Measurement of the nonlinear refractive index n2 of electronically excited terthiophene

The nonlinear refractive index n2 of excited terthiophene (3T) dissolved in 1,4-dioxane (c=1 mM) has been determined using the z-scan technique at 1047 nm. After excitation with ps-pulses at 349 nm, 3T exhibits a negative n2 of −1.1×10−14 cm2/W. This value, caused by the percentage of excited molecules only, is more than one order of magnitude higher than that of the non-excited 3T. The strong enhancement is due to a resonant S2←S1 transition. By employing a time-resolved $$pump–probe technique, the corresponding excited state absorption (ESA) of 3T was observed at 1047 nm.