Laser ablation of various polymers has been studied using a beam of 100 fs laser pulses at a wavelength of 800 nm. For a beam focussed by a plano-convex lens, high-quality machining of holes with diameters 20-40 μm and depths 300-400 μm (aspect ratio greater than 10) has been achieved. When projection patterning is carried out with a rectangular pinhole, arrays of micro-strings have been observed deep in the bulk of the sample. The micro-strings have diameters as small as 2 μm (about three times the wavelength) and lengths greater than 10 mm (aspect ratio 5000). The achievement of the high aspect ratios and the formation of the arrays of micro-strings are discussed.