Modification of the spontaneous emission rate of Eu3+ ions close to a thin metal mirror

We have experimentally determined how the spontaneous emission rate of Eu3+ ions depends upon both the distance from, and thickness of, a silver film. We show that as the silver film thickness is reduced (< 100 nm) the well established influence of the mirror on the spontaneous emission rate is further modified. By comparing our data to theory we show that this is due to the availability of an additional decay channel, the surface-plasmon polariton on the far side of the metal film.