Mark C. Brant

Nonlinear absorption study of a C60-toluene solution.

Nonlinear absorption at 532 nm in a C60–toluene solution by using 8-ns and 30-ps laser pulses is reported. The transmittance for both pulse widths is fluence dependent. A five-level model of C60 is described that yields excellent agreement with both pulse-width data sets for incident fluences as high as approximately 1 J/cm2. Additional phenomena observed at higher fluences indicate that other mechanisms may be active and contribute to optical limiting in this regime. The application of C60 as an optical limiter material is discussed.

Polymer host materials for optical limiting

Two polymer systems including polymer elastomers and gels have been studied as host materials for optical limiting applications. Both systems have high laser damage thresholds (LDT), typically 20 to 35 times higher than commercial PMMA bulk materials. For the polymer elastomers, Epotek optical epoxy 301-2 and 310, the LDT increases with an increase of the molecular flexibility. We speculate that the thermo-mechanical fracture may be the mechanism for the laser induced damage. For the hydrogel system, the LDT increases with increasing water content. The mobility of the water plays a key role in determining the LDT by facilitating laser energy dissipation and self-healing. It appears that the polymer elastomer and hydrogel systems both have potential for high power laser applications.

Chitosan-based hydrogels : A new polymer-based system with excellent laser-damage threshold properties

Two types of chitosan hydrogel systems have been prepared that have a laser damage threshold (LDT) up to 35 times higher than commercial PMMA bulk materials. For these samples, the LDT increases with increasing water content. The mechanism of laser damage and the contribution of water to their high laser damage resistance have been examined. DSC measurements indicate water within the hydrogels exist in various states, each with different laser damage resistance properties. These various states play a key role in determining the LDT by controlling the dissipation of laser energy and providing a mechanism for self-healing. This preliminary research shows that polymer hydrogels have potential for high power laser applications because they combine good mechanical integrity due to the polymer frame and good energy dissipation and healing characteristics due to the molecular mobility of water. These two traits allow for bulk shape-retaining films with high laser damage thresholds and potential reversibility in damage processes.

Structure-optical property relationships of porphyrins

Porphyrins are attractive compounds for optical applications. We have been investigating the relationship between molecular structure and optical properties of a number of porphyrin compounds. Structural variations explored include insertion of metal ions, extension of conjugation, halogenation and alkylation either at the pyrrole position or the meso-aryl groups. The characterization of these chromophores includes measurement of UV/Vis, fluorescence and fluorescence lifetimes. Furthermore, we have investigated their nonlinear absorption, excitation dynamics. The significant factors influencing limiting behavior appear to be the heavy atom effect, electron donating and withdrawing substituents conformation distortion and changes in conjugation. Detailed understanding will be gained from measurements of photophysical parameters underlying limiting behavior.

Development of novel two-photon absorbing chromophores

There has been much interest in the development of two-photon absorbing materials and many efforts to understand the nonlinear absorption properties of these dyes but this area is still not well understood. A computational model has been developed in our lab to understand the nanosecond nonlinear absorption properties that incorporate all of the measured one-photon photophysical parameters of a class of materials called AFX. We have investigated the nonlinear and photophysical properties of the AFX chromophores including the two-photon absorption cross-section, the excited state cross-section, the intersystem crossing quantum yield, and the singlet and triplet excited state lifetimes using a variety of experimental techniques that include UV-visible, fluorescence and phosphorescence spectroscopy, time correlated single photon counting, ultrafast transient absorption, and nanosecond laser flash photolysis. The model accurately predicts the nanosecond nonlinear transmittance data using experimentally measured parameters. Much of the strong nonlinear absorption has been shown to be due to excited state absorption from both the singlet and triplet excited states. Based on this understanding of the nonlinear absorption and the importance of singlet and triplet excited states we have begun to develop new two-photon absorbing molecules within the AFX class as well as linked to other classes of nonlinear absorbing molecules. This opens up the possibilities of new materials with unique and interesting properties. Specifically we have been working on a new class of two-photon absorbing molecules linked to platinum poly-ynes. In the platinum poly-yne chromophores the photophysics are more complicated and we have just started to understand what drives both the linear and non-linear photophysical properties.

