^a Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, 1406 West Green Street, Urbana, IL 61801, USA

^b Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, 405 N. Mathews, Urbana, IL 61801, USA

^c Physikalisches Institut, University of Wuerzburg, Am Hubland, Wuerzburg 97074, Germany

^d Department of Physics, Ohio State University, 174 W. 18th Avenue, Columbus, OH 43210, USA