Measurement of the nonlinear refractive index of highly scattering aerosols

The quality of air significantly impacts both the quality of life and the health of individuals. Femtosecond laser filament-induced nonlinear spectroscopy effectively measures both aerosol concentration and composition. Specifically, the nonlinear refractive index coefficient of the atmosphere directly influences the nonlinear propagation of femtosecond lasers in the air. The presence of aerosol particles in the atmosphere, particularly water droplets, may affect this nonlinear refractive index coefficient. However, the measurement of the nonlinear refractive index coefficient of highly scattering aerosols has not yet been reported. In this paper, a method to obtain the nonlinear refractive index coefficients of aerosols based on spectral changes is presented. Experiment measured the n₂ coefficient of the air and water vapor aerosols respectively. Experimental results show that the n₂ coefficients are 2.5 × 10⁻¹⁹ cm²/W and 2.4 × 10⁻¹⁹ cm²/W respectively for air with incident energy of 48 μJ and 68 μJ, the n₂ coefficient are 2.5 × 10⁻¹⁹ cm²/W and 2.3 × 10⁻¹⁹ cm²/W respectively for aerosol with attenuation coefficients of 0.029 dB/cm. When the concentration of aerosols was increased to an attenuation coefficient of 0.045 dB/cm, the nonlinear refractive index coefficient of the aerosols was 3.1 × 10⁻¹⁹ cm²/W. The experimental results indicated that low concentrations of aerosols did not affect the nonlinear refractive index coefficient of air, but as the concentration increased to a certain level, the nonlinear refractive index coefficient of air increased. This work provides a simpler and faster technical route for measuring the n₂ coefficient of gaseous media, offers a new approach to the problem of measuring the nonlinear refractive index of thick, highly scattering media, and addresses the shortcomings of the z-scan.