1. Semiconductor Physics Department and Institute of Applied Research, Vilnius University, Sauletekio ave. 9-III, LT-10222 Vilnius, Lithuania
2. Baltic Institute of Advanced Technology, Sauletekio ave. 15, LT-10224 Vilnius, Lithuania
3. Institute of Photonics and Optoelectronics, National Taiwan University, No. 1, Sec. 4, Roosevelt rd., Taipei 10617, Taiwan