Theoretical Study Of Beam Transformations By Volume Diffraction

Laser beams can be manipulated by volume diffractive elements in addition to conventional optical elements like mirrors, lenses, and beam splitters. Conventional optical elements can be described by applying the basic laws of reflection and refraction at the surfaces of the elements. Even diffraction by surface gratings utilizes relatively simple mathematics. This is to be contrasted with the volume diffraction, which requires coupled wave theory in the slowly varying envelope approximation (SVEA) to obtain accurate results. Efficient spatially distributed diffraction of laser beams is possible due to the high coherence of laser light, and it occurs at specific resonant Bragg conditions. This research work is inspired and driven by the successful development of recording technology for robust, high-efficiency volume Bragg gratings (VBGs) in photo-thermo-refractive (PTR) glass. Mostly VBGs of the reflective type are discussed in this dissertation. Starting with an analysis of electro-magnetic wave propagation in layered media, we have reformulated Fresnel and volume reflection phenomena in terms of a convenient parameter – strength of reflection. The influence that the different non-uniformities inside a VBG have on its spectral properties has been examined. One important result of this work is the proposal of moiré VBG and the derivation of an analytical expression for its bandwidth. A multiplexed VBG used as a coherent combiner is discussed as well. Beam distortion via transmission through and/or reflection by a heated VBG due to residual absorption is analyzed

Advanced FBG fabrication for challenging applications

This thesis describes work performed for both advancing the fabrication process of fibre Bragg gratings and for a potential application for gratings inscribed by a system employing the improved methods. The aim of the discussed research is to increase the potential for fabricated grating variations whilst maintaining a grating accurate to the initial design and compensating for errors in fabrication. In addition, simulations for a system for detecting nanoparticles is sought, making use of the improvements to the fabrication setup and highlighting an example of the improved fabrication system’s flexibility. As a part of the development of the fabrication system, an increase in the efficiency of grating fabrication is a desired result, primarily via the reduction of grating iterations required to produce a high quality grating. To perform this, automated control of an optical vector analyser was created, for use in a feedback process in which the grating is observed during inscription and compared to simulations for the detection of fabrication errors. This will not only increase the likeness of gratings to their design but will also assist in repeatability of inscription by the system. Other improvements to the fabrication system have also been progressed, such as the inclusion of an optional interferometer inscription head and the ability to fabricate gratings of up to one metre in length. These two discussed factors will increase the flexibility of grating designs that can be written, enabling a greater variety of grating-based research to be available from a single fabrication setup. Potential fabrication designs to demonstrate the capabilities of the system were investigated, leading to the development of a simulation for a nanoparticle detector. Observations of the transmission spectrum as nanoparticles pass through the beam path in an orthogonally aligned microchannel demonstrate alterations to the spectrum which may be used to identify the presence of nanoparticles and some select attributes. How the changes in the wavelength of a valley within the spectrum are affected by variations in parameters is overviewed as well as improvements to the simulations to further develop the accuracy to a physical detection system

Photosensitive Materials: Optical properties and applications

Les matériaux photosensibles sont des matériaux (organique ou inorganique) dont l’indice de réfraction peut être localement modifié lorsqu’ils sont soumis à une excitation lumineuse dont une partie de l’énergie est absorbée par ce matériau. La majorité des travaux réalisés dans ce domaine a longtemps concerné des applications d’optique guidée (fibre ou guides planaires). L’apparition récente de nouveaux matériaux a rendu possible l’utilisation de ce phénomène de photosensibilité dans des composants optiques massifs en permettant notamment la réalisation d'hologrammes de volume à hautes performances. Dans le cadre de ces travaux, un verre inorganique a fait l’objet d’études approfondies : il s’agit d’un verre photo-thermo-réfractif (PTR), pour lequel la variation d’indice est obtenue par exposition à un rayonnement ionisant suivie d’un traitement thermique. La nature même du matériau lui procure des propriétés compatibles avec les contraintes des applications lasers à haute énergie.La première partie de ce manuscrit présente donc l’ensemble des travaux qui ont été réalisés sous ma responsabilité dans le but de mettre en évidence l’inter-relation qui existe entre les propriétés optiques (photosensibilité, absorption, diffusion, interaction laser-matière, propriétés spectrales) et les propriétés structurales de ces verres. La deuxième partie de ce manuscrit présente différentes applications de ces verres d’un point de vue composants, que ce soit pour du filtrage à bande très étroite, la fabrication de masques de phase volumiques, l’étirement ou la compression d’impulsions ultra-courtes ou le développement de nouveaux designs de sources lasers. Enfin, la troisième partie de ce manuscrit présente l’ensemble des réalisations dans lesquelles j’ai été impliqué durant ces 10 dernières années, que ce soit en termes de production scientifique, de management de projets ou d’encadrement de recherches

