In 1920, Leon Brillouin discovered a new kind of light scattering – Brillouin scattering – which occurs as a result of the interaction of light with a transparent material’s temporal periodic variations in density and refractive index. Many advances have since been made in the study of Brillouin scattering, in particular in the field of fiber optics. An in-depth investigation of Brillouin scattering in optical fibers has been carried out in this thesis, and the theory of stimulated Brillouin scattering (SBS) and combined Brillouin gain and loss has been extended. Additionally, several important applications of SBS have been found and applied to current technologies. Several mathematical models of the pump-probe interaction undergoing SBS in the steady-state regime have emerged in recent years. Attempts have been made to find analytical solutions of this system of equations, however, previously obtained solutions are numerical with analytical portions and, therefore, qualify as hybrid solutions. Though the analytical portions provide useful information about intensity distributions along the fiber, they fall short of describing the spectral characteristics of the Brillouin amplification and the lack of analytical expressions for Brillouin spectra substantially limits the utility of the hybrid solutions for applications in spectral measurement techniques. In this thesis, a highly accurate, fully analytic solution for the pump wave and the Stokes wave in Brillouin amplification in optical fibers is given. It is experimentally confirmed that the reported analytic solution can account for spectral distortion and pump depletion in the parameter space that is relevant to Brillouin fiber sensor applications. The analytic solution provides a valid characterization of Brillouin amplification in both the low and high nonlinearity regime, for short fiber lengths. Additionally, a 3D parametric model of Brillouin amplification is proposed, which reflects the effects of input pump and Stokes powers on the level of pump wave depletion in the fiber, and acts as a classification tool to describe the level of similarity between various Brillouin amplification processes in optical fibers. At present, there exists a multitude of electro-optic modulators (EOM), which are used to modulate the amplitude, frequency, phase and polarization of a beam of light. Among these modulators, phase modulation provides the highest quality of transmitted signal. As such, an improved method of phase-modulation, based on the principles of stimulated Brillouin scattering, as well as an optical phase-modulator and optical phase network employing the same, has been developed. Due to its robustness, low threshold power, narrow spectrum and simplicity of operation, stimulated Brillouin scattering (SBS) has become a favourable underlying mechanism in fiber-based devices used for both sensing and telecommunication applications. Since birefringence is a detrimental effect for both, it is important to devise a comprehensive characterization of the SBS process in the presence of birefringence in an optical fiber. In this thesis, the most general model of elliptical birefringence in an optical fiber has been developed for a steady-state and transient stimulated Brillouin scattering (SBS) interaction, as well as the combined Brillouin gain and loss regime. The impact of the elliptical birefringence is to induce a Brillouin frequency shift and distort the Brillouin spectrum – which varies with different light polarizations and pulse widths. The model investigates the effects of birefringence and the corresponding evolution of spectral distortion effects along the fiber, and proposes regimes that are more favourable for sensing applications related to SBS – providing a valuable prediction tool for distributed sensing applications. In recent years, photonic computing has received considerable attention due to its numerous applications, such as high-speed optical signal processing, which would yield much faster computing times and higher bandwidths. For this reason, optical logic has been the focus of many research efforts and several schemes to improve conventional logic gates have been proposed. In view of this, a combined Brillouin gain and loss process has been proposed in a polarization maintaining optical fiber to realize all-Optical NAND/NOT/AND/OR logic gates in the frequency domain. A model describing the interaction of a Stokes, anti-Stokes and a pump wave, and two acoustic waves inside a fiber, ranging in length from 350m-2300m, was used to theoretically model the gates. Through the optimization of the pump depletion and gain saturation in the combined gain and loss process, switching contrasts of 20-83% have been simulated for different configurations.

