Gap levels of Ti3+ on Nb or Li sites in LiNbO3 :(Mg): Ti crystals and their effect on charge transfer processes

Abstract

The Ti dopant occupying Li or Nb sites and the charge transfer processes induced by thermochemical reduction and optical bleaching treatments have been investigated in LiNbO3 systems using optical absorption and EPR. The Ti3+ centers, built preferentially at Nb sites in heavily Mg-codoped crystals, are shown to have absorption bands at 1.62+/-0.08 eV and 2.65+/-0.25 eV, which are similar or slightly redshifted compared to Ti3+ centers at Li sites in LiNbO3:Ti crystals. The Ti-Nb(4+/3+) gap level plays an important role in the trapping of electron-polarons in LiNbO3, double-doped with Mg and Ti; in particular, an enhanced optical detrapping sensitivity for pumping in the 1.3-2.8 eV range is observed, which may be relevant for applications in integrated optics. Evidence indicating the possible existence of bipolarons involving the Ti dopant is presented.