Ytterbium doped tantalum pentoxide nanowire waveguide lasers

Abstract

Ytterbium (Yb) has been widely studied in several host materials and device forms due to its broad gain bandwidth, long excited-state lifetime, and simple quasi-three energy level structure. Yb doped tantalum pentoxide (Yb:Ta₂O₅) waveguides are favorable for laser applications since Ta₂O₅ has promising optical properties including high refractive index (≈2.1), wide transmission spectrum (350 nm to 9 μm) and suitability to host rare-earth ions. In this work, a nanowire waveguide design with taper and coupler section was demonstrated. Thin Yb:Ta₂O₅ films were fabricated by plasma assisted reactive magnetron sputtering (PARMS) from high purity (99.999%) Ta metal and Yb metal targets at low temperature (< 70°C). The PARMS process allows Yb doping concentration to be easily adjusted, leading to a new flexible deposition method for not only Yb:Ta₂O₅ but other doped materials. Nanoscale waveguide fabrication processes of strip loaded and buried channel waveguides on SiO₂ using electron beam lithography and inductively couple plasma reactive ion etching (RIE-ICP) were demonstrated and the first Yb:Ta₂O₅ anisotropic RIE-ICP etching process was developed. Buried channel waveguide in polymer fabricated by nano-imprint was also demonstrated, offering low-cost and mass production of nanoscale waveguide. A 6.7 mm long strip loaded Yb:Ta₂O₅ nanowire waveguide laser with 1.25 atomic percentage Yb doping concentration was demonstrated with a laser cavity formed of bare polished end facets only, with 12% Fresnel reflectivity. Single mode lasing was observed between 1015 nm and 1030 nm in strip loaded waveguides with nanowire of 400 nm width when end pumped by a 977 nm laser diode. The launched power threshold and the slope efficiency were measured to be ≈ 12 mW and ≈ 38%. The lasing wavelength of such waveguide can be switched by tuning the pump coupling position at the coupler section, implying a potential tuning mechanism.The results presented in this work demonstrate the capability of Yb:Ta₂O₅ for being a waveguide laser, show the great potential for being used in mass-producible, CMOS compatible integrated optical circuits.

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Identifiers

ORCID for Cornelis De Groot: orcid.org/0000-0002-3850-7101

ORCID for Katrina Morgan: orcid.org/0000-0002-8600-4322

ORCID for Ruomeng Huang: orcid.org/0000-0003-1185-635X

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