Microscopic Foundation of the Förster Excitonic Energy Transfer Process

We present a derivation of Förster coupling between two quantum dots by solving the equation of motion for the material excitation simultaneously with Maxwells equations. This allows us to identify the Förster process with the short-range contribution of the coupling that is mediated by the total electric field. The dependence of the coupling on the distance R of the dots is ∝ R—3, i.e. the rates follow the well-known R—6 behaviour. For distances between the sites longer than a quarter of the wavelength of light there is a cross-over to coupling due to the retarded light field, which is ∝ R—1, i.e. the rates are ∝ R—2.