Silicon and Germanium Suspended Waveguides for the Mid-Infrared

Both silicon and germanium are very interesting materials for mid-infrared applications, particularly sensing, due to their low material losses at long wavelengths. Silicon is low loss up to 8 μm, and germanium up to 15 μm [1]. To fully exploit these long wavelength transmission ranges of Si and Ge, and to build Si and Ge long wavelength photonic circuits, cladding materials need to have the same or larger transmission wavelength ranges than Si or Ge. One solution is to remove a lossy substrate and create Si and Ge suspended structures with air cladding. In the case of SOI, buried oxide can be removed by HF [2,3], and for the Ge-on-Si platform, Si substrate can be removed by TMAH. In this paper we report on the fabrication and characterization of suspended Ge waveguides at a wavelength of 3.8 μm, and on Si waveguides operating at a wavelength of 7.7 μm, both of which are the longest reported wavelengths for the two platforms.