Unraveling the complex nature of optical nonlinearity in water-for biophotonics applications

Water plays an essential role in biological systems, and understanding its nonlinear optical properties is crucial for advancements in biomedical applications. In laser-based medical treatments, water's absorption and refraction significantly influence the precision and safety of procedures. Additionally, water's nonlinear refractive behavior is critical in biophotonics, particularly in two- and three-photon microscopy, where it impacts imaging resolution and depth in tissues. Various parameters, such as laser wavelength, pulse duration, and pulse repetition frequency, significantly influence the nonlinear optical properties of water. From this perspective, we investigate how the nonlinear refraction of water changes with varying laser pulse repetition rates. A conventional single-beam z-scan experimental setup, equipped with a femtosecond pulsed laser with tunable repetition rate, was employed for this investigation. Our findings reveal three distinct regimes of nonlinear refraction in water, highlighting the complex interaction between electronic and thermal contributions to the nonlinearity. We observed unusual z-scan traces at certain pulse repetition rates, primarily attributed to the competition between multiple contributions to the nonlinear processes. This preliminary investigation provides a deeper insight into the complex nonlinear optical behavior of water. These findings are vital for improving laser-based applications in biological tissues, enhancing microscopy techniques, and developing better models for light propagation in aqueous environments. This research also helps to improve the safety, precision, and efficacy of laser-based medical procedures and bio-imaging.