Amplified spontaneous emission (ASE) in dye-doped mesostructured silica films, deposited directly over fused silica substrates, is demonstrated with low thresholds. Due to functionalization of the silica network with phenyl groups, the refractive index of the active layer is sufficiently increased to impart waveguiding properties that lead to highly efficient ASE. Such functionalization eliminates the need for a separate low-refractive-index layer between the active film and the substrate, as is often used to achieve waveguiding properties with silica mesostructured materials, though which is typically accompanied by significant optical losses that reduce overall ASE performance. By forming a high-quality waveguide without the need for a separate low-refractive-index layer, significantly improved properties are obtained, with ASE thresholds as low as 8 kW cm−2. Solid-state 2D NMR spectroscopy measurements establish the compositions and structures of the active mesostructured film at a molecular level, in particular that phenyl functionalization of the mesochannel surfaces promotes the homoge- neous dispersion of the laser dye species over nanoscale dimensions in the materials.