Thermal and non-thermal intensity dependent optical nonlinearities in ethanol at 800 nm, 1480 nm, and 1560 nm

Intensity dependent self-action of a continuous wave (CW) or pulsed optical beam can lead to spatial or spectral effects upon propagation through a nonlinear medium, which can be described as an intensity dependence of the refractive index, known as self-phase modulation (SPM). In this work, we revisit the nonlinear optical propagation of a CW and a CW mode-locked (CW-ML) high repetition rate [∼ megahertz (MHz)] laser propagating through pure ethanol in regions of very low optical absorption (800 nm) or very high absorption (1480 nm, 1560 nm). Spatial and spectral SPM and Z-scan experiments were performed to clarify the origin of the third-order nonlinear optical response in the different optical excitation regimes. From spatial SPM and Z-scan at either CW or CW-ML MHz regime, a thermal nonlinear response was determined to be the origin of the nonlinearity at 800 nm or 1480 nm, 1560 nm region, with the nonlinear refractive index response of the order of 10−4−10−9cm2/W. From the spectral SPM, the non-thermal origin of the nonlinearity, arising from electronic or nuclear processes in the ethanol, was determined, and values of the order of 10−13−10−16cm2/W were obtained, depending upon the spectral region. The results were supported by theoretical calculations using the nonlinear Schrodinger equation for the spectral behavior or Fresnel–Kirchhoff integral for the spatial results and clarify some of the misinterpreted results reported in the literature, besides complementing other nonlinear refraction data available at different wavelengths from 355 nm to 1560 nm.