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Register a new userWatching charge separation in nanoantennas by ultrafast point-projection electron microscopy
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Watching the motion of electrons on their natural nanometre length- and femtosecond time scales is a fundamental goal and an open challenge of contemporary ultrafast science. Optical techniques and electron microscopy currently mostly provide either ultrahigh temporal or spatial resolution, yet, microscopy techniques with combined space-time resolution need further development. Here we create an ultrafast electron source by plasmon nanofocusing on a sharp gold taper and implement this source in an ultrafast point-projection electron microscope. This source is used, in an optical pump - electron probe experiment, to study ultrafast photoemission from a nanometer-sized plasmonic antenna. We show that the real space motion of the photoemitted electrons and residual holes in the metal is probed with 20-nm spatial resolution and 25-fs time resolution. This is a step forward towards time-resolved microscopy of electronic motion in nanostructures.
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The ultrafast response of metals to light is governed by intriguing non-equilibrium dynamics involving the interplay of excited electrons and phonons. The coupling between them gives rise to nonlinear diffusion behavior on ultrashort timescales. Here, we use scanning ultrafast thermo-modulation microscopy to image the spatio-temporal hot-electron diffusion in a thin gold film. By tracking local transient reflectivity with 20 nm and 0.25 ps resolution, we reveal two distinct diffusion regimes, consisting of an initial rapid diffusion during the first few picoseconds after optical excitation, followed by about 100-fold slower diffusion at longer times. We simulate the thermo-optical response of the gold film with a comprehensive three-dimensional model, and identify the two regimes as hot-electron and phonon-limited thermal diffusion, respectively.
Ultrafast Electron Microscopy (UEM) has been demonstrated to be an effective table-top technique for imaging the temporally-evolving dynamics of matter with subparticle spatial resolution on the time scale of atomic motion. However, imaging the faster motion of electron dynamics in real time has remained beyond reach. Here, we demonstrate more than an order of magnitude (16 times) enhancement in the typical temporal resolution of UEM by generating isolated 30 fs electron pulses, accelerated at 200 keV, via the optical-gating approach, with sufficient intensity for efficiently probing the electronic dynamics of matter. Moreover, we investigate the feasibility of attosecond optical gating to generate isolated subfemtosecond electron pulses, attaining the desired temporal resolution in electron microscopy for establishing the Attomicroscopy to allow the imaging of electron motion in the act.
Femtosecond electron microscopy produces real-space images of matter in a series of ultrafast snapshots. Pulses of electrons self-disperse under space-charge broadening, so without compression, the ideal operation mode is a single electron per pulse. Here, we demonstrate for the first time femtosecond single-electron point projection microscopy (fs-ePPM) in a laser-pump fs-e-probe configuration. The electrons have an energy of only 150 eV and take tens of picoseconds to propagate to the object under study. Nonetheless, we achieve a temporal resolution with a standard deviation of 120 fs, combined with a spatial resolution of 100 nm, applied to a localized region of charge at the apex of a nanoscale metal tip induced by 30 fs 800 nm laser pulses at 50 kHz. These observations demonstrate real-space imaging of reversible processes such as tracking charge distributions is feasible whilst maintaining femtosecond resolution. Our findings could find application as a characterization method, which, depending on geometry could resolve tens of femtoseconds and tens of nanometres. Dynamically imaging electric and magnetic fields and charge distributions on sub-micron length scales opens new avenues of ultrafast dynamics. Furthermore, through the use of active compression, such pulses are an ideal seed for few-femtosecond to attosecond imaging applications which will access sub-optical cycle processes in nanoplasmonics.
In the quest for dynamic multimodal probing of a materials structure and functionality, it is critical to be able to quantify the chemical state on the atomic and nanoscale using element specific electronic and structurally sensitive tools such as electron energy loss spectroscopy (EELS). Ultrafast EELF, with combined energy, time, and spatial resolution in a transmission electron microscope, has recently enabled transformative studies of photo excited nanostructure evolution and mapping of evanescent electromagnetic fields. This article aims to describe the state of the art experimental techniques in this emerging field and its major uses and future applications.
Point Projection Microscopy (PPM) is used to image suspended graphene using low-energy electrons (100-200eV). Because of the low energies used, the graphene is neither damaged or contaminated by the electron beam. The transparency of graphene is measured to be 74%, equivalent to electron transmission through a sheet as thick as twice the covalent radius of sp^2-bonded carbon. Also observed is rippling in the structure of the suspended graphene, with a wavelength of approximately 26 nm. The interference of the electron beam due to the diffraction off the edge of a graphene knife edge is observed and used to calculate a virtual source size of 4.7 +/- 0.6 Angstroms for the electron emitter. It is demonstrated that graphene can be used as both anode and substrate in PPM in order to avoid distortions due to strong field gradients around nano-scale objects. Graphene can be used to image objects suspended on the sheet using PPM, and in the future, electron holography.
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