Size Corrections during Ultrafast Laser Induced Refractive Index Changes in Bulk Transparent Materials

Three-dimensional structuring of bulk dielectric materials usually requires focusing through airdielectric interfaces. Consequently, depth-dependent spherical aberration appears. This determines an elongation of the energy deposition area and restricts the structuring accuracy. We discuss here strategies for counteracting wavefront distortion effects which occur during ultrafast laser induced changes of refractive index. The proposed approaches are based on programmable spatio-temporal pulse shaping and have the objective of concentrating the laser energy on minimal spatial scales. Using adaptive spatial tailoring of ultrashort laser pulses, spherical aberrations can be dynamically corrected, in synchronization with the writing procedure. This facilitates optimal writing of homogeneous longitudinal waveguides over significant lengths. We also show that temporal forming of ultrafast laser pulses restricts the energy spread, leading to a higher confinement. We indicate the role of reduced nonlinearity in plasma formation as a control factor coupling the spatial and temporal response of the material. The size corrections enable higher processing accuracy.