Threshold and Linear Dispersion of the Plasma Resonance in Thin Ag Films

The plasma resonance in thin continuous Ag films has been studied by real-time optical spectroscopy. We show that the plasma resonance can be excited by light only after the third monolayer starts growing. Moreover, we demonstrate a linear blue-shift of the resonance as a function of the inverse of the film thickness. The observed blue-shift is reproduced by use of a recent microscopic model.