🔬 optics

Temporal characterization of femtosecond electron pulses inside ultrafast scanning electron microscope

This paper presents an all-optical, in situ method for characterizing femtosecond electron pulses in an ultrafast scanning electron microscope, demonstrating that pulse durations ranging from 0.5 to 2.7 ps can be achieved and further optimized by reducing the photoemission photon energy to minimize initial electron energy spread.

Original authors: Kamila Moriová, Petr Koutenský, Marius Constantin Chirita Mihaila, Martin Kozák

Published 2026-02-24

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Kamila Moriová, Petr Koutenský, Marius Constantin Chirita Mihaila, Martin Kozák

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to take a photograph of a hummingbird's wings in mid-flap. If your camera shutter is too slow, you'll just get a blurry mess. To see the details, you need a shutter that opens and closes incredibly fast.

In the world of science, Ultrafast Electron Microscopes (UEMs) are like super-powered cameras that use tiny particles called electrons instead of light to take pictures of atoms and molecules. These microscopes are used to film chemical reactions and material changes happening in "slow motion" (but actually, they are happening in quadrillionths of a second).

However, there's a catch: How fast is the shutter?

If the "electron shutter" (the pulse of electrons) is too long, the image blurs. For years, scientists in these microscopes had to guess how fast their shutter was, or they had to use complex, expensive equipment that didn't work well with the specific type of microscope they were using.

This paper by Kamila Moriová and her team is like inventing a new, simple way to measure exactly how fast that shutter is, right inside the microscope, without needing to take the machine apart.

The Problem: The "Blurry" Electron

Think of an electron pulse like a crowd of runners starting a race.

The Start: They all start at the same time.
The Race: As they run, the faster ones pull ahead, and the slower ones lag behind. Also, because they all have the same negative charge, they push each other away (like magnets repelling), spreading the crowd out even more.
The Result: By the time they reach the finish line (the sample), the "packet" of runners has stretched out. If the packet is too long, the picture is blurry.

Scientists need to know exactly how long this "packet" is to know how sharp their pictures will be.

The Solution: The "Optical Gate"

The team developed a clever trick using light to measure the electron pulse.

Imagine the electron runners are running through a tunnel. The scientists set up a giant, invisible "light gate" across the tunnel. This gate isn't a solid wall; it's a standing wave of laser light (like a ripple in a pond that isn't moving).

Here is the magic:

When the electron runners hit this light gate, they get a tiny push (a "nudge") from the light, causing them to swerve slightly to the side.
The scientists can control exactly when the light gate opens and closes.

The Experiment:
They send the electron runners through the gate at different times relative to the light.

If the light gate is open when the whole electron packet is there: The whole packet gets nudged, and they all swerve.
If the light gate is open for only a split second: Only the electrons that happen to be there at that exact moment get nudged.

By watching how many electrons get nudged at different times, the scientists can reconstruct the shape of the electron packet. It's like shining a flashlight through a foggy window at different angles to figure out exactly how thick the fog is.

The Big Discovery: Tuning the "Gun"

The paper also found a way to make the electron runners start out more organized.

They used a special "gun" (a Schottky emitter) to shoot the electrons. They realized that the color of the laser light used to fire the gun mattered a lot.

The Old Way (Blue/UV Light): This gave the electrons a lot of extra energy, like kicking a soccer ball with a sledgehammer. The ball flies off at all different speeds, causing the packet to spread out quickly.
The New Way (Green/Red Light): They switched to a lower-energy laser (515 nm instead of 257.5 nm). This is like gently tapping the ball. The electrons come out with much more uniform speeds.

The Result:
By using this gentler tap, the electron packet stayed tight and didn't spread out as much.

At high speeds (30 keV), they achieved a pulse duration of 0.5 picoseconds (half a trillionth of a second).
At lower speeds, they got 2.7 picoseconds.
Most importantly, by changing the laser color, they cut the "spread" in half, making the microscope capable of taking much sharper, faster "movies" of the atomic world.

Why This Matters

Before this, if you wanted to film a chemical reaction at the atomic level, you had to guess if your camera was fast enough. Now, scientists can:

Measure the speed of their electron pulse directly inside the machine.
Tune the machine to make the pulses shorter and sharper.
See faster things: This allows us to watch things like electrons jumping between atoms or molecules changing shape in real-time, which is crucial for designing better solar cells, faster computers, and new medicines.

In short, they built a speedometer for the world's fastest camera, and they figured out how to make the camera even faster by adjusting the lens.

1. Problem Statement

Ultrafast Electron Microscopy (UEM) is a critical tool for visualizing dynamic processes at atomic scales with high temporal resolution. However, achieving high time resolution requires generating short electron pulses and, crucially, accurately measuring their duration in situ.

