The attempt for achieving optical gain from colloidal QDs has been a subject for extensive study in the past two decades. Previous studies in this field have only shown population inversion which was limited for short time scales (typically less than 2ns). Also, in many cases the involvement of multiexcitons in the process was required, due to the two-fold degeneracy of the lowest energy electronic excitation.

We studied two quantum dots systems, the first is of Te-doped CdSe/CdS nanorods and the second is of poorly-passivated CdS nanocrystals, both of which exhibit strong trap states-related emission in room temperature. Using emission depletion measurements, we showed these quantum dots behave as a 3-level system, as was indicated by a 50% threshold for population inversion. Moreover, we were able to measure for the first time population inversion with a lifetime proportional to the radiative lifetime (tens of nanoseconds).