Critical separation for efficient Tm3+ -Tm3+ energy transfer evidenced in nanostructured Tm3+: Al2O3 thin films.

Nanostructured amorphous Al oxide (a-Al2O3) thin films doped with Tm3+ were synthesized by alternate pulsed laser deposition. The Tm3+ ions have been deposited in layers with in-depth separation ranging from 0.75 to 6 nm. The films show two broad emission bands originated from the Tm3+ 3H4-->3F4 and 3F4-->3H6 transitions. Their intensity increases at a similar rate and the lifetimes are not modified as the layer separation decreases down to 1.5 nm, suggesting that there is no concentration quenching. At the critical value of 1.5 nm the onset of Tm3+ -Tm3+ 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 3F4-->3H6 transition is much higher than that for the 3H4-->3F4 transition, evidencing quenching of the 3H4-->3F4 transition through a cross-relaxation mechanism. The control of the Tm3+ 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.