that are expected to be easier to fabricate, and at the same time acquire improved propagation characteristics of the guided mode. In particular, it is demonstrated that the guided modes are significantly screened by the extended metal at the bottom of the structure. This is important for the design of highly integrated optics as it provides the opportunity to place two waveguides close together without significant cross-talk. This thesis also investigates the use of plasmonic nanowires to construct a Fabry-Pérot resonator/interferometer. It is shown that the resonance effect can be achieved with the appropriate resonator length and gap width. Typical quality factors of the Fabry- Pérot cavity are determined and explained in terms of radiative and dissipative losses. The possibility of using a nanowire resonator for the design of plasmonic filters with close to ~100% transmission is also demonstrated. It is expected that the results obtained in this thesis will play a vital role in the development of high resolution near field microscopy and spectroscopy, new measurement techniques and devices for single molecule detection, highly integrated optical devices, and nanobiotechnology devices for diagnostics of living cells.

Carbon dioxide is an important gas for life on Earth. But as human activities have been expanding throughout modern history, the CO2 concentration in the atmosphere is increasing. High concentrations of carbon dioxide can lead to various consequences, such as climate change and poor air quality both indoors and outdoors. It is therefore of importance to detect this gas, in order to understand our environment, and to avoid health impacts that it may cause. Non-dispersive infrared sensors are widely used in CO2 sensing and are based on optical absorption technology. This thesis investigates the optical performance of suspended waveguides for non-dispersive infrared sensors, with regard to different material qualities, i.e. monocrystalline and polycrystalline silicon, and geometries of these waveguides. The waveguides that are studied in this thesis consist of splitters, and at the end of each splitter a grating coupler that projects the IR radiation perpendicularly from the plane of the chip. Measurements are conducted to evaluate the IR radiation propagation loss of the waveguides and their feasibility for sensing carbon dioxide. It has been found that longer waveguides suffer from high propagation losses. When comparing the polycrystalline silicon with monocrystalline silicon waveguides, it has been observed in the measurements that the IR radiation propagates better in monocrystalline silicon waveguides than in polycrystalline silicon because of their crystal structures. The measured propagation loss in polycrystalline silicon waveguides is less than the loss obtained for the monocrystalline silicon waveguides, although some intensities from the grating couplers are excluded in the calculations, due to low signal strength. It is also concluded that the studied waveguides are feasible for detecting carbon dioxide with a concentration of 1%. Further investigation regarding the feasibility of gas sensing using lower concentrations of CO2 would be interesting for future work.

O incessante aumento do volume de informações produzido por uma sociedade cada vez mais informatizada tem elevado drasticamente os requisitos quanto ao desenvolvimento de dispositivos capazes de suportar velocidades de operação cada vez mais elevadas em tamanhos cada vez mais reduzidos. No entanto, a contínua redução do tamanho desses dispositivos, celebrado através da lei de Moore, também produz um indesejável aumento na produção de calor durante a operação dos mesmos, comprometendo seu desempenho global. Uma alternativa promissora para aliviar, ou mesmo superar, estas limitações é oferecida pelos dispositivos ópticos integrados. No entanto, todo esse avanço esbarrava no fato de que as dimensões de tais dispositivos estavam restringidas fundamentalmente ao que é largamente conhecido como limite de difração (LD). Uma maneira de contornar essa limitação é obtida através da utilização de Plásmon Poláritons de Superfície, ou SPPs, que, de maneira simplificada, são ondas que se propagam ao longo da superfície de um condutor depositado sobre um dielétrico. Estas são essencialmente ondas de luz que são localizadas na superfície por causa de sua interação com os elétrons livres do condutor. Nesta interação, os elétrons livres respondem coletivamente oscilando em ressonância com a onda de luz. No presente trabalho, o fenômeno de geração de SPPs é estudado teoricamente e aplicado na modelagem de diversas estruturas de interesse científico e tecnológico, tais como acopladores direcionais e ressoadores. O objetivo principal é a obtenção de estruturas capazes de proporcionar propagação de SPPs por longas distâncias, permitindo, assim, estender ainda mais o leque de possíveis aplicações. As estruturas são investigadas prioritariamente no COMSOL Multiphysics, um aplicativo baseado em elementos finitos que permite solução vetorial de problemas eletromagnéticos. Os resultados obtidos até o momento permitem afirmar que o conceito de SPP de longa distância (long range SPP, LRSPP) podem ser aplicados com sucesso a estruturas geometricamente complexas como os ressoadores em anel e acopladores direcionais.
