CC BY 4.0 UnportedGallacher, K.Ortolani, M.Rew, K.Ciano, C.Baldassarre, L.Virgilio, M.Scalari, G.Faist, J.Gaspare, L.D.I.Seta, M.D.E.Capellini, G.Grange, T.Birner, S.Paul, D.J.2021-11-302021-11-302020https://oa.tib.eu/renate/handle/123456789/7561https://doi.org/10.34657/6608The waveguide losses from a range of surface plasmon and double metal waveguides for Ge/Si1-xGex THz quantum cascade laser gain media are investigated at 4.79 THz (62.6 µm wavelength). Double metal waveguides demonstrate lower losses than surface plasmonic guiding with minimum losses for a 10 µm thick active gain region with silver metal of 21 cm-1 at 300 K reducing to 14.5 cm-1 at 10 K. Losses for silicon foundry compatible metals including Al and Cu are also provided for comparison and to provide a guide for gain requirements to enable lasers to be fabricated in commercial silicon foundries. To allow these losses to be calculated for a range of designs, the complex refractive index of a range of nominally undoped Si1-xGex with x = 0.7, 0.8 and 0.9 and doped Ge heterolayers were extracted from Fourier transform infrared spectroscopy measurements between 0.1 and 10 THz and from 300 K down to 10 K. The results demonstrate losses comparable to similar designs of GaAs/AlGaAs quantum cascade laser plasmon waveguides indicating that a gain threshold of 15.1 cm-1 and 23.8 cm-1 are required to produce a 4.79 THz Ge/SiGe THz laser at 10 K and 300 K, respectively, for 2 mm long double metal waveguide quantum cascade lasers with facet coatings. © 2020 OSA - The Optical Society. All rights reserved.enghttps://creativecommons.org/licenses/by/4.0/530waveguide lossesdouble metal waveguidessilicon foundry compatible metalArticle