Imaging A Nanoscale Electromagnetic Field

A new imaging technique can probe evanescent electromagnetic fields induced around nanostructures, reports Brett Barwick, David J. Flannigan, and Ahmed H. Zewail of California Institute of Technology (Nature 2009, 462, 902). Understanding and manipulating this phenomenon is important for developing applications such as optical sensing and plasmonics. The researchers split a train of femtosecond laser pulses into two arms, one of which was used to excite a carbon nanotube or silver nanowire and generate a light field around the structure. The other arm was used to generate electron packets for an ultrafast electron microscope, which was also trained on the nanostructures. When the electrons interacted with the light field, some of them absorbed photons, gaining energy. By tracking the higher energy electrons, the researchers were able to generate a “snapshot” of the light field created around the nanostructure, with subnanometer spatial and femtosecond time resolution. Barwick and colleagues call the new approach photon-induced near-field electron microscopy (PINEM).