Short-pulse-induced thermal lensing effect in 1,2-dichloroethane and 1,2-dibromoethane

1,2-dichloroethane (${{\rm C}_2}{{\rm H}_4}{{\rm Cl}_2}$) and 1,2-dibromoethane (${{\rm C}_2}{{\rm H}_4}{{\rm Br}_2}$) show linear absorption (LA) and stimulated light scattering in response to the excitation by 820 nm 18 fs laser pulses. Using the $Z$-scan technique, with these pulses separated by 12.2 ns (much shorter than both samples’ thermal diffusivity time constants ${\tau _{\rm th}}$’s) and extended over 41 ms (considerably longer than ${\tau _{\rm th}}$’s), we investigated the thermal lensing effect of both samples and found that LA dominates their heating. In addition, relative to ${{\rm C}_2}{{\rm H}_4}{{\rm Br}_2}$, ${{\rm C}_2}{{\rm H}_4}{{\rm Cl}_2}$ shows more/less thermal lensing effect before/after the exposure time passes ${\tau _{\rm th}}$’s. This means that the thermal lensing effect is more severely reinforced by heating and stabilized by thermal diffusivity in ${{\rm C}_2}{{\rm H}_4}{{\rm Cl}_2}$ than in ${{\rm C}_2}{{\rm H}_4}{{\rm Br}_2}$. Notably, the Kerr lensing effect is negligible in this study, although it is noticeable in a $Z$-scan study with 17 ps laser pulses.