1. Unité Mixte de Physique, CNRS-Thales, 1 Av. Augustin Fresnel, Campus de l'Ecole Polytechnique, 91120 Palaiseau, France and Université Paris-Sud, 91405 Orsay, France
2. Physics Department and Institute for Nanoscience and Engineering, University of Arkansas, Fayetteville, Arkansas 72701, USA
3. Laboratory of Dielectric Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China