Experimental Demonstration of Germanium-on-Silicon Slot Waveguides at Mid-Infrared Wavelength

We first demonstrated a slot waveguide based on a Ge-on-Si (GOS) platform with a 3 μm thickness of the Ge in the mid-infrared wavelength range at 4.2 μm. We numerically designed the slot waveguide to have a large field confinement in the slot for sensing application. Based on the design, we fabricated the GOS slot waveguide with a slot gap of 200 nm, which is coupled with in-out grating couplers. We characterized the propagation loss of the waveguides by the cut-back method and carefully compared it with channel waveguides. In fundamental TE mode, the propagation loss in the channel waveguide and the slot waveguide were quite similar (5.20 and 4.86 dB/cm, respectively), whereas the field confinement was much higher in the slot waveguides. Additionally, we simulated and analyzed the loss of the devices in terms of radiation, sidewall roughness scattering, material absorption by free-carrier absorption (FCA), and CO2. These results strongly suggest that the fabricated slot waveguide based on the GOS platform would be one of the promising building blocks for the optical gas sensor platform.