[1] Univ Calif Santa Barbara, Dept Elect & Comp Engn, Santa Barbara, CA 93106 USA
[2] Univ Calif Santa Barbara, Mat Dept, Santa Barbara, CA 93106 USA
[3] Univ Calif Santa Cruz, Elect Engn Dept, Santa Cruz, CA 95064 USA
[4] Univ Calif Berkeley, Dept Mech Engn, Berkeley, CA 94720 USA