SPIE Proceedings [SPIE 6th International Symposium on Advanced Optical Manufacturing and Testing Technologies (AOMATT 2012) - Xiamen, China (Thursday 26 April 2012)] 6th International Symposium on Advanced Optical Manufacturing and Testing Technologies: Design, Manufacturing, and Testing of Smart Structures, Micro- and Nano-Optical Devices, and Systems - Effect of Er doping on the electronic structure and optical properties of ZnO