Rare-earth ion doped potassium yttrium double tungstate, RE:KY(WO 4 ) 2 , is a promising candidate for the realization of on-chip lasers and amplifiers. Two major bottlenecks difficult the realization of compact, high-contrast devices. Firstly, the crystal can only be grown on a lattice matched substrate, leading to a low (<2•10 -2 ) refractive index contrast between core and cladding. Secondly, the required thickness for the high-index contrast waveguides, ~1 µm, makes a lapping and polishing

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... pproach very challenging. In this work we propose a novel polishing stop that will permit to accurately control the final thickness of the KY(WO 4 ) 2 waveguide within a few tens of nanometers. A 1 mm thick KY(WO 4 ) 2 substrate is flip-chip bonded with an adhesive layer onto a SiO 2 substrate. Afterwards a low temperature pulsed laser deposited (PLD) Al 2 O 3 layer -with the desired final thickness of the KY(WO 4 ) 2 waveguide core -is deposited on top of the assembly. The sample is then thinned using a multistep lapping and polishing procedure. Earlier work with a polishing stop made from SiO 2 , showed a decrease of the polishing speed with a factor 3-4, allowing the termination of the process within a tolerance of a few tens of nanometers.