Study of resonant nonlinearities with time-resolved fluorescence using picosecond train of pulses

The Z-scan technique has been used to study the nonlinearities of a variety of photosensitive materials. In resonant condition, the excited states determine the nonlinear properties. Using a Qswitched and modelocked Nd:YAG laser, that delivery trains with about 20 strong modelocked pulses with 70ps duration spaced by 13ns at 532 nm, we are able to resolve the dynamic of several nonlinear effects such as saturable and reverse saturable absorption. Here, looking at the fluorescence, we have mapped the first singlet excited state dynamic. A five or three-energy-level model were applied to describe the dynamic of these effects. The absorption cross-section, lifetimes of excited states and intersystem crossing rate could be obtained. Introduction The Z-scan technique has been used to study the nonlinear properties in a variety of photosensitive materials. Traditionally, this technique is used extensively to study refractive and absorptive nonlinearities. In resonant condition, the excited states determine the nonlinear properties. Recently, we have introduced a simple experimental technique to study the refractive and the absorcive nonlinear dynamics with pulse train [1, 2]. By this technique, we are able to determine several important parameters from the nonlinear sample such as cross-sections and lifetime dynamics of the excited states. Since several interesting nonlinear materials presents fluorescence, here we also extended our pulse train method to map the excited state dynamics looking at the fluorescence behavior. Materials and Methods There are several materials with interesting nonlinear effects that can be used in devices. The performance of these devices relies on the type and the magnitude of these nonlinearities. For example, there are materials were the nonlinear effects suffer saturation. In this case, there is two type of opposite behavior: one presents a saturation of the absorption (SA) and other a reverse saturation of the absorption (RSA). These nonlinear effects play important role in several devices such as in optical limiters, switches and memory, for example. Using a Q-switched and modelocked Nd:YAG laser, that delivery trains with about 20 strong pulses with 70ps duration spaced by 13ns at 532nm, we can resolve the dynamics of resonant nonlinear effects in the picosecond and nanosecond time scale. Several resonant nonlinear material exhibits long excited states lifetime. Indeed, a cumulative nonlinearity can take place in a sample if the lifetime of one excited state, responsible for one nonlinear process, is longer than the separation of the excitation pulses. Looking at the fluorescence we also can observe a cumulative effect if any slow or dark (non-fluorescent) state take place in the transition process during the pulse train interaction. We have observed that pulse train fluorescence (PTF) can be used as complementary measurements with absorptive and refractive ones to obtain the dynamic of nonlinear effects due to their high sensitivity and low noise. To demonstrate this new method, experiments have been carried out on a fluorescent porphyrin (PPhs) solution. This molecule presents excited singlet and triplet states. Porphyrins are macro-cyclic aromatic molecules having four pyrrole rings occupying position at four corners of square and connected by a unsaturated bridge. To assure structure stability, the PPh ring presents a central part constituted by either two protons or a metal ion, M2+. The porphyrins belong to a class of important material for their potential XXVI ENFMC Annals of Optics Volume5 2003 application in photo-dynamic therapy (PDT), for example. Considerable effort has been directed towards creating new efficient porphyrins for PDT application. These molecule presents excited singlet and triplet states. The triplet state play the most important role to the mechanism of action of drugs base on porphyrins molecules. A five-energy-level model were applied to describe the dynamic of the effects.