Optimization of the Energy Deposition in Glasses with Temporally-Shaped Femtosecond Laser Pulses

International audienceBulk machining of glasses with femtosecond laser pulses enables the fabrication of embedded optical functions. Due to the nonlinear character of the laser-matter interaction, structural modifications can occur within the focal region. To reach a full control of the process, ways of controlling the deposition of the laser energy inside the material have to be unveiled. From static and time-resolved pictures of bulk-excitation of a-SiO2 and borosilicate glass, we show that particular laser temporal shapes such as picosecond sequences can better confine the energy deposition than the femtosecond sequence by reducing the propagation artifacts

Time-Resolved Observation of Energy Deposition in Fused Silica by Ultrashort Laser Pulses in Single and Cumulative Regime

International audienceWhen femtosecond laser pulses are focused in the bulk of transparent materials (glasses), deposition of energy on a restricted volume can occur owing to the non linear character of the laser matter interaction. As a consequence, the possibility to generate micrometer-sized structural modifications arises. Those local changes are often associated with a minute variation in the refractive index which, when positive, enables the fabrication of light guiding components in three dimensions through simple laser translation. Although the first corresponding experimental demonstration approaches fifteen years of age, the complete picture of the dynamics and the proc- esses leading to the local refractive index changes has still to be drawn to reach an optimal control of the laser-induced modification process. In this report, the laser-dielectric interaction is followed on an ultrashort time scale with the help of a unique time-resolved side-imaging technique allow- ing for absorption and phase contrast detection. Experimental observation of an absorptive elec- tronic cloud in the first moments of the interaction along with the launch of a pressure wave after a few ns is reported. These physical objects are shown to be reliable indicators of the success of the energy transfer to the lattice which largely depends on the pulse temporal envelope

Transformation photo-assistée de diélectriques pour l’optique par laser à impulsions ultra-brèves : études des mécanismes microscopiques

Local refractive index changes (RIC) are the building blocks of laser-induced optical functions in bulk transparent materials, where the use of a fused silica as a target material plays a paramount role. Depending on the regime of laser interaction ultra-short pulses can induce positive isotropic refractive index changes (usually denoted as type I) or produce self-arranged nano-scale layered structures resulting in form birefringence (type II). In this thesis we have studied two objectives related to these material transformations. From the one side, we qualitatively determined the effects of the focused ultra-short laser pulses on the fused silica and borosilicate glasses. With the independent control of the energetic dose, pulse duration and focusing conditions, the isotropic type I and birefringent type II traces could be performed with the certain optical properties. Finally, complex polarization sensitive devices were designed and fabricated. From the other side, as these types of RIC have consequences in the functionality and the performances of 3D embedded optical devices, an investigation of the laser-induced structures is particularly useful. We applied photoluminescence and Raman microscopy (RM) to investigate defect formation and glass network reorganization paths. The proposed spectroscopy study distinguishes type I and type II regions by presence and distribution of silicon clusters and non-bridging oxygen hole centers (NBOHC). RM reveals signs of compaction of the glass network in the RIC regions. At the same time, zones with high concentration of NBOHC where no visible RIC and densification signs were detected. Assuming that these zones are precursors of permanent visible modification, we propose a scenario of cold defect-assisted densification realized in type I irradiation regime. This, thereby, revises the densification paths in fused silicaLe changement local de l'indice de réfraction (CLIR) est l’élément constitutif des fonctions optiques créées par laser dans des matériaux transparents. Selon le régime de l'interaction et en particulier en fonction de la dose d’énergie déposée, de la durée de l'impulsion laser et des conditions de focalisation on peut induire un CLIR isotrope et positif ou produire à l’échelle nanométrique des structures auto-arrangées présentant une biréfringence. Ce changement est essentiel pour les applications photoniques intégrées utilisant des matériaux comme la silice, ce qui été démontré dans la thèse par réalisation des composants optiques allant du simple guide canal enterré à des dispositifs sensibles à la polarisation. En parallèle au développement d’applications photoniques nous avons étudiés les mécanismes microscopiques à l’origine de la modification des propriétés optiques des diélectriques utilisés. Nous avons appliqué les techniques de micro photoluminescence et microspectroscopie Raman pour étudier la formation des défauts ponctuels et des chemins de réorganisation de la structure du verre. Ces modifications de structure de verres jouent un rôle important dans le changement des propriétés électroniques de la silice fondue et, par conséquent, de ses constantes optiques. L’ensemble de ces résultats spectroscopiques nous a permis de revisiter les schémas de densification dans la silice et de proposer un scénario de densification assistée par les défauts générés suite à la relaxation des excitons auto-piégé