Plasmonic band gap cavities

Ankara : The Department of Physics and the Institute of Engineering and Science of Bilkent University, 2008.Thesis (Ph.D.) -- Bilkent University, 2008.Includes bibliographical references leaves 46-51.Surface plasmon polaritons (SPP’s) are trapped electromagnetic waves coupled to free electrons in metals that propagate at the metal-dielectric interfaces. Due to their surface confinement and potential in sub-wavelength optics, SPP’s have been extensively studied for sensing and nanophotonic applications. Dielectric structures and metallic surfaces, both periodically modulated, can form photonic band gaps. Creating a defect cavity region in the periodicity of dielectrics allows specific optical modes to localize inside a cavity region. However, despite the demonstration of numerous plasmonic surfaces and unlike its dielectric counterparts, low index modulation in metallic surfaces limits the formation of plasmonic defect cavity structures. This thesis describes new approaches for plasmonic confinement in a cavity through the use of selective loading of grating structures as well as through the use of Moiré surfaces. In our first approach, we demonstrate that a high dielectric superstructure can perturb the optical properties of propagating SPPs dramatically and enable the formation of a plasmonic band gap cavity. Formation of the cavity is confirmed by the observation of a cavity mode in the band gap both in the infrared and the visible wavelengths. In addition to the confinement of SPP’s in the vertical direction, such a cavity localizes the SPP’s in their propagation direction. Additionally, we have demonstrated that such biharmonic grating structures can be used to enhance Raman scattering and photoluminescence (PL). Using biharmonic grating structure 105 times enhancement in Raman signal and 30 times enhancement in PL were measured. Furthermore, we show that metallic Moiré surfaces can also serve as a basis for plasmonic cavities with relatively high quality factors. We have demonstrated localization and slow propagation of surface plasmons on metallic Moiré surfaces. Phase shift at the node of the Moiré surface localizes the propagating surface plasmons in a cavity and adjacent nodes form weakly coupled plasmonic cavities. We demonstrate group velocities around v = 0.44c at the center of the coupled cavity band and almost zero group velocity at the band edges can be achieved. Furthermore, sinusoidally modified amplitude about the node suppresses the radiation losses and reveals a relatively high quality factor for plasmonic cavities.Kocabaş, AşkınPh.D

Multiplexed optical fibre sensors for civil engineering applications

Fibre-optic sensors have been the focus of a lot of research, but their associated high cost has stifled their transferral from the laboratory to real world applications. This thesis addresses the issue of multiplexing, a technology that would lower the cost per unit sensor of a sensor system dramatically. An overview of the current state of research of, and the principles behind, multiplexed sensor networks is given. A new scheme of multiplexing, designated W*DM, is developed and implemented for a fibre Bragg grating (FBG) optical fibre sensor network. Using harmonic analysis, multiplexing is performed in the domain dual to that of the wavelength domain of a sensor. This scheme for multiplexing is compatible with the most commonly used existing schemes of WDM and TDM and thus offers an expansion over, and a resultant cost decrease from, the sensor systems currently in use. This research covered a theoretical development of the scheme, a proof of principle, simulated and experimental analysis of the performance of the multiplexed system, investigation into sensor design requirements and related issues, fabrication of the sensors according to the requirements of the scheme and the successful multiplexing of eight devices (thus offering an eightfold increase over current network capacities) using this scheme. Extensions of this scheme to other fibre sensors such as Long Period Gratings (LPGs) and blazed gratings were also investigated. Two LPGs having a moiré structure were successfully multiplexed and it was shown that a blazed Fabry Perot grating could be used as a tuneable dual strain/refractive index sensor. In performing these tests, it was discovered that moiré LPGs exhibited a unique thermal switching behaviour, hereto unseen. Finally the application of fibre sensors to the civil engineering field was investigated. The skill of embedding optical fibre in concrete was painstakingly developed and the thermal properties of concrete were investigated using these sensors. Field tests for the structural health monitoring of a road bridge made from a novel concrete material were performed. The phenomenon of shrinkage, creep and cracking in concrete was investigated showing the potential for optical fibre sensors to be used as a viable research tool for the civil engineer

Optical Fiber Interferometric Sensors

The contributions presented in this book series portray the advances of the research in the field of interferometric photonic technology and its novel applications. The wide scope explored by the range of different contributions intends to provide a synopsis of the current research trends and the state of the art in this field, covering recent technological improvements, new production methodologies and emerging applications, for researchers coming from different fields of science and industry. The manuscripts published in the Special issue, and re-printed in this book series, report on topics that range from interferometric sensors for thickness and dynamic displacement measurement, up to pulse wave and spirometry applications

