[1] Univ Calif Santa Barbara, Dept Phys, Santa Barbara, CA 93106 USA
[2] Univ Calif Santa Barbara, Dept Mech Engn, Santa Barbara, CA 93106 USA
[3] Ecole Polytech Fed Lausanne, Microsyst Space Technol Lab, CH-2002 Neuchatel, Switzerland
[4] Tech Univ Denmark, Dept Micro & Nanotechnol, DK-2800 Lyngby, Denmark
[5] Univ Calif Santa Barbara, Dept Mat, Santa Barbara, CA 93106 USA