“Giant” CdSe/CdS core/shell nanocrystals (NCs) were synthesized with thick CdS shell (15 monolayers), and the x-ray diffraction (XRD) measurement indicates there is a zinc blende phase in the thick CdS shell, whereas it transformed into wurtzite phase under 5 min radiation with a 400 nm, 594 μJ∕cm² femtosecond (fs) laser beam. The evolution of the NCs’ spontaneous emission under the fs laser radiation was recorded with a Hamamatsu streak camera. The as-synthesized NCs exhibit an amplified spontaneous emission (ASE) at 530 nm, which comes from a bulk-like CdS shell due to the interfacial potential barrier, which could slow down the relaxation of holes from the shell to the core. After being annealed by an fs laser, the ASE of the g-NCs is transferred from a bulk-like CdS shell to a quantum-confined CdSe core because the phase transformation determined with the XRD measurement could remove the interfacial barrier. Besides the ASE at 643 nm, two shorter-wavelength ASE peaks at 589 and 541 nm, corresponding to optical transitions of the second (1P) and third (1D) electron quantization shells of the CdSe core, also appear, thus indicating that Auger recombination is effectively suppressed.materials