Dissertationen zum Thema „Brillouin scattering“

Acoustic modes guided by thin-film metal superlattices have been investigated using Brillouin spectroscopy. Samples were grown on both single-crystal sapphire and fused silica substrates by alternately sputtering two different metals to yield a total thickness in the range 0.3 - 0.5 μm. Structural and chemical characterization of the polycrystalline films was performed using x-ray diffraction. Rutherford backscattering and optical interferometry. Thermally excited acoustic waves in the metal film create a surface ripple which weakly interacts with light incident from a single mode argon laser. A tandem Fabry-Perot consisting of two synchronized 3-pass cavities is used to measure the frequency shift of light which is inelastically scattered from acoustic waves. The contrast ratio of this interferometer exceeds 10¹⁰ and provides sufficient stray light rejection to detect the surface Rayleigh wave and as many as 13 higher order acoustic modes. The elastic stiffness constants of the anisotropic superlattices were estimated by fitting the measured acoustic mode velocities to a parameterized acoustic model. A comparison is made between these elastic constants and those predicted from the properties of the separate bulk constituents. The dependence of bilayer wavelength on the elastic properties of both Cu/Nb and Mo/Ta superlattices over the range of roughly 10 to 200 Å was determined. The unexpected softening of Cu/Nb superlattices within a range of bilayer wavelengths near 20 Å which was reported previously is qualitatively similar to the measurements reported here. It is shown that the elastic stiffness coefficient with the largest variation is c₄₄. The stiffness variations determined for the Mo/Ta samples are much smaller than for Cu/Nb. It is suggested that this is due to either structural differences (Cu/Nb is fcc-bcc and Mo/Ta is bcc-bcc) or the smaller interfacial lattice mismatch for Mo/Ta. Interfacial strain is found to be strongly correlated with the stiffness variations of the Mo/Ta samples. However, the underlying cause of these variations in stiffness remains anomalous. This dissertation also reports the first observations of Love waves and Stoneley waves by Brillouin scattering. The purely transverse Love waves guided by Cu/Nb films were detected by elasto-optic scattering from the evanescent acoustic strain in the sapphire substrate. The stiffness coefficient c₁₂ of the hexagonally symmetric metal film cannot be determined by the other guided acoustic waves which ripple the surface. Molybdenum in contact with fused silica is predicted to support a Stoneley wave which is guided by the interface. The lowest order Sezawa made guided by a molybdenum film was found to evolve to the Stoneley wave as the film becomes thicker. These measurements together with measurements of the surface Rayleigh wave show that the stiffness of the sputtered metal films is quite homogeneous and independent of film thickness.

Orientador: Paulo Clóvis Dainese Júnior<br>Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin<br>Made available in DSpace on 2018-08-29T15:49:49Z (GMT). No. of bitstreams: 1 FlorezPenaloza_OmarEnrique_M.pdf: 35304158 bytes, checksum: f1b7fbb7d6036f3186dd9378d06e850e (MD5) Previous issue date: 2015<br>Resumo: O Espalhamento Brillouin e um processo nao linear oriundo da interacao entre ondas opticas e acusticas. Este efeito foi amplamente estudado em fibras mono-modo e mais recentemente em uma grande variedade de micro-estruturas fotonicas. A habilidade de se fabricar estruturas que podem confinar ambas as ondas opticas e acusticas em dimensoes menores que o comprimento de onda criou novas oportunidades de se estudar a interacao foton-fonon. Um aspecto em particular que se torna importante em sistemas de alto confinamento e o efeito de deslocamento de borda (Shifting-Boundary), alem do efeito Elasto-Optico, mais bem entendido. Micro-fios de Silica sao ideais para estudar estes efeitos. Primeiro, quando seu diametro e menor que o comprimento de onda, o campo eletrico na superficie do guia de onda aumenta significativamente. Em segundo lugar, a interface ar-silica apresenta um alto contraste de indice de refracao, o que aumenta o espalhamento devido ao efeito de deslocamento de borda. Finalmente, a geometria cilindrica simples permite um calculo analitico da eficiencia do espalhamento Brillouin, considerando as perturbacoes tanto Elasto-Opticas como de Deslocamento de Borda. Nesta tese, estudamos teorica e experimentalmente o espalhamento Brillouin em microfios de Silica. Amostras com diametros de 0.6 a 3 ¿Êm foram fabricadas e caracterizadas utilizando um sistema de deteccao heterodina para os espalhamentos co-propagante e contra-propagante. Para o espalhamento Brillouin co-propagante, usamos a tecnica de bombeio e prova para induzir excitacao impulsiva dos modos acusticos proximos da frequencia de corte. Espalhamento devido a ondas acusticas do tipo Rayleigh foi observado e extensivamente caracterizado. Estas ondas sao particularmente interessantes, ja que a maior parte da energia acustica e concentrada proxima da superficie do guia de onda, o que aumenta a contribuicao do efeito de Deslocamento de Borda. Desenvolvemos estudos teoricos extensivos dos modos opticos e acusticos na geometria cilindrica, e aplicamos a teoria de modos acoplados para calcular a eficiencia de espalhamento