Engineering pump-probe schemes with short fs pulses has made feasible to imprint and probe electronic coherence in nanosystems. The first steps of the response to short fs laser pulses often consist in ultrafast coherent energy and charge migration. The understanding and control of these first steps open the way to the development of efficient novel materials for photovoltaics and information processing applications. We report on coherent energy transfer and charge migration using quantum dynamical studies of two kinds of nanosystems made of several subunits: semi-conducting CdSe quantum dot (QD) dimers and arrays linked by short dithiol molecules and small gold clusters functionalized with a chromophore. To effectively induce coherent energy transfer and charge migration between the nanosystem subunits, the laser pulse needs to access electronic states of a mixed local and charge transfer (CT) character with enough local character to be effectively optically excited and enough CT character to initiate energy and charge transfer to neighboring subunits. We showed[1] that this is the case in small stoichiometric gold Au11 and Au20 clusters functionalized by a bipyridine ligand when photoexcited by a few fs UV pulse. Quantum dynamical simulations are carried by numerically solving the time-dependent Schroedinger equation (TDSE) for a large band of excited electronic states computed at the TDDT level. We show that up to 0.4 |e| charge can be transferred from the chromophoric subunit to the gold core during the pulse. When the pulse is over, the charge is coherently beating between the two subunits around the transferred average value. We next report on the coherent dynamics in CdSe QD dimers and arrays for which the situation is quantitatively different because the optically active local electronic excited states do not exhibit enough charge transfer character for a significant transfer to occur during a few fs laser pulse. For this larger system, we model the quantum dynamics by solving the TDSE for a semi-empirical excitonic model Hamiltonians. We show that efficient coherent transfer can be engineered by tailoring QD dimers[2]. We show that coherent charge and energy transfer occur after the pulse is over, either by a dipole-dipole coupling between two excitons localized on different dots, or by Coulombic coupling between local and CT excitonic states.

Ultrafast coherent energy and charge transfer in nanosystems by fs laser pulses: A quantum dynamical study of CdSe QD dimers and of functionalized gold clusters

2019 Spring National Meeting

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