Origin of Thermal Nonlinear Refraction in Alcohols Revealed by Spatial Self-Phase Modulation under Excitation at 1560 nm

Abstract We investigated the relationship between molecular structure and nonlinear thermal responses of alcohols with different numbers of carbon atoms in their main chains using spatial self-phase modulation experiments at 1560 nm. The studied materials included the following aliphatic primary alcohols: methanol, ethanol, 1-propanol, 1-butanol, 1-hexanol, 1-heptanol, and 1-octanol, in addition to isopropanol and phenylmethanol. The absolute values of the effective nonlinear refractive indices, |n2eff|, ranged from 9.33 × 10–5 cm2/W for methanol to 3.10 × 10–5 cm2/W for octanol, with the linear absorption coefficient, α0, linking the molecular architecture to the thermal nonlinearity. The decrease in |n2eff| follows the reduction in the volumetric concentration of O–H oscillators, which decreases as the carbon chain length increases. We also performed control experiments at 800 nm, where all solvents are transparent and no spatial self-phase modulation rings were observed, supporting the predominantly thermal origin of the phenomenon at 1560 nm. These findings highlight resonant linear absorption as a central link between the chemical structure of the alcohols and their thermal nonlinear responses, strongly influencing the magnitude of the thermal effect across the investigated series.