Approaches for Optimizing and Tuning the Optical Limiting Response of Phthalocyanine Complexes
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Mat. Res. Soc. Symp. Proc. Vol. 374 @1995 Materials Research Society
In this paper we highlight some recent results from our laboratories aimed at optimizing and tuning the optical limiting response of phthalocyanine complexes. We extend the heavy-central-metal approach to naphthalocyanine complexes and show that it can be effective in enhancing the optical limiting of these extended ring molecules. We have been investigating approaches for red-shifting the optical limiting response of phthalocyanine based materials into the red and near-infrared spectral region. We show below that the extended ring size and donor substitution in octaalkoxynaphthalocyanine complexes results in strong optical limiting response in the red region of the spectrum. Another objective of our work has been aimed at the optimal use of the optical limiting capabilities of phthalocyanines. In a preliminary demonstration of the "bottleneck" limiter concept, we show that an appropriately designed multiple plate limiter, that permits pumping of a large excited-state population throughout the interaction region prior to damage, provides a large enhancement in pulse suppression, without an increase in linear absorbance, when compared to a single homogeneous nonlinear absorber element. OPTIMIZING AND TUNING PHTHALOCYANINES FOR OPTICAL LIMITING Enhanced Limiting in Heavy-Metal Naphthalocyanine Complexes In recent work, 3 it was shown that the use of central metal atoms of increased atomic number in phthalocyanine complexes leads to an increase in the effective excited-state absorption cross section and the optical limiting response, for nanosecond duration, 532 nm laser pulses. For example, molecules containing Sn, Pb, or In were shown to be more effective than those containing Si or Al. This enhancement results from two characteristics of the photophysics of phthalocyanine molecules: 1) at 532 nm, the absorption cross section for the triplettriplet absorption is typically larger than that of the excited singlet-singlet absorption that occurs promptly upon excitation; and 2) an increase of the central metal atomic number leads to an increase in the S1 to T1 intersystem crossing rate, thus an increase in the triplet population during the pulse, resulting from the increase in the spin-orbit coupling for ring orbitals due to mixing with the metal (the heavy-atom effect). An additional benefit of utilizing the triplet state is that the typically long (10's to 100's of microseconds) triplet lifetimes permit efficient limiting of comparably long duration pulses. It was also shown in earlier work 4 that an extended ring phthalocyanine derivative, silicon bis(tri-n-hexylsiloxy) naphthalocyanine (SiNc(OSiHex3) 2) exhibited stronger optical limiting at 532 nm than chloroaluminumphthalocyanine. Hence we were motivated to investigate whether the heavy-atom approach could also be used to enhance the limiting of naphthalocyanine complexes. To explore the utility of the heavy-atom approach to enhanced limiting in naphthalocyanines we prepared a series of co
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