Focused-ion-beam nano-structured rib channel waveguides in KY(WO₄)₂ for laser applications

Bulk $KY(WO_4)_2$ (hereafter KYW) laser crystals doped with rare-earth ions are recognized to be among the most promising host materials for obtaining novel solid-state lasers. The rare-earth ions $RE^{3+}$ are easily incorporated in the KYW structure by replacing the $Y{3+}$ ions, resulting in a stoichiometric active material with high absorption and emission cross-sections [1]. The realization of waveguiding structures with such materials is of great interest for future active integrated optics devices. In order to achieve highly confined single-mode propagation and low optical losses, we have developed a new KYW:Yb layer co-doped with $Lu{3+}$ and $Gd{3+}$ions. A special composition of $KLu_{0.253}Gd_{0.13}Yb_{0.017}Y_{0.6}W$ has provided reproducible crack-free layers with a refractive-index contrast as high as 7.5 x $10^{-3}$, which is the highest value reported for KYW:Yb / KYW composite structures. Rib waveguides obtained by reactive ion etching (RIE) are characterized by propagation losses below 1 dB/cm at the $Yb^{3+}$-fluorescence wavelength (λ ≈ 1020 nm) and by a high optical confinement. As a consequence, they offer a great potential for the realization of efficient nano-structured waveguides such as Bragg grating filters or even DBR/DFB (Distributed Bragg Reflection/ Distributed FeedBack) micro-lasers. Here, the first promising results concerning the realization of Bragg filters on KYW by focused ion beam (FIB) etching are reported. This tool is very powerful for producing nano-structures in a large variety of materials. In this work, different thin conductive layers have been tested and a 20-nm-thick aluminum layer has been chosen to reduce charging effects. Then, FIB parameters have been optimised to etch periodical nano-scale patterns with a high aspect ratio at a high speed. The dimensions and the shape of the grating have been fixed by numerical simulations to obtain efficient rib-based Bragg filters at λ ≈ 1020 nm. Such gratings, with a periodicity as low as 250 nm and a high aspect ratio, have been milled on the KYW:Gd,Lu,Yb / KYW ribs. Current work concentrates on the optical characterization and optimization of these first Bragg filters produced in a double tungstate material.