Low-loss germanium strip waveguides on silicon for the mid-infrared

TLDR

Mid‑infrared photonics on silicon requires low‑loss integrated waveguides, yet experimental realization of monocrystalline germanium waveguides on silicon has not been reported. The authors demonstrate a germanium strip waveguide fabricated on a silicon substrate. The waveguide is engineered for single‑mode TM transmission at 5.8 µm from quantum‑cascade lasers and its propagation loss was quantified using the Fabry–Perot resonance method. The device achieves a minimum propagation loss of 2.5 dB cm⁻¹, with a 90° bend loss of 0.12 dB at a 115 µm radius.

Abstract

Mid-infrared photonics in silicon needs low-loss integrated waveguides. While monocrystalline germanium waveguides on silicon have been proposed, experimental realization has not been reported. Here we demonstrate a germanium strip waveguide on a silicon substrate. It is designed for single mode transmission of light in transverse magnetic (TM) polarization generated from quantum cascade lasers at a wavelength of 5.8 μm. The propagation losses were measured with the Fabry–Perot resonance method. The lowest achieved propagation loss is 2.5 dB/cm, while the bending loss is measured to be 0.12 dB for a 90° bend with a radius of 115 μm.