Lee W. Tutt

Optical limiting with C60 in polymethyl methacrylate

We demonstrate optical limiting for the C(60) fullerene in polymethyl methacrylate (PMMA) as a solid polymer host. It is shown that the optical-limiting behavior is consistent with excited-state absorption (reverse saturable absorption) as a mechanism. We suggest that a higher threshold for optical limiting compared with that of C(60) in toluene is due to nonlinear scattering for the liquid. The performance of C(60) in PMMA is compared with that in chloroaluminum phthalocyanine, N-methylthioacridone, Kings complex, and ruthenium Kings complex in PMMA. Optical damage thresholds are reported.

Picosecond investigations of optical limiting mechanisms in King's complex

We have investigated the nonlinear optical mechanisms responsible for optical limiting of both picosecond and nanosecond 532-nm optical pulses in the organometallic compound cyclopentadienyliron carbonyl tetramer (Kings complex). For fluences below ~200 mJ/cm 2 , picosecond pump-probe measurements in solutions of the Kings complex reveal a prompt reverse saturable absorption (RSA) that recovers with a time constant of 120 ps. We attribute this RSA to excited-state absorption within the singlet system of the Kings complex, and we demonstrate that the RSA is completely characterized by a simple three-level model. We find, however, that the material parameters extracted from these picosecond measurements cannot account for the strong optical limiting previously observed in identical solutions of this compound using nanosecond excitation at higher fluences. Picosecond measurements at fluences greater than 200 mJ/cm 2 reveal the onset of an additional loss mechanism that appears ~1 ns after excitation. The magnitude of this loss depends on both the laser repetition rate and the solvent, indicating that the loss is not directly related to the intrinsic properties of the Kings complex but is most likely thermal in origin. Using nanosecond excitation pulses, we have performed angularly resolved transmission and reflection measurements, which reveal strong forward- and backward-induced scattering at these fluences. Furthermore, when the Kings complex is incorporated in a solid host, we observe negligible induced scatter and the response is completely described by the singlet parameters extracted from the picosecond measurements. These observations indicate that the nanosecond optical limiter response of solutions of Kings complex is dominated by thermally induced scattering.

Optical limiting with reverse saturable absorbers

We have used a physically realistic model to investigate the spatial and temporal behavior of reverse saturable absorption and optical limiting in organometallic cluster compounds. An algorithm was developed to solve numerically the model and is used for the case of a collimated beam to determine various material parameters by fitting theory to experimental optical limiting data. Using these parameters, the algorithm was then extended to the case of converging and/or diverging beams.

Optimization of optical limiting properties of organometallic cluster compounds

Ground and excited state absorption spectra along with optical limiting measurements have been made on a series of systematically altered organometallic cluster compounds. Measurements using 8 ns pulses at 532 nm on methylene chloride solutions of iron-tricobalt organometallic cluster complexes (70% nominal transmission) show limiting throughputs of 350-700 mJ/cm2 without focusing. The optical limiting properties are shown to be minimally dependent on counter-ion substitution indicating little perturbation in the cluster bonding. Optical limiting is shown to be strongly dependent on ligand substitution. Ground state and excited state absorption spectra reveal broad features indicating the materials can be used as limiters over a large spectral range (>lOOnm) . Temporal pulse narrowing of 8 ns pulses to 5 ns at 532 nm was observed, indicating subnanosecond intersystem crossing. The lifetime of the excited state was determined to be 115 ns, much longer than the pulse length, indicating that efficient limiting occurs for longer pulses. Optimization of optical limiting properties is discussed.

Picosecond measurements of optical nonlinearities in King's complex and synthesized analogues

We have measured the photodynamics of reverse saturable absorption (RSA) in solutions of cyclopentadienyliron carbonyl tetramer (Kings complex) using picosecond pump-probe techniques. Similar preliminary measurements in solutions of synthesized variations of the Kings complex indicate that the excited state transition responsible for the observed RSA is most likely a second d-d transition within the metal core of the molecule. On time scales of hundreds of picoseconds, the observed RSA in the Kings complex is well characterized by a three-level rate-equation, singlet-state absorption model, where the excited-state cross section is greater than that of the ground state. On nanosecond timescales and at fluences above 200 mJ.cm-2, however, we observe the onset of a response that is consistent with a thermally induced scattering process. Further evidence of this scattering is provided by angularly-resolved measurements of the transmitted and back-scattered signals for nanosecond excitation. When the Kings complex is incorporated in a solid host negligible scatter was observed and the response is completely described by the singlet parameters extracted from the picosecond measurements. The observation of, scatter from solution, together with a time- resolved decay to the ground state that is rapid (approximately 120 ps) and largely nonradiative in this molecule, indicate that solutions of Kings complex may provide a mechanism for efficiently generating thermal nonlinearities on a subnanosecond timescale.