A single slot micro-ring structure for simultaneous CO2 and CH4 gas sensing Weijia Zhang, Xin Zhang, Xiaowei Zhang, Han Jin, Qinghui Jin and Jiawen Jian Eur. Phys. J. Appl. Phys., 82 3 (2018) 30502 Published online: 10 October 2018 DOI: 10.1051/epjap/2018180093