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Los efectos de la luz lenta y luz rápida (SFL) han mostrado unas capacidades excepcionales sobre el control dinámico de la velocidad de la luz en diferentes medios. Una de las motivaciones más estimulantes redica en la potente aplicación de estos sistemas en el marco del procesado fotónico de señales de radio frecuencia (RF). En esta tesis doctoral, se evalúan las prestaciones de las plataformas de SFL actuales para desarrollar múltiples tareas que se requieren en el campo de la fotónica de microondas (MWP) con el valor añadido de sintonizabilidad y operación en banda ancha. En esta contexto, el scattering de Brillouin estimulado (SBS) tanto en fibra estándar como en fibra mantenedora de polarización (PMF), de redes Bragg (FBG), amplificadores ópticos de semiconductor (SOA) y cristales fotónicos (PhC) han sido las tecnologías bajo estudio. Desde escalas del orden de km hata mm, estas plataformas de SFL representan la evolución hacia la consolidación de componentes y subsistemas de MWP en circuitos fotónicos integrados (PIC). Diversos modelos analíticos y numéricos se han desarrollado con el objetivo de entender los procesos físicos que goboernan la porpagación a través de las diferentes plataformas de SFL, así como para describir los enfoques de MWP propuestos. Además, a través de las plataformas presentadas se ha llevado a cabo el análisis de las prestaciones de dos de las funcionalidades clave que se requieren para el procesado fotónico de señales de microondas, desfasadores sintonizables y retardos verdaderos (TTD). Se ha propuesto un sistema de TTD basado en la llamada técnica de sintonización separada de la portadora (SCT) a través de los efectos de SBS en fibrras estándr. Se ha evaluado la interacción del SBS en PMF con el propósito de desarrollar redes de Brillouin dinámicas (DBG), cuya fase generada ha sido fruto de estudio. Por otro lado, también se ha demostrado un sistema de densado distribuido basado en la reflexión continua de un pulso estrecho a lo la / Sancho Durá, J. (2012). Photonic-assisted RF Signal Processing based on Slow and Fast Light Technological Platforms [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/16471

Vibrations of nuclei in crystals govern various properties such as thermal expansion, phase transitions, and elasticity, and the quasiharmonic approximation (QHA) is the simplest nontrivial approximation which includes the effects of vibrational anharmonicity into temperature dependent observables. Nonetheless, the QHA is often implemented with additional approximations due to the complexity of computing phonons under arbitrary strains, and the generalized QHA, which employs constant stress boundary conditions, has not been completely developed. Here we formulate the generalized QHA, providing a practical algorithm for computing the strain and other observables as a function of temperature and true stress. We circumvent the complexity of computing phonons under arbitrary strains by employing irreducible second order displacement derivatives of the Born-Oppenheimer potential and their strain dependence, which are efficiently and precisely computed using the lone irreducible derivative approach. We formulate two complementary strain parametrizations: a discretized strain grid interpolation and a Taylor series expansion in symmetrized strain. We illustrate the quasiharmonic approximation by evaluating the temperature and pressure dependence of select elastic constants and the thermal expansion in thoria (ThO₂) using density functional theory with three exchange-correlation functionals. The convergence of the two complementary strain parametrizations is evaluated for the computed thermal expansion. The temperature dependent lattice parameter and thermal expansion computed within the QHA is compared with experimental measurements. The QHA results are compared to measurements of the elastic constant tensor using time domain Brillouin scattering and inelastic neutron scattering. We then demonstrate the generalized quasiharmonic approximation in a non-cubic material, ferroelectric lead titanate, computing the temperature and stress dependence of the full elastic constant tensor. The irreducible derivative approach is employed for computing strain dependent phonons using finite difference, explicitly including dipole-quadrupole contributions. We use density functional theory, computing all independent elastic constants and piezoelectric strain coefficients at finite temperature and stress. There is good agreement between the quasiharmonic approximation and the experimentally measured lattice parameters close to 0 K. The quasiharmonic approximation overestimates the measured temperature dependence of the lattice parameters and elastic constant tensor, demonstrating that a higher level of strain dependent anharmonic vibrational theory is needed. The next material we study is zirconium nitride, employing the quasiharmonic approximation with the irreducible derivative approach to compute the phonons and thermal expansion. Density functional theory is used with two exchange-correlation functionals. We investigate the difference between the measured and computed optical phonon branches, showing that volume effects, two-phonon scattering, and nitrogen vacancies do not explain the discrepancy between the measurement and computation. The temperature dependent lattice parameter is computed within the QHA, where the thermal expansion is overestimated as compared with existing experimental measurements.

