Adaptive spatio-temporal techniques for smart ultrafast laser processing of optical glasses
Résumé
Ultrafast lasers emerged as efficient tools to process transparent materials on minimal scales. Localized refractive index changes due to laser-induced nonlinear structural matter rearrangements can then serve as building blocks for embedded optical functions. The requirements of a desired photonic response involve precise adjustments of the refractive index which usually depends on the material relaxation paths. For example, it can result in both positive and negative index variations, the latter being detrimental for waveguiding applications. These facts impose specific limitations to the photoinscription process. Additionally, if at low photon doses isotropic refractive index changes are induced via soft electronic alterations, in more energetic regimes corresponding to thermo-mechanical photoinscription effects, ultrafast laser radiation can generate an intriguing nanoscale spontaneous arrangement, leading to form birefringence and modulated index patterns. The formation dynamics of refractive index structures and the characteristics times of the driving factors can be visualised using time-resolved microscopy techniques. This determines a guideline for intelligently improving the irradiation outcome using tailored irradiation and regulated energy delivery. Considering the above observations, advanced strategies are then required to improve the irradiation results. Recently, new beam manipulation concepts were developed which allow a modulation of the energy feedthrough according to the material transient reactions, enabling thus a synergetic interaction between light and matter and, therefore, optimal results. Considering the potential of optical functionalization we discuss the possibility of controlling laser-induced modifications of transparent materials employing automated temporal pulse shaping. Examples of adaptive design of refractive index changes in glasses will be shown, accompanied by concepts of efficient processing approaches. This involves an engineering aspect related to simultaneous processing of structural modifications in 3D arrangements where a feasible solution is represented by dynamic spatial beam shaping techniques and wavefront engineering. The approach has a dual aspect and includes corrections for beam propagation errors and spatial intensity distributions in desired patterns. Adding the possibility of laser-induced birefringence and the associated anisotropic light scattering properties characteristic to the nanoscale self-arrangements, photowritten structures can be arranged in patterns generating complex propagation and polarization effects.