Adaptive control on laser-induced refractive index changes for 3D optical functions in bulk optical glasses
Résumé
Strong nonlinear absorption promotes ultrafast lasers as potential tools for 3D processing of transparent materials, particularly for optical functionalization by refractive index engineering. This typically involves thermo-mechanical and structural rearrangements of the dielectric matrix. Challenges are related to the time-effectiveness of irradiation, correct beam delivery and the influence of material properties on the exposure outcomes. Mastering the irradiation result may open the path for new applications involving 3D optical devices. Particularly for light-guiding applications it is suitable to master positive refractive index changes in a time-efficient manner, where the result depends on deposited energy and its relaxation paths. In this respect several irradiation concepts based on adaptive optics in spatial and temporal domains were developed. We review here some of the applications from various perspectives. A physical aspect is related to time-synchronized energy delivery tuned on material transient reactions, enabling a synergetic interaction between light and matter and, therefore, optimal results. Examples are given concerning refractive index flip in thermally expansive glasses by thermo-mechanical regulation, energy confinement by nonlinear control, accompanied by time-resolved investigations of the refractive index change. A second engineering aspect is related to processing efficiency. We give insights into 3D parallel photoinscription techniques and beam-delivery corrections utilizing dynamic wavefront engineering. Additionally, energetic domains of ultrafast laser radiation can generate a spontaneous nanoscale material arrangement, leading to form birefringence and modulated index patterns. Using birefringence and the deriving anisotropic optical properties, polarization sensitive devices were designed and fabricated. The polarization sensitivity allows particular light propagation and confinement properties in 3D structures.