Role of stress in the chemical etching of fused silica exposed to low-energy femtosecond laser pulses

Recently, it was demonstrated that femtosecond lasers pulses with energies below the ablation threshold locally enhance the etching rate of fused silica: regions that are exposed to the laser beam are etched faster. This remarkable property has been used for fabricating a variety of micro-structures like fluidic channels, tunnels or more complex devices, like mechanical flexures. The physical effect causing the etching-rate local enhancement is still debated and various hypotheses have been proposed among which localized densification models seem to prevail. In that context, we recently demonstrated that the amount of deposited energy plays a very important role. It was found that for laser repetition rates where no cumulative effects are observed, there exists an optimal amount of energy deposited to achieve the fastest etching rate. These observations suggest that the stress introduced during laser exposure plays an important role in the processing of fused silica with low energy ultrafast pulses. In this paper, we investigate the stress distributions in various laser patterns and how this stress distribution can account for various effects observed during processing such as a local etching enhancement, the occurrence of cracks in dense patterns made of multiple lines and finally, the presence of stress-induced birefringence.