Machining of micrometer-scale high aspect ratio features with single femtosecond laser pulses

The authors characterize femtosecond laser single-pulse machining of deep, micrometer-diameter holes and long, micrometer-width channels in fused silica by the use of spherical, cylindrical, and aspheric singlet lenses. Repositionable spherical lenses form an adjustable beam expander that also provides a means of minimizing-or deliberately introducing-spherical aberration (SA) in the focal region by controlling the beam divergence at the asphere. Inserting cylindrical lenses creates a line focus for machining channel patterns parallel to the sample surface and at any depth within the bulk of the sample. The effects of controlled SA and pulse energy on the depth of round-focus holes and line-focus channels are studied. Holes less than 1 mu m in diameter but with depths exceeding 30 mu m are observed in the case of strong positive SA. Channel patterns from similar to 1 to 3 mu m wide, up to 2000 mu m long, and with depths of 6-40 mu m can also be machined with a single pulse, depending on the lens configuration and pulse energy. For the highest pulse energies studied, channel features exhibit phenomena such as bifurcations and multiple, separated focal regions along the beam path, indicating a possible complex interplay between SA, self-focusing, and filamentation. The authors also present experiments on attempting to form arrays of closely spaced, parallel microfluidic channels in fused silica by KOH etching of line-focus features for highly parallelized microfluidic applications.