Enhancement and sub-picosecond dynamics of optical non-linearities of excited-states: trans-stilbene in solution

Using degenerated four wave mixing techniques and optical Kerr effect we studied the optical non-linearities of excited t-stilbene solution. Strong enhancement of the non-linear signal between the excited and ground states is observed and related to the third order non-linear susceptibility ( χ(3)) taking into account resonance conditions. Sub-picosecond studies of the X(3) dynamics show, depending on the experimental conditions, a fast decay time (in the range of < 1 ps) associated to electronic processes and a slower decay time (10–100 ps), depending on the solvent, associated with the reorientation time τor of the molecules in solution. We show that vibrational relaxation and solvent cooling may control the dynamics of χ(3).