Brillouin scattering has been verified to be an effective mechanism in temperature and strain sensing. This kind of sensors can be applied to civil structural monitoring of pipelines, railroads, and other industries for disaster prevention. This thesis first presents a novel fiber sensing scheme for long-span fully-distributed pressure measurement based on Brillouin scattering in a side-hole fiber. After that, it demonstrates that Brillouin frequency keeps linear relation with temperature up to 1000°C; Brillouin scattering is a promising mechanism in high temperature distributed sensing. A side-hole fiber has two longitudinal air holes in the fiber cladding. When a pressure is applied on the fiber, the two principal axes of the fiber birefringence yield different Brillouin frequency shifts in the Brillouin scattering. The differential Brillouin scattering continuously along the fiber thus permits distributed pressure measurement. Our sensor system was designed to analyze the Brillouin scattering in the two principal axes of a side-hole fiber in time domain. The developed system was tested under pressure from 0 to 10,000 psi for 100m and 600m side-hole fibers, respectively. Experimental results show fibers with side holes of different sizes possess different pressure sensitivities. The highest sensitivity of the measured pressure induced differential Brillouin frequency shift is 0.0012MHz/psi. The demonstrated spatial resolution is 2m, which maybe further improved by using shorter light pulses. / Master of Science

Yes / The gold standard substrate for the ex vivo expansion of human limbal stem cells (LSCs) remains the human amniotic membrane (HAM) but this is not a defined substrate and is subject to biological variabil-ity and the potential to transmit disease. To better define HAM and mitigate the risk of disease transmis-sion, we sought to determine if decellularisation and/or c-irradiation have an adverse effect on culture growth and LSC phenotype. Ex vivo limbal explant cultures were set up on fresh HAM, HAM decellularised with 0.5 M NaOH, and 0.5% (w/v) sodium dodecyl sulfate (SDS) with or without c-irradiation. Explant growth rate was measured and LSC phenotype was characterised by histology, immunostaining and qRT-PCR (ABCG2, DNp63, Ki67, CK12, and CK13). Ƴ-irradiation marginally stiffened HAM, as measured by Brillouin spectromicroscopy. HAM stiffness and c-irradiation did not significantly affect the LSC phe-notype, however LSCs expanded significantly faster on Ƴ-irradiated SDS decellularised HAM (p < 0.05) which was also corroborated by the highest expression of Ki67 and putative LSC marker, ABCG2. Colony forming efficiency assays showed a greater yield and proportion of holoclones in cells cultured on Ƴ-irradiated SDS decellularised HAM. Together our data indicate that SDS decellularised HAM may be a more efficacious substrate for the expansion of LSCs and the use of a c-irradiated HAM allows the user to start the manufacturing process with a sterile substrate, potentially making it safer.

Generating, miniaturizing and controlling spin waves on the nanometer scale is of great interest for magnonics. For instance, this holds the prospect of exploring wave-based logic concepts and reduced Joule heating, by avoiding charge transport, in spin-wave circuitry. In this work, a novel approach is for the first time confirmed experimentally, which allows confining spin-wave transport to nanometre-wide channels defined by magnetic domain walls. This is investigated for different domain wall types( 90deg and180deg Néel walls) in two material systems of polycrystalline Ni81Fe19 and epitaxial Fe. The study covers the thermal, linear and non-linear regime utilizing micro- focused Brillouin light scattering microscopy complemented by micromagnetic simulations. An initially linear dispersion dominated by dipolar interactions is found for the guided spin waves. These are transversally confined to sub-wavelength wide beams with a well-defined wave vector along the domain wall channel. In the non-linear regime, higher harmonic generation of additional spin-wave beams at the sides of the domain wall channel is observed. Furthermore, the possibility to shift the position of the domain wall over several microns by small magnetic fields is demonstrated, while maintaining its spin-wave channeling functionality. Additionally, spin-wave transmittance along domain walls, which change direction at the edges of the structure as well as between interconnected walls of identical and different type is studied. Characterization of spin-wave transmission through interconnected domain walls is an important step towards the development of magnonic circuitry based on domain wall(-networks). With respect to developing flexible and scalable spin-wave sources, the second part of this thesis addresses auto-oscillations in spin Hall oscillators (based on a Pt / Ni81Fe19 bilayer) of tapered nanowire geometry. In these systems, a simultaneous formation of two separate spin-wave bullets of distinct localization and frequency has been indicated. This spin-wave bullet formation is con- firmed experimentally and investigated for different driving currents. Subsequently, control over these bullets by injecting external microwave signals of varying frequency and power is demon- strated, switching