Unconventional z-scan signatures in water arising from the interplay of repetition rate dependent electronic and thermal nonlinearities

Document Type

Article

Publication Title

Journal of Molecular Liquids

Abstract

Nonlinear refraction has spawned significant scientific interest owing to its diverse applications across numerous domains. Understanding the influence of various extrinsic parameters on the nonlinear refraction of materials is essential for incorporating these materials into optoelectronic devices and for other applications. However, reports on the systematic investigation of the interplay of electronic and thermal nonlinearities are scarce. In this context, we present the excitation pulse repetition rate dependent electronic to thermal nonlinearity switching effects revealed through z-scan technique. Specifically, we investigated these effects in de-ionized water using a repetition rate tunable femtosecond laser operating at a wavelength of 1030 nm. A remarkable modification in the nature of the nonlinear refraction is observed when the repetition rate of the excitation laser is varied. Below 10 kHz repetition rate, the observed nonlinear refraction can be primarily attributed to electronic contributions, while above 75 kHz repetition rate, thermal contributions become dominant. More interestingly, we identified a novel regime (repetition rates between 10 kHz and 75 kHz), where the nonlinear refraction exhibited is caused by the interplay of electronic and thermal nonlinearities. This led to the observation of unconventional closed aperture z-scan signatures that have not been reported earlier in water. The electronic and thermal contributions to the observed nonlinear refraction at different repetition rates were numerically evaluated. This research provides deeper insight into the complex nature of nonlinear refraction at low, moderate, and high laser pulse repetition rates. Such insights are crucial for measurements or applications involving water and intense lasers such as multiphoton microscopy, confocal fluorescence microscopy, and laser-assisted therapies.

DOI

10.1016/j.molliq.2025.127181

Publication Date

5-1-2025

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