Size dependent oscillator strength of CdSe quantum dots determined by nanophotonic control

Abstract

Control over spontaneous emission of quantum dots is important for many applications, especially as light sources in photonic materials and nanostructures. Colloidal CdSe quantum dots have recently generated enormous interest because of the tunability of their emission energy with particle diameter over the entire visible range. The strength of the interaction of a quantum dot with the light field is determined by the oscillator strength, making the oscillator strength a crucial parameter for successful inhibition and enhancement of spontaneous emission. This paper presents measurements of the oscillator strength and quantum efficiency of ensembles of colloidal CdSe quantum dots as a function of emission energy or equivalently quantum dot size. It is the first time the oscillator strength is determined from emission, giving highly accurate values that can be used to test theories. The total decay rate is measured as a function of normalised LDOS for different sizes of quantum dots in our ensemble. A linear relation between total decay rate and LDOS is found, as expected from Fermi's golden rule. The total decay rate increases with emission energy, in agreement with recent literature. This increase is mostly due to an increase in nonradiative decay rate. The oscillator strength is nearly independent on emission energy suggesting that the quantum dots are in the strong confinement regime.