Non-Linear Optical Titanyl Arsenates Crystal Growth and Properties

Doktorsavhandling, 2000

Crystals are appreciated not only for their appearance, but also for their unique physical properties which are utilized by the photonic industry in appliances that we come across every day. An important part of enabling the technical use of optical devices is the manufacture of crystals. This dissertation deals with a specific group of materials called the potassium titanyl phosphate (KTP) family, known for their non-linear optical and ferroelectric properties. The isomorphs vary in their linear optical and dielectric properties, which can be tuned to optimize device performance by forming solid solutions of the different materials. Titanyl arsenates have a wide range of near-infrared transmission which makes them useful for tunable infrared lasers. The isomorphs examined in the present work were primarily RbTiOAsO4 (RTA) and CsTiOAsO4 (CTA) together with the mixtures RbxCs1-xTiOAsO4 (RCTA). Large-scale crystals were grown by top seeding solution growth utilizing a three-zone furnace with excellent temperature control. Sufficiently slow cooling and constant upward lifting produced crystals with large volumes useable for technical applications. Optical quality RTA crystals up to 10 ´ 12 ´ 20 mm were grown. The greater difficulty in obtaining good crystals of CTA led to the use of mixed RCTA materials. The mixing of rubidium and cesium in RCTA is more favorable to crystal growth than the single components in pure RTA and CTA. Mixed crystals are rubidium-enriched and contain only 20 - 30 % of the cesium concentration in the flux. The cesium atoms show a preference for the larger cation site. The network structure is very little affected by the cation substitution; consequently, the non-linear optical properties of the Rb-rich isomorphic mixtures of RTA and CTA can be expected to remain intact. Crystallographic methods utilizing conventional X-ray tubes, synchrotron radiation and neutron diffraction have been employed to investigate the properties of the atomic structures. It was shown that, instead of two cation sites, there is a considerable distribution of the cations over additional sites related by pseudosymmetry. The cation disorder is temperature dependent and seems to be universal for the KTP family. Charge density maps for RTA reflect the anisotropy of non-linear susceptibility, which is greater in the directions with strong locally anti-symmetric components of the residual electron density. Accordingly, the Ti-O and As-O covalent bonds can be related to the linear and non-linear susceptibility of these materials.