Silicon-germanium alloy for infrared GRIN materials

Silicon (n~3.4) and germanium (n~4.0) are harmless and conventional infrared optical materials. SiGe alloys, mixed crystals of Si and Ge, permit a flexible Si/Ge ratio in the crystals, which results in a refractive-index gradient. We report on the Travelling Liquidus-Zone (TLZ) method for growing both gradient and homogeneous concentration SiGe crystals. The wavelength dependences of the refractive indices from 2.0 to 20 μm of the 20 mm diameter Si1-XGeX (x=0.32 and 0.63), Si and Ge disks were measured using an ellipsometry. An infrared lens system made of germanium and homogeneous concentration SiGe crystals has been designed.