Dual mode microwave deflection cavities for ultrafast electron microscopy

\u3cp\u3eThis paper presents the experimental realization of an ultrafast electron microscope operating at a repetition rate of 75 MHz based on a single compact resonant microwave cavity operating in a dual mode. This elliptical cavity supports two orthogonal TM\u3csub\u3e110\u3c/sub\u3e modes with different resonance frequencies that are driven independently. The microwave signals used to drive the two cavity modes are generated from higher harmonics of the same Ti:Sapphire laser oscillator. Therefore, the modes are accurately phase-locked, resulting in periodic transverse deflection of electrons described by a Lissajous pattern. By sending the periodically deflected beam through an aperture, ultrashort electron pulses are created at a repetition rate of 75 MHz. Electron pulses with τ = (750 ± 10) fs pulse duration are created with only (2.4 ± 0.1) W of microwave input power; with normalized rms emittances of ϵ\u3csub\u3en,x\u3c/sub\u3e = (2.1 ± 0.2) pm rad and ϵ\u3csub\u3en,y\u3c/sub\u3e = (1.3 ± 0.2) pm rad for a peak current of I\u3csub\u3ep\u3c/sub\u3e = (0.4 ± 0.1) nA. This corresponds to an rms normalized peak brightness of B n p , rms = ( 7 ± 1 ) × 10 6 A/m\u3csup\u3e2\u3c/sup\u3e sr V, equal to previous measurements for the continuous beam. In addition, the FWHM energy spread of ΔU = (0.90 ± 0.05) eV is also unaffected by the dual mode cavity. This allows for ultrafast pump-probe experiments at the same spatial resolution of the original TEM in which a 75 MHz Ti:Sapphire oscillator can be used for exciting the sample. Moreover, the dual mode cavity can be used as a streak camera or time-of-flight electron energy loss spectroscopy detector with a dynamic range >10\u3csup\u3e4\u3c/sup\u3e.\u3c/p\u3

High-quality, high-repetition rate, ultrashort electron bunches generated with an RF-cavity

In collaboration with FEI Company, we are studying the possi- bility of using microwave TM110 streak cavities in combination with a slit, to chop a continuous electron beam into 100 fs elec- tron bunches.We have shown that this can be done with minimal increase in transverse emittance and longitudinal energy spread. Furthermore, these bunches are created at a repetition rate of 3 GHz. Accurately synchronized to a mode-locked laser system, this allows for high-frequency pump-probe experiments with the beam quality of high-end electron microscopes. At Eindhoven University of Technology, we will soon implement such a cavity in a 200 keV Tecnai, which should result in high- frequency ultrafast (S)TEM with sub-ps time-resolution while maintaining the atomic spatial resolution of the TEM

Beam pulsing device for use in charged-particle microscopy

A charged-particle microscope comprising: - A charged-particle source, for producing a beam of charged particles that propagates along a particle-optical axis; - A sample holder, for holding and positioning a sample; - A charged-particle lens system, for directing said beam onto a sample held on the sample holder; - A detector, for detecting radiation emanating from the sample as a result of its interaction with the beam; - A beam pulsing device, for causing the beam to repeatedly switch on and off so as to produce a pulsed beam, wherein the beam pulsing device comprises a unitary resonant cavity disposed about said particle-optical axis and having an entrance aperture and an exit aperture for the beam, which resonant cavity is embodied to simultaneously produce a first oscillatory deflection of the beam at a first frequency in a first direction and a second oscillatory deflection of the beam at a second, different frequency in a second, different direction. The resonant cavity may have an elongated (e.g. rectangular or elliptical) cross-section, with a long axis parallel to said first direction and a short axis parallel to said second direction

Time-of-flight electron energy loss spectroscopy using TM110 deflection cavities

We demonstrate the use of two TM110 resonant cavities to generate ultrashort\u3cbr/\u3eelectron pulses and subsequently measure electron energy losses in a time-of-flight type of setup. The method utilizes two synchronized microwave cavities separated by a drift space of 1.45 m. The setup has an energy resolution of 1262 eV FWHM at 30 keV, with an upper limit for the temporal resolution of 2.760.4 ps. Both the time and energy resolution are currently limited by the brightness of the tungsten filament electron gun used. Through simulations, it is shown that an energy resolution of 0.95 eV and a temporal resolution of 110 fs can be achieved using an electron gun with a higher brightness. With this, a new method is provided for time-resolved electron spectroscopy without the need for elaborate laser setups or expensive magnetic spectrometers

Theory and particle tracking simulations of a resonant radiofrequency deflection cavity in TM110 mode for ultrafast electron microscopy

We present a theoretical description of resonant radiofrequency (RF) deflecting cavities in TM 110 mode as dynamic optical elements for ultrafast electron microscopy. We first derive the optical transfer matrix of an ideal pillbox cavity and use a Courant-Snyder formalism to calculate the 6D phase space propagation of a Gaussian electron distribution through the cavity. We derive closed, analytic expressions for the increase in transverse emittance and energy spread of the electron distribution. We demonstrate that for the special case of a beam focused in the center of the cavity, the low emittance and low energy spread of a high quality beam can be maintained, which allows high-repetition rate, ultrafast electron microscopy with 100 fs temporal resolution combined with the atomic resolution of a high-end TEM. This is confirmed by charged particle tracking simulations using a realistic cavity geometry, including fringe fields at the cavity entrance and exit apertures.

Discovering hidden material properties of MgCl\u3csub\u3e2\u3c/sub\u3e at atomic resolution with structured temporal electron illumination of picosecond time resolution

\u3cp\u3eA combination of atomic resolution phase contrast electron microscopy and pulsed electron beams reveals pristine properties of MgCl\u3csub\u3e2\u3c/sub\u3e at 1.7 Å resolution that were previously masked by air and beam damage. Both the inter- and intra-layer bonding in pristine MgCl\u3csub\u3e2\u3c/sub\u3e are weak, which leads to uncommonly large local orientation variations that characterize this Ziegler–Natta catalyst support. By delivering electrons with 1–10 ps pulses and ≈160 ps delay times, phonons induced by the electron irradiation in the material are allowed to dissipate before the subsequent delivery of the next electron packet, thus mitigating phonon accumulations. As a result, the total electron dose can be extended by a factor of 80–100 to study genuine material properties at atomic resolution without causing object alterations, which is more effective than reducing the sample temperature. In conditions of minimal damage, beam currents approach femtoamperes with dose rates around 1 eÅ\u3csup\u3e−2\u3c/sup\u3e s\u3csup\u3e−1\u3c/sup\u3e. Generally, the utilization of pulsed electron beams is introduced herein to access genuine material properties while minimizing beam damage.\u3c/p\u3