Size dependent oscillator strength and quantum efficiency of CdSe quantum dots determined by controlling the local density of states

We study experimentally time-resolved emission of CdSe quantum dots in an environment with a controlled local density of states (LDOS). The decay rate is measured versus frequency and as a function of distance to a mirror. We observe a linear relation between the decay rate and the LDOS, allowing us to determine the size-dependent quantum efficiency and oscillator strength. We find that the quantum efficiency decreases with increasing emission energy mostly due to an increase in nonradiative decay. For the first time, we manage to obtain the oscillator strength of the important class of CdSe quantum dots. The oscillator strength varies weakly with frequency in agreement with behavior of quantum dots in the strong confinement limit. Surprisingly, the measured absolute values are a factor of 5 below theoretically calculated values. Our results are relevant for applications of CdSe quantum dots in spontaneous emission control and cavity quantum electrodynamics.