Stoichiometry and Domain Structure of KTP-Type Nonlinear Optical Crystals

Abstract

In recent years the growth technologies of only a few inorganic oxide crystals, such as BBO (BaB 2 O 4), LBO (LiB 3 O 5), and the KTP (KTiOPO 4) group of isomorphic compounds, have matured to a degree allowing their extensive integration into commercial laser systems in the form of nonlin-ear optical (NLO) and electrooptic (EO) devices. The KTP-type crystals are ferroelectrics at room temperature. They are also well known for their large birefringence, high NLO and EO coefficients, wide acceptance angles, thermally stable phase-matching properties, and relatively high damage threshold. These properties make them especially useful for high-power wavelength-conversion applications, such as the second-harmonic generation (SHG) and optical parametric oscillations (OPO), as well as for electrooptic phase modulation and Q-switching. Lately, a great deal of effort has been put into the development of periodically poled KTP (PPKTP) devices based on quasi-phase-matched (QPM) wavelength conversion. However, both birefringent and QPM properties of KTP crystals depend on their structural characteristics, such as morphology, chemical composition, point defect distribution (stoichiometric and impurities), and particularly the ferroelectric domain structure, which are closely related to the specific crystal growth parameters. Current research includes studies of nonstoichiometry and distribution of point defects, e.g., vacancies and impurities, as well as the basic mechanisms underlying ferroelectric domain formation during KTP crystal growth and cool-down. By controlling the stoichiometry and achieving single-domain growth of bulk crystals it is also possible to create as-grown periodic domain structures useful for nonlinear QPM applications. 20.1 Background Potassium titanyl phosphate (KTP) belongs to the family of isomorphic compounds with generic composition of MTiOXO 4 , where X = {P or As} and M = {K, Rb, Tl, NH 4 or Cs (for X = As only)} [20.1]. All crystals of this family, including their solid solutions, are orthorhombic and belong to the noncentrosymmetric point group mm2