Nonlinear optical properties of water from 1150 nm to 1400 nm

Understanding the optical properties of water is critical to both laser-tissue interactions as well as setting ocular laser safety standards. The nonlinear properties of water are responsible for supercontinuum generation; however, these effects are poorly understood for wavelengths longer than 1064 nm. A previous study suggested that the supercontinuum generation may convert retinal-safe femtosecond near-infrared pulses with wavelengths longer than 1064 nm into visible wavelength pulses that are above the maximum permissible exposure limit as defined by ANSI Z136.1-2014. To address this knowledge gap, we extend the Z-scan technique in distilled water to wavelengths between 1150 nm to 1400 nm, where linear absorption is strong. Utilizing wavelength tunable, nominally 100 fs laser pulses, we observe wavelength dependence of the nonlinear optical properties of water. The nonlinear refractive index at 1150 nm was consistent with measurements taken at 532 nm in previous studies, and was observed to increase at longer wavelengths. The nonlinear absorption was positive for wavelengths between 1150 nm and 1350 nm and reversed to saturable absorption at 1400 nm. Saturable absorption poses a previously unanticipated eye safety risk as current ocular laser safety standards assume strong absorption at 1400 nm. These results expand our current understanding of the nonlinear optical properties of water to wavelengths in the 1150 nm to 1400 nm region, and inform efforts to revise national and international exposure limits to account for retinal hazards due to nonlinear effects.