Multiphoton microscopy based on four-wave mixing of colloidal quantum dots

We demonstrate a novel multiphoton imaging modality based on the detection of four-wave mixing (FWM) from colloidal nanoparticles. This third-order signal is excited and detected in resonance with the ground-state excitonic transition of CdSe/ZnS quantum dots. We measure transversal and axial resolutions of 140 and 590 nm, respectively, significantly improved compared to the one-photon diffraction limit. The coherent FWM signal is detected using a heterodyne technique which rejects incoherent background for improved image contrast compared to fluorescence methods. As sensitivity limit, we estimate a minimum number of 10 dots/√HZ detectable in the focal volume