1. Beckman Laser Institute, University of California Irvine, Irvine, California 92612, USA
2. China State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, China
3. Department of Biomedical Engineering, University of California Irvine, Irvine, California 92697, USA