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Studies of Rb ion-exchanged KTiOPO4 waveguides by x-ray diffraction, SIMS, electron and proton microprobe analysis and comparison with optical refractive index profiles

KTP crystals doped with rubidium (Rb) or similar ions are potentially valuable in the fabrication technology for active optical waveguides and other integrated optical components. The optical transparency, damage threshold, electro-optic coefficients and nonlinear properties suggest that it could become a material of preference. Simple waveguides and domain-reversed devices have already been tested. The composition and structure of the mixed crystals RbxK1-xTiOPO4, formed in the ion-exchanged waveguiding layer, is not well understood. In this paper the penetration depth of Rb and barium (Ba) from a mixed Rb/Ba exchange melt into the crystal is explored by SIMS, and by proton and electron microprobe. Rb is shown to diffuse deep into the crystal to form an exponential or semi-Gaussian concentration and refractive index profiles depending on the processing conditions. Ba is found to facilitate the Rb penetration but could be detected only at the surface of the waveguiding layer at the end of the process. Fabrication of flux-grown mixed crystals RbxK1-xTiOPO4 and comparison with a fully doped Rb:KTP sample of the same composition enabled an important structural difference related to the sites of the Rb ions to be noticed. The use of x-ray techniques has indicated that the ion-exchange process induces considerable strain in the surface layer, possibly with significant implications for active device performance.