1. State Key Laboratory for Infrared PhysicsShanghai Institute of Technical PhysicsChinese Academy of Sciences 500 Yu‐tian Road Shanghai 200083 China
2. Department of PhysicsNational Cheng Kung University 1 Ta‐Hsueh Road Tainan 70101 Taiwan
3. Department of Physical and Chemical SciencesUniversity of L'Aquila via Vetoio L'Aquila (AQ) 67100 Italy
4. CNR‐IMM Istituto per la Microelettronica e MicrosistemiChinese Academy of Sciences VIII strada 5 Catania I‐95121 Italy