Abstract

Polydiacetylenes (PDA) are model materials as quasi one-dimensional semiconductors. Their optical properties, which have been extensively investigated in the past years, display large nonlinearities associated to the π electron delocalization along the backbone [1]. The dynamics of the PDA excitons has been studied under various experimental configurations, and has allowed to determine the relaxation associated to the exciton self-trapping mechanism [2-4]. The dephasing properties of the excitons has also been measured using incoherent wave-mixing [5] and coherent photon echo techniques [6]. A determining mechanism for the coherent relaxation processes in PDA systems is the influence of the molecular motion on the electronic delocalization. The most important contribution comes from the vibrations of the backbone which are strongly coupled to the excitonic wavefunction. Recently it has been shown that quantum beats with 23 fs period, associated to the C=C vibration modes, appear during the polarization dephasing process [6]. The aim of the present paper is to study the wave-packet dynamics associated to this backbone coherent motion, using ultrashort optical pulses. The wave-packet motion is probed in the pump-probe configuration as reported in the case of molecular systems [7].

© 1996 Optical Society of America