Transformation photo-assistée de diélectriques pour l optique par laser à impulsions ultra-brèves (études des mécanismes microscopiques)

Le changement local de l'indice de réfraction (CLIR) est l élément constitutif des fonctions optiques créées par laser dans des matériaux transparents. Selon le régime de l'interaction et en particulier en fonction de la dose d énergie déposée, de la durée de l'impulsion laser et des conditions de focalisation on peut induire un CLIR isotrope et positif ou produire à l échelle nanométrique des structures auto-arrangées présentant une biréfringence. Ce changement est essentiel pour les applications photoniques intégrées utilisant des matériaux comme la silice, ce qui été démontré dans la thèse par réalisation des composants optiques allant du simple guide canal enterré à des dispositifs sensibles à la polarisation. En parallèle au développement d applications photoniques nous avons étudiés les mécanismes microscopiques à l origine de la modification des propriétés optiques des diélectriques utilisés. Nous avons appliqué les techniques de micro photoluminescence et microspectroscopie Raman pour étudier la formation des défauts ponctuels et des chemins de réorganisation de la structure du verre. Ces modifications de structure de verres jouent un rôle important dans le changement des propriétés électroniques de la silice fondue et, par conséquent, de ses constantes optiques. L ensemble de ces résultats spectroscopiques nous a permis de revisiter les schémas de densification dans la silice et de proposer un scénario de densification assistée par les défauts générés suite à la relaxation des excitons auto-piégésLocal refractive index changes (RIC) are the building blocks of laser-induced optical functions in bulk transparent materials, where the use of a fused silica as a target material plays a paramount role. Depending on the regime of laser interaction ultra-short pulses can induce positive isotropic refractive index changes (usually denoted as type I) or produce self-arranged nano-scale layered structures resulting in form birefringence (type II). In this thesis we have studied two objectives related to these material transformations. From the one side, we qualitatively determined the effects of the focused ultra-short laser pulses on the fused silica and borosilicate glasses. With the independent control of the energetic dose, pulse duration and focusing conditions, the isotropic type I and birefringent type II traces could be performed with the certain optical properties. Finally, complex polarization sensitive devices were designed and fabricated. From the other side, as these types of RIC have consequences in the functionality and the performances of 3D embedded optical devices, an investigation of the laser-induced structures is particularly useful. We applied photoluminescence and Raman microscopy (RM) to investigate defect formation and glass network reorganization paths. The proposed spectroscopy study distinguishes type I and type II regions by presence and distribution of silicon clusters and non-bridging oxygen hole centers (NBOHC). RM reveals signs of compaction of the glass network in the RIC regions. At the same time, zones with high concentration of NBOHC where no visible RIC and densification signs were detected. Assuming that these zones are precursors of permanent visible modification, we propose a scenario of cold defect-assisted densification realized in type I irradiation regime. This, thereby, revises the densification paths in fused silicaST ETIENNE-Bib. électronique (422189901) / SudocSudocFranceF

Polarization-Dependent Scattering of Nanogratings in Femtosecond Laser Photowritten Waveguides in Fused Silica

International audienceThe properties of polarization-selective, light-guiding systems upon subwavelength nanogratings formation in the case of type II refractive index traces induced by femtosecond laser pulses in bulk fused silica were studied. Polarization-dependent scattering is analyzed both in simulation using a finite-difference, time-domain method and in experiments. We argue that the polarization-sensitive optical guiding of type II waveguides is due to polarization-dependent scattering of nanogratings. Optical designs can then be suggested where the guiding efficiency of type I traces can be combined with type II anisotropies. A low-loss waveguide polarizer is demonstrated based on the modulation of the evanescent field emerging from type I waveguides using polarization-dependent scattering of neighboring nanogratings

Ultrafast laser photoinscription of polarization sensitive devices in bulk silica glass

International audienceUltrashort pulsed laser irradiation of bulk fused silica may result under specific energetic conditions in the self-organization of subwavelength material redistribution regions within the laser trace. The modulated structures have birefringent properties and show unusual anisotropic light scattering and reflection characteristics. We report here on the formation of waveguiding structures with remarkable polarization effects for infrared light. The photoinscription process using 800 nm femtosecond laser pulses is accompanied by third harmonic generation and polarization dependent anisotropic scattering of UV photons. The photowritten structures can be arranged in three-dimensional patterns generating complex propagation and polarization effects due to the anisotropic optical properties

Structural modifications in bulk fused silica after interaction with ultrashort laser pulses