Optical Properties of TMDC Monolayers and Their Heterostructures

Recently, the 2D semiconductors represented by transition-metal dichalcogenides (TMDCs) received strong attention owing to a number of important features. These include the large exciton binding energies relevant for room-temperature applications, the valley pseudo-spin degree of freedom and polarization sensitivity, and the overall strong light-matter interactions at the monolayer limit which motivate their use for optoelectronics. Furthermore, van-der-Waals stacking of different 2D crystals results in out-of-plane heterostructures, which can, in addition to the inherited properties of the individual layered constituents, even exhibit tailored properties caused by the strong influence of the environment and hybridization of atomic orbitals. Accordingly, a mixture of unique and novel properties can arise. Ultimately, in contrast to conventional semiconductor heterostructure growth, there is no direct need for lattice matching or a fixed orientation during assembly. Nonetheless, the relative orientation between different or similar 2D lattices may indeed be important for the composed stack’s features. Thus, the vast number of possible combinations of 2D-materials with each other, as well as with substrates and with conventional semiconductors or molecular materials, offer huge opportunities for engineering and tailoring the material stack properties to meet the demand of a specific application. To do that effectively and systematically, several questions need to be addressed, to the answering of which the following studies contribute with important insights focusing on the optical properties of semiconductor monolayers and van-der-Waals stacks. In this work, four central chapters discuss excitonic signatures in different TMDC 2D structures, shedding light on the role of the environment and stacking configuration, as well as on the quasi-particle energy-momentum dispersion, valley polarization as well as light-matter interactions. Firstly, the influence of the surroundings on the fundamental properties of 2D-semiconductor monolayers, such as the energetics, the exciton–phonon coupling, exciton–exciton annihilation and exciton diffusion, is addressed based on time-integrated and time-resolved photoluminescence spectroscopy. Thereby, the important role of hexagonal BN as substrate or capping layer and as encapsulant is discussed. Following that, a better understanding of high-symmetry alignments for bilayers is gained employing epitaxially grown tungsten-disulfide samples with two distinct and deterministically obtained configurations. While formerly only the symmetry of the stack itself was considered, the study presented here shows that also the symmetry of the surrounding has to be considered as it can lift the degeneracy between the layers. Thereby, the out-of-plane symmetry break renders homobilayers in fact heterojunctions. Moreover, the aspect of spin–valley and spin–layer locking has been discussed for the natural and artificial bilayer type, with eyes towards valleytronic applications. In contrast to high-symmetry stacks, investigations on arbitrarily stacked heterostructures are at the starting point for explorations on the impact of moiré patterns and interlayer hybridization. Here, a preliminary study on a tungsten-based heterostructure exhibits pronounced spectral features attributed to such interlayer effects, besides the occurrence of conventional intra- and interlayer excitons. Next, regarding linewidth improved encapsulated monolayers, a unique access to the excitonic energy–momentum dispersion is demonstrated with the help of angle-resolved spectrospopy. The analysis of Fourier-space-resolved emission and reflection spectra hereby facilitate the ongoing discussion of dispersion relations in 2D semiconductors. The so far unrivalled optical measurements show novel experimental evidence of meV strong excitonic dispersion within the light cone in support of theories discussed in the literature. Furthermore, Fourier-space spectroscopy delivers a tool to identify the radiative patterns of bright and partially dark excitonic states and provided evidence for the phonon-sidebands in agreement with the prediction in the literature. Finally, improvements of the light–matter interactions and of the emission behavior towards optoelectronic applications are crucial, taking further into account challenges in the integration of van-der-Waals materials in established silicon-, III/V semiconductors- or fiber-based technology. Therefore, a nanostructured photonic substrate landscape for lateral confinement of optical fields and vertical enhancement of coupling of light into and out of 2D-materials is investigated as a prototype structure for possible nanophotonic applications

Functional optical surfaces by colloidal self-assembly: Colloid-to-film coupled cavities and colloidal lattices