The continuous growth of knowledge produced by a society with increasing access to information technologies has demanded the development of communication devices capable of supporting high processing speeds at more and more reduced sizes. Nevertheless, the continuous reduction of the size of these devices, celebrated by the Moore\'s law, has also produced an undesirable increase of heat produced during the operation of the device itself, compromising its overall performance. A promising alternative to alleviate, or even overcome, these limitations has been offered by photonic integrated circuits. However, all the advance of photonic devices was restricted to what is known as diffraction limit. A fascinating way of circumventing this limit is now available to the scientific community, and consists in the generation of Surface Plasmon Polariton (SPP) waves. In a simplified manner, SPP waves are waves that propagate along a metal/dielectric interface. These waves are essentially localized at the metal/dielectric interface because of the interaction of light with free electrons of the metal. In this interaction, the free electrons respond collectively and oscillate resonantly with the incident light. In the present work, the phenomenon of SPP generation is theoretically investigated and applied to the modeling of several structures, such as directional couplers and resonators. The primary goal of this work is to design structures capable propagating SPP waves for long distances, known as long range SPP (LRSPP). The structures are investigated mostly with COMSOL Multiphysics, a finite elements based software that allows for the vectorial solution of electromagnetic problems. The results obtained so far are extremely encouraging, and prove that the LRSPP concept can be successfully applied to geometrically complex structures, such as couplers and ring resonators.

Le contrôle de la propagation des ondes élastiques repose principalement sur la conception de milieu artificiel à base de matériaux structurés pour obtenir une ingénierie avancée de la dispersion de la propagation. Au cours de la thèse, la dispersion du mode (S0) dans des membranes micro-structurées à base d’AlN a été numériquement investiguée et les applications qui en découlent explorées. Il est mis en évidence le lien fort entre la dispersion du mode et la sensibilité aux perturbations externes en combinant la membrane d’AlN avec une couche de SiO2 structurée en rubans. En particulier, il est montré qu’il est possible d’obtenir un TCF=0 pour les résonateurs sans presque aucune dégradation du coefficient K2. Il est montré qu’il est possible d’ouvrir des bandes interdites avec une largeur de l’ordre de 50% en structurant l’AlN sous forme de rubans ou en utilisant des piliers pour former un PhnC. Sur cette base, des designs de cavités et de guides d’ondes sont proposés et leurs performances sont étudiées en fonction des paramètres géométriques. Il est également proposé un nouveau design de cavité basé sur l’introduction d’un défaut résonant dans le PhnC sous forme de disque de dimension très petite par-rapport à la taille de la cellule élémentaire. Le défaut permet d’introduire des modes quasi-plats dans le diagramme de bande et permet en conséquence la conception d’une nouvelle génération de dispositifs phononiques robustes pour des applications en traitement du signal et capteurs. Les structures optimales sont utilisées pour la conception de capteur de champs magnétiques, une sensibilité de 5% est obtenue pour le mode localisé dans le cas d’un disque magnéto-élastique
The control of elastic wave propagation relies mainly on the design of artificial media based on structured materials to achieve advanced propagation dispersion engineering. During the thesis, the dispersion of the mode (S0) in micro-structured membranes based on AlN was numerically investigated and the resulting applications explored. The strong link between mode dispersion and sensitivity to external disturbances is highlighted by combining the AlN membrane with a layer of SiO2 structured into strips. In particular, it is shown that it is possible to obtain a TCF = 0 for the resonators without any degradation of the K2 coefficient. It is shown that it is possible to open wide band-gaps of 50% by structuring the AlN in the shape of strips or using pillars to form a PhnC. On this basis, designs of cavities and waveguides are proposed and their performances are studied according to the geometrical parameters. It is also proposed a new cavity design based on the introduction of a resonant defect with a disc shape in the PhnC and presenting very small size in comparison to the unit cell. The defect makes it possible to introduce quasi-flat modes in the band diagram and consequently allows the design of a new generation of phononic devices for signal processing and sensor applications. The optimal structures are used to design a magnetic field sensor design, a sensitivity of 5% is obtained for the localized mode in the case of defect based on magneto-elastic thin film

