Optical limiting properties of fullerenes and related materials

Optical limiting properties of fullerene C₆₀ and different C₆₀ derivatives (methano-, pyrrolidino-, and amino-) towards nanosecond laser pulses at 532 nm were studied. The results show that optical limiting responses of the C₆₀ derivatives are similar to those of the parent C₆₀ despite their different linear absorption and emission properties. For C₆₀ and the derivatives in room-temperature solutions of varying concentrations and optical path length, the optical limiting responses are strongly concentration dependent. The concentration dependence is not due to any optical artifacts since the results obtained under the same experimental conditions for reference systems show no such dependence. Similarly, optical limiting results of fullerenes are strongly dependent on the medium viscosity, with responses in viscous media weaker than that in room-temperature solutions. The solution concentration and medium viscosity dependencies are not limited to fullerenes. In fact, the results from a systematic investigation of several classes of nonlinear absorptive organic dyes show that the optical limiting responses are also concentration and medium viscosity dependent. Interestingly, however, such dependencies are uniquely absent in the optical limiting responses of metallophthalocyanines. In classical photophysics, the strong solution concentration and medium viscosity dependencies are indicative of significant contributions from photoexcited-state bimolecular processes. Thus, the experimental results are discussed in terms of a significantly modified five-level reverse saturable absorption mechanism.; Optical limiting properties of single-walled and multiple-walled carbon nanotubes toward nanosecond laser pulses at 532 nm were also investigated. When suspended in water, the single-walled and multiple-walled carbon nanotubes exhibit essentially the same optical limiting responses, and the results are also comparable with those of carbon black aqueous suspension. For solubilized carbon nanotubes, where the solubilization is due to a functionalization of the nanotubes with highly soluble polymers, homogeneous solutions of the nanotubes can be prepared for optical limiting measurements. The results show that the optical limiting of the solubilized carbon nanotubes are quite different from those of the carbon nanotube aqueous suspensions. The nanotubes exhibit significantly weaker optical limiting responses in homogeneous solutions than in suspensions. Mechanistic implications of the experimental results are discussed.