High efficiency and nearly cubic power dependence of below-band-edge photoluminescence in water-soluble, copper-doped ZnSe/ZnS quantum dots.

Three-photon absorption (3PA) and three-photon-excited photoluminescence (3PE-PL) of ZnSe/ZnS and copper-doped ZnSe/ZnS core-shell quantum dots (QDs) in aqueous solutions have been unambiguously determined by Z-scan and 3PE-PL measurements with 200-fs laser pulses at 1000 nm. The 3PA cross-section is as high as 3.5 x 10(-77) cm6 s2 photon(-2) for 4.1 nm-sized, copper-doped ZnSe/ZnS QDs, while their below-band-edge PL is found to be nearly cubic dependent on the excitation intensity, with efficiency enhanced by approximately 20 fold compared to the undoped ZnSe/ZnS QDs.