Large optical phase shifts in hydrogels written with femtosecond laser pulses: elucidating the role of localized water concentration changes

Abstract

We describe the treatment of new hydrogels with nine different chemical compositions by femtosecond laser writing. The phase change induced in the wavefront when passing through the written areas was measured in all of these materials. The induced phase change is negative, which is attributed to the higher presence of water in the written regions and confirmed via Raman spectroscopy. The largest induced change in a single layer at 100 mm/s was −3.69 waves at 543 nm. These results show a strong dependence of the phase change on the concentration of some components and their molar ratio. We propose that some components are essential for the nonlinear energy absorption (“dopants”), while other components (“quenchers”) are essential in redirecting the absorbed energy to cause chemical reactions that profoundly change the polymer structure.