Physical processes at nanoscale (one millionth of a millimeter) started to attract strong interest from researchers in the past decade, since manufacturing and observation of such small objects became possible. In particular, plasmonics -the physics of light interaction with metallic structures at dimensions much smaller than that of the visible light wavelength -is one of the hot topics, offering numerous applications and intriguing effects. Plasmonic devices are suggested to substitute current electronic and photonic components due to their potential for miniaturizing signal processing devices including sensors, lasers, and other components for integrated optics. Moreover, high electromagnetic energy concentration in plasmonic structures gives possibility for achieving a nonlinear optical response at reasonably low signal power levels, thus providing opportunities for light manipulation and control. The purpose of this thesis is to study the light propagation in plasmonic waveguiding structures, and to reveal the fundamental effects in both linear and nonlinear plasmonic waveguides. We focus our analysis on the possibility of tuning and control over light propagation in such structures at the nanoscale. Chapter I gives a general introduction to the subject, and discusses the present state of the art in plasmonics and possible future developments in the field. The chapter also provides a detailed discussion of aims and purposes of this thesis. In Chapter II we discuss the basic electrodynamic properties of media, thereby we give an introduction to the theoretical methods and concepts employed in the analysis of original results carried out in the scope of this work (Chapters III-VI). We provide a classification of media, and give brief description of the main properties of each material type, including dispersion, second and third order nonlinear effects. The chapter is concluded with an introduction to the physics of surface plasmon-polaritons. We start our analysis of plasmonic waveguiding structures with the study of a linear planar metal-dielectric-metal waveguide, Chapter III. We show that the mode structure and corresponding dispersions strongly depend on the waveguide width and losses. We provide a classification of modes in the structure, and reveal that in lossy systems propagating and evanescent modes become mixed and cannot be distinguished from each other. We also observe mode transformation with the increase of loss strength, occurring due to several bifurcation scenarios. In Chapter IV we study multilayer metal-dielectric structures with linear chirp of the structure period. We analyze light propagation in both short and long structures. For short structures we find the spectrum of eigen-modes and reveal that practically all structures with linear gradient of period have equidistant states, corresponding to the Wannier-Stark ladder and manifesting the existence of plasmonic Bloch oscillations. For long structures we apply an asymptotic analytical method and find a novel regime of beam dynamics. In particular, we show that for long linearly chirped metal-dielectric structures it is possible to achieve different directions of energy flow at different edges of the structure, so that a paraxial beam propagating in such structure will curl. Chapters V and VI discuss nonlinear effects of plasmon propagation in waveguiding structures. First, in Chapter V we study both third and second order nonlinear processes in plasmonic systems. We show that in planar metal-dielectric{u00AC}metal waveguides the third order nonlinear optical response leads to the nonlinear dispersion of guided modes, depending on the input power level. Moreover, we reveal the symmetry breaking bifurcation, which occurs at certain power levels. We also address a more general question of plasmonic beam propagation in non-linear plasmonic wavegudies and derive an asymptotic analytical model describing beam propagation in such structures. We employ our model to the study of the plasmonic beam propagation at the metal -nonlinear dielectric interface and prove our theory with numerical simulations. We show that at certain power levels the plasmon-soliton is formed, however due to very strong losses in plasmonic structures the soliton propagates for very short distances. As for the second order nonlinear processes, we study the possibility of plasmon to plasmon frequency conversion in metal-dielectric-metal waveguide supporting plasmonic modes of different symmetry. Our analysis shows that the phase-matching condition between modes with different symmetry can be satisfied. This indicates the possibility of efficient nonlinear processes, including parametric amplification of plasmons. We discuss the conditions for plasmon-to-plasmon frequency conversion. In Chapter VI we study the plasmon propagation in nonlinear tapered waveguides. We show that for certain taper shapes it is possible to achieve the effective compensation of plasmon amplitude decay. Depending on this condition we reveal three regimes of plasmonic beam 'propagation in nonlinear tapers, and show that the tapering allows solitons to propagate for reasonably long distances, and at particular conditions three-dimensional nanofocusing in two-dimensional waveguides is possible. Finally we conclude the thesis with a brief summary of the main results and the outlook for the future of plasmonics in Chapter VII.

