Analysis Of The NIR Laser Gulse

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The visible pulse (gating pulse) enters the microscope, and together with the ultrafast electron pulses illuminate the nanostructure specimen (gold nanoparticle). The visible laser pulse kept at low power (~1.8 mJ/cm2) to avoid saturation. At the spatiotemporal overlapping, the coupling between the visible and electron pulses takes place. The signature of this coupling can be revealed by measuring the electron energy spectrum using the electron energy spectrometer attached to the microscope. The measured electron spectrum is shown in blue line curve in Fig. 2A. The white reddish part of the spectrum represents the ZLP while the shaded blue part shows the gated electron spectrum.
The temporal profile of the “original” ultrafast electron pulse
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The acquired spectrogram is plotted in Fig. 3C. In fact, the information about the gated electron pulse duration is carried on the spectral modulation and the cross-correlation temporal profile of the gated electron pulse, the original electron pulse spectrum (ZLP) and/or the coupling spectrum between these two electron pulses with the NIR laser pulse. Retrieving of the gated pulse by tracing the modulation of the gated electron pulse spectrum (the side peaks around the ZLP plotted in green line in Fig. 3B) will be so difficult since it is always attached to the ZLP spectrum. Therefore, we chose to retrieve this information from the cross-correlation temporal profile of the coupling between these two electron pulses with the NIR laser pulse. In this case, we fitted the electron energy spectrum in the absence of the NIR pulse (τ_NIR=-1000fs ) to each electron energy spectrum of the spectrogram measured at different instants of the NIR pulse arrival time (τ_NIR). The fitted spectra are subtracted from the spectrogram leaving behind the energy spectrum of the coupling between the NIR laser pulse and the two (original and gated) electron pulses. The temporal cross-correlation profile of this coupling can be calculated by integrating each spectrum at different NIR pulse delay (τ_NIR). This cross-correlation temporal profile (average of six scans), is calculated and plotted in Fig.3 D (black connected circles). It carries the information of the gated electron pulse duration (appears as a dip in the cross-correlation profile) on top of the

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