Cesium-germanium halide salts forming nonlinear optical crystals

摘要

A family of cesium-germanium halide salts have utility as nonlinear optical crystals in applications including electro-optics and optical frequency conversion. These salts have the general formula CsGeCl. sub.x Br.sub.y I.sub.z, in which x, y, and z equal 0 through 3, inclusive, and where x+y+z=3. In contrast to well-known oxygen-containing materials, such as LiNbO.sub.3, these cesium-germanium halide salts exhibit optical nonlinearity with an extremely wide transparency range (without significant absorption) at wavelengths from about 0.4 &mgr;m to greater than 25 &mgr;m (covering the entire MWIR and LWIR regions). Based on Ge. sup.2+ ions, these halides have non-centrosymmetric perovskite structures. The halide ions form a strongly coordinating octahedral environment about the Ge.sup.2+ ions and resist migration in an electric field. The Ge2+ ions, however, are shifted slightly off center in the tetragonal phase, leading to a net electric dipole moment in each unit cell. The alignment of these dipole moments into ferroelectric domains can be controlled through applied electric fields. By aligning all dipole moments into a single ferroelectric domain, these cesium-germanium halides have utility for electro-optic and birefringently phase-matched frequency conversion devices. Alternately, periodic poling of the ferroelectric domains allows these materials to be used for quasi-phase- matched frequency conversion. Cesium-germanium halide salts, which can be isolated from aqueous solutions as salts of GeX.sub.3 .sup.- anions, are water-soluble, which facilitates large-aperture crystal growth.