Suspended Germanium Waveguide for Infrared Wavelengths

Silicon-on-insulator (SOI) is the dominant platform in near-infrared silicon photonics. However, the high absorption of SiO2 at wavelengths above 4 μm limits the range at which this material can be used in the mid-infrared wavelengths (2 – 20 μm). In this paper we review our recent investigation on suspended devices, building on our previous work based on suspended silicon devices with sub-wavelength grating (SWG) lateral cladding at 3.8 μm, followed by our more recent results at 7.67 μm. This platform which uses the well-known fabrication techniques of SOI, only requires a single lithographic etch step and it allows the use of the whole transparency range of the waveguide core material. Due to this, it could be a good candidate for sensing applications since the design covers a very large wavelength range. As the bottom cladding of the waveguide is air, the waveguide can be designed so that the mode is squeezed to increase the evanescent field proportion and therefore the interaction with a surrounding analyte.