para cada modo acustico. Um estudo da eficiencia de espalhamento em funcao do diametro foi feito, ajudando a entender melhor os mecanismos que determinam a evolucao do espectro Brillouin. Finalmente, fomos capazes de identificar modos nos quais o processo de espalhamento e dominado pelo efeito de Deslocamento de Borda, e modos nos quais o efeito dominante e o Elasto-Optico. Este entendimento pode contribuir no projeto de estruturas nas quais estes efeitos sao somados ou cancelados, e pode ser usado como outro mecanismo para controlar o processo de espalhamento Brillouin<br>Abstract: Brillouin scattering is a nonlinear process that arises from the interaction between op- tical and acoustic waves. This effect has been widely studied in standard single-mode fibers and more recently in a variety of photonics microstructures. The ability to fab- ricate structures that can confine both optical and acoustic waves in sub-wavelength dimensions has created new opportunities to study photon-phonon interaction. One particular aspect that becomes important in high-confinement systems is the effect of shifting boundaries, in addition to the better-understood elasto-optic effect. Silica mi- crowires are ideal systems to study these effects. First, when its diameter is smaller than the wavelength, the electric field overlaps strongly with the waveguide surface. Second, the air-silica interface presents high index contrast, which enhances scattering due to shifting boundary effect. Finally, the simple cylindrical geometry allows an- alytical calculation of Brillouin scattering efficiency considering both elasto-optic and shifting-boundary perturbations. In this thesis, we studied theoretically and experimentally Brillouin scattering in silica microwires. Samples with diameter ranging from 0.6 to 3 ?m were fabricated and char- acterized using heterodyne detection for both backward and forward Brillouin scattering. For forward Brillouin scattering, we used the pump and probe technique to induce impul- sive excitation of acoustic modes near cutoff. Scattering due to Rayleigh acoustic waves was observed and extensively characterized. These waves are particularly interesting as most of the acoustic energy is concentrated close to the waveguide surface, therefore enhancing the shifting boundary contribution. Theoretically, we developed extensive studies of optical and acoustic modes in cylindrical geometry, and applied coupled-mode theory to calculate the scattering efficiency for each acoustic mode. A study of the scat- tering efficiencies as a function of diameter was performed, helping better understand the mechanisms that determined the evolution of the Brillouin spectrum. Finally, we were able to identify modes in which the scattering process is dominated by shifting- boundary effect and modes in which elasto-optic dominates. This understanding may help design structures in which these effects add or cancel each other, and can be used as another mechanism to control Brillouin scattering process<br>Mestrado<br>Física<br>Mestre em Física<br>1142161/2012<br>CAPES

In a world where science is constantly challenged to solve problems of increasing complexity, light is paving new ways to gather information about the physical properties of matter. Among these properties, elasticity is becoming fundamental in the understanding and the diagnosis of several diseases. Current solutions to gather mechanical information, however, measure the response of a material to an applied excitation, which makes them invasive and limited by a low spatial resolution. In contrast with these techniques, Brillouin spectroscopy offers the unique solution to retrieve stiffness information from the spectrum of the light scattered by inherent thermal acoustic waves. The combination of Brillouin spectroscopy with confocal microscopy has yielded a confocal Brillouin microscope able to perform mechanical imaging in a non-invasive manner. This was used to investigate two different biological problems: on the one hand the stiffness variations in specific endothelium cells of the eye, aiming at a better understanding of the mechanisms responsible for glaucoma, and on the other the characterisation of the mechanical structures of blood vessels, which could provide fundamental information regarding the formation of atherosclerotic plaques. Following an investigation on the optimal geometry that minimises the spectral broadening caused by the collection of photons over a range of scattering angles, high resolution Brillouin imaging was obtained in a confocal backscattering arrangement. To the best of our knowledge this thesis presents, for the first time, sub-cellular Brillouin images. In particular, in vitro Brillouin images of single HUVEC cells were acquired to investigate the cell's mechanical response to the application of the Latrunculin-A drug. This analysis, together with the finding of a linear correlation between the Brillouin modulus and the standard Young's modulus, validates the technique as a feasible means of measuring stiffness. Following this assessment, Brillouin images of normal and diseased vessels were acquired showing that the atherosclerotic plaques had a lower stiffness compared to both diseased and healthy vessel walls. These results might encourage the application of confocal Brillouin microscopy as the tool of choice for the investigation of the arterial biomechanics.

