Optimization of fs-laser-induced voxels in nonlinear materials via over-correction of spherical aberration.

In this Letter, over-correction of spherical aberration is used to counteract nonlinear effects such as Kerr self-focusing and plasma effects, resulting in more spherical and small-sized femtosecond laser-inscribed voxels within nonlinear materials. By strategically redirecting marginal focusing rays toward the beginning of the laser modification zone, the induced plasma prevents any rays from causing a structural modification beyond this zone, irrespective of any focus elongation caused by nonlinear effects. The method has been effectively validated across a range of materials, including ZnS, ZnSe, BIG, GeS₄, and SiO₂. A significant outcome is the achievement of quasi-spherical and (sub-)micrometer voxels in highly nonlinear materials. These findings open avenues for single-mode active waveguides and high-resolution patterning within nonlinear materials. The experiments are performed using a microscope objective equipped with a correction collar, a widely available tool in laboratories, highlighting the potential and versatility of the technique.