Femtosecond laser micromachining in ophthalmic hydrogels: spectroscopic study of materials effects

Femtosecond laser micromachining relies on tightly focused, ultrashort laser pulses to locally modify material properties through nonlinear absorption, and is finding applications in the field of vision correction. Here we study the material effects of femtosecond laser micromachining in hydrogels to understand the mechanisms of laser-induced refractive index (RI) changes. Single layer dense line patterns were inscribed successively into the middle of hydrogels using a 405 nm femtosecond laser. A maximum phase change of ∼1.2 waves could be obtained at 543 nm with only 60 mW beam intensity at the focal volume. The phase change profile, fitted to a photochemical scaling model from earlier study, indicated that the micromachining process was mainly dominated by two-photon absorption. A confocal micro-Raman system was custom-designed to quantify the structural changes in written regions, especially the local water content. Change in the local water content exhibited three distinctive stages as a function of beam intensity. Below the optical breakdown threshold, a significant increase of local water content within the written layer was observed, while all Raman signatures remained the same. We posited that the negative RI changes were likely due to the generation of free radicals, followed by water permeation in the modified volume. This increase of local water content, likely presented as free water, was further confirmed by thermogravimetric analysis. Fourier-transform infrared spectroscopy was also used to gain insights into the chemical changes in the depolymerization stage.