Manuscript received December 1, 2007; revised March 2, 2008. This work
was supported in part by the U.S. Department of Energy and in part by the
National Science Foundation under grants.
The authors are with the Department of Physics and Astronomy, University
of California, Los Angeles, CA 90095 USA (e-mail: huangck@ee.ucla.edu).
Digital Object Identifier 10.1109/TPS.2008.924455