The Gap: While Ultrafast Transmission Electron Microscopes (UTEMs) utilize high-resolution spectrometers to measure pulse durations via electron-light interactions (e.g., Photon-Induced Near-field Electron Microscopy or PINEM), these techniques are difficult to implement in Ultrafast Scanning Electron Microscopes (USEMs).
The Challenge: USEMs operate at lower electron energies (1–30 keV) and do not transmit electrons through a sample, making inelastic scattering techniques (like EELS) ineffective. Consequently, pulse durations in USEMs have historically been estimated via theoretical predictions rather than direct measurement. Furthermore, existing direct measurement techniques often require complex modifications to the microscope or are limited to specific energy ranges.

2. Methodology

The authors implemented an all-optical method for the direct temporal characterization of femtosecond electron pulses within a commercial USEM (ThermoFischer Verios 5 UC).

Experimental Setup:
- Electron Source: A Schottky field emitter tip illuminated by ultraviolet (257.5 nm, 4.8 eV) or visible (515 nm, 2.4 eV) femtosecond laser pulses to trigger photoemission.
- Interaction Region: Inside the SEM chamber, an optical standing wave (optical grating) is generated by two counter-propagating 1030 nm laser pulses (220 fs duration).
- Detection: Scattered electrons are detected by a hybrid-pixel Timepix3 detector.
- Timing: Two independent optical delay lines control the relative arrival time between the electron pulse and the optical standing wave.
Physical Mechanism:
- The method relies on elastic ponderomotive scattering. As electrons traverse the oscillating electromagnetic field of the standing wave, they experience a ponderomotive force ( $F \propto \nabla I$ ), which deflects them from their original trajectory.
- The magnitude of the deflection depends on the intensity of the standing wave and the temporal overlap between the electron pulse and the laser pulse.
Data Analysis:
- Instead of traditional cross-correlation time traces, the authors employed a laser-streaking approach.
- They measured the transverse velocity distribution ( $v_\perp/c$ ) of scattered electrons as a function of the time delay.
- By integrating electron counts above a specific transverse velocity threshold (to filter out the laser pulse pedestal) and fitting the resulting profile with a Gaussian function, they reconstructed the electron pulse duration (Full Width at Half Maximum, FWHM).

3. Key Contributions

First In-Situ Characterization in USEM: Demonstrated a direct, all-optical method to measure electron pulse durations inside a scanning electron microscope without requiring high-resolution spectrometers or major hardware modifications.
Broad Energy Range Applicability: Validated the technique across a wide range of electron kinetic energies (1–30 keV), covering the standard operating range of SEMs.
Optimization of Initial Energy Spread: Investigated the impact of the photoemission photon energy on pulse duration. By switching from 257.5 nm (4.8 eV) to 515 nm (2.4 eV), the authors reduced the excess energy of emitted electrons, thereby minimizing the initial energy spread ( $\Delta E_i$ ).
Quantitative Modeling: Developed a semi-empirical model (Equation 5) to describe pulse broadening as a function of electron energy, separating contributions from the acceleration region and the field-free drift region.

4. Key Results

Pulse Durations Achieved:
- Under optimal conditions (negligible Coulomb interactions, ~0.1 electrons/pulse), the method achieved pulse durations ranging from 0.5 ps at 30 keV to 2.7 ps at 5.5 keV using 257.5 nm photoemission.
- Using the optimized 515 nm photoemission, the initial energy spread was reduced by a factor of 2 (from $\approx 1.2$ eV to $\approx 0.6$ eV). This resulted in significantly shorter pulses, particularly at lower energies where dispersion is dominant.
Space-Charge Effects:
- The study confirmed that increasing the number of electrons per pulse (from ~0.1 to ~0.8) leads to significant space-charge broadening. For 15 keV electrons, the pulse duration increased from 1.1 ps (low density) to 3.5 ps (high density).
Linearity and Thresholds:
- The maximum scattering angle was found to scale linearly with laser pulse energy, confirming the absence of saturation effects in the measured regime.
- A minimum laser pulse energy of ~500 nJ was identified as necessary to overcome detector pixel resolution limits, though the experiments used 20 $\mu$ J.

5. Significance

Enabling True 4D-SEM: This work bridges a critical gap in ultrafast microscopy by providing a reliable, direct measurement tool for USEMs. This allows researchers to move from theoretical estimates to experimentally verified temporal resolution.
Optimization of Experiments: By enabling in-situ characterization, researchers can now optimize experimental parameters (such as laser trigger wavelength and beam current) to generate the shortest possible electron pulses, directly improving the temporal resolution of pump-probe experiments.
Versatility: The technique is applicable to a wide range of electron energies and does not require the complex spectrometer setups needed for UTEMs, making high-resolution temporal characterization accessible to a broader community of SEM users.
Fundamental Physics: The study provides new insights into the interplay between initial energy spread, space-charge effects, and pulse broadening in low-energy electron beams, validating theoretical models of electron dynamics in field emission sources.