Faculty Candidate TEM Joint Chemistry/Physics Seminar: Ultrafast coherent control of electrons, phonons and plasmons via electromagnetic fields

Controlling the ultrafast evolution of elementary excitations and their interaction in regime of quantum confinement is crucial for understanding the dynamic behaviour of low-dimensional systems under non-equilibrium conditions, which plays a fundamental role in deciphering the mechanism governing their chemical and physical functions. Although an enormous effort has been devoted to the comprehension of these phenomena, the capability of investigating their dynamics is hindered by the difficulty of simultaneously studying their evolution in space and time at the appropriate scales. The traditional characterization techniques and the steady-state theoretical models are both inadequate for describing their non-equilibrium behaviour. Instead, a novel approach for visualizing these phenomena with high temporal and spatial resolution, together with momentum and energy selection, is indispensable to fully exploit their potential.

 

Ultrafast electron microscopy (UEM) has recently been developed with the capability of performing time-resolved imaging, diffraction and electron-spectroscopy [1]. The high scattering cross-section for electron/matter/light interactions, high spatial resolution (down to the atomic scale), ultrafast temporal resolution and high energy selectivity of UEM represent the key elements that make this technique a unique tool for the dynamic investigation of complex phenomena.

 

In this talk, I will address several recent studies highlighting for each case the challenges that were overcome and the main scientific contribution. In particular, ultrafast diffraction, which provides atomic-scale resolution at a femtosecond time scale, is used to unveil the effect of the reduced dimensionality on the non-equilibrium dynamics of optically-excited lattice vibrations (phonons) and their transport regimes [2,3]. Also, by adopting the near-field variant of UEM, called photon-induced near-field electron microscopy (PINEM) [4], I will describe a generalized method for the longitudinal and transverse coherent manipulation of free electrons with attosecond precision by the interaction with an appropriately-synthesized electromagnetic field [5]. 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 energy-momentum states of the electrons. This effect is the result of coherently constructive and destructive phase modulation of the electron wave-function while varying the relative phase between the two driving optical pulses.

 

The highly inter- and multi-disciplinary approach presented here will pave the way for unprecedented insight into the non-equilibrium phenomena of advanced materials, and should play a decisive role in the rational design and engineering of materials for future applications.