Four-wave-mixing imaging and carrier dynamics of PbS colloidal quantum dots

We demonstrate coherent multiphoton imaging of PbS colloidal quantum dots based on the detection of their transient resonant four-wave-mixing (FWM) emission under 150 fs pulsed laser excitation. Background-free imaging of PbS quantum dots in resonance with their ground-state excitonic absorption at 1.2 mu m wavelength is shown, with intrinsic optical sectioning and a spatial resolution better than the one-photon diffraction limit, promising for deep tissue microscopy. The four-wave-mixing efficiency is found to be resonant to the ground-state absorption maximum, which is a prerequisite for spectral multiplexing. The measured FWM dynamics is consistent with the presence of charged exciton and biexiton Auger recombination in the 10-100 ps time scale, phonon-assisted exciton thermalization in the nanosecond time scale, and excitonic recombination in the microsecond time scale.