A Publication of Indian Society for Radiation and Photochemical Sciences

Metalloporphyrins are promising materials for optical limiting possessing strong nonlinear absorptio~ and high third order-optical nonlinearitiescombined with ultrafast response time in the picosecond and femtosecond time scales. Structural modifications like heavy atom effect and introduction of donor-acceptor groups are known to vary the excited state properties in these molecules. We discuss the effects of these modifications on the optical limiting properties and excited state dynamics in phosphorous(V) tetratolylporphyrin and tin(IV) tetratolylporphyrin. 1.Optical Limiting (OL) and Nonlinear Absorption With the advent of high power laser sources over wide range of wavelengths and pulse widths, protection of sensors and eyes has posed a major challenge in the last decade. In this context optical limiters have received significant attention. Optical limiters are devices that strongly attenuate optical beams at high intensities while exhibiting high transmittance at low intensities, thus providing safety to sensors in general and to human eyes in particular. An ideal limiter exhibits a linear transmission below a threshold and a constant above it. Optical limiting (OL)has been achieved by means of various nonlinear optical mechanisms. Although there are a great variety of processes leading to optical limiting, most of them can be divided into two categories. First one is the energy-spreading type of processes and the other are the energy-absorbing type of processes. The mechanisms like self-focusing, self-defocusing, induced scattering, induced-refraction, induced aberration, and photorefraction result in spreading of the incident energy. The processes like excitedsta:te absorption, two-photon absorption, and free-carrier absorption result in the absorption of the energy [lJ. Among the above mentioned processes nonlinear absorption is very promising for the development of materials for' optical limiting. Nonlinear absorption is defined as an increase / decrease in the absorption coefficient with increasing intensity. The nonlinear increase in absorption coefficient is termed as.reverse saturation of absorption (RSA) while the decrease in absorption coefticient is termed as saturation of absorption (SA). Recent investigations indicate that materials with RSA behavior are 9I1eof the potential candidates for optical limiting applications [2,3] as they give a response that is closer to that of the ideal opticalliiniter. SA in various materials has been studied for applications in laser pulse compression and laser amplification [4]. The optical limiting capability of different materials is characterized by the parameter known as limiting threshold (11/2).Limiting threshold is defined as the input intensity at which the transmittance of the material becomes half of the linear transmittance. An intense laser pulse redis tributes the molecular population between the ground and excited states causing a transient modification in the optical properties of the material. The dependence of transmission in organic molecules on light intensity can yield considerable information on the molecular level system that can lead to variety of nonlinear optical processes like limiting, switching and bistability. Organic materials with delocalized electrons are of a great deal of importance because of their large nonlinear optical absorption coefficients, architectural flexibility, and ease of fabrication. Metalloporphyrins form an important class of electronic materials because of the large n-electron conjugation over two-dimensional molecular structure [5]. In organic molecules, RSA arises mainly due to two different processes named excited state absorption (ESA) and two-photon absorption (TPA). Since the mechanism of limiting in such materials depends on the absorption of excited molecules, it is important to characterize the excited state dynamics and evaluate parameters like excited state absorption cross-sections, lifetimes etc. so as to optimize their properties for the realization of a functional device [6,7]. Structural modifications to the porphyrin ring can be expected to result in molecules