1. Center for Alternative Fuels, Engines and Emissions (CAFEE), Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia 26506-6106, USA
2. Department of Aerospace Engineering, University of Maryland, College Park, Maryland 20742, USA
3. Department of Physics, Umeå University, Umeå 90187, Sweden
4. Department of Applied Physics and Electronics, Umeå University, 901 87 Umeå, Sweden
5. Center for Combustion Energy, Tsinghua University, Beijing 100084, China