P.O. Box 217, 7500 AE Enschede, The Netherlands. Rare-earth-ion doped materials, such as the monoclinic potassium double tungstates, are recognized as.
Focused-ion-beam Nanostructured Photonic Cavities for Integrated Lasers in Crystalline Double Tungstate Channel Waveguides F. Ay, D. Geskus, I. Iñurrategui, S. Aravazhi, and M. Pollnau Integrated Optical MicroSystems Group, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
Rare-earth-ion doped materials, such as the monoclinic potassium double tungstates, are recognized as important candidates for applications in active photonics, providing high absorption and emission cross sections, especially when doped with Yb3+. Recently, laser emission has been obtained in KGdxLu1-x(WO4)2 (or KGLW) with slope efficiencies up to 82.3% [1] in planar and 71% [2] in channel waveguides. In this work, we report utilization and optimization of the focused ion beam (FIB) milling technique for fabrication of grating structures to obtain integrated active photonic devices. The ridge waveguides in the KGLW:Yb3+ layers were aligned along the Ng optical axis of the material to make use of the large emission cross-section of the active ions when operated in TE polarization. The active film thickness was 1.92 µm, etched down by a depth of 1.2 µm using Ar+ milling [3]. The resulting ridge waveguides had widths varying between 5 and 8 µm. Finally, a 1-µm-thick Si3N4 film was deposited by plasma-enhanced chemical vapour deposition as a top cladding layer to increase the overlap of the optical mode with the active waveguide region. A procedure for FIB milling of deep grating structures in KGLW:Yb3+ was developed by optimizing the distribution of the pixels’ location and variation of dwell time along each grating period. Re-deposition effects were significantly reduced and grating structures more than 4 µm in depth with an improved total sidewall angle of about 5º were achieved [4], see Fig. 1 (a) and (b). Gratings with a period of 0.89 µm and a total length of 4.48 µm were defined for testing at wavelengths around 0.98 µm.
(a) (b) (c) Fig. 1 (a) Integrated Fabry-Pérot microcavity for optical investigation realized by optimized FIB milling; (b) SEM image of the grating structure forming the Fabry-Pérot cavity; (c) measured (dots) and modelled (line) performance of laser cavity.
The integrated cavities designed for laser operation consisted of a single grating on one side, while on the other side a polished waveguide facet was FIB-milled. The total cavity length was 4.5 mm. The active waveguide cavity was pumped via the polished facet by a Ti:Sapphire laser tuned to 932 nm in TE polarization. Lasing was observed for the cavity involving 12-µm-wide grating structures. The slope efficiency of the laser was 33% versus launched pump power, see Fig. 1 (c). Modelling of the laser emission from on-chip cavities was performed with a spatially resolved, quasi-three-dimensional steady-state rate-equation model. The theoretical model was fitted to the measured results to obtain the relevant parameters. For both lasers the reflectivity of the FIB-polished in-coupling facet was estimated to be 9%. The reflectivity of the grating structure of the laser is estimated to be 40%. FIB milling of deep grating structures in KGLW:Yb3+ has been optimized to achieve the first on-chip integrated laser in crystalline potassium double tungstate. An integrated waveguide laser with 33% slope efficiency was demonstrated in KGLW:Yb3+. Further performance increase of these on-chip waveguide lasers is possible by better mode confinement and laterally extended gratings and polished end-facets. References [1] D. Geskus, S. Aravazhi, E. Bernhardi, C. Grivas, S. Harkema, K. Hametner, D. Günther, K. Wörhoff, and M. Pollnau, “Low-threshold, highly efficient Gd3+, Lu3+ co-doped KY(WO4)2:Yb3+ planar waveguide lasers,” Laser Phys. Lett. 6, 800-805 (2009). 2 D. Geskus, S. Aravazhi, K. Wörhoff, and M. Pollnau, “High-power, broadly tunable, and low-quantum-defect KGd1-xLux(WO4)2:Yb3+ channel waveguide lasers,” Opt. Express 18, 26107-26112 (2010). [3] D. Geskus, S. Aravazhi, C. Grivas, K. Wörhoff, and M. Pollnau, “Microstructured KY(WO4)2:Gd3+, Lu3+, Yb3+ channel waveguide laser”, Opt. Express 18, 8853-8858 (2010). 4 I. Iñurrategi, F. Ay, D. Geskus, S. Aravazhi, V. Gadgil, K. Wörhoff, and M. Pollnau, “Bragg gratings in crystalline waveguides fabricated by focused ion beam milling,” in Proceedings of the 2nd International Workshop on FIB for Photonics, 8-9 (2010).