International audienceRefractive index changes are the building blocks of laser-induced optical functions in bulk transparent materials, for example, in fused silica. Depending on the regime of laser interaction, focused ultrashort pulses could induce either positive or negative isotropic refractive index changes (usually denoted as type I) or produce nanoscale self-arranging layered structure resulting in form birefringence (type II regime) [1]. As these types of refractive index changes have consequences in the functionality and performances of 3D optical devices (e. g. embedded waveguides), an investigation of the laser-induced structures is particularly useful [2]. We propose Photoluminescence (PLM) and Raman microscopy (RM) to characterize refractive index changes and to provide insights into the responsible mechanisms. Revealed by PLM, electronic changes in the form of defects appearing in the fused silica, alters its electronic structure, and consequently its optical constants. Alternatively RM allows observing reorganizations of fused silica network and, connected to it, changes in density and refractive index. We equally explore primary stages of birefringent regions where we observe that nanoscale reorganization is preceded by voids formation due to self-focusing effects.. The accumulation of microexplosions leads finally to a regular periodic alternance of high and low density layers. Therefore, investigation of single and multiple pulse irradiation effects allows to emphasize several key factors defining laser modification, particularly its dependence on the energetic dose, pulse duration and focusing conditions. The proposed spectroscopy study distinguishes type I and type II regions by and NBOHC (?) defect centers formation and distribution. Taking into account that one of the mechanisms of defect generation is the liberation of molecular oxygen from the silica network, the presence of these defects in the regions of oxygen deficiency [1] could be interpreted as a result of oxygen migration during interaction. RM reveals signs of densification in the region of refractive index changes. At the same time zones with high concentration of non-bridging oxygen hole centers (NBOHC) with no visible refractive index changes and densification signs were detected. We propose that this defect generation occurs as a result of bond breaking after self-trapped exciton relaxation. If this process is efficient enough, non-bridging atoms favored by local increase of temperature could create new bondings forming network, corresponding to compacted silica. This investigation provides, thus, key elements in further elucidating the nature of ultrafast laser induced changes in the bulk of dielectrics. References. 1. R. Taylor, C. Hnatovsky, and E. Simova, "Applications of femtosecond laser induced self-organized planar nanocracks inside fused silica glass," Laser and Photon. Rev. 2, 26-46 (2008) 2. K. Mishchik et al., "Nanosize structural modifications with polarization functions in ultrafast laser-irradiated bulk fused silica," Optics Express, 18, 24809-24824, (2010)

Patterning linear and nonlinear optical properties of photosensitive glasses by femtosecond structured light

We report on structured light-induced femtosecond direct laser writing (DLW) under tight focusing in non-commercial silver-containing zinc phosphate glass, which leads to original patterns of fluorescent silver clusters. These fluorescence topologies show unique features of frustrated diffusion of charged species, giving rise to distorted silver cluster spatial distributions. Fluorescence and second harmonic generation correlative microscopy demonstrate the realization of structured light-induced direct laser poling, resulting from a laser-induced permanent and stable electric field buried inside the modified glass. Thus, structured light-induced DLW remarkably enables both linear and nonlinear patterning. This work highlights the interest of optical phase engineering to obtain nontrivial beam profiles and subsequent photo-induced patterns that cannot be reached under Gaussian beam irradiation.Femtosecond Laser Interaction and Nanostructurin

Investigation and Control of Ultrafast Laser-Induced Nanoscale Patterns in Bulk Fused Silica

International audienceRefractive index changes in a-SiO2 consist of positive index variations or regular nanoscale patterns. We spectroscopically reveal the electronic and structural transformation of glass in the self-organized structures, indicating bond breaking and oxygen deficiency. We equally propose a method of real time control of nanogratings formation under the action of ultrashort laser pulse with variable envelopes. Application as polarizing optical devices is discussed

Investigation and control of ultrafast laser-induced nanoscale patterns in bulk dielectric materials

International audienceLocal refractive index changes are the building blocks of laser-induced 3D optical functions in bulk transparent materials, e.g. fused silica. Depending on the regime of interaction, focused ultrashort pulses could induce either positive or negative isotropic smooth refractive index changes (usually denoted as type I) or produce regular nanoscale self-arranging layered structures resulting in form birefringence (type II nanogratings - Fig. 1), a regime which is regularly observed in fused silica [1,2]. Particularly the latter phenomenon is a spectacular and intriguing physical manifestation that allows the development of embedded polarization functions. A spectroscopic study is proposed to reveal the particular electronic and structural transformation of glassy matter in the self-organized subwavelength structures, indicating bond breaking and the abundance in oxygen deficiency. As the spontaneous arrangement is intermediated by electronic excitation, we equally propose a method of real time control and optimization of nanogratings formation in bulk fused silica under the action of ultrashort laser pulse with programmable variable envelopes. Relying on the advantage on the intrinsic anisotropies, the application potential in terms of polarization sensitive optical devices is discussed