Guest-host optical limiting elastomers with high laser damage threshold

We report the results of the laser damage threshold of an undoped optical grade epoxy elastomer and compare its performance to that of an epoxy glassy thermoset and polymethylmethacrylate (PMMA) thermoplastic. Tests were performed in a tight focus, f/5 geometry, with 7 ns pulses at 532 nm. The elastomer exhibits a damage threshold more than 600 times greater than its glassy thermosetting analog and over 20 times greater than PMMA. We describe the importance that thermomechanical properties have on improving laser damage resistance in organic polymers. Optical limiting experiments are performed with matrices doped with silicon naphthalocyanine and zinc octabromotetraphenyl porphyrin as the chromophores. We demonstrate the advantage of using an elastic matrix to improve the performance of solid-state organic optical limiters.

Guest-host optical limiters with high-laser-damage threshold

Conventional guest-host optical limiting materials utilize either a liquid solvent or solid as the matrix for nonlinear absorbing chromophore dopants. Concentration gradients of the chromophore in the matrix can improve optical limiting performance. However, low viscosity liquid solutions can not retain a concentration gradient while polymer solid matrices damage at low laser fluences. We report on a novel approach of using an elastic polymer and viscoelastic gels for guest- host optimal limiting matrices. We achieve high bulk laser damage thresholds in the hosts and maintain a concentration gradient of the chromophore. By softening the epoxy we significantly enhance its bulk laser damage threshold. We characterize this effect by measuring the damage threshold as a function of viscoelastic properties. In addition, optical limiting was demonstrated in all the hosts doped with nonlinear phthalocyanine chromophores.

Nonlinear absorption in modified porphyrins

We study one free base and seven metallo- octabromotetraphenylporphyrins by several techniques. In a pico-second pump-probe experiment, we monitor the transient transmission of each sample up to 11 ns after it is irradiated by an intense laser pulse. Combined with the results from time-resolved fluorescence spectroscopy, we propose a simple model to interpret the transmission data. We attribute the reduction in the transmission to triplet state absorption and extract the triplet state absorption cross-sections, as well as the lifetimes from the time dependent transmission data. In a separate experiment where the transmission of a nanosecond laser pulse is measured with various input energies, our measurement in the cross- sections predicts the correct optical limiting behavior. We assess the overall optical limiting performance of all 8 samples by direct comparison with C60 at the same ground state transmission.

Reverse saturable absorption properties of C60 complexes

Complex formations in C60 solutions and the reverse saturable absorption (RSA) properties have previously been observed. In a wide survey of solvents conducted in our laboratory, several solvents exhibited complexing behavior with C60 when analyzed with UV-Vis and fluorescence spectroscopy. We have studied the formation of complexes in C60/toluene solutions when benzonitrile, N-methyl-pyrrolidinone, and triphenylamine, and diethylaniline were added. Formation of the complexes was followed with UV-Vis spectroscopy. Several plots of the complexes were done to determine the equilibrium constant and the change in molar extinction coefficient in each system. We report the behavior of the reverse saturable absorption (RSA) properties as a function of complexing agent and concentration.

Photoluminescence in C60 solutions

The nonlinear absorptive properties of C60 are of significant interest especially in the near IR. In this paper we describe spectral emission measurements on C60 solutions. In toluene solutions the laser induced emission in the fluence regime of 3 to 1500 J/cm2 excited by nanosecond and picosecond pulses is measured. In C60/toluene solutions a broad double feature is observed with peaks at 692 nm and 732 nm matching the published fluorescence spectra of C60. No evidence for breakdown is apparent in the C60 solutions though there is in the neat toluene. A survey study to determine solvent effects on the electronic states of C60 is reported. Thirteen solvents were chosen from among those in which C60 is soluble to give a wide range of dielectric constant and polarity. The molar extinction coefficient and fluorescence is measured in these solutions. A large solvent induced increase of the near IR molar extinction coefficient is observed in most solvents relative to toluene. Evidence for complex formation is observed. The changes of these spectra relative to that observed for toluene solutions indicate that solvent induced change of the quantum efficiency of triplet production is possible.