Optical Limiting Processes in Derivatized Fullerenes and Porphyrins/Phthalocyanines
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ABSTRACT We review our results from spectral studies of the ultrafast excited-state absorption in fullerenes and derivatized fullerenes. These results allow determination of both the spectral response of reverse saturable absorption (RSA) nonlinearities such as optical limiting (OL) in fullerenes, and the dynamical response for different morphologies. We have investigated the effects of thin film and various sol-gel glass environments on the nanosecond OL and femtosecond dynamics of derivatized fullerenes. These data provide evidence of decay pathways which compete with the intersystem crossing to a triplet from the initial singlet states. With appropriate processing, however, the OL response of derivatized-fullerene sol-gel glasses can be enhanced to approach that of the same molecule in solution, while significantly enhancing the optical damage threshold. The optical limiting of these derivatized fullerenes is compared with that of various porphyrin and phthalocyanine molecules. INTRODUCTION Optical power limiting (OL) occurs when the optical transmission of a material decreases with increasing laser fluence [1]. This property is desirable for providing protection to optical sensors (such as human eyes) from the high fluence output of modern pulsed lasers. One mechanism for obtaining OL is reverse saturable absorption (RSA), which occurs when excited states formed through optical pumping of the ground state have a higher absorption cross-section than the ground state [1, 2]. The fullerene C 60 has demonstrated broadband optical limiting [2, 3, 4] due to broad singlet and triplet excited-state absorptions that extend throughout the visible and near infrared, in regions where the ground-state absorption cross-section (ae) is small.[3, 5, 6, 7] Limiting for nanosecond and longer pulses has been attributed to rapid (650 ps)[6, 8] intersystem crossing to a triplet excited-state which has an absorption cross-section, cIT > c0 over a broad wavelength range and a lifetime of microseconds[7] to milliseconds[6] depending on the exposure to quenchers such as oxygen. [7] Selective derivatization across a bond joining two hexagons in the C 60 molecule (to form a 6,6 fullerene adduct) allows these materials to be optimized for solubility and consequent ease of processing [9]. In fact, sol-gel glasses with a wide range of linear transmissions [10, 11] can be obtained due to enhanced solubility. This 6,6 derivatization also extends the OL further into the infrared as a consequence of additional ground-state absorption at long wavelengths. [3, 12] To extend the maximum laser pulse energy that the optical limiter can sustain before catastrophic optical damage as well as to improve the ease of incorporating the limiter into an optical system, many researchers have sought to disperse limiting materials into a solid-state matrix. 110, 11, 13] However, early studies [10, 12] showed that the induced absorptions have much faster decay dynamics in the solid state. Consequently, the optical limiting response of fullerenes i
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