Feasibility of a Pulsed Ponderomotive Phase Plate for Electron Beams

Abstract

We propose a scheme for constructing a phase plate for use in an ultrafast Zernike-type phase contrast electron microscope, based on the interaction of the electron beam with a strongly focused, high-power femtosecond laser pulse and a pulsed electron beam. Analytical expressions for the phase shift using the time-averaged ponderomotive potential and a paraxial approximation for the focused laser beam are presented, as well as more rigorous quasiclassical simulations based on the quantum phase integral along classical, relativistic electron trajectories in an accurate, non-paraxial description of the laser beam. The results are shown to agree well unless the laser beam is focused to a waist size below a wavelength. For realistic (off-the-shelf) laser parameters the optimum phase shift of −π/2 is shown to be achievable. When combined with RF-cavity based electron chopping and compression techniques to produce electron pulses, a femtosecond regime pulsed phase contrast microscope can be constructed. The feasibility and robustness of the scheme are further investigated using the simulations, leading to motivated choices for design parameters such as wavelength, focus size and polarization.

Design of an ultrafast pulsed ponderomotive phase plate for cryo-electron tomography

“… 34 , 36 , 37 Moreover, the spatial interactions range from micrometers to millimeters, which reach the physical bounds of reasonable interactions within the limited space and accessibility of electron microscope columns. A recent report 38 discussing the use of radiofrequency cavity-generated electron pulses at high repetition rates ranging from 56 to 420 MHz showed that laser pulses operating at central wavelengths of 800 and 1,500 nm with pulse durations of 100 and 300 fs could reach the required phase shifts at much smaller spatial constraints than the ones discussed in this article. Importantly, the smaller beam waists presented in that report dramatically decreased the required pulse energy compared with the energies discussed here.…”