1. Nanoscale Quantum Photonics Laboratory, RIKEN Cluster for Pioneering Research, Saitama 351-0198, Japan
2. Quantum Optoelectronics Research Team, RIKEN Center for Advanced Photonics, Saitama 351-0198, Japan
3. National Institute for Materials Science, Ibaraki 305-0044, Japan
4. Department of Materials Engineering, The University of Tokyo, Tokyo 113-8656, Japan