Femtosecond micro-machining of hydrogels: parametric study and photochemical model including material saturation

We report on the effects of various laser system parameters on the magnitude of phase change induced inside hydrogel-based contact lens materials in the two-photon absorption limit via a laser induced refractive index (LIRIC) technique. In comparison with near infrared writing at 1035 nm where four-photon absorption process dominates, blue writing at 405 nm allows for the achievement of a decent amount of phase change (one wave) with a low power and a fast scan speed due to a more efficient two photon absorption process. Efficacy of the LIRIC process could be further improved by taking advantage of intermediate repetition rate laser pulses instead of high repetition rate (> 60 MHz) pulses or low repetition rate (< 500 KHz) pulses. A generally applicable photochemical model based on multiphoton absorption mechanism and pulse overlapping effect in two dimensions is proposed to predict the scaling behavior of the induced phase change. A modified photochemical model incorporating a saturation factor is developed to account for the behavior at large phase shifts. The modified photochemical model also helps explain the inapparent dependence of the phase change on numerical aperture (NA) at low irradiation doses and the observed sub-linear inverse dependence on scan speed.