Nanostructured amorphous Al oxide $\left(a\text{-}{\mathrm{Al}}_2{\mathrm{O}}_3\right)$ thin films doped with ${\mathrm{Tm}}^{3+}$ were synthesized by alternate pulsed laser deposition. The ${\mathrm{Tm}}^{3+}$ ions have been deposited in layers with in-depth separation ranging from 0.75 to $6\mathrm{nm}$ . The films show two broad emission bands originated from the ${\mathrm{Tm}}^{3+}{}^3\mathrm{H}_4\to {}^3\mathrm{F}_4$ and ${}^3\mathrm{F}_4\to {}^3\mathrm{H}_6$ transitions. Their intensity increases at a similar rate and the lifetimes are not modified as the layer separation decreases down to $1.5\mathrm{nm}$ , suggesting that there is no concentration quenching. At the critical value of $1.5\mathrm{nm}$ the onset of ${\mathrm{Tm}}^{3+}–{\mathrm{Tm}}^{3+}$ energy transfer is evidenced by a sharp decrease of the emission intensity and lifetime. Below this critical value, the rate at which the intensity increases for the ${}^3\mathrm{F}_4\to {}^3\mathrm{H}_6$ transition is much higher than that for the ${}^3\mathrm{H}_4\to {}^3\mathrm{F}_4$ transition, evidencing quenching of the ${}^3\mathrm{H}_4\to {}^3\mathrm{F}_4$ transition through a cross-relaxation mechanism. The control of the ${\mathrm{Tm}}^{3+}$ ions in the nanometer scale allows evidencing the onset of energy transfer processes among ions and offers a route to optimize compact photonic gain integrated devices.