碩士
國立成功大學
光電科學與工程學系
104
Antennas are important elements of wireless information transmission technologies. In radio engineering, antennas refer to devices converting electric currents to radio waves and, vice versa. However, nano-antenna has more advantageous characteristic, such as localized surface plasmon resonance, surface enhancement of light and high sensitivity to the changing of refractive index of surrounding. These advantages bring it to have the potential applications in optical computing and molecule sensing. We built up single-mode rib waveguides and nano-antennas on the silicon-on-insulator (SOI) substrate by using semiconductor manufacturing process. The resonance wavelength of nano-antennas was designed to be at 1550 nm based on the literature. The fabrication results, challenges and corresponding solutions were showed in this thesis. In optical measurement, nano-antenna was excited by different wavelengths and polarizations of different guiding modes of waveguide. The response of the antenna was also showed and discussed in this thesis. It could be seen that the antenna was reacting with different excitation but the resonance peak was different from our expectations. It may be due to the induced charge of substrate.

abstract: This thesis summarizes the research work carried out on design, modeling and simulation of semiconductor nanophotonic devices. The research includes design of nanowire (NW) lasers, modeling of active plasmonic waveguides, design of plasmonic nano-lasers, and design of all-semiconductor plasmonic systems. For the NW part, a comparative study of electrical injection in the longitudinal p-i-n and coaxial p-n core-shell NWs was performed. It is found that high density carriers can be efficiently injected into and confined in the core-shell structure. The required bias voltage and doping concentrations in the core-shell structure are smaller than those in the longitudinal p-i-n structure. A new device structure with core-shell configuration at the p and n contact regions for electrically driven single NW laser was proposed. Through a comprehensive design trade-off between threshold gain and threshold voltage, room temperature lasing has been proved in the laser with low threshold current and large output efficiency. For the plasmonic part, the propagation of surface plasmon polariton (SPP) in a metal-semiconductor-metal structure where semiconductor is highly excited to have an optical gain was investigated. It is shown that near the resonance the SPP mode experiences an unexpected giant modal gain that is 1000 times of the material gain in the semiconductor and the corresponding confinement factor is as high as 105. The physical origin of the giant modal gain is the slowing down of the average energy propagation in the structure. Secondly, SPP modes lasing in a metal-insulator-semiconductor multi-layer structure was investigated. It is shown that the lasing threshold can be reduced by structural optimization. A specific design example was optimized using AlGaAs/GaAs/AlGaAs single quantum well sandwiched between silver layers. This cavity has a physical volume of 1.5×10-4 λ03 which is the smallest nanolaser reported so far. Finally, the all-semiconductor based plasmonics was studied. It is found that InAs is superior to other common semiconductors for plasmonic application in mid-infrared range. A plasmonic system made of InAs, GaSb and AlSb layers, consisting of a plasmonic source, waveguide and detector was proposed. This on-chip integrated system is realizable in a single epitaxial growth process.
Dissertation/Thesis
Ph.D. Electrical Engineering 2012

Wave propagation is a common phenomenon in aircraft structures resulting from high velocity transient loadings like bird hit, gust etc. Apart from understanding the behavior of structures under such loading, wave propagation analysis is also important to gain knowledge about their high frequency characteristics, which have several applications. The applications include structural health monitoring using diagnostic waves and control of wave transmission for reduction of noise and vibration. Transient loadings with high frequency content are associated with wave propagation. As a result, the higher modes of the structure participate in the response. Finite element (FE) modeling for such problem requires very fine mesh to capture these higher modes. This leads to large system size and hence large computational cost. Wave propagation problems are usually solved in frequency domain using fast Fourier transform (FFT) and spectral finite element method is one such technique which follows FE procedure in the transformed frequency domain. In this thesis, a novel wavelet based spectral finite element (WSFE) is developed for wave propagation analysis in finite dimension structures. In WSFE for 1-D waveguides, the partial differential wave equations are reduced to a set of ODEs using orthogonal compactly supported Daubechies scaling functions for temporal approximation. The localized nature of the Daubechies basis functions allows finite domain analysis and imposition of the boundary conditions. The reduced ODEs are usually solved exactly, the solution of which gives the dynamic shape functions. The interpolating functions used here are exact solution of the governing differential equation and hence, the exact elemental dynamic stiffness matrix is derived. Thus, In the absence of any discontinuities, one element is sufficient to model 1-D waveguide