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Thursday seminars - Phase-shaped ultrafast transmission electron microscopy for nano-optics

Thursday, April 21 2022, 11.00 am, CEMES (coffee served at 10.30 am), + Zoom

by Hugo Lourenço-Martins
CNRS researcher at CEMES, Toulouse

 

 

Zoom link :
https://cnrs.zoom.us/j/94504560688?pwd=b0g1QUo1eldvUTQ2djMza1ZwQ2orUT09

 

Electron energy-loss spectroscopy (EELS) in a transmission electron microscope (TEM) is a powerful technique to probe optical and electronic excitations with a sub-nanometer spatial resolution. However, probing spontaneous losses, this technique : (1) remains intrinsically unable to detect phase-related properties (e.g., dichroism), (2) does not provide time-domain access to ultrafast processes and (3) is restricted in its spectral resolution even for the most advanced electron sources available (10-100 meV).

Here, recent developments in the field of ultrafast transmission electron microscopy (UTEM) promise to overcome these limitations by probing laser-excited optical modes with femtosecond electron pulses. Specifically, in a stroboscopic laser-pump/electron probe scheme, an optically excited sample is measured with photo-emitted ultrashort electron wave-packets [3] and scanning their relative time delay gives access to the involved ultrafast dynamics. Such an instrument thus combines the spatial resolution of a conventional TEM (nm) with the unrivaled spectral and temporal resolutions provided by ultrafast lasers (resp. sub-meV and hundreds of femtoseconds), and therefore offers unique capabilities to probe optical fields at the nanoscale.

In this seminar, I will demonstrate how this technique - called photon-induced near-field electron microscopy (PINEM) - can be used to analyze the optical response of individual micro- and nano-resonators directly at the nanoscale and with a femtosecond resolution. I will first show that combining phase-shaping techniques applied to the laser or the electron beam enable to extract phase related quantities in PINEM experiments.

Then, I will present our boundary element method (BEM)-based data analysis which enable us to extract from optical near-field maps, the magnitude and relative phase of each modes of a plasmon resonator excited by the femtosecond pump laser, as illustrated on the figure.

Finally, I will theoretically and experimentally demonstrate that the plasmonic optical near-field can be coherently manipulated by pumping the system with two phase-locked optical pulses of different wavelength, which enables to create a complex beating pattern between two different plasmonic modes in the same single nano-resonator.