Femtosecond Laser Applications in Glasses – FLAG

Femtosecond direct writing opens an unprecedented gateway to shape in 3D the linear and non-linear optical properties of glassy media and with functionalities unattainable with other processes. Exposing silica-based glasses to laser light induces numerous phenomena through multi-photon absorption (via defects states or inter-band absorption). These excitations lead to photo-induced reactions and thus yield various permanent changes in macroscopic physical properties of the glass. Furthermore, the pulse duration of IR femtosecond laser (100 fs) allows delivering finite quantities of fluence into a controlled volume of material and making easy the production of high peak power densities such as 100 TW/cm² (after focusing tightly the laser spot). As a result, the development of this type of ultra-fast laser has prompted the investigation of many nonlinear physical phenomena, like e.g. multiphoton absorption or electron plasma formation. Accordingly, it is an attractive candidate for many applications since it is naturally highly localized laser-induced effects (non-linear effect) such as ablation, photo-structural changes and consequently changes in refractive index. Two recent breakthroughs, in the field of femtosecond induced modifications in glass are at the root of our project. The first one is our recent discovery of photo-induced chirality in silica glass by a femtosecond laser linearly polarized. In addition, due to laser beam asymmetry, the interaction is sensitive to the direction of writing (non-reciprocal writing), which is another original finding. The second breakthrough is from our closest collaborators at the University of Southampton who discovered other asymmetric structures under the same conditions. These are self-organized nanogratings (200 nm periodicity, 20 nm thick), which are in fact the smallest structures ever created by light. From the authors, these gratings arise from modulation of chemical composition (probably oxygen redistribution) and this would be related to stationary density waves in the plasma produced by coupling between plasma oscillations and light waves. In addition, these structures are likely at the root of other experimental unusual facts like strong birefringence (80%) and negative index change. These experimental observations left a number of open questions yet that should be addressed for moving efficiently towards innovative applications. Consequently, the first part of the project aims at understanding the laser-induced transformations in silica-based glasses, their types and their organization. Hence, the understanding of this type of photosensitivity will allow to correctly assess its industrial potential and to exploit the qualities of the interaction fully such as high linear birefringence (>10-2), polarized luminescence, strong dichroism, large negative or positive index change (up to 10-2) and extraordinary good thermal stability (above 1000°C). We have thus foreseen to study new innovative applications like elaboration of special lenses for hollow beam (atomic teleportation), non uniform retardation plates (e.g. for aberration compensation), volume Bragg gratings (e.g. for output laser stabilization, chirp pulse amplification), birefringent elements for LCD. It is obvious that this technology will be a source of employment. The laser may be a tool of choice if it is properly controlled (and it is the second objective of the project) for the realization of 3D micro optical devices but also for mastering the photo-precipitation of nanoparticles. FLAG is an international project, which consists in a collection of several specific supports. With the PRES label, we formalized the fundamental core of our project at the root of this project. With the support of the Essonne French Department (ASTRE procedure) we have completed our project by imparting it a direction towards innovations (inclusion of companies in the network i.e. 3S photonics, Thales R&T, Thales Laser). With the RTRA, it is a multi-center link that is financed through a postdoctoral researcher using all means of the network. Finally, the Integrated Action Programs (PAI also called PHC) from the Ministry of foreign affairs and the FP7-PEOPLE-IIF fellowship, have given an international dimension to the project promoting exchanges with ORC-Southampton (UK), Shanghai and Wuhan (Chine) and Sydney (Australia).