Volume 86, June 2024, 102805Author links open overlay panel, , , , Highlights•

Phase plates improve image contrast in phase contrast electron microscopy.

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The laser phase plate is the first phase plate design which provides stable and maximal contrast enhancement.

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Scanning transmission electron microscopy methods may be alternatives to using phase plates.

Abstract

Although defocus can be used to generate partial phase contrast in transmission electron microscope images, cryo-electron microscopy (cryo-EM) can be further improved by the development of phase plates which increase contrast by applying a phase shift to the unscattered part of the electron beam. Many approaches have been investigated, including the ponderomotive interaction between light and electrons. We review the recent successes achieved with this method in high-resolution, single-particle cryo-EM. We also review the status of using pulsed or near-field enhanced laser light as alternatives, along with approaches that use scanning transmission electron microscopy (STEM) with a segmented detector rather than a phase plate.

Section snippetsHistory of phase plates in transmission electron microscopy

A phase plate generates image contrast for transparent (non-absorbing) objects, which can be achieved only imperfectly by defocusing the image. Phase plates generate contrast by introducing a relative phase shift between the scattered component of the electron beam's wave function and the component that has not been scattered by the sample (see Figure 1). This can be achieved by applying a phase shift at the center of a diffraction plane of the transmission electron microscope (TEM), where the

The laser phase plate

The laser phase plate (LPP) applies a 90° phase shift to the unscattered wave using a high-intensity laser beam, which is focused in the diffraction plane. The interaction between the oscillating electromagnetic field of the laser beam and the charge of the electron results in a repulsive effective potential known as the ponderomotive potential [8,9]. This potential (like any other) imparts a phase shift to a through-going electron wave function. The LPP stands in contrast to previous phase

Scanning transmission electron microscopy

While scanning-transmission electron microscopes are most commonly used for dark-field imaging and for electron energy-loss spectroscopy, they can also be used to produce images from which one computationally recovers the phase of the exit wave. Ptychography, originally conceived as a way to solve the “phase problem” in X-ray crystallography [35], is readily implemented in a scanning transmission electron microscope (STEM) that is equipped with a pixel-array area detector [36,37]. An

Declaration of competing interest

J.J.A., R.M.G., and H.M. are inventors on U.S. Patent No. 10,395,888 and U.S. Patent Pending No. 17/640,255.

Acknowledgments

This work was supported by the US National Institutes of Health (grant no. 2R01GM126011), Chan Zuckerberg Initiative through the Silicon Valley Community Foundation, Gordon and Betty Moore Foundation (grant no. 9366), and a cooperative research and development agreement (CRADA) with Thermo Fisher Scientific (award number AWD00004352). Grant no. 2R01GM126011 and award number AWD00004352 were administered at Lawrence Berkeley National Laboratory under Contract No. DE-AC02-05CH11231. P.N.P.

References (45)R. Danev et al.Using the Volta phase plate with defocus for cryo-EM single particle analysis

eLife

(Jan. 2017)

R. DanevRoutine sub-2.5 Å cryo-EM structure determination of GPCRs

Nat Commun

(Jul. 2021)

P.W. Smorenburg et al.Polarization-dependent ponderomotive gradient force in a standing wave

Phys Rev A

(Jun. 2011)

J.J. Axelrod et al.Observation of the relativistic reversal of the ponderomotive potential

Phys Rev Lett

(May 2020)

R. Haindl et al.Femtosecond tunable-wavelength photoassisted cold field emission

Appl Phys B

(Mar. 2023)

J.L. Reynolds et al.Nanosecond photoemission near the potential barrier of a Schottky Emitter

Phys Rev Appl

(Jan. 2023)

J.W. LauLaser-free GHz stroboscopic transmission electron microscope: components, system integration, and practical considerations for pump–probe measurements

Rev Sci Instrum

(Feb. 2020)

K.A. H et al.Feasibility of a pulsed ponderomotive phase plate for electron beams

New J Phys

(Feb. 2023)

D.X. Du et al.Design of an ultrafast pulsed ponderomotive phase plate for cryo-electron tomography

Cell Rep Methods

(Jan. 2023)

M.C. Chirita Mihaila et al.Transverse electron-beam shaping with light

Phys Rev X

(Sep. 2022)

S. Meier et al.Coulomb interactions and the spatial coherence of femtosecond nanometric electron pulses

ACS Photonics

(Sep. 2022)

N. BachCoulomb interactions in high-coherence femtosecond electron pulses from tip emitters

Struct Dyn

(Jan. 2019)

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