Germanium is a semiconductor in group IVA of the periodic table. It has an atomic number of 32, an atomic weight of 72.59, and a density of 5.32 Mg/m 3. Its melting temperature is 937.4 C. Germanium ...

So silicon dioxide based glass has a very different permittivity function than silicon metal. Silicon like Germanium is somewhat transparent in SWIR, but mostly opaque elsewhere.