title: Long-lived population inversion in isovalently doped quantum dots.
abstract: Optical gain from colloidal quantum dots has been desired for several decades since their discovery. While gain from multiexcitations is by now well-established, nonradiative Auger recombination limits the lifetime of such population inversion in quantum dots. CdSe cores isovalently doped by one to few Te atoms capped with rod-shaped CdS are examined as a candidate system for enhanced stimulated emission properties. Emission depletion spectroscopy shows a behavior characteristic of 3-level gain systems in these quantum dots. This implies complete removal of the 2-fold degeneracy of the lowest energy electronic excitation due to the large repulsive exciton-exciton interaction in the doubly excited state. Using emission depletion measurements of the trap-associated emission from poorly passivated CdS quantum dots, we show that 3-level characteristics are typical of emission resulting from a band edge to trap state transition, but reveal subtle differences between the two systems. These results allow for unprecedented observation of long-lived population inversion from singly excited quantum dots.
authors.affiliation: Department of Physics of Complex Systems, Weizmann Institute of Science , Rehovot 7610001, Israel.
authors.forename: Ohr
authors.initials: O
authors.lastname: Lahad
authors.forename: Noga
authors.initials: N
authors.lastname: Meir
authors.forename: Iddo
authors.initials: I
authors.lastname: Pinkas
authors.forename: Dan
authors.initials: D
authors.lastname: Oron
keywords: isovalent doping; optical gain; quantum dots; stimulated emission; trap emission