Future developments in nanophotonics require facile, inexpensive and parallelizable fabrication methods and need a fundamental understanding of the spectroscopic properties of such nanostructures. These challenges can be met through colloidal self-assembly where pre-synthesized colloids are arranged over large areas at reasonable cost. As so-called colloidal building blocks, plasmonic nanoparticles and quantum dots are used because of their localized light confinement and localized light emission, respectively. These nanoscopic colloids acquires new hybrid spectroscopic properties through their structural arrangement. To explore the energy transfer between these nanoscopic building blocks, concepts from physical optics are used and implemented with the colloidal self-assembly approach from physical chemistry. Through an established synthesis, the nanocrystals are now available in large quantities, any they receive the tailored spectroscopic properties through directed self-assembly. Moreover, the tailored properties of the colloids and the use of stimuli-responsive polymers allow a functionality that goes beyond current developments. The basics developed in this habilitation thesis can lead to novel functional devices in the field of smart sensors, dynamic light modulators, and large-area quantum devices.:1 Abstract 2 2 State of the art 4 2.1 Metallic and semiconductive nanocrystals as colloidal building blocks 4 2.2 Concept of large-scale colloidal self-assembly 7 2.3 Functional optical nanomaterials by colloidal self-assembly 9 2.4 Scope 13 2.5 References 14 3 Single colloidal cavities 20 3.1 Nanorattles with tailored electric field enhancement 20 4 Colloidal -to-film coupled cavities 31 4.1 Template-assisted colloidal self-assembly of macroscopic magnetic metasurfaces 31 4.2 Single particle spectroscopy of radiative processes in colloid-to-film-coupled nanoantennas 50 4.3 Active plasmonic colloid-to-film coupled cavities for tailored light-matter interactions 65 5 Colloidal polymers 74 5.1 Direct observation of plasmon band formation and delocalization in quasi-infinite nanoparticle chains 74 6 Colloidal lattice 84 6.1 Hybridized guided-node resonances via colloidal plasmonic self-assembled grating 84 6.2 Mechanotunable surface lattice resonances in the visible optical range by soft lithography templates and directed self-assembly 94 6.3 Tunable Circular Dichroism by Photoluminescent Moiré Gratings 103 7 Conclusion and perspective 112 8 Appendix 113 8.1 Further publications during the habilitation period 113 8.2 Curriculum vitae of the author 116 9 Acknowledgments 117 10 Declaration 118Zukünftige Entwicklungen in der Nanophotonik erfordern einfache, kostengünstige und parallelisierbare Herstellungsmethoden und benötigen ein grundlegendes Verständnis der spektroskopischen Eigenschaften solcher Nanostrukturen. Diese Herausforderungen können durch kolloidale Selbstorganisation erfüllt werden, bei der kostengünstige und zuvor synthetisierte Kolloide großflächig angeordnet werden. Als sogenannte kolloide Bausteine werden wegen ihrer lokalisierten Lichtfokussierung unterhalb der Beugungsbegrenzung plasmonische Nanopartikel sowie wegen ihrer lokalisierten Lichtemission Quantenpunkte verwendet. Diese nanoskopischen Kolloide werden in dieser Habilitationsschrift verwendet und durch Selbstanordnung in ihre gewünschte Nanostruktur gebracht, die neue hybride Eigenschaften aufweist. Um den Energietransfer zwischen diesen nanoskopischen Bausteinen zu untersuchen, werden Konzepte aus der physikalischen Optik verwendet und mit dem kolloidalen Selbstorganisationskonzept aus der physikalischen Chemie großflächig umgesetzt. Durch eine etablierte Synthese sind die Nanokristalle nun in großen Mengen verfügbar, wobei sie durch gerichtete Selbstorganisation die gewünschten spektroskopischen Eigenschaften erhalten. Darüber hinaus ermöglicht die Verwendung von stimulierbaren Polymeren eine Funktionalität, die über die bisherigen Entwicklungen hinausgeht. Die in dieser Habilitationsschrift entwickelten Grundlagen können bei der Entwicklung neuartiger Funktionsgeräte im Bereich für intelligente Sensorik, dynamischer Lichtmodulatoren und großflächiger Quantengeräte genutzt werden.:1 Abstract 2 2 State of the art 4 2.1 Metallic and semiconductive nanocrystals as colloidal building blocks 4 2.2 Concept of large-scale colloidal self-assembly 7 2.3 Functional optical nanomaterials by colloidal self-assembly 9 2.4 Scope 13 2.5 References 14 3 Single colloidal cavities 20 3.1 Nanorattles with tailored electric field enhancement 20 4 Colloidal -to-film coupled cavities 31 4.1 Template-assisted colloidal self-assembly of macroscopic magnetic metasurfaces 31 4.2 Single particle spectroscopy of radiative processes in colloid-to-film-coupled nanoantennas 50 4.3 Active plasmonic colloid-to-film coupled cavities for tailored light-matter interactions 65 5 Colloidal polymers 74 5.1 Direct observation of plasmon band formation and delocalization in quasi-infinite nanoparticle chains 74 6 Colloidal lattice 84 6.1 Hybridized guided-node resonances via colloidal plasmonic self-assembled grating 84 6.2 Mechanotunable surface lattice resonances in the visible optical range by soft lithography templates and directed self-assembly 94 6.3 Tunable Circular Dichroism by Photoluminescent Moiré Gratings 103 7 Conclusion and perspective 112 8 Appendix 113 8.1 Further publications during the habilitation period 113 8.2 Curriculum vitae of the author 116 9 Acknowledgments 117 10 Declaration 11