Nonlinear absorption measurements of intraocular lens polymers by integrating Z-scan

Abstract

An ultrashort pulsed laser processing knowledge of the amount and distribution of the absorbed energy in the workpiece is essential. For transparent dielectrics, energy input by ultrashort laser pulses occurs by nonlinear absorption in the bulk material. Even at high intensities, the penetration depth is high and for most cases decays with an unknown function. Additionally, nonlinear absorptivity dramatically changes depending on the number of incident pulses due to incubation of the workpiece. This is usually attributed to chemical modifications or generated voids inside the workpiece. Particularly, polymers are strongly influenced by incubation preventing full understanding of ablation at industrial processes. Especially, the Polymethylmethacrylate (PMMA) copolymer analyzed in this paper is of high interest for ophthalmic applications and requires high processing quality and efficiency. To gain detailed insight into interaction of polymers with multiple subsequent picosecond laser pulses, investigations on nonlinear absorptivity have to be carried out. Therefore, the determination of nonlinear transmittance and reflectance by a modified Z-scan setup is presented. Laser pulses at 1064 nm wavelength, which are commonly used for production, are applied with pulse energies up to 130 μJ. Measuring the absorptivity depending on the pulse number is enabled by integrating spheres included in the setup. Up to 100 lasers pulses which are scattered by the rough surface, caused by ablation of previous pulses, are taken into account by collecting the scattered proportion by the spheres. A significant change of transmittance and reflectance is observed depending on the pulse energy and pulse number. The influence of incubation on nonlinear absorption is quantified by fitting the recorded data to the models known from literature. The analytical model of three-photon absorption shows best agreement and is considered as the dominant absorption process. The corresponding nonlinear absorption coefficients are quantified in dependence of the pulse number showing significant increase for the first six pulses.