Ultrafast lensing effect of beta-carotene in n-hexane solution

An ultrafast lensing effect of photo-induced beam focusing and defocusing was observed for dilute all-trans-beta-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 beta-carotene. Similarity in lensing intensity dependence between the ultrafast effect and a slower thermal effect was also examined. (C) 1997 Elsevier Science B.V.