Femtosecond laser micromachining exploits the nonlinear absorption of focused ultrashort laser pulses to induce localized and permanent modifications of a transparent dielectric material. The range of applications is wide, encompassing direct inscription of optical waveguides, holographic storage of information, and fabrication of refractive micro-optic components. Recently, the capability to use this method to process biocompatible hydrogels, interesting for producing, e.g., contact lenses or intraocular lenses, was demonstrated. In this paper, the authors perform a detailed investigation on the femtosecond laser processing of hydrogels with different chemical composition. They show the possibility to inscribe high-refractive-index-contrast traces, corresponding to a phase delay of several wavelengths, with just a single irradiation pass. They study the dependence of the induced optical modifications on the chemical composition of the hydrogel, proposing that some specific dopants can catalyze the energy absorption from the laser pulses, thus enhancing the response to the laser treatment. The results of this study may enable a faster production of refractive optical components, such as GRIN lenses, in these types of hydrogels. This opens interesting perspectives for direct inscription, by the use of a femtosecond laser, of visual correction devices tailored to the needs of individual patients.
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