Germanium has the highest index of refraction among commonly used infrared transmitting materials . It  is a very popular material for systems operating in the 3-5um or 8-12um spectral region. Germanium has nearly the highest density of the infrared transmitting material and this should be taken into account when  designing  for weight-restricted  systems.  Ge is subject to thermal runaway,  which  means that the hotter it gets,  the more  the  absorption  increases.  We produce Germanium optics as mirror,lens,window,etc