of any length. This elemental stiffness matrix can be assembled to obtain the global matrix as in FE and after solution, the time domain responses are obtained using the inverse wavelet transform. The developed technique circumvents several serious limitations of the conventional FFT based Spectral Finite Element (FSFE). In FSFE, the wave equations are reduced to ODEs using FFT for time approximation. The remaining part of the formulation is quite similar to that of WSFE. The required assumption of periodicity in FSFE, however, does not allow modeling of finite length structures. It results in “wrap around” problem, which distorts the response simulated using FSFE and a semi-infinite (“throw-off”) element is required for imparting artificial damping. This artificial damping occurs as the “throw off” element allows leakage of energy. In some cases, a very high damping can also be considered instead of “throw off” element to remove wrap around effects. In either cases, the damping introduced is much larger than any inherent damping that may be present in the structure. It should also be mentioned that even in presence of the artificial damping, a larger time window is required for removing the distortions completely. The developed WSFE method is completely free from such problems and can efficiently handle undamped finite length structures irrespective of the time window considered. Apart from this, FSFE allows imposition of only zero initial condition and in contrary any initial conditions can be used in WSFE. Though FSFE has problem in modeling finite length undamped structures for time domain analysis, it is well suited for performing frequency domain study of wave characteristics, namely, the determination of spectrum and dispersion relations. WSFE is also capable of extracting these frequency dependent wave properties, however only up to a certain fraction of the Nyquist frequency. This constraint results from the loss in frequency resolution due to the increase in time resolution in wavelet analysis, where the basis functions are bounded both in time and frequency. A price has to be paid in frequency domain in order to obtain a bound in the time domain. The consequence of this analysis is to impose a constraint on the time sampling rate for the simulation with WSFE, to avoid spurious dispersion. WSFE for 2-D waveguides are formulated using Daubechies scaling functions for both temporal and spatial approximations. The initial and boundary conditions, however, are imposed using two different methods, which are wavelet extrapolation technique and periodic extension or restraint matrix respectively. The 2-D WSFE is bounded in both the spatial directions unlike 2-D FSFE, which is essentially unbounded in one spatial direction. Apart from this, 2-D WSFE is also free from “wrap around” problem similar to 1-D WSFE due to the localized nature of the basis functions used for temporal approximation. In this thesis, WSFE is developed for isotropic 1-D and 2-D waveguides for time and frequency domain analysis. These include elementary rod, Euler-Bernoulli and Timoshenko beams in 1-D modeling, and plates and axisymmetric cylinders in 2-D modeling. The wave propagation responses simulated using WSFE for these waveguides are validated using FE results. The advantages of the proposed technique over the corresponding FSFE method are also highlighted all through the numerical examples. Next part of the thesis involves the extension of the developed WSFE technique for modeling composite and nano-composite structures to study their wave propagation behavior. Due to their anisotropic nature, analysis of composite structures, particularly high frequency transient analysis is much more complicated compared to the corresponding metallic structures. This is due to the presence of stiffness coupling in these structures. Superior mechanical properties of composites, however, are making them integral parts of an aircraft and thus they often experience such short duration, high velocity impact Loadings. Very few literatures report the response of composite structures subjected to such high frequency excitations. Here, WSFE is formulated for a higher order composite beam with axial, flexural, shear and contractional degrees of freedom. WSFE is also formulated for composite plates using classical laminated plate theory with axial and flexural degrees of freedom. Simulations performed using these WSFE models are used to study the higher order and elastic coupling effects on the wave propagation responses. Carbon nanotubes (CNTs) and their composites are attracting a great deal of experimental and theoretical research world-wide. The recent trend in the literature shows a great interest in the dynamic and wave characteristics of CNTs and nano-composites because of their several applications. In most of these applications, CNTs are used in the embedded form as it does not requires precise alignment of the nano-tubes. In addition, the extraordinary mechanical properties of CNTs are being exploited to achieve high strength nano-composite. Apart from the experimental studies and atomistic simulation to study the mechanical properties of CNTs and nano-composites, continuum modeling is also receiving much attention, mainly due to its computational viability. In this thesis, a 1-D WSFE is formulated for multi-wall carbon nanotube (MWNT) embedded composite modeled as beam using higher order layer-wise theory. This theory allows to model partial interfacial