1. Plasma Technology Research Centre, Physics Department, University of Malaya, 50603 Kuala Lumpur, Malaysia
2. Institute of Physics, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile
3. Department of Electrical Engineering, Ruhr University Bochum, Bochum 44780, Germany
4. Department of Physics, West Virginia University, Morgantown, West Virginia 26506, USA