Active control of bright electron beams with RF optics for femtosecond microscopy

Abstract

A frontier challenge in implementing femtosecond electron microscopy is to gain precise optical control of intense beams to mitigate collective space charge effects for significantly improving the throughput. Here, we explore the flexible uses of an RF cavity as a longitudinal lens in a high-intensity beam column for condensing the electron beams both temporally and spectrally, relevant to the design of ultrafast electron microscopy. Through the introduction of a novel atomic grating approach for characterization of electron bunch phase space and control optics, we elucidate the principles for predicting and controlling the phase space dynamics to reach optimal compressions at various electron densities and generating conditions. We provide strategies to identify high-brightness modes, achieving ∼100 fs and ∼1 eV resolutions with 106 electrons per bunch, and establish the scaling of performance for different bunch charges. These results benchmark the sensitivity and resolution from the fundamental beam brightness perspective and also validate the adaptive optics concept to enable delicate control of the density-dependent phase space structures to optimize the performance, including delivering ultrashort, monochromatic, high-dose, or coherent electron bunches.

Nonequilibrium dynamics of spontaneous symmetry breaking into a hidden state of charge-density wave

“…Due to the good thermal isolation in this sample setting, the experiments show no visible dissipation of the pump energy absorbed into the materials over the observation window (2 ns). To optimize the probe beam brightness, we deliver intense electron pulses (~10 6 e/pulse) at 100 keV generated from a silver photo-cathode just below the virtual cathode limit 30 , 31 . The phase space of the space-charge-dominated pulse is actively manipulated through placing a radio-frequency cavity in the optical column acting as the longitudinal lens 30 , 32 (Fig.…”

“…1b ) to reach ≈100 fs pulse duration. This is accomplished under a condition that its transverse phase space, controlled through a series of magnetic lenses, produces a highly collimated beam 33 with a beam coherence length up to 40 nm 30 . A high-quality pattern produced by this fs coherent scattering setup is shown in Fig.…”

Direct imaging of plasma waves using ultrafast electron microscopy

“…Ultimately, the image resolution is limited by the probe beam transverse emittance, which is estimated to be ∼0.02 mm⋅mrad. 28 This gives a resolution at least on the 10 nm scale, as demonstrated by charactering the step-edge of the mesh; see the supplementary material . Time-compression of the pulses to 100 fs, however, leads to increased energy spread degrading the resolution to ∼100 nm.…”

“…In a significant departure from the earlier approach, 1 the new UEM reported here employs a high-flux photo-electron gun, operated near the virtual cathode limit, to boost the beam brightness. 28 To gain sensitivity to the field, low-energy beams (25 keV) and ≈10 5 electrons/pulse at 10 kHz are employed. The spatiotemporal resolutions are optimized utilizing an RF cavity functioning as a longitudinal focusing lens.…”

Direct visualization of electromagnetic wave dynamics by laser-free ultrafast electron microscopy

“…For the former, the intrinsic nanosecond duration of the electron pulse largely restricts the temporal resolution. For the latter, further optimization of the photoemitted electron pulse using microwave compression (25,26), terahertz compression (27)(28)(29)(30)(31), active control via radio frequency (RF) optics (32,33), or photon gating (34,35) can extend the temporal resolution into the deep femtosecond regime, which finds vast applications in studying the transient structures and morphologies of inorganic and organic materials (13,19,36). Therefore, the laser-actuated photoemission scheme is currently the primary method for UEM.…”