[ES] Los efectos no lineales son herramientas valiosas en el procesamiento óptico. El obje-tivo de esta Tesis es contribuir con las nuevas arquitecturas y métodos a este campo, en particular al control de la polarización de la luz con luz y filtrado óptico de señales de microondas. La manipulación de las propiedades de la polarización de la luz en medios guiados es crucial en muchos sistemas ópticos clásicos y cuánticos. Sin embargo, la capacidad de la tecnología actual para definir con precisión el estado de polarización de determina-das longitudes de onda está lejos del nivel de madurez conseguido en el control de la amplitud. En el capítulo 3, se presenta un nuevo enfoque para el control totalmente óptico del estado de polarización con selectividad en longitud de onda, basado en el cambio del retardo fase por medio del stimulated Brillouin scattering. Los experimen-tos muestran que se puede llegar a cualquier punto de la esfera de Poincaré desde un estado de polarización de entrada arbitrario con tan solo una ligera variación en la amplitud de la señal (<2.5 dB). A diferencia de otros esquemas de procesamiento Bri-llouin, la degradación de la figura de ruido es pequeña (1.5 dB para una rotación completa en la esfera, 2pi). Este controlador de polarización completamente óptico puede forjar el desarrollo de nuevas técnicas basadas en la polarización en comunica-ciones ópticas, ingeniería laser, detección, sistemas cuánticos y sondeo basado en luz de sistemas químicos y biológicos. La segunda área de interés de la tesis se centra en el filtrado fotónico de microondas. La fotónica proporciona una implementación alternativa a los filtros de microondas. Las características proporcionadas por el scattering de Brillouin son muy atractivas para el diseño de filtro con especificaciones competitivas. El capítulo 4 está dedicado a los nuevos esquemas para el filtrado fotónico de microondas basado en SBS. En parti-cular, se presenta un método para mejorar la pendiente de los filtros fotónicos de mi-croondas basados en Brilouin. Esta mejora se logra mediante la combinación de las respuestas en ganancia y atenuación del Brillouin sobre la señal modulada en fase. Los resultados experimentales muestran una respuesta paso banda que exhibe una pendiente de 16.7 dB por octava, lo que corresponde con una mejora de 3 veces en comparación con la respuesta Lorentziana natural de la ganancia Brillouin. Sin embargo, la necesi-dad de 3 ondas de bombeo, es decir tres osciladores de microondas, incrementan la complejidad del sistema y dificulta la capacidad de ajuste. Para superar estas limita-ciones, se propone una segunda técnica para mejorar la pendiente de un filtro fotónico de microondas basado en scattering de Brillouin estimulado, el cual mantiene una fácil sintonización. Esta propuesta se basa en la dependencia de la polarización de la ganan-cia del Brillouin en fibras birrefringentes. La presencia de dos respuestas ortogonales de ganancia/atenuación Brillouin en fibras birrefringentes da como resultado dos res-puestas del filtro, que pueden ser sustraídas en un fotodetector balanceado para elimi-nar el lento decaimiento de la respuesta de ganancia natural Lorentziana del Brillouin. Los resultados experimentales muestran que se puede obtener una pendiente del filtro de 8.3 dB/oct. Finalmente, el documento de tesis proporciona conclusiones y actividades futuras abiertas por este trabajo de doctorado.<br>[CAT] Els efectes no lineals son ferramentes valuoses en el processament òptic. L'objectiu d'aquesta tesi es contribuir amb les noves arquitectures i mètodes a aquest camp, en particular al control de la polarització de la llum amb llum i filtrar òptic de senyals de microones. La manipulació de les propietats de la polarització de la llum en mitjans guiats es cru-cial en molts sistemes òptics clàssics i quàntics. No obstant això, la capacitat de la tecnologia actual per definir amb precisió l'estat de polarització de determinades lon-gituds d'ona està lluny del nivell de maduresa aconseguit en el control de l'amplitud. En el capítol 3, es presenta un nou enfocament per al control totalment òptic de l'estat de polarització amb selectivitat en longitud d'ona, basat en el canvi del retard de fase mitjançant el stimulated Brillouin scattering. Els experiments mostren que es pot arri-bar a qualsevol punt de l'esfera de Poincaré des d'un estat de polarització d'entrada arbitrari amb tant sols una lleugera variació de l'amplitud de la senyal (<2.5 dB). A diferencia d'altres esquemes de processament Brillouin, la degradació de la figura de soroll es petita (1.5 dB per a una rotació completa en l'esfera, 2pi). Aquest controlador de polarització completament òptic pot forjar el desenvolupament de noves tècniques basades en la polarització en comunicacions òptiques, enginyeria làser, detecció, sis-temes quàntics y sondeig basat en llum de sistemes químics i biològics. La segona àrea d'interès de la tesi es centra en el filtrar fotònic de microones. La fo-tònica proporciona una implementació alternativa als filtres de microones. Les