Sub-nanometric Au nanoclusters are known to act as very efficient sensitizers for the luminescent emission of Er³⁺ ions in silica through a non-resonant broad-band energy-transfer mechanism. In the present work the energy-transfer process is investigated in detail by room temperature photoluminescence characterization of Er and Au co-implanted silica systems in which a different degree of coupling between Er³⁺ ions and Au nanoclusters is obtained. The results allow us to definitely demonstrate the short-range nature of the interaction in agreement with non-radiative energy-transfer mechanisms. Moreover, an upper limit to the interaction length is also set by the Au–Au intercluster semi-distance which is smaller than 2.4 nm in the present case.