Thin film for optical applications, light-emitting structure using the same and the fabrication method thereof are disclosed. The present invention provides a silica or silica-related thin film for optical applications in which silicon nanoclusters and rare earth elements are co-doped. The average size of the silicon nanoclusters is less than 3 nm and the concentration of the rare earth elements is less than 0.1 atomic %. The ratio of the rare earth element concentration to that of silicon nanoclusters is controlled to range from 1 to 10 in the thin film. The thin film emits light by exciting the rare earth elements through electron-hole recombinations in the silicon nanoclusters. According to the present invention, the conditions such as the size and concentration of the silicon nanoclusters, the concentration of the rare earth element, and their concentration ratio are specifically optimized to fabricate optical devices with better performance.