Spectroscopy of Tm3+-doped CaF2 waveguiding thin films grown by Liquid Phase Epitaxy

Abstract

Thulium-doped calcium fluoride (Tm:CaF2) single-crystalline films are grown on (100)-oriented undoped bulk CaF2 substrates by Liquid Phase Epitaxy (LPE) using LiF as a solvent. Their spectroscopic properties are studied in details. For ~2 at.% Tm-doped films, the active ions are predominantly isolated and are located in oxygen-assisted trigonal sites, C3v(T2). With increasing the Tm doping level, ion clustering is promoted but it is slowed down as compared to bulk crystals. For ~6 at.% Tm-doped films, the majority of active ions form clusters resulting in a “glassy-like” spectroscopic behavior (smooth and broad spectral bands). For such layers, the maximum stimulated-emission cross-section for the 3F4 → 3H6 transition is 0.14 × 10−20 cm2 at 1856 nm, the luminescence lifetime of the 3F4 state is 21.70 ms and the emission bandwidth exceeds 180 nm. The transition probabilities of Tm3+ ions are determined using the Judd-Ofelt theory. The waveguiding properties of the Tm:CaF2 films are confirmed. Highly-doped Tm:CaF2 epitaxial films are promising for waveguide lasers at ~2 μm.