Towards a Rationalization of Ultrafast Laser-Induced Crystallization in Lithium Niobium Borosilicate Glasses: The Key Role of The Scanning Speed

Femtosecond (fs)-laser direct writing is a powerful technique to enable a large variety of integrated photonic functions in glass materials. One possible way to achieve functionalization is through highly localized and controlled crystallization inside the glass volume, for example by precipitating nanocrystals with second-order susceptibility (frequency converters, optical modulators), and/or with larger refractive indices with respect to their glass matrices (graded index or diffractive lenses, waveguides, gratings). In this paper, this is achieved through fs-laser-induced crystallization of LiNbO3 nonlinear crystals inside two different glass matrices: a silicate (mol%: 33Li2O-33Nb2O5-34SiO2, labeled as LNS) and a borosilicate (mol%:33Li2O-33Nb2O5-13SiO2-21B2O3, labeled as LNSB). More specifically, we investigate the effect of laser scanning speed on the crystallization kinetics, as it is a valuable parameter for glass laser processing. The impact of scanning energy and speed on the fabrication of oriented nanocrystals and nanogratings during fs-laser irradiation is studied. Fs-laser direct writing of crystallized lines in both LNS and LNSB glass is investigated using both optical and electron microscopy techniques. Among the main findings to highlight, we observed the possibility to maintain crystallization during scanning at speeds ~ 5 times higher in LNSB relative to LNS (up to ~ 600 μm/s in our experimental conditions). We found a speed regime where lines exhibited a large polarization-controlled retardance response (up to 200 nm in LNSB), which is attributed to the texturation of the crystal/glass phase separation with a low scattering level. These characteristics are regarded as assets for future elaboration methods and designs of photonic devices involving crystallization. Finally, by using temperature and irradiation time variations along the main laser parameters (pulse energy, pulse repetition rate, scanning speed), we propose an explanation on the origin of 1) crystallization limitation upon scanning speed, 2) laser track width variation with respect to scanning speed, and 3) narrowing of the nanogratings volume but not the heat-affected volume

Démonstration d'un contrôle de l'anisotropie des propriétés optiques induites par irradiation d'un laser IR fs dans des matériaux organiques : biréfringence, di-atténuation, GSH, photoluminescence

L'écriture directe par laser femtoseconde dans les matériaux a été étudiée en raison de ses avantages en micro-technologie. La découverte de l'effet de polarisation trouvé dans le contrôle de l'orientation de la cristallisation dans le verre d'oxydes LiNbSi a été une découverte exceptionnelle. Outre la longueur d'onde, la durée des impulsions, leur énergie et leur taux de répétition, la polarisation joue ainsi également un rôle essentiel étendant la dimensionalité du contrôle des processus à l'échelle nanométrique. De notre point de vue, nous avons étendu ce type de recherche, du monde inorganique au monde organique dans divers types de matériaux organiques parmi les cristaux et les verres. Des propriétés telles que la génération de seconde harmonique (GSH), la biréfringence linéaire (LB), le dichroïsme ou la di-atténuation linéaire (LD) ou la photoluminescence, ont été créés par irradiation laser. Toutes indiquent lors de ces processus que des espèces asymétriques ont été créés. Plus encore, la formation de GSH implique qu'une symétrie d'inversion a été brisée comme dans le cristal d'α-glycine. La création de luminescence a été trouvée dans de nombreux matériaux organiques après une irradiation laser femtoseconde. Cela montre une certaine généralité du mécanisme d'interaction entre le laser fs et le matériau organique. Le point culminant de cette thèse est la découverte de l'anisotropie de la luminescence et le contrôle de l'orientation de celle-ci par la polarisation, incluant l'écriture, l'effacement et la réécriture. Cette étude est la démonstration que l'action de la polarisation de la lumière sur le contrôle de l'anisotropie peut être réalisé dans un matériau organique et à l'échelle moléculaire.Femtosecond laser direct writing in materials has been studied due to its advantages in micro-processing. The discovery of polarization effect found in controlling the crystallization orientation in LiNbSi oxide glass was an outstanding finding. Besides the wavelength, the pulse duration, the pulse energy, and the repetition rate, the polarization also plays an essential role in extending the dimension of control at the nanoscale. We extended such a research from inorganic to organic world in various kinds of organic materials among crystals and glasses. Properties such as photoluminescence, second harmonic generation (SHG), Linear birefringence (LB), Linear dichroism or linear di-attenuation (LD) were discovered after laser irradiation. All indicate that asymmetric species have been created. SHG formation e.g. like in α-glycine crystal, implies in addition that inversion symmetry has been broken. Luminescence creation has been found in many organic materials after femtosecond laser irradiation, which seems to indicate a general interaction mechanism between fs laser and organic material. The highlight of this thesis is the discovery of the anisotropy of luminescence and the control of its orientation by light polarization, including writing, erasing, and rewriting. This study performs a step forwards to demonstrate that light polarization effect and anisotropy control by polarization can be realized in organic material and at a molecular scale

Usable Analytical Expressions for Temperature Distribution Induced by Ultrafast Laser Pulses in Dielectric Solids

This paper focuses on the critical role of temperature in ultrafast direct laser writing processes, where temperature changes can trigger or exclusively drive certain transformations, such as phase transitions. It is important to consider both the temporal dynamics and spatial temperature distribution for the effective control of material modifications. We present analytical expressions for temperature variations induced by multi-pulse absorption, applicable to pulse durations significantly shorter than nanoseconds within a spherical energy source. The objective is to provide easy-to-use expressions to facilitate engineering tasks. Specifically, the expressions are shown to depend on just two parameters: the initial temperature at the center denoted as T00 and a factor Rτ representing the ratio of the pulse period τp to the diffusion time τd. We show that temperature, oscillating between Tmax and Tmin, reaches a steady state and we calculate the least number of pulses required to reach the steady state. The paper defines the occurrence of heat accumulation precisely and elucidates that a temperature increase does not accompany systematically heat accumulation but depends on a set of laser parameters. It also highlights the temporal differences in temperature at the focus compared to areas outside the focus. Furthermore, the study suggests circumstances under which averaging the temperature over the pulse period can provide an even simpler approach. This work is instrumental in comprehending the diverse temperature effects observed in various experiments and in preparing for experimental setup. It also aids in determining whether temperature plays a role in the processes of direct laser writing. Toward the end of the paper, several application examples are provided

Photoluminescence Creation in CYTOP Optical Fiber by Femtosecond Laser Direct Writing

Spatial-selective photoluminescence in visible range was induced in the core of CYTOP fibers by femtosecond laser direct writing. This implemented optical property may have potential applications for luminescence-based fiber sensing for biomedical and environmental fields. © Optica Publishing Group 2022

Space-selective creation of photonics functions in a new organic material: Femtosecond laser direct writing in Zeonex glass of refractive index change and photoluminescence

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