HDR defense Ciro d'Amico"Structuring and functionalization of materials by ultrashort laser: from the fundamental response to the application".

"Structuring and functionalization of materials by ultrashort laser: from the fundamental response to the application".

Abstract

The direct laser structuring of the volume of a transparent material is based on a concept of direct irradiation, which involves delivering the energy of a focused pulse, without losses, to the point of impact (focus), after passing through a dielectric interface, and then depositing the beam's energy into the material in a zone surrounding the point of impact (confocal region). An ultrashort laser pulse is a perfectly suitable tool for this irradiation scheme. Although the material is typically perfectly transparent at the carrier wavelength of the pulse, the increase in intensity due to focusing triggers nonlinear absorption processes at the focal volume, generating free charge carriers in the conduction band by crossing the material's energy gap. Following these processes, the energy stored in the focal volume can be substantial enough to induce significant chemical, structural, and thermodynamic changes in the material, leading to a local modification corresponding to a permanent change in the refractive index. Translating the focal region in space and combining different scanning strategies can determine a 3D energy deposition pattern in a design that can be very complex, creating 3D optical functions with many potential applications.

This type of volume micro/nano-structuring essentially relies on controlling the density of the energy of the pulse deposited locally in the material. A low or moderate energy density generates a gentle increase in the refractive index, while a high concentration of energy can result in a complete breakdown of the material, potentially leading to the creation of empty nano-cavities. The modal characteristics of the photo-induced index modification can be controlled by adjusting the focusing geometry, as well as the spatiotemporal envelope of the laser pulse and its energy. The need for complete control over the deposited energy density, with respect to the irradiated material, to both achieve optimized optical functions and progress towards increasingly smaller spatial scales, drives us to gain an in-depth understanding of the dynamics of material modification during and after interaction with the laser pulse. These dynamics can be studied using advanced spectroscopy and imaging techniques based on pump-probe schemes.

As an Associate Professor at the Hubert Curien Laboratory, I address all these aspects of laser material structuring processes. My research activity can be divided into two main areas. The first area, which has a more applicative focus, involves mastering the irradiation process through the spatial and temporal shaping of the ultrafast laser pulse to transform the material and control its optical, mechanical, and chemical properties. The second area, with a more fundamental focus, involves using the irradiation process to study the response of the material (spectroscopy, pump-probe experiments) and generate secondary sources (plasma photoluminescence, THz radiation, etc.) that can be used to analyze and understand the interaction mechanisms and thus optimize the structuring process.