Speaker
Description
In the transmission electron microscope (TEM), high temporal resolution can be achieved using short electron pulses, typically in a pump-probe stroboscopic scheme. Temporal resolution can be described in terms of a convolution of pump and probe pulses, in addition to jitter arising from random phase shifts in their arrival times. The primary methods for generating short electron pulses in the TEM are photoemission and beam blanking. In photoemission, a short (fs to µs) ultraviolet laser pulse strikes a photocathode in the electron gun, resulting in electron emission through either the photoelectric effect or laser-triggered field emission. The use of fs lasers is often costly, requiring complex instrument modifications, and beam instability can result from laser pointing fluctuations on the cathode. In beam blanking, the electron gun remains unchanged, while a continuous electron beam is chopped into short pulses by sweeping it rapidly over a narrow slit placed before the sample. Even a “fast” electrostatic beam blanker is limited by the capacitive response of the electrostatic elements, which constrains the temporal resolution to nanoseconds or longer [1, 2].
Here, we show how a travelling wave radiofrequency (RF) deflection cavity combined with a small aperture can be used to chop a continuous electron beam to generate short electron pulses. In this approach, the constant beam is swept across the aperture, producing a pulsed beam. A second correcting RF cavity is operated to zero out the time-dependent transverse momenta introduced by the first deflecting RF cavity, in a way that maintains the original peak brightness and energy spread. Such an RF-driven electron beam pulser can generate short electron pulses with a tunable repetition rate up to 12 GHz [3, 4]. By retaining the original field-emission source, pulsed operation is enabled while maintaining the intrinsic beam brightness, transverse coherence, and energy spread of the continuous electron beam. The present implementation represents a next-generation design of the pulser incorporating substantial RF and optical upgrades, automation-compatible software, and improved control over pulsed spot size and beam shape.
The Ultra-Fast Pulser (UFP) unit from Euclid Techlabs LLC, with optics from CEOS GmbH, was installed on a FEI Titan G2 60–300 TEM to enable time-resolved experiments when combined with an ultrafast pump, in any standard operating mode of imaging, diffraction, or spectroscopy. The unit, which can generate electron beam pulses as short as 10 ps is positioned at the gun exit to preserve the original TEM performance when no RF signal is applied. When activated, it can be operated in either RF (2–6 GHz) or Pulse Picker mode (1 MHz - 100 MHz). The capability to pulse the electron beam over a wide range of frequencies opens new opportunities for investigating ultrafast phenomena in functional materials, including phase transitions, magnetization dynamics, and other electrically or microwave-driven excitations. Furthermore, pump–probe experiments can be performed using a synchronized microwave sample holder, enabling phase-locked excitation and stroboscopic investigation of GHz-frequency dynamics with high temporal precision.
References
1. Ultramicroscopy 176, 2017, 63-73.
2. Ultramicroscopy 184, 2018, 8-17.
3. Ultramicroscopy 235, 2022, 113497.
4. Sci. Adv. 6, 2020, eabc3456.