Department of Applied Physics, Stanford University, CA 94305, USA. mmeena@ntu.edu.sg|Department of Applied Physics, Stanford University, CA 94305, USA. mmeena@ntu.edu.sg|Department of Applied Physics, Stanford University, CA 94305, USA. mmeena@ntu.edu.sg|Department of Applied Physics, Stanford University, CA 94305, USA. mmeena@ntu.edu.sg|Department of Applied Physics, Stanford University, CA 94305, USA. mmeena@ntu.edu.sg