Imaging light with electrons

In recent years, notions of the ultrafast, the ultraintense, and the ultrasmall have been recurring themes in physics as those envelopes have been relentlessly pushed to reveal new phenomena. Caltech’s Brett Barwick, David Flannigan, and Ahmed Zewail have combined all three notions into a new technique they dub photon-induced near-field electron microscopy. PINEM exploits the fact that free–free interactions of electrons and photons are greatly enhanced when a third body, like a nanostructure, is present and when the electrons are more energetic than the photons. The physicists illuminated a carbon nanotube with an intense femtosecond laser pulse that generated an evanescent plasmonic field at the CNT’s surface. Simultaneously, a similar-duration pulse of 200-keV electrons from an electron microscope traversed the sample. During the few-hundred-attosecond interaction time, some of those electrons absorbed energy quanta from the 2.4-eV photon field. By selecting only those electrons that gained energy, the researchers could image the evanescent surface field with the spatial resolution of electron microscopy. That field extends about 50 nm into the vacuum from the dark surface of the roughly 150-nm-diameter CNT. As shown in the images, Zewail and colleagues also monitored the temporal decay of the surface field by varying the delay times between the exciting laser pulse and the probing electron pulse, from zero (top) to 400 fs (bottom) and beyond. With tunable and temporally controlled light pulses, PINEM enables visualization of dynamical optical responses of various nanostructures. (B. Barwick, D. J. Flannigan, A. H. Zewail, Nature 462, 902, 2009.) —Stephen Benka