Determination of cross-relaxation efficiency based on spectroscopy in thulium-doped rare-earth sesquioxides

Abstract

Tm3+ doped cubic rare-earth sesquioxides have been recognized as excellent laser materials operating at ∼ 2 μm. Upon 3H4 excitation by commercially available laser diode around 800 nm, a cross-relaxation (CR) process enables efficient Tm3+ laser originated from 3F4→3H6 transition. The CR process, (Tm1:3H4, Tm2:3H6) → (Tm1:3F4, Tm2: 3F4), cuts one excitation photon into two photons of 2 μm with quantum efficiency of 200%. As a result, efficient CR is needed. The CR efficiency is usually determined based on the emission lifetime of 3H4. However, the CR between the nearest Tm3+ is too fast to emit, overrating the emission lifetime and underrating the CR efficiency. Here, we propose a spectroscopic method for determination of CR efficiency based on emission intensity ratio. Tm3+ doped cubic Y2O3, Lu2O3, Sc2O3, YScO3, LuScO3 and YLuO3 sesquioxides are prepared by solid-state reaction. Near infrared emission spectra and decay curves are studied as a function of Tm3+ concentration. The relationship between CR efficiency and emission intensity ratio is established. Large Stark splittings of 1002 cm−1 and 511 cm−1 are observed for the 3H6 ground state and the 3F4 excited state of Tm3+, respectively in Sc2O3:Tm3+, leading to a long-wave emission at 2.15 μm. The comparison of these six samples reveals that Sc2O3: Tm3+ has the largest CR parameter and Lu2O3:Tm3+ has the highest quantum efficiency for populating the 3F4 from the 3H4 at the same Tm3+ concentration.