One- and two-photon induced QD-based energy transfer and the influence of multiple QD excitations.

Due to the increased use of quantum dots (QDs) in diverse laser microscopies, it is interesting to study the excitation pump power and excitation wavelength dependence of QD-based energy transfer (ET) processes. The ET in QD conjugates with phthalocyanines (Pcs) was studied with femtosecond time-resolved pump-probe spectroscopy upon one- and two-photon excitation. At the used excitation wavelengths only the QDs are excited and become the energy donors. Due to the matched spectral overlap of QD photoluminescence and Pc absorption, the ET occurs on a picosecond time scale. The ET process shows strong pump power dependence whereby an increase in excitation power results in multiple QD excitations and in shorter excited state lifetimes on the QDs due to Auger relaxation. As a result, high excitation pump power leads also to an accelerated ET to the acceptor molecules from the initially multiply excited states of the QDs. Excited state quenching studies as function of pump power suggest that ET occurs mainly from the lowest one-exciton state (n = 1) and only to a minor extent from the multiply excited states (n > 1). For the short-lived, multiply excited states the ET competes inefficiently with Auger recombinations and energy transfer efficiencies of phi(ET)(n=1>) approximately 20%, phi(ET)(n=2>) approximately 7%, phi(ET)(n=3>) < or = 2% were obtained. Also after two-photon excitation the ET efficiency is highest from the one-exciton state. The experimentally determined ET efficiencies were compared with theoretical ET efficiencies upon multiple excitations. In both cases the ET efficiency decreases with the increase in excitation pump power.