Broad band antireflection coating on silicon from 1.5 to 6 μm spectral band

Silicon is one of the easily available, low cost and stable material transmitting in 1.5-6 μm spectral band. Silicon's optical application includes thermal imaging, motion sensors and forward looking infrared (FLIR) technology. In this work a broad band antireflection coating on silicon substrate for 1.5-6 μm spectral band is reported. In the design, a gradient index approach is used which is a very useful tool to achieve broad band antireflection coating. To realize this gradient index profile, the refractive index is assumed to increase from 1.45 (thorium fluoride chosen as low index material) to 3.42 of silicon in five discrete optimized steps. The design was conceived with thorium fluoride as first layer next to air and each other four gradient layers were replaced by two equivalent layer combination of silicon and thorium fluoride. This nine layer coating deposited by e-beam evaporation system has a total thickness of only 879 nm. Experimental results shows average transmission 96% in 1.5-6 μm region on a 5 mm thick silicon substrate. This coating also has 96.3% transmission in the 1.54 μm eye safe laser and 96.5% average transmission in 3-5 μm wave bands. The coating was environmentally stable, tested as per MIL standard.