Independent control of aspect ratios in the axial and lateral cross sections of a focal spot for three-dimensional femtosecond laser micromachining

We theoretically and experimentally show that independent control of aspect ratios of cross-sectional shapes of a focal spot in both axial and lateral directions can be achieved for three-dimensional (3D) femtosecond laser micromachining by the use of a combination of a slit beam shaping technique and a temporal focusing technique. The simultaneous employment of the spatial and temporal beam shaping techniques allows us to achieve isotropic resolution in 3D space even for an objective lens of low numerical aperture. We also present analytical expressions of the peak-intensity distributions near the focus for the spatiotemporally focused femtosecond laser beams with and without utilizing the slit beam shaping technique.