1. Physics Department, De La Salle University, 2401 Taft Avenue, Manila, Philippines, 0922
2. Department of Applied Physics, University of Fukui, 3-9-1 Bunkyo, Fukui, Japan, 910-8507
3. Research Center for Development of Far-Infrared Region, University of Fukui, 3-9-1 Bunkyo, Fukui, Japan, 910-8507