Thermal refraction in ionic liquids induced by a train of femtosecond laser pulses

• We measure the thermal lens effect in a set of ionic liquids with Z-scan.
• We study the cation (anion) influence when combined with [Tf2N]− ([BMIM]+).
• We identify the anions/cations that enhance and minimize the thermal lens effect.
• We provide absorption coefficients and, when possible, thermal conductivities.
• We calculate, when possible, the thermo-optic coefficients.

The Z-scan technique is used to characterize the nonlinear refraction induced by a train of ultrashort laser pulses (80 fs, repetition rate 80.75 MHz) for a set of ionic liquids as a function of their structural parameters such as cation, anion type and alkyl chain length in the cation. The nonlinear refractive index change is originated by linear absorption processes which cause a nonlocal and inhomogeneous increase of temperature in the irradiated region of the sample that behaves as a thermal lens. The thermal refraction, according to the thermal lens model, is mainly related to three physical properties of the medium, which are the absorption at the excitation wavelength, the thermal conductivity and the thermo-optic coefficient. We explore the influence of anions and cations on these properties and on the thermal lens strength. The cationic influence was the object of study for the ILs based on the 1-butyl-3-methyl-imidazolium cation. The anionic influence was studied for the ILs based on the bis(trifluoromethylsulfonyl)imide anion. The obtained results show that both cation and anion affect significantly the thermal lens strength. The thermal refraction is 1.6 times higher in 1-butyl-3-methyl-imidazolium chloride than in 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide, and 3.3 times higher in trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide than in 1-butylpyridinium bis(trifluoromethylsulfonyl)imide. Optical absorption at 810 nm is influenced by both ionic parts, whereas the anion hardly determines the magnitude of the thermal conductivity.