Laser-Based Zernike Plate For Phase Contrast Transmission Electron Microscopy

Recent technological advances in transmission electron microscopy (TEM) enabled the reconstruction of biological macromolecules with near-atomic resolution. The macromolecules embedded in vitreous ice are transparent to the electron beam, but impart a phase to the transmitted electron wave. This phase is conventionally detected by defocusing the imaging system, which, however, offers only limited contrast at lower spatial frequencies. This drawback makes it challenging to reconstruct particles smaller than 100 kDa, or those exhibiting significant structural variability. The phase of the electron wave function can be detected without defocusing using a Zernike phase plate to retard the transmitted electron wave by a quarter wave relative to the scattered wave. However, building an electron retarder has proven difficult due to charging and gradual destruction of material objects exposed to the electron beam. We report our progress towards building a Zernike phase plate for TEM using the ponderomotive potential of an intense laser field. With no material objects inserted in the electron stream, this approach offers a stable, controllable phase shift and negligible electron loss. For practical use in life science TEM, the phase plate must operate continuously, which requires reaching an intensity of a few hundred GW/cm2 with a continuous-wave (CW) laser system. We experimentally demonstrate that CW laser field can be amplified to the intensity of more than 40 GW/cm2 in a high finesse, high numerical aperture near-concentric Fabry-Pérot optical resonator. Our numerical model shows that the phase shift profile created by the fundamental mode of such resonator can function as a nearly ideal Zernike phase plate for protein structure studies.