Influence of vapor transport equilibration on spectroscopic parameters of Er ³⁺ in LiNbO ₃ crystals

A number of X-, Y- and Z-cut Er(/Yb) : LiNbO ₃ crystals with six different doping levels of rare earth ions were annealed at 1110°C for 120 h using the vapor transport equilibration (VTE) technique. At room temperature, α-polarized or unpolarized absorption spectra of these VTE-treated crystals and corresponding as-grown crystals were recorded in the wavelength range 280-1700 nm. The absorption characteristics of the VTE-treated crystals are summarized and compared with those of the as-grown crystals. The (Li ⁺ + RE ³⁺)/Nb ⁵⁺(RE ³⁺ = Er ³⁺ or Er ³⁺ + Yb ³⁺) ratios in the VTE-treated Er(/Yb) : LiNbO ₃ crystals were evaluated using the optical absorption edge or Raman linewidth of the 153 cm ^-1 E(TO) phonon. Based upon the measured spectral data, spectroscopic parameters of Er ³⁺ ions in as-grown and VTE-treated crystals were calculated using the Judd-Ofelt (J-O) theory. The influences of E ³⁺ doping level, type of crystal cut and VTE treatment on the key spectroscopic parameters, including integrated absorption coefficient, oscillator strength, J-O parameters and radiative lifetimes of three dominant emission manifolds ( ⁴I _13/2 (∼1530 nm), ⁴F _9/2 (∼660 nm) and ⁴S _3/2 (∼560 nm)), are discussed and summarized and compared with the results for as-grown crystals. The results for as-grown crystals are also compared with those reported previously. The present theoretical result concerning the VTE effect on the 1530 nm lifetime is in rough agreement with the experimental result reported previously.