Novel multi-photon microscopy based on resonant nonlinear optics of colloidal quantum dots

We demonstrate a novel imaging modality based on the detection of resonant four-wave mixing emitted by colloidal quantum dots. This third-order signal is excited and detected at a visible wavelength in resonance with an electronic transition, and results in a significant improvement of the spatial resolution with respect to a twophoton fluorescence microscope. In particular, we have investigated the dependence of the transient reduced absorption of the sample induced by the illumination (photobleaching) on the exposure time and excitation intensity, and its influence to the fourwave mixing signal