Dr. Giovanni Maria Vanacore, University of Milano-Bicocca
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Abstract: The interaction between light and electrons can be exploited for generating radiation, such as in synchrotrons and free electron lasers, or for controlling electron beams for the dynamical investigation of materials and molecules. Using electromagnetic fields, the coherent control of an electron wave function can be pushed to unexplored timescales, enabling new applications in lightassisted quantum devices and diagnostics at extremely small timescales. In this contribution, I will describe a novel method for the coherent longitudinal and transverse phase manipulation of a free-electron wave function. Using appropriately synthesized optical light fields I will demonstrate how to modulate the energy, linear momentum and orbital angular momentum (vorticity) of the electron wave function with sub-fs precision. A relativistic pulsed electron beam was made to interact with an appropriately synthesized electromagnetic field. The energy-momentum exchange resulting from the electron-field interaction was directly mapped via momentum-resolved ultrafast electron energy-loss spectroscopy in an ultrafast transmission electron microscope (UEM). First, I will show how it is possible to coherently manipulate the longitudinal phase of a free-electron wave function. We used a semi-infinite light field configuration composed of a sequence of two mutually phase-locked light pulses impinging on a mirror synchronous with the electron pulse and delayed in time by fractions of the optical cycle [1]. Then, I will demonstrate that also the transverse electron’s phase profile can be efficiently manipulated when using localized fields coupled to lightinduced collective electronic modes (surface plasmon polaritons, SPPs). To show this effect, we have generated an ultrafast vortex electron beam by means of a spatially-confined optical field carrying Orbital Angular Momentum (OAM) as generated by the excitation of chiral SPPs [2]. The formation of chiral plasmons relies on the spin-to-OAM conversion from circularly-polarized light in non-paraxial scattering from a nanoscale cavity. The quantized inelastic coupling between a free-electron and a chiral SPP is thus responsible for an efficient transfer of a nonzero topological charge and helical phase distribution from the near-field to the electron wave function. Finally, the ability to access the phase profile of low-energy quantized plasmonic excitations directly resulted from the development of a new ultrafast time-domain holographic imaging technique [3]. With this method we were able to capture attosecond/nanometer-resolved phase-sensitive movies of rapidly evolving localized electromagnetic fields in plasmonic structures. The potential of our approach for longitudinal and transverse phase modulation at the sub-fs timescale – and below – should pave the way to achieve unprecedented insights into non-equilibrium phenomena in advanced quantum materials, and should play a decisive role in the rational design and engineering of future photonics and electronics applications.
Zoom Link: https://arizona.zoom.us/j/5716629626
Please contact Karina Valdez at kvaldez@email.arizona.edu for Zoom password.