the oscillator into single-mode operation. Three synchronized auto-oscillatory states are observed, which can be selected by the frequency of the externally imprinted signal. This synchronization results in linewidth reduction and frequency-locking of the individual bullet modes. Simultaneously the bullet-amplitude is amplified and is found to scale as P2/3 with the injected microwave power P. This amplification and control over position and frequency of the spin-wave bullets is promising for the development of microwave amplifiers/detectors and spin- wave sources on the nanoscale based on spin Hall oscillators.:1 Introduction 1 2 Theoretical background 4 2.1 Energy density of thin film ferromagnets and domain(wall) formation 2.2 Magnetizationdynamicsinthinfilmferromagnets 11 2.2.1 Spin-wavedispersioninthelinearregime 13 2.2.2 Magnetizationdynamicsinthenon-linearregime 17 2.3 SpinHallOscillators 21 2.3.1 Spin Hall effect and spin transfer torque in a ferromagnet/heavy-metal bi- layersystem 21 2.3.2 Characteristics of magnetization auto-oscillations 25 2.3.3 Improvement of monochromaticity, coherence and output power by injec- tionlocking 28 3 Materials and Methods 31 3.1 ElectronBeamLithography,EBL 31 3.2 Ni81Fe19 microstructures 32 3.3 Femicrostructures 34 3.4 TaperedspinHalloscillators 35 3.5 Micro-focused Brillouin Light Scattering Spectroscopy, μBLS 36 3.5.1 μBLSspatialresolution 40 4 Experimental results 43 4.1 Spin-wave dynamics in multi-domain magnetic configurations 43 4.1.1 Spin-wave dynamics of 180◦ Néel walls in rectangular elements 44 4.1.2 Spin-wave dynamics of 90◦ Néel walls in square elements 63 4.1.3 Spin-wave dynamics of interconnected Néel walls in Fe wires 76 4.2 Auto-oscillationintaperedwiregeometries 88 4.2.1 Initial static magnetic configuration and effective field 89 4.2.2 Thermally excited dynamics and spectral properties 91 4.2.3 Direct microwave excitation of spin-wave dynamics 93 4.2.4 Auto-oscillatoryresponse 96 4.2.5 Microwaveamplificationandinjectionlocking 104 5 Summary and outlook 114 Own publications 118 Bibliography 120 Acknowledgement 141 A Appendix 143 A.1 Splitting process in magnetic domains confined by domain walls 143 A.2 reconfigurable remanent states in square structures stabilized by local ion irradiation 144 A.3 Domain wall displacements induced by a scanning laser beam 145 A.4 Magnetic Force Microscopy investigation of the domain wall type and width 147 A.5 Micromagnetic simulations: problem definition and analysis 149 A.6 Current dependence of auto-oscillations in the tapered SHO 152 A.7 Fabrication of Ni81Fe19 microstructures for spin waves in domain walls 153

Dans le cadre des études sur l’endommagement laser liées au projet Mégajoule, nous analysons le couplage entre l’auto-focalisation induite par effet Kerr et la rétrodiffusion Brillouin stimulée pour des impulsions de durée nanoseconde se propageant dans des échantillons de silice. L’influence de la puissance d’entrée, des modulations de phase ou d’amplitude ainsi que la forme spatiale du faisceau sur la dynamique de filamentation est discutée. Nous montrons qu’une modulation d’amplitude appropriée divisant l’impulsion incidente en train d’impulsions de l’ordre de la dizaine de picosecondes supprime l’effet Brillouin pour toute puissance incidente mais réduit notablement la puissance laser disponible. A l’inverse, des impulsions modulées en phase avec une largeur spectrale comparable peuvent subir de la filamentation multiple et une auto-focalisation à distance plus courte causées par des instabilités modulationnelles. Nous démontrons cependant l’existence d’une largeur spectrale critique à partir de laquelle la rétrodiffusion peut être radicalement inhibée par une modulation de phase, même pour des fortes puissances. Cette observation reste valide pour des faisceaux de forme carrée avec des profils spatiaux plus larges, qui s’auto-focalisent beaucoup plus rapidement et se brisent en filaments multiples sur de courtes distances. L’inclusion de la génération de plasma pour limiter la croissance des ondes pompe et Stokes est finalement abordée. / As part of the studies on laser damage linked to the Megajoule project, we analyze the coupling between the Kerr induce self-focusing and the stimulated Brillouin backscattering pour nanosecond optical pulses propagating in silica samples. The influence of the incident power, phase or amplitude modulations as well as the spatial profile of the pulse of the filamentation dynamic is discussed. We show that an appropriate amplitude modulation dividing the incident pulse in pulse trains of picosecond durations suppresses the Brillouin effect for any incident power but noticeably reduces the available average laser power. On the contrary, phase modulated pulses with a comparable spectral width can undergo multiple filamentation and self-focusing at a shorter distance, caused by modulational instabilities. We demonstrate however the existence of a critical spectral bandwidth from which the backscattering can be radically inhibited by a phase modulation, even for high powers. This conclusion remains valid for spatially broader squared pulses, which self-focus earlier and break into multiple filaments at shorter distances. The inclusion of plasma generation to limit the growth of pump and Stokes waves is finally addressed.