An ultrafast lensing effect of photo-induced beam focusing and defocusing was observed for dilute all-trans-β-carotene solutions in n-hexane. Sub-picosecond pump and probe pulses were used with a low power semi-resonant excitation at 390 nm and a non-resonant probe at 780 nm. High speed averaging with intensity-modulated mode-locked pulse trains successfully provided a double exponential response where the pre-exponential factors had different signs. The time constants were 0.2 and 10 ps, respectively corresponding to those of two excited states of β-carotene. Similarity in lensing intensity dependence between the ultrafast effect and a slower thermal effect was also examined.
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