Joint Physics Dept - MtSE Seminar

February 25th, Monday

When electrons meet light: Attosecond coherent control of matter waves for low-dimensional photonics

Dr. Giovanni Vanacore

Institute of Physics, Laboratory for Ultrafast Microscopy and Electron Scattering,

Ecole Polytechnique Federal de Lausanne, Switzerland

(Experimental Condensed Matter/Materials Physics, Host: Federici)

Time: 11:45 am - 12:45 pm with 11:30 am tea time

Room: ECE 202

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 light-assisted quantum devices and diagnostics at extremely small timescales, such as those governing intramolecular electronic motions and nuclear processes.

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 attosecond precision.

A relativistic pulsed electron beam was made to interact with an appropriately synthesized electromagnetic field. The field was generated either by a sequence of two fs laser pulses reflected at the surface of a mirror (semi-infinite field), or by the coherent superposition of the surface plasmon polaritons (SPPs) optically-generated from nanofabricated structures (near field). The energy-momentum exchange resulting from the electron-field interaction was directly mapped via momentum-resolved ultrafast electron energy-loss spectroscopy. When the two phase-locked light pulses were delayed by fractions of the optical cycle, we observed coherent oscillations in the electrons energy-momentum states. This effect is the result of coherent constructive and destructive phase modulation of the electron wave function while varying the relative phase between the two driving optical pulses.

In addition, our method offers the possibility to manipulate the phase-controlled interaction of the electrons with both a semi-infinite light field and a plasmon polariton propagating on a plasmonic waveguide. Here, I will describe the case of SPPs generated at the edge of a circular nanocavity carved in a Ag layer deposited on a Si3N4 thin film, and demonstrate that in the case of circularly-polarized illumination the resulting near-field distribution transiently creates a vortex plasmon carrying a welldefined orbital angular momentum (OAM), which can be efficiently transferred to the interacting electrons as a result of the coherent interaction.

The potential of our approach for longitudinal and transverse phase modulation at the attosecond 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.