Optimization of off-axis electron holography performed with femtosecond electron pulses.

We report on electron holography experiments performed with femtosecond electron pulses in an ultrafast coherent Transmission Electron Microscope based on a laser-driven cold field emission gun. We first discuss the experimental requirements related to the long acquisition times imposed by the low emission/probe current available in these instruments. The experimental parameters are first optimized and electron holograms are then acquired in vacuum and on a nano-object showing that useful physical properties can nevertheless be extracted from the hologram phase in pulsed condition. Finally, we show that the acquisition of short exposure time holograms assembled in a stack, combined with a computer-assisted shift compensation of usual instabilities encountered in holography, such as beam and biprism wire instabilities, can yield electron holograms acquired with a much better contrast paving the way to ultrafast time-resolved electron holography.

Development of a high brightness ultrafast Transmission Electron Microscope based on a laser-driven cold field emission source.

We report on the development of an ultrafast Transmission Electron Microscope based on a cold field emission source which can operate in either DC or ultrafast mode. Electron emission from a tungsten nanotip is triggered by femtosecond laser pulses which are tightly focused by optical components integrated inside a cold field emission source close to the cathode. The properties of the electron probe (brightness, angular current density, stability) are quantitatively determined. The measured brightness is the largest reported so far for UTEMs. Examples of imaging, diffraction and spectroscopy using ultrashort electron pulses are given. Finally, the potential of this instrument is illustrated by performing electron holography in the off-axis configuration using ultrashort electron pulses.

From patterned optical near-fields to high symmetry acoustic vibrations in gold crystalline platelets.

Noble metal particles allow enhanced interaction with light and efficient light to heat conversion. In the present paper, we report on non-linear optical spectroscopy of individual gold crystalline platelets and address two of the energy relaxation steps following optical excitation of the metallic nano-objects. In particular, at short timescales we show that optical excitation yields intense two-photon photoluminescence at particular locations of the gold platelets. Our experimental results are interpreted with numerical simulations based on the Green Dyadic Method. Subsequent conversion from optical to thermal energy triggers acoustic vibrations that modulate the optical response of the nano-object on a 10 ps-100 ps timescale. We address the different contributions to the damping of the associated mechanical oscillations focusing on the high frequency thickness vibrations (100 GHz) of these nanometer-thin metallic structures.

Charge distribution induced inside complex plasmonic nanoparticles.

We developed a versatile numerical technique to compute the three-dimensional charge distribution inside plasmonic nanoparticles. This method can be easily applied to investigate the charge distribution inside arbitrarily complex plasmonic nanostructures and to identify the nature of the multipolar plasmon modes involved at plasmonic resonances. Its ability to unravel the physical origin of plasmonic spectral features is demonstrated in the case of a single gold nanotriangle and of a gold nano-antenna. Finally, we show how the volume charge distribution can be used to define and compute the first terms of the multipolar expansion.

Acoustic vibration modes and electron-lattice coupling in self-assembled silver nanocolumns.

Using ultrafast spectroscopy, we investigated electron-lattice coupling and acoustic vibrations in self-assembled silver nanocolumns embedded in an amorphous Al2O3 matrix. The measured electron-lattice energy exchange time is smaller in the nanocolumns than in bulk silver, with a value very close to that of isolated nanospheres with comparable surface to volume ratio. Two vibration modes were detected and ascribed to the breathing and extensional mode of the nanocolumns, in agreement with numerical simulations.

Optical control of the coherent acoustic vibration of metal nanoparticles.

Optical control of the coherent breathing vibrations of silver nanospheres is demonstrated using a high-sensitivity femtosecond pump-probe technique in a double-pump pulse configuration. Oscillation of the fundamental mode that usually dominates the time-domain vibrational response can thus be stopped, permitting observation of the first order radial mode and determination of its properties. These are found to be in agreement with the predictions of the model of an elastic sphere embedded in an elastic matrix.

Direct measurement of the single-metal-cluster optical absorption.

The absorption of a single isolated metal cluster is directly measured using a novel far-field optical technique based on modulation of its position. Single gold nanoparticles with average diameters down to 5 nm, dispersed on a transparent substrate, are optically detected and their absolute absorption cross section determined.

Electron-phonon scattering in metal clusters.

Electron-lattice energy exchanges are investigated in gold and silver nanoparticles with sizes ranging from 30 to 2.2 nm embedded in different environments. Femtosecond pump-probe experiments performed in the low-perturbation regime demonstrate a strong increase of the intrinsic electron-phonon interaction for nanoparticles smaller than 10 nm due to a confinement effect.