Nonlinear Absorption of 1030-nm Ultrashort Laser Pulses in Polymethyl Methacrylate

Abstract

The nonlinear interaction of 250-fs laser pulses at a wavelength of 1030 nm with polymethyl methacrylate is investigated using an optimized z-scan technique. It is shown that the mechanism of pulse attenuation for peak intensities below 0.65 TW/cm² can be described as four-photon absorption with the coefficient β₄ = (35 ± 5) cm⁵/TW³. The nonlinear refractive index n₂ = (7.6 ± 0.4) × 10⁻⁴ cm²/TW and the critical power for the onset of self-focusing P_cr = (1.42 ± 0.08) MW are determined and these results are used to demonstrate the correspondence between the calculated shift of the nonlinear focal point and the location of the laser-modified regions.