Nonlinear single-crystal fibers of undoped or Nd3+-doped niobates: growth by LHPG, spectroscopy and second harmonic generation

The renewal of second order non-linear crystals is very strong in laser materials optics. We are involved in the search of new systems based upon the highly non-linear niobate crystal family. Among these crystals, Ba2NaNb5O15 (BNN) and K3Li2-xNb5+xO15+2x (KLN, 0.15 < x < 0.5) with a tungsten bronze-type structure are very efficient materials for the obtention of, respectively, green and blue laser light by frequency doubling or by self-frequency doubling after doping by Nd3+ ions. Unfortunately, BNN and KLN are difficult to grow by conventional techniques such as Czochralski without cracks caused by a strong lattice change occurring during cooling of the crystals. In addition, microtwins in BNN crystals and compositional inhomogeneities in KLN ones are often encountered. The obtention of good quality and crackless BNN and KLN single crystals doped with different amounts of Nd2O3 is reported. The crystals were grown by the laser heated pedestal growth (LHPG) technique. This method allowed to obtain good quality and homogeneous crystals of BNN and KLN overcoming the problems encountered with other growth techniques. More particularly, for BNN crystals, a structural transition between orthorhombic (space group: Pba2) and tetragonal symmetry (space group: P4bm) was found to occur for a Nd3+ amount between 2 and 3 at% avoiding the microtwinning which disturbs the optical properties of the crystals. Low temperature spectroscopy revealed that Nd3+ ions substitute both Ba2+ and Na+ ions in the pentagonal and square sites of the tungsten bronze-type structure of BNN whereas they substitute only K+ ions in the pentagonal sites of that of KLN. Non-critical phase matching at room temperature was also investigated for both materials. (C) 1997 Elsevier Science B.V.