High-speed