Biology subjects

Du, D. X.

Publications and source records attributed to Du, D. X..

3 recordsLinked to original sources

bioRxiv2025

Pulsed-laser lensing for phase modulation in electron microscopy

Phase contrast electron microscopy is fundamental for visualizing unstained biological specimens. Advances in electron detection have not yet overcome the low contrast caused by weak scattering. Here, we demonstrate that an orthogonal pulsed laser-electron beam interaction produces a pronounced peak phase shift of 430 radians through ponderomotive defocusing, leading to a maximum angular deflection of 45 {micro}rad. Experiments encompassing a variety of probe pulse energies and pump positions verified the properties of the electron pulses in a range of pulse durations from 5.8 {+/-} 1.9 ps to 13.4 {+/-} 0.9 ps and a width of 15.0 {+/-} 2.6 {micro}m at the interaction region. The stability of the beam was also tested across 10 hours of cumulative acquisition time, with only small variations in laboratory conditions resulting in a gradually shifting baseline measurement. Pulsed laser lensing of the electron beam offers the potential for refinement in phase shift and electron beam shaping with careful consideration to the overlap between laser and electron pulses. Calculations of phase shifts across a wide experimental envelope show that poorly chosen laser parameters can generate large incoherent distributions at both 30 keV and 300 keV. Thus, a delicate balance between laser and electron widths and pulse durations must be struck to adequately achieve uniform phase shifts, particularly when singling out specific beamlets in the back-focal-plane.

biophysics↗

bioRxiv2024

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

Cryo-electron tomography (cryo-ET) is a powerful modality for resolving cellular structures in their native state. While single-particle cryo-electron microscopy (cryo-EM) excels in determining protein structures purified from recombinant or endogenous sources, cryo-ET suffers from low contrast in crowded cellular milieux. A novel experimental approach to enhance contrast in cryo-ET is to manipulate the phase of scattered pulsed electrons using ultrafast pulsed photons. Here, we outline the experimental design of a proof-of-concept electron microscope and demonstrate synchronization between electron packets and laser pulses. Further, we show ultrabright photoemission of electrons from an alloy field emission tip using femtosecond ultraviolet pulses. These experiments pave the way towards exploring the utility of the ponderomotive effect using pulsed radiation to increase phase contrast in cryo-ET of subcellular protein complexes in situ, thus advancing the field of cell biology.

biophysics↗

bioRxiv2022

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

Ponderomotive phase plates have shown temporally consistent phase contrast is possible within electron microscopes via high fluence static laser modes resonating in Fabry-Perot cavities. Here, we explore using pulsed laser beams as an alternative method of generating high fluences. We find through forward-stepping finite element models that picosecond-or-less interactions are required for meaningful fluences phase shifts, with higher pulse energies and smaller beam waists leading to the predicted higher fluences. An additional model based on quasiclassical assumptions is used to discover the shape of the phase plate by incorporating the oscillatory nature of the electric field. From these results, we find the transient nature of the laser pulses removes the influence of Kapitza-Dirac diffraction patterns that appear in the static resonator cases. The addition of a second laser aligned 90{degrees} to the first induces anisotropy to the shape of the phase plate. By incorporating a shifting-electron-beam algorithm, the effects of a finite electron beam crossover are also simulated. A total pulse energy of 8.7 J is enough to induce the required {pi}/2 phase shift for Zernike-like phase microscopy. As a brief thought experiment, we also explore the usage of high frequency lasers in a standard electron emission scheme to see if a pulsed electron beam is even necessary. Ultimately, frequency requirements limit the laser to nanosecond pulse durations, causing the required pulse energies to reach unreasonable levels before adequate phase shifts are achieved.

biophysics↗