PICOSECOND INVESTIGATIONS OF OPTICAL LIMITING MECHANISMS IN KINGS COMPLEX

We have investigated the nonlinear optical mechanisms responsible for optical limiting of both picosecond and nanosecond 532-nm optical puls es in the organometallic compound cyclopentadienyliron carbonyl tetram er (King's complex). For fluences below approximately 200 mJ/cm2, pico second pump-probe measurements in solutions of the King's complex reve al a prompt reverses saturable absorption (RSA) that recovers with a t ime constant of 120 ps. We attribute this RSA to excited-state absorpt ion within the singlet system of the King's complex, and we demonstrat e that the RSA is completely characterized by a simple three-level mod el. We find, however, that the material parameters extracted from thes e picosecond measurements cannot account for the strong optical limiti ng previously observed in identical solutions of this compound using n anosecond excitation at higher fluences. Picosecond measurements at fl uences greater than 200 mJ/cm2 reveal the onset of an additional loss mechanism that appears approximately 1 ns after excitation. The magnit ude of this loss depends on both the laser repetition rate and the sol vent, indicating that the loss is not directly related to the intrinsi c properties of the King's complex but is most likely thermal in origi n. Using nanosecond excitation pulses, we have performed angularly res olved transmission and reflection measurements, which reveal strong fo rward- and backward-induced scattering at these fluences. Furthermore, when the King's complex is incorporated in a solid host, we observe n egligible induced scatter and the response is completely described by the singlet parameters extracted from the picosecond measurements. The se observations indicate that the nanosecond optical limiter response of solutions of King's complex is dominated by thermally induced scatt ering.