caracte-rístiques proporcionades per el scattering de Brillouin son molt atractives per al dis-seny de filtres amb especificacions competitives. El capítol 4 està dedicat als nous esquemes per al filtrat fotònic de microones basat en SBS. En particular, es presenta un mètode per a millorar la pendent dels filtres fotònics de microones basats en Bri-llouin. Aquesta millora s'aconsegueix mitjançant la combinació de les respostes en guany i atenuació del Brillouin sobre la senyal modulada en fase. Els resultats experi-mentals mostren una resposta pas banda que exhibeix una pendent de 16.7 dB per octava, el que correspon amb una millora de 3 vegades en comparació amb la resposta Lorentziana natural del guany Brillouin. Tot i això, la necessitat de 3 ones de bom-beig, es a dir tres oscil·ladors de microones, incrementen la complexitat del sistema i dificulta la capacitat d'ajust. Per superar aquestes limitacions, es proposa una segona tècnica per millorar la pendent d'un filtre fotònic de microones basat en scattering de Brillouin estimulat, el qual manté una fàcil sintonització. Aquesta proposta es basa en la dependència de la polarització del guany del Brillouin en fibres birefringents. La presència de dos respostes ortogonals de guany/atenuació Brillouin en fibres birefrin-gents dona com a resultat dos respostes del filtre, que poden ser sostretes en un fotode-tector balancejat per eliminar el lent decaïment de la resposta de guany natural Lo-rentziana del Brillouin. Els resultats experimentals mostren que es pot obtenir una pendent del filtre de 8.3 dB/oct. Finalment, el document de tesi proporciona conclusions i activitat futures obertes per aquest treball de doctorat.<br>[EN] Nonlinear effects are valuable tools in the field of optical processing. This Thesis is aimed at contributing with new architectures and methods to this field, in particular to the light-by-light control of polarization and optical filtering of microwave signals. The manipulation of the polarization properties of light in guided media is crucial in many classical and quantum optical systems. However, the capability of current technology to finely define the state of polarization of particular wavelengths is far from the level of maturity in amplitude control. In Chapter 3, a new approach for all-optical control of the state of polarization with wavelength selectivity based on the change of the phase retardance by means of stimulated Brillouin scattering is present-ed. Experiments show that any point on the Poincaré sphere can be reached from an arbitrary input state of polarization with little variation of the signal amplitude (< 2.5 dB). Unlike other Brillouin processing schemes, the degradation of the noise figure is small (1.5 dB for a full 2pi rotation). This all-optical polarization controller can forge the development of new polarization-based techniques in optical communication, laser engineering, sensing, quantum systems and light-based probing of chemical and biological systems. The second area of interest of the Thesis is photonic microwave filtering. Photonics provides an alternative implementation of microwave filters. The features provided by Brillouin scattering are very attractive to design filters with competitive specifications. Chapter 4 is devoted to new schemes for photonic microwave filtering based on SBS. In particular, a method to enhance the filter slope of Brillouin-based photonic microwave filters is presented. This improvement is achieved by the combination of Brillouin gain and loss responses over phase modulated signals. The experimental results show passband responses exhibiting a slope of 16.7 dB per octave, which corresponds with a 3-fold improvement in comparison to the natural Lorentzian response for the same gain. However, the need of three pump waves, i.e. three microwave oscillators, increases the system complexity and make tunability more difficult. To overcome these limitations, a second technique to enhance the slope of a photonic microwave filter based on stimulated Brillouin scattering is proposed, that maintains easy tunability. It relies on exploiting the polarization dependence of Brillouin gain in birefringent fibers. The presence of two orthogonal Brillouin gains/loss in birefringent fibers results in two filter responses that can be subtracted in a balanced photodetector to remove the slow Lorentzian decay of the natural Brillouin gain response. Experimental results show that a filter slope of 8.3 dB/oct can be obtained. Finally, the Thesis document provides conclusions and future activities opened by this PhD work.<br>Samaniego Riera, DP. (2019). OPTICAL PROCESSING BASED ON BRILLOUIN SCATTERING [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/124820<br>TESIS

Copies of author's previously published articles inserted. Bibliography: p. 169-176. Develops a one and three dimensional transient model of Stimulated Brillouin Scattering to investigate the transient regime of Stimulated Brillouin Scattering and recognize different transient phenomena that affect its performance.