  This letter is the first demonstration of material modification using tightly
focused femtosecond laser vortex beams. Double-charge femtosecond vortices were
synthesized with the polarization-singularity beam converter described in Ref
[1] and then focused using moderate and high numerical aperture optics (viz.,
NA = 0.45 and 0.9) to ablate fused silica and soda-lime glasses. By controlling
the pulse energy we consistently machine high-quality micron-size ring-shaped
structures with less than 100 nm uniform groove thickness.
