Consistent nonlinear optical refractive index $$n_2$$ measurement of porcine crystalline lens and its surrounding in the 650–900 nm range

Abstract

Abstract The nonlinear optical properties of transparent ocular media have been shown to alter the focused intensity distribution of femtosecond laser pulses, potentially affecting the precision of laser ophthalmic surgery. In this study, the nonlinear refractive index $$n_2$$ of porcine aqueous humor, crystalline lens and vitreous humor was measured using two complementary techniques: the standard Z-scan-derived D4 $$\sigma$$ method and the phase-object imaging method. Both were performed with a tunable femtosecond laser source across the visible to near-infrared range (650–900 nm). All measured $$n_2$$ values were found to be close to $$2 \times 10^{-20}\,m^2/W$$ within experimental uncertainties, with no measurable nonlinear absorption detected. Notably, the phase-object technique proved particularly well-suited for slightly scattering or heterogeneous media, such as the freshly extracted crystalline lens, when calibrated against a reference medium (here, water). These experimental results allow for a quantitative numerical assessment of pulse and beam degradation due to nonlinear refraction during procedures like cataract surgery, as well as an evaluation of its potential impact on photodisruption geometry.