Optical limiting performance, third-order nonlinearity χ<sup>(3)</sup>, and nonlinear absorption properties have been investigated in a new class of azoarene phosphorus (V) porphyrins with charge transfer (CT) states. The introduction of axial azoarene groups into the phosphorus porphyrin structure is found to reduce the limiting threshold by a factor of 2 and lead to a rise in the second hyperpolarizability by 1 order of magnitude in the picosecond time regime and by 2 orders of magnitude in the nanosecond regime. The experimental data show reverse saturation of absorption in the nanosecond time regime and a saturation of the nonlinear absorption above a fluence of 0.5 J/cm<sup>2</sup> in the picosecond regime. The presence of the CT state reduces saturation of excited-state absorption (ESA) in the <i>S</i><sub>1</sub> → <i>S</i><sub><i>n</i></sub> transition through the <i>S</i><sub>1</sub> →CT transition. Faster CT → <i>T</i><sub>1</sub> transition increases the ESA from <i>T</i><sub>1</sub> → <i>T</i><sub><i>n</i></sub> states in the nanosecond regime. A self-consistent theoretical analysis based on rate equations is used to estimate the high-lying excited-state lifetimes and absorption cross sections from the experimental results.
© 2002 Optical Society of America
(140.3360) Lasers and laser optics : Laser safety and eye protection
(190.0190) Nonlinear optics : Nonlinear optics
(190.4360) Nonlinear optics : Nonlinear optics, devices
(190.4710) Nonlinear optics : Optical nonlinearities in organic materials
P. Prem Kiran, D. Raghunath Reddy, Bhaskar G. Maiya, Aditya K. Dharmadhikari, G. Ravindra Kumar, and Narayana Rao Desai, "Enhanced optical limiting and nonlinear absorption properties of azoarene-appended phosphorus (V) tetratolylporphyrins," Appl. Opt. 41, 7631-7636 (2002)