Professor Zewail is credited with porting the femtosecond pump-probe experiments to the realm of Transmission Electron Microscopy [ 1]. The pulsed beam required to operate in Ultrafast stroboscopic mode usually consists of a femtosecond laser aimed at the TEM cathode to produce a photo-generated electron pulse with a duration down to a few hundred femtoseconds.
The use of a laser to generate short electron pulses has two main drawbacks, complexity, and limited beam coherence in pulsed mode. Both these drawbacks can be eliminated by chopping the continuous beam instead of modifying the gun to produce a pulsed beam. Chopping can be achieved with RF technology [ 2, 3].
This contribution discusses a compact RF-cavity based design capable of generating ultrashort electron pulses without degrading the native coherence of the gun. A further, fast electrostatic beam blanker (ESBB) serves as a pulse picker, for flexible control over the electron pulse sequence and repetition rate on the specimen.
In a conventional Ultrafast setup, the time delay between pumping and probing is straightforwardly set using an optical delay stage, here the synchronization between the sample pumping laser and the probing electron pulse must be done electronically. Successful synchronization, down to the sub-picosecond regime is demonstrated using Photon-Induced Near-Field Electron Microscopy [ 4]. PINEM as it’s known visualizes the interaction of a probing electron with the evanescent field created around the sample by the pumping laser pulse. As a consequence of this interaction the electron will lose or gain one or multiple quanta of energy which can be seen in the Energy Loss & Gain spectrum (EELS & EEGS) as seen in figure 1.
Fig. 1.
A typical PINEM signature. Intensity from the zero-loss peak is funneled into the loss and gain regions as electrons gain or lose one or multiple quanta of energy while interacting with an evanescent field around the sample.