Temporal characterization of femtosecond electron pulses inside ultrafast scanning electron microscope

In this work, we present the implementation of all-optical method for directly measuring electron pulse duration in an ultrafast scanning electron microscope. Our approach is based on the interaction of electrons with the ponderomotive potential of an optical standing wave and provides a precise in situ technique to characterize femtosecond electron pulses at the interaction region across a wide range of electron energies (1-30 keV). By using single-photon photoemission of electrons by ultraviolet femtosecond laser pulses from a Schottky emitter we achieve electron pulse durations ranging from 0.5 ps at 30 keV to 2.7 ps at 5.5 keV under optimal conditions where Coulomb interactions are negligible. Additionally, we demonstrate that reducing the photon energy of the femtosecond pulses used for photoemission from 4.8 eV (257.5 nm) to 2.4 eV (515 nm) decreases the initial energy spread of emitted electrons, leading to significantly shorter pulse durations, particularly at lower electron energies.

💡 Research Summary

In this work the authors introduce an all‑optical technique for directly measuring the duration of femtosecond electron pulses inside an ultrafast scanning electron microscope (USEM). The method exploits the ponderomotive potential created by an optical standing wave formed by two counter‑propagating 1030 nm laser pulses. When an electron traverses this standing‑wave grating it experiences a force proportional to the spatial gradient of the light intensity, which deflects the electron out of its original trajectory. By recording the transverse velocity component (v⊥/c) of the electrons as a function of the relative arrival time between the electron packet and the optical grating, the temporal profile of the electron pulse can be reconstructed.

Electron packets are generated by single‑photon photoemission from a Schottky nanotip inside a ThermoFisher Versios 5 UC SEM. Two photo‑emission wavelengths are employed: 257.5 nm (4.8 eV) and 515 nm (2.4 eV). The former provides higher photon energy, while the latter reduces the initial kinetic‑energy spread of the emitted electrons. The electron kinetic energy is tunable from 5.5 keV to 30 keV. To avoid space‑charge broadening, the average number of electrons per laser shot is kept below one, and repetition rates of 50 kHz and 500 kHz are used to improve signal‑to‑noise.

Inside the microscope chamber the two 1030 nm pulses are focused to a 10 µm spot 9 mm downstream of the tip, intersecting at an angle of ≈1 mrad to create a high‑contrast intensity grating. Independent optical delay lines control the timing of the electron pulse and the standing‑wave field with sub‑femtosecond precision. The scattered electrons are detected with a Timepix3 hybrid‑pixel detector, which records the spatial distribution of the deflected beam for a series of delay settings.

Data analysis proceeds by selecting electrons whose transverse velocity exceeds a threshold (v⊥/c > 0.002). For each delay the number of such electrons is summed, yielding a cross‑correlation trace that reflects the convolution of the electron pulse shape, the 220 fs laser pulse envelope, and the spatial overlap of the electron with the optical focus. Because the electron pulse durations of interest (0.5–2.7 ps) are significantly longer than both the laser pulse and the transit time through the focus, the trace can be fitted with a Gaussian function without additional angular filtering. The full‑width‑at‑half‑maximum (FWHM) of the fit provides the electron pulse duration. Numerical simulations based on a fifth‑order Runge‑Kutta integration of the relativistic equation of motion corroborate the experimental traces and allow extraction of the dependence on electron energy, laser intensity, and initial energy spread.

The measured pulse durations span from 0.5 ps (30 keV) to 2.7 ps (5.5 keV) under conditions where Coulomb interactions are negligible. When the photo‑emission wavelength is switched from 257.5 nm to 515 nm, the initial energy spread of the electrons is reduced, leading to noticeably shorter pulses at low kinetic energies (up to ~30 % reduction for 5–10 keV electrons). This demonstrates that the photon energy used for photo‑emission is a critical parameter for minimizing temporal broadening, especially in the low‑energy regime where velocity dispersion dominates.

A phenomenological model (Eq. 5) that incorporates the electron kinetic energy, the initial energy spread, and the laser‑induced ponderomotive interaction fits the experimental data across the full energy range. The model predicts that, for a given laser intensity, the pulse duration scales approximately with the square root of the electron kinetic energy, reflecting the reduced velocity spread at higher energies.

Key insights from the study are: (1) Ponderomotive scattering in a standing‑wave optical grating provides a simple, in‑situ, all‑optical probe of electron pulse duration without requiring high‑resolution electron spectrometers; (2) Reducing the photon energy of the photo‑emission laser narrows the initial electron energy distribution, which directly translates into shorter electron pulses, especially at low kinetic energies; (3) Maintaining sub‑single‑electron conditions effectively eliminates space‑charge effects, allowing the measured durations to reflect the intrinsic temporal properties of the source.

The technique opens a pathway for routine, real‑time characterization of electron pulse lengths in USEM setups, facilitating optimization of source parameters for ultrafast imaging, diffraction, and spectroscopy. Moreover, the ability to generate and verify sub‑picosecond electron packets at 1–30 keV energies expands the applicability of ultrafast electron microscopy to surface‑sensitive studies, nanophotonic device probing, and emerging attosecond‑scale electron manipulation schemes.