The Potassium Titanyl Phosphate Structure Field: A Model for New Nonlinear Optical Materials

Potassium titanyl phosphate (KTP) is unique in its overall qualifications for second-order nonlinear and electrooptic processes with a large hyperpolarizability, an excellent temperature window, a wide wavelength range for phase matching, and outstanding crystal stability. Until recently, only KTP and its arsenic analogue, potassium titanyl arsenate (KTA), had been structurally characterized. The structural properties of KTP make it possible to design and synthesize a large structural field (currently over 40 members) to fine-tune and modify optical properties. An unusual opportunity exists to develop a quantitative model for second-order nonlinear optic (NLO) effects that can handle large structural distortions in extended structures. Current nonlinear susceptibility models concur that the microscopic hyperpolarizability of an octahedral metal center increases with distortion of its geometry and that the Ti=O (titanyl) metal center is primarily responsible for KTP's optical nonlinearity. All models assume that bulk NLO properties can be defined in terms of local structural perturbations involving either the O-Ti-O trans bonds or the TiO6 group, ignoring cation (K+ in KTP) and secondary atom (P or As) contributions. Since the observed optical properties are particularly sensitive to crystal quality, this review focuses on the synthesis and ion-exchange chemistry and then considers chemical modification of the KTP structure and the consequent structural and optical implications in light of the present NLO structure/property models.