Nonlinear optical Properties of Pentaazadentate Expanded Porphyrins and Application in Optical Limiting
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between the Soret and Q bands in visible region provides a property to allow a high linear transmittance at low laser powers in the ground state of the molecules. It has been established from the dependence of the transmittance on the incident intensity that different nonlinear optical effects occur in the excited states [4-5]. It provides the possibility of utilizing nonlinear abopin r optical
absorption and / or refraction to achieve high
density which limits the amount of the maximum output energy at high laser powers. We also report here the optical limiting based on the reverse saturable absorption (RSA) and nonlinear refraction for ns and continuous laser at different conditions. A composite optical limiter based on the RSA and self-refraction is also performed. EXPERIMENTS Asymmetric pentaazadentate expanded porphyrin metal complexes have been synthesized and characterized [6]. The complexes were dissolved in different solvents to form a series of solutions with concentrations ranging from 10-5 to 10-3 mol/L. The linear absorbances of the samples were measured on a Hitachi model 557 UV-VIS Spectrophotometer. 41 Mat. Res. Soc. Symp. Proc. Vol. 479 01997 Materials Research Society
Third-order nonlinear optical susceptibilities X(3) of each solution were measured by using degenerate four-wave mixing in a phase-conjugate geometry ( DFWM ) as described earlier [7]. Nonlinear absorption measurements were performed with a Continuum Np70 Nd:YAG laser system that consists of a seed-injected, Q-switched oscillator producing 8 ns pulses with repetition rate 10 Hz and a mode-locked oscillator with a single pulse selector producing 23 ps pulses. The average number of pulses is 10 times. The Nd:YAG output beam, whose frequency was doubled to 532 nm, was focused into a 2mm path-length sample cell by a 9 cm focal-length lens. The measured l/e 2 diameters at the focus for the ns and ps beams were 551tm and 571tm, respectively. The laser beam was divided into two beams. One was used to monitor the incident laser energy, the other was focused into the sample cell. Energy of both beam was detected by two Rjp-735 energy detectors. The plots of energy transmittance versus incident fluences were given for compounds I, II, I11and IV with initial linear transmissions of 70%, 81%, 77% and 64% respectively in chloroform which has a negligible nonlinearity.
Nonlinear refraction measurements were performed by the Z-scan technique [8] using the laser system above provide to produce 8 ns pulses at 532 nm, which were divided into two beams. One was used to monitor the variation of incident light intensity, and the other was focused into the sample cell of 2 mm path-length by a 14 cm focal-length lens. Energy of the beam passing the aperture with diameter of 3 mm at the far field and the other monitoring beam were detected by two Rjp-735 energy detectors. The pure refraction normalized Z-scan
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