Size corrections during ultrafast laser induced refractive index changes in bulk transparent materials
Résumé
Three-dimensional structuring of bulk dielectric materials usually requires focusing through air-dielectric interfaces and, consequently, depth-dependent spherical aberration appears. This deter-mines 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 in-duced changes of refractive index. The proposed strategies 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 dy-namically 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 in-dicate 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.