Photon-induced near-field electron microscopy (PINEM): theoretical and experimental
Electron imaging in space and time is achieved in microscopy with timed (near relativistic) electron packets of picometer wavelength coincident with light pulses of femtosecond duration. The photons (with an energy of a few electronvolts) are used to impulsively heat or excite the specimen so that the evolution of structures from their nonequilibrium state can be followed in real time. As such, and at relatively low fluences, there is no interaction between the electrons and the photons; certainly that is the case in vacuum because energy–momentum conservation is not possible. In the presence of nanostructures and at higher fluences, energy–momentum conservation is possible and the electron packet can either gain or lose light quanta. Recently, it was reported that, when only electrons with gained energy are filtered, near-field imaging enables the visualization of nanoscale particles and interfaces with enhanced contrast (Barwick et al 2009 Nature 462 902). To explore a variety of applications, it is important to express, through analytical formulation, the key parameters involved in this photon-induced near-field electron microscopy (PINEM) and to predict the associated phenomena of, e.g., forty-photon absorption by the electron packet. In this paper, we give an account of the theoretical and experimental results of PINEM. In particular, the time-dependent quantum solution for ultrafast electron packets in the nanostructure scattered electromagnetic (near) field is solved in the high kinetic energy limit to obtain the evolution of the incident electron packet into a superposition of discrete momentum wavelets. The characteristic length and time scales of the halo of electron–photon coupling are discussed in the framework of Rayleigh and Mie scatterings, providing the dependence of the PINEM effect on size, polarization, material and spatiotemporal localization. We also provide a simple classical description that is based on features of plasmonics. A major part of this paper is devoted to the comparisons between the theoretical results and the recently obtained experimental findings about the imaging of materials and biological systems.
Additional Information© 2010 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. Received 15 September 2010; Published 17 December 2010. This work was supported by the National Science Foundation and the Air Force Office of Scientific Research in the Center for Physical Biology funded by the Gordon and Betty Moore Foundation.We are indebted to Professor Archie Howie for his genuine interest in our work and for numerous stimulating discussions with AHZ. In one of the correspondences on the subject, he sent us prior to publication his independent work on the analytical analysis of the energy loss/gain processes, which will be published in a special proceeding issue honoring Professor C Colliex. We are grateful to Professor García de Abajo for sending his publications and for his interest in our work [1, 21, 22, 47], and we appreciate the helpful discussions with Professors John Spence and Harry Atwater, and Dr Eyal Feigenbaum. The experimental effort of Drs Brett Barwick and David Flannigan of this laboratory stimulated this theoretical work, and we have benefited from numerous discussions on PINEM.
Published - Park2010p12457New_J._Phys.pdf