Epitaxial Growth of KTiOAsO4 by Pulsed Laser Deposition for Nonlinear Frequency Conversion

Twin-photon and triple-photon generations require pump laser intensities of several hundred MW cm^–2. Micrometer-sized crystalline waveguides allow such interactions to be achieved at relatively low energies by taking advantage of the confinement of light. The materials selected to achieve these phenomena in the present work are from the titanyl phosphate family. The chosen architecture is a micrometric layer of KTiOAsO₄ (KTA) grown on a KTiOPO₄ (KTP) substrate forming a planar waveguide in a first step. These two isostructural materials have a low lattice parameter mismatch (around 2%), making it possible to achieve high-quality epitaxial growth. Due to the refractive index of KTA being higher than that of KTP (Δn ≈ 0.043 at 1500 nm), this couple of materials could allow light to be guided in the deposited layer. To grow the KTA film, the chosen technique is pulsed laser deposition (PLD), which allows a nonlinear complex oxide layer of optical quality to be grown. In this study, we show that the growth is well oriented for layers with a thickness up to 200 nm after deposition under 32 mTorr of oxygen and a laser fluence of 0.8 J cm^–2, forming epitaxial layers. Then, after annealing at 650 °C for 14 h, the crystal state is improved as well as the surface. However, our calculations show that the layer’s thickness has to be equal to 1.68 μm to achieve second-harmonic generation (SHG) at telecom wavelengths, which is the next step of this research. In this study we demonstrate the feasibility of KTA epitaxy over KTP by PLD with good epitaxial quality, good orientation, and theoretical phase-matching conditions for a planar KTA waveguide.