shear stress transfer, which normally occurs due to improper dispersion of CNTs in nano-composites. The effects of different matrix materials and fraction of shear stress transfer on the wave characteristics are studied. The responses obtained using other beam theories are also compared. The beam modeling does not allow capturing the radial motions of the CNT, which are important for several applications. These can be effectively captured by modeling the CNT using a 2-D axisymmetric model. Hence, a 2-D WSFE model is constructed to capture the high frequency characteristics of single-walled carbon nanotubes (SWNTs). The response of SWNT simulated using the developed model is validated with experimental and atomistic simulation results reported in the literature. The comparison are done for dispersion relation and also radial breathing mode frequencies. The effects of geometrical parameters, namely the radius and the wall thickness of the SWNT on the higher radial, longitudinal and coupled radial-longitudinal vibrational modes are analyzed. These behaviors are studied in both time and frequency domains. Such time domain analyses of finite length SWNT are not possible with the Fourier transform based techniques reported in literature, although, such analyses are important particularly for sensor applications of SWNT. Spectral finite element method is very much suited for solution of inverse problems like force reconstruction from the measured wave response. This is because the technique is based on the concept of transfer function between the displacements (output) and applied forces (input). In the present work, WSFE is implemented for identification of impact force from the wave propagation responses simulated with FE and used as surrogate experimental results. The results show that WSFE can accurately reconstruct the impulse load applied to 1-D waveguides which include rod, Euler-Bernoulli beam and connected 2-D frame, even with highly truncated response. This is unlike FSFE, where the accuracy of the identified force depends largely on the time window of the measured responses. The detection of damage from the wave propagation analysis is another class of inverse problems considered in this thesis and is of utmost importance in the area of aircraft structural health monitoring. Here, the detection scheme is based on arrival time of the waves reflected from the damage. A novel detection technique based on wavelet filtering is proposed here and it is shown to work efficiently even in the presence of noise in the measured wave responses. Detection of damage requires an efficient damage model to simulate the mode of structural failure. In this regard, two spectrally formulated wavelet elements are proposed, one to model isotropic beam with through-width notch and the second to model composite beam with embedded de-lamination. In the first case, the response of the damaged beam is considered as the perturbation of the undamaged response and the linear perturbation analysis leads to a completely new set of dynamic stiffness matrix. In the second case, the delamination is modeled by subdividing the de-laminated region into separate waveguides and full damage model is established by imposing the kinematics. These models help to simulate wave propagation in such damaged beams to study the effect of damage on the wave response. Noise and vibration are often transmitted from the source to the other parts of the structure in the form of wave propagation. Thus, control of such wave transmission is essential for reduction of noise and vibration, which are the main cause of discomfort and in many cases cause failure of structure. Here, techniques for both passive and active controls of wave are proposed. For active control, a closed loop system is modeled using WSFE with magnetostrictive actuator for control of axial and flexural wave propagations in connected isotropic 1-D waveguides. The feedback is negative velocity and/or acceleration measured at different sensor points. A very new application of CNT reinforced composite for passive control of vibration and wave response is explored in this thesis. For this, a novel concept of nano-composite inserts is proposed. This insert can be made from CNTs dispersed in polymer. The high stiffness of the inserts helps to regulate the power flow in the form of wave propagation from the point of application of the loads to other parts of the structures. The length of the insert, volume fraction of CNTs and position are changed to achieve the required reduction in wave amplitudes. The entire thesis is split up into eight chapters. Chapter 1 presents a brief introduction, the motivation and objective of the thesis. Chapters 2 and 3 give a detail account of wavelet spectral finite element formulation for 1-D and 2-D isotropic waveguides, while Chapter 4 gives the same for composite waveguides. Chapter 5 brings out essential wave characteristics in carbon nanotubes and nano-composite structures, while Chapters 6 and 7 exclusively deal with application of WSFE to some real world problems. The thesis ends with summary and directions of future research. In summary, the thesis has brought out several new aspects of wave propagation in isotropic, composite and nano-composite structures. In addition to establishing wavelet spectral finite element as a useful tool for wave propagation analysis, several new techniques are presented, several new algorithm are proposed and several new concepts are explored.