Abstract

Material modification by ultrashort laser pulses represents a promising tool for a variety of technological applications. Nevertheless the mechanisms of femtosecond laser ablation are still not well understood. A striking feature of femtosecond laser ablation is the extremely sharp threshold; i.e. an increase of the laser fluence by less than one percent leads to the removal of a macroscopic amount of the material. ^1-4 Trying to understand the physical nature of this process we have conducted a detailed investigation of the final ablation morphology produced on GaAs( 100) and Si(100) surfaces by single intense fs-laser pulses using optical interference, differential interference contrast (DIC), atomic force (AFM) and scanning electron (SHM) microscopies. The experimental conditions are the same as for our dynamic studies.^1-5 Figure I shows as an example an optical image of an ablation crater on GaAs (laser fluence F = 0.3 J/cm² = 1.4^* F_abl,, τ_laser = 100 fs, λ = 800 n m, laser focus 150 × 75 μm FWFIM) and the detailed inspection of the crater boundary by SEM and AFM. The ablation crater manifests itself as a well-defined ring in the optical DIC image and the jump of the in terference lines in the optical interferogram indicating that at the crater boundary a 40 nm thick layer of material is removed. The ablation craters appear to be flat: its depth shows only a weak dependence on the laser fluence. The crated boundary consists of a rim with a lateral extension of only 50-100 nm and a height of 100-200 nm. Both the height and the width of the rim vary along the crater boundary in an irregular way, although sometimes we have observed a system of very sharp and high needles periodically spaced along the rim. The craters on GaAs produced with the same laser fluence 1. 4^*F_abl under two different focusing conditions (laser focus 150 × 75 μm and 30 × 15 μm FWHM) are found to be almost identical. The craters on Si (laser fluence F = 0.49J/cm² = 1.4^* F_abl, τ_laser = 100 fs, λ = 800 nm, laser focus 150 × 75 μm FWHM) have a depth of only 7-10 nm at the crater boundary and possess a remarkable hill in the center of the crater. Despite of these differences the crater boundary on Si is qualitatively similar to that on GaAs, it also consists of a rim with a typical width of 200-400 nm and a height of 20-30 nm being also large compared to the crater depth.

© 2002 Optical Society of America