3D adaptive spatio-temporal control of laser-induced refractive index changes in optical glasses

The nonlinear absorption character determines a high potential of ultrafast laser pulses for 3D processing of transparent materials, particularly for optical functions. This is based on refractive index engineering involving 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 results. Particularly for light-guiding applications it is suitable to master positive refractive index changes in a time-efficient manner, considering that the result depends on the deposited energy and its relaxation paths. To address these challenges 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 temporal pulse shaping and time-synchronized energy delivery tuned to material transient reactions, enabling thus a synergetic interaction between light and matter and, therefore, optimal results. Examples will be given concerning refractive index flip in thermally expansive glasses by thermo-mechanical regulation and energy confinement by nonlinear control. A second engineering aspect is related to processing efficiency. We give insights into beam-delivery corrections and 3D parallel complex photoinscription techniques utilizing dynamic wavefront engineering. Additionally, in energetic regimes, ultrafast laser radiation can generate an intriguing nanoscale spontaneous arrangement, leading to form birefringence and modulated index patterns. Using the birefringence properties and the deriving anisotropic optical character, polarization sensitive devices were designed and fabricated. The polarization sensitivity allows particular light propagation and confinement properties in 3D structures.