Femtosecond laser induced step-like structures inside transparent hydrogel due to laser induced threshold reduction.

In the area of laser material processing, versatile applications for cutting glasses and transparent polymers exist. However, parasitic effects such as the creation of step-like structures appear when laser cutting inside a transparent material. To date, these structures were only described empirically. This work establishes the physical and chemical mechanisms behind the observed effects and describes the influence of process and material parameters onto the creation of step-like structures in hydrogel, Dihydroxyethylmethacrylat (HEMA). By focusing laser pulses in HEMA, reduced pulse separation distance below 50 nm and rise in pulse energy enhances the creation of unintended step-like structures. Spatial resolved Raman-spectroscopy was used to measure the laser induced chemical modification, which results into a reduced breakdown threshold. The reduction in threshold influences the position of optical breakdown for the succeeding laser pulses and consequently leads to the step-like structures. Additionally, the experimental findings were supplemented with numerical simulations of the influence of reduced damage threshold onto the position of optical breakdown. In summary, chemical material change was defined as cause of the step-like structures. Furthermore, the parameters to avoid these structures were identified.

ROWIAK GmbH mission statement is: “Our developments support ophthalmologists around the world in diagnosing and treating their patients. Solid innovation, research and development are the basis of our work. Ultra-precise femtosecond lasers and highly sensitive opto-electronics are our tools.” Thus, ROWIAK is going to sell ophthalmic laser systems and equipment. The aim of our manuscript has a very fundamental character. Thus, there are no competing interests of ROWIAK GmbH along with any other relevant declarations relating to employment, consultancy, patents, products in development, or marketed products, etc. We confirm that the affiliation to ROWIAK GmbH does not alter our adherence to all PLOS ONE policies on sharing data and materials. Laser Zentrum Hannover is a publicly funded research institute and not a commercial company.