Characterization of Fs-laser induced unintended, periodic structures in hydroxy-ethyl-methacrylat (HEMA)

In ophthalmic surgery fs laser pulses are used as a precise and safe cutting tool. At specific processing parameters, however, an interesting phenomenon of unintended periodic structures inside the tissue can be observed. In this study, a transparent polymer served as ocular phantom material for further investigations. A Femtosecond laser with MHz-repetition rate and pulse energies below 200 nJ was used. The size of the durable material change caused by applied fs-single pulses was measured in regard to the pulse energy. Furthermore, lines were cut inside the material with different laser spot distances and laser pulse energies. The creation and enhancement of unintended step-like structures could be related to a decrease of spot distance and further increase of pulse energy. Cutting planes inside the material resulted also in a formation of step like structures. For planes the step-like structures were formed with different orientation in the x-y-plane in regard to the used line distance between two applied lines. A maximum step height of 75 mu m was measured using our setup. Those periodic structures are unwanted for any application in the field of laser material processing, ophthalmology or biomedical sectioning. Hence, investigations of the parameters which trigger this phenomenon are of great interest.