Long-lived electronic polarization and nonlinear optical effects of fluorescent molecules in solution

Summary form only given. Preparation and probing of coherent superposition of multiple quantum states leading to the generation of quantum coherences in condensed phase systems has been the topic of intense research for over a decade and is one of the most interesting and least understood of the quantum phenomena. Evolution of ultrashort broadband excitation sources has over the years provided us with new tools for revisiting problems in every field pertaining to light matter interaction. One such problem is the formation of electronic coherence through the reversible non-radiative electronic coupling and superposition of electronic states. Understanding the role of electronic coherence in condensed phase systems is central to elucidating the mechanism of energy transfer in photosynthetic pigment protein complexes [1] and several tubular nanostructures [2]. The recent observation of long-lived electronic coherence in chemical and biological system at room temperatures [3] forces one to reconsider the existing theories on energy transfer and devise an efficient and smart way in designing of synthetic light harvesting systems.Here we explore the linear and nonlinear optical response of solvated fluorescent molecules under saturation conditions by studying the changes in fluorescence and stimulated emission upon interacting with a pulse pair generated by pulse shaper assisted phase and amplitude modulation. In particular we focus on the observation of long lived oscillations that are detected in the stimulated emission signal during an interferometric measurement. Interferometric time-delay scans are measured while simultaneously monitoring the fundamental, fluorescence and stimulated emission, as shown in Fig. 1a. The pump pulse is fixed at time zero while the probe pulse is scanned. All signals are normalized to unity at time zero when the system is being interrogated by a single transform-limited pulse. The early portion of the scan (for IJ <; 60 fs) has strong- interferometric modulation caused by the linear optical interference between the laser pulses. The different asymptotic levels of fluorescence and stimulated emission are due to the nonlinear effect of strong field excitation. The most interesting feature is the observed long lived oscillations in the stimulated emission signal which persists beyond ~150 fs. The observed π phase shift of the delayed modulation indicates that the physical source of the observed long lived oscillations is most likely caused by the induced linear polarization of the medium induced by the electronic coherence between ground and excited state. A simulation (using phenomenological formulas) of the stimulated emission showing the saturation, appearance and phase shift of the signal is shown in Fig 1b.