Site-selective spectroscopy of Er3+: Ti : LiNbO3 waveguides

Starting from previous investigations in LiNbO3 bulk crystals, we studied t he optical properties of Er3+ ions in Ti:LiNbO3 channel waveguides and inve stigated the waveguide-specific lattice environment of the Er3+ ions ("site s") caused by the doping method used and the presence of a large number of Ti4+ ions. For that purpose the method of combined excitation-emission spec troscopy was applied for the first time to waveguides at low temperatures. Comparing the spectroscopic results obtained for the green, red, and near-I R luminescence (lambda approximate to 550, approximate to 650 and approxima te to 980 nm) under direct (450 nm), 2-step (980 nm), and 3-step (1.5 mum) laser excitation, we found several distinguishable Er3+ sites which in term s of energy levels and relative numbers are similar to those in bulk materi al, but exhibit significantly different upconversion efficiencies and stron gly inhomogeneously broadened transitions. Moreover, we were able to distin guish isolated and cluster Er3+ sites by their characteristic excitation an d emission transition energies and studied the respective excitation/relaxa tion channels. The cluster sites are most efficient in the up-conversion pr ocess, especially under 3-step excitation. Using accepted microscopic model s for Er3+ and Ti4+ incorporation into the LiNbO3 crystal lattice, the site distribution and up-conversion mechanisms are elucidated and their consequ ences for laser applications in different spectral regions are discussed.