Optical control of in-plane domain configuration and domain wall motion in ferroelectric and ferroelastic

The sensitivity of ferroelectric domain walls to external stimuli makes them functional entities in nanoelectronic devices. Specifically, optically driven domain reconfiguration with in-plane polarization is advantageous and thus highly sought. Here, we show the existence of in-plane polarized sub-domains imitating a single domain state and reversible optical control of its domain wall movement in a single-crystal of ferroelectric BaTiO3. Similar optical control in the domain configuration of non-polar ferroelastic material indicates long-range ferroelectric polarization is not essential for the optical control of domain wall movement. Instead, flexoelectricity is found to be an essential ingredient for the optical control of the domain configuration and hence, ferroelastic materials would be another possible candidate for nanoelectronic device applications

Structuration sub-micronique de matériauxtransparents à haut rapport d'aspect parfaisceaux laser ultra-rapides non-diffractifs:dynamique et regimes d'interaction

This thesis is focused on the controllability of laser-induced refractive indexchanges at sub-micron level over long dimensions i.e., with high aspect ratiosand sections on the nanoscale. To this end, we explore non-diffractive zerothorderultrafast Bessel beams and factors contributing to energy confinementbeyond the diffraction limit. Laser processing of transparent materials usingnon-diffracting beams offers a strong advantage for high aspect ratio submicronstructures inside the bulk in view of nanophotonics and nanofluidicsapplications. We present the role of various focusing conditions and laserparameters on material modification in bulk fused silica and explore the differentinteraction regimes. This thesis tackles mostly the moderate focusingconditions as they offer a stable interaction regime backed up dispersion engineeringover a large range of laser parameters. The laser pulse durationwas found to be key in defining the type of laser induced refractive indexor structural modification. For instance, machining using femtosecond laserpulses results in increased refractive index structures whereas picosecondlaser pulses result in uniform void i.e., low index structures.To acquire better control over the laser energy deposition and precision,a range of physical mechanisms responsible for the laser induced damage innon-diffractive excitation conditions have been observed experimentally andfurther interrogated by simulations indicating a critical role of light scatteringon carriers. Time-resolved pump-probe microscopy measurements witha sub-picosecond temporal and sub-micron spatial resolution allow accessto the instantaneous excitation and relaxation dynamics. Dynamic opticaltransmission and phase contrast offer complementary information of eitherelectronic and glass matrix response. Primarily, ultrafast dynamics of freecarriers was studied as the electron mediated energy transfer to the lattice iskey to the subsequent material transformation. Role of instantaneous excitationat different laser pulse durations and energies is outlined. Then completecarrier dynamics is presented at different laser parameters. Particularly dynamicsin conditions of positive refractive index structures and uniform voidsis indicating two different paths of electronic relaxation and energy deposition:a fast defect mediated relaxation for positive index structures and slowthermomechanical relaxation for nanosize void structures. Finally, by correlatingthe results of time resolved studies, simulations and post-irradiatedphotoluminescence results, we formulate potential formation scenarios for thepositive refractive index and low index or uniform void structures.Cette thèse se concentre sur la contrôlabilité de l'indice de réfraction au niveau submicronique par changements d'indice induits par laser sur de longues dimensions i.e., avec des hauts rapports d'aspect _élèves et des sections à l'_échelle nanométrique. A cette fin, nous explorons les faisceaux ultracourts de Bessel non-diffractifs d'ordre zéro et les facteurs qui contribuent au confinement de l'_énergie au-delà de la limite de diffraction. Le traitement par laser de matériaux transparents à l'aide de faisceaux non diffractifs offre un avantage important pour les structures submicroniques en volume de haut rapport d'aspect _a des _ns applicatives en nanophotonique et en nanofluidique. Nous présentons l'effet de différentes conditions de focalisation et de paramètres laser sur la modification de la silice fondue, explorant ainsi les différents régimes d'interaction. Cette thèse aborde essentiellement des conditions modérées de focalisation car elles offrent un régime d'interaction stable sur une large gamme de paramètres laser, permettant l'ingénierie de la dispersion. La durée de l'impulsion laser s'est révélée ^être essentielle dans la définition du type de modification de l'indice de réfraction ou de modification structurale. Par exemple, l'usinage utilisant des impulsions laser femtosecondes entraine une augmentation des structures d'indice de réfraction alors que les impulsions laser picosecondes engendrent une cavité uniforme i.e., des structures de faible indice.Pour acquérir un meilleur contrôle et une meilleure précision du dépôt d'_énergie laser, un ensemble de mécanismes physiques responsables des dommages induits par laser dans des conditions d'excitation non-diffractives a été observé expérimentalement et examiné par des simulations indiquant le rôle essentiel de la diffusion de la lumière sur les _électrons. Des mesures de microscopie pompe-sonde résolues en temps avec une résolution temporelle sub-picoseconde et spatiale submicronique donnent accès _à l'excitation et à la relaxation dynamique instantanées. La transmission optique dynamique et le contraste de phase offrent des informations complémentaires sur la réponse _électronique ou sur celle de la matrice vitreuse. La dynamique ultrarapide des porteurs libres a _été particulièrement _étudiée puisque le transfert d'_énergie des _électrons vers le réseau est la clé de transformation ultérieure du matériau. Le rôle de l'excitation instantanée pour différentes durées et énergie d'impulsion laser est exposé. Ainsi, la dynamique complète des porteurs de charge est présentée pour différents paramètres du laser. En particulier, la dynamique d'obtention de structures d'indice de réfraction positif et des cavités uniformes indique deux chemins différents de relaxation électronique et de dépôt de l'_énergie: une relaxation rapide par l'intermédiaire de défauts pour les structures d'indice positif et une relaxation thermomécanique lente pour les cavités nanométriques. Enfin, en corrélant les résultats des études résolues en temps, les simulations et les résultats de photoluminescence après irradiation, nous formulons des scénarios potentiels de formation de l'indice de réfraction positif ainsi que des structures d'indice faible ou de vides uniformes

Femtosecond Laser-Induced Damage Characterization of Multilayer Dielectric Coatings

The laser-induced damage threshold (LIDT) of optical components is one of the major constraints in developing high-power ultrafast laser systems. Multi-layer dielectric (MLD) coatings-based optical components are key parts of high-power laser systems because of their high damage resistance. Therefore, understanding and characterizing the laser-induced damage of MLD coatings are of paramount importance for developing ultrahigh-intensity laser systems. In this article, we overview the possible femtosecond laser damage mechanisms through damage morphologies in various MLD optical coatings tested in our facility. To evaluate the major contributions to the coating failure, different LIDT test methods (R-on-1, ISO S-on-1 and Raster Scan) were carried out for a high reflective hybrid Ta2O5/HfO2/SiO2 MLD mirror coating at a pulse duration of 37 fs. Different LIDT test methods were compared due to the fact that each test method exposes the different underlying damage mechanisms. For instance, the ISO S-on-1 test at a higher number of laser pulses can bring out the fatigue effects, whereas the Raster Scan method can reveal the non-uniform defect clusters in the optical coating. The measured LIDT values on the sample surface for the tested coating in three test methods are 1.1 J/cm2 (R-on-1), 0.9 J/cm2 (100k-on-1) and 0.6 J/cm2 (Raster Scan) at an angle of incidence of 45 deg. The presented results reveal that the performance of the tested sample is limited by coating defects rather than fatigue effects. Hence, the Raster Scan method is found to be most accurate for the tested coating in evaluating the damage threshold for practical applications. Importantly, this study demonstrates that the testing of different LIDT test protocols is necessary in femtosecond regime to assess the key mechanisms to the coating failure

Spatio-temporal dynamics of non-diffractive ultrafast laser excitation and nanostructuring in bulk silica glass

International audienc

““Slow” development of nanoscaled voids in ultrafast laser micro-explosions in bulk dielectrics

International audienc

3D Nano-Fabrication Using Controlled Bessel-Glass Interaction in Ultra-fast Modes

International audienceWith a quest to develop rapid and reliable three-dimensional nanofabrication techniques for photonicand fluidic applications, we exploited the potential of zero-order Bessel beams for fabricatinghigh aspect ratio nano-structures in fused silica glass. In particular, the energetic conditions towardsstable and uniform nanovoid fabrication were explored, correlating the laser pulse energy and temporalduration. The underlying carrier and structural dynamics revealed through multiscale timeresolvedmicroscopy techniques in the case of Bessel pulse excitation of silica glass were also studied.We discovered different material relaxation pathways corresponding to material modificationsof different morphologies such as increased and decreased refractive index structures

Ultrafast imaging of free carriers: a controlled dynamics with chirped nondiffractive Bessel beams

International audienceNonlinear propagation of intense ultrafast laser pulses inside transparent materials has a strong influence on the fabrication quality and accuracy for 3D laser-material processing. Due to their ability to maintain near-constant fluence profiles over an appreciable distance along the propagation direction in linear and nonlinear media, ultrafast Bessel beams are ideal sources for high aspect ratio sub-micron structuring applications. We report here on the interaction of transparent materials, especially fused silica, with ultrafast non-diffractive beams of moderate cone angle at various laser energies and pulse durations and define their impact on photoinscription regimes, i.e. formation of isotropic and non-isotropic (positive and negative) refractive index structures. The laser pulse duration was observed to be key in deciding the type of the structures via excitation efficiency. To understand the significant mechanisms for forming these different structures, the free carrier behavior as a function of laser pulse duration and energy was studied by capturing instantaneous excitation profiles using time-resolved microscopy. Time-resolved imaging and simulation studies reveal that low carrier densities are generated for ultrashort pulses leading to soft positive index alterations via presumably non-thermally induced structural transitions via defects. On the other hand, the high free carrier density generation in the case of longer pulse durations leads to a hydrodynamic expansion resulting in high aspect ratio sub-micron size wide voids. Delayed ionization, carrier defocusing and lower nonlinear effects are responsible for the confinement of energy, resulting in efficient energy deposition on-axis

Role of free carriers excited by ultrafast Bessel beams for submicron structuring applications

International audienceUltrafast Bessel beams are ideal sources for high aspect ratio submicron structuring applications because of their nondiffracting nature and higher stability in nonlinear propagation. We report here on the interaction of ultrafast Bessel beams at various laser energies and pulse durations with transparent materials (fused silica) and define their impact on photoinscription regimes, i.e., formation of positive and negative refractive index structures. The laser pulse duration was observed to be key in deciding the type of the structures via excitation efficiency. To understand the relevant mechanisms for forming these different structures, the free carrier behavior as a function of laser pulse duration and energy was studied by capturing instantaneous excitation profiles using time-resolved microscopy. Time-resolved imaging and simulation studies reveal that low carrier densities are generated for ultrashort pulses, leading to soft positive index alterations via presumably nonthermally induced structural transitions involving defects. On the other hand, the high free carrier density generation in the case of longer pulse durations leads to hydrodynamic expansion, resulting in high aspect ratio submicron- size wide voids. Delayed ionization, carrier defocusing, and lower nonlinear effects are responsible for the confinement of energy, resulting in efficient energy deposition on-axis

High aspect ratio submicron fabrication in bulk dielectrics with non-diffractive ultrafast Bessel beams; dynamics and interaction regimes

International audienceNonlinear propagation of intense ultrafast laser pulses inside transparent materials has a strong influence on the fabrication quality and accuracy in 3D laser-material processing. The ability to maintain near-constant intensity profiles over an appreciable distance along the propagation direction, sustaining nonlinear absorption, recommends ultrafast Bessel beams for high aspect ratio submicron structuring applications. We discuss here the characteristics of the interaction of transparent materials, especially fused silica, with ultrafast non-diffractive beams of moderate and high cone angle at various laser energies and pulse durations. We define their impact on photoinscription regimes, i.e. formation of isotropic and non-isotropic refractive index structures (Fig. 1) and the stability limits. The laser pulse duration was observed to be key in deciding the type and morphology of the structures. In particular, high aspect ratio void-like structures with submicron cores (down to 100 nm) over hundreds of microns are produced when using laser pulse of longer pulse duration, highlighting the important contribution of delayed ionization and light diffusion on excited carriers. On the contrary, for the same energy, smooth refractive index modified structures are produced for short laser pulses. To understand the formation mechanisms of these structures, we studied the ultrafast dynamics of excitation around the modification threshold using time-resolved microscopy and spectral imaging techniques. We reveal various relaxation mechanisms leading to either permanent refractive index changes accompanied by structural characteristic defect markers or to hydrodynamic phenomena and cavitation in hot states. We observe fast carrier trapping in structural matrix deformations for soft positive index changes accompanied by the formation of non-bridging oxygen centers and long living carriers characteristic of "phase" transition and hydrodynamic expansion for void-like domains. To illustrate the respective material changes we compare the ultrafast laser material interaction mechanisms in terms of energy deposition and relaxation characteristics in fused silica associated with Bessel beams [1]. Potential application in deep drilling and opto-fluidics are discussed

Time-resolved dynamics of ultrafast Bessel and Gaussian beam propagation and energy deposition in transparent materials

Propagation of high intense ultrafast laser pulses inside transparent materials has a strong influence in microfabrication quality and accuracy in the field of 3D laser-material processing. Concepts of tight focusing or non-diffractive propagation carry in consequence a strong potential. Knowledge of the propagation of Gauss and Bessel beams at the focus where the laser-material interaction regime can sustain for either short or higher propagation distances becomes essential. We study the material interaction with ultrafast Bessel and Gaussian beams at different energy and focusing conditions and their impact in the various photoinscription regimes, i.e. formation of isotropic type-I refractive index structures or non-isotropic nanoscale modulated type II structures. To understand the formation mechanisms of these different interaction regimes, we propose probing the ultrafast dynamics of these structures using time-resolved microscopy and spectral imaging in fused silica. Single and multi pulse measurements around the damage threshold can give information about the relaxation mechanisms leading to permanent refractive index changes accompanied by structure characteristic defect markers. Also we figure out key mechanisms in excitation and relaxation dynamics associated with Gaussian and Bessel beam propagation