We present recent results on the occurrence of an Er fluorescence sensitization induced by ultra-small Au clusters (NC) incorporated in Er-doped silica by a low fluence ion implantation; it is shown that the energy transfer from the metal cluster to the Er ions critically depends on the evolution of the Au metal clustering process, starting from dispersed metal atoms up to the formation of nanometer-sized clusters. To investigate sub-nanometric Au structures and dopant ions (Au, Er) dispersed into the matrix, extended X-ray absorption fine structure spectroscopy has been used because, by measuring the local site around the dopant (Er, Au) independently of any long-range order considerations, it can detect both few-atom clusters and the possible presence of dopant dispersed into the matrix. This investigation allowed us to directly follow the early stages of Au nucleation, where clusters nearly have a the critical radius (i.e. aggregates formed by two to three atoms), and to relate the Au NC size with the Er3+ photoluminescence (PL) enhancement. The defect distribution and radiation damage recovery have been investigated using positron annihilation spectroscopy; the effect of the radiation damage recovery on the Er site and PL is also reported.