Enhanced reverse saturable absorption and optical limiting in heavy-atom- substituted phthalocyanines

The reverse saturable absorption and the optical-limiting response of metal phthalocyanines can be enhanced by use of the heavy-atom effect. Phthalocyanines containing heavy-metal atoms, such as In, Sn, and Pb, show a nearly factor-of-2 enhancement in the ratio of effective excited-state to ground-state absorption cross sections compared with those containing lighter atoms, such as Al and Si. In an f/8 optical geometry, homogeneous solutions of heavy-metal phthalocyanines, at 30 % linear transmission, limit 8-ns 532-nm laser pulses to ' 3,uJ (the energy for 50 % probability of eye damage) for incident energies as high as 800,IJ. Materials that exhibit reverse saturable absorption (RSA) are currently of interest for use in optical-limiting devices for protection of sensors and eyes from energetic light pulses. RSA can occur when states with an absorption cross section (ire) in excess of the ground-state cross section (ag) are produced. RSA in the visible spectrum that is due to elec-tronic excitation was reported for several classes of organic chromophores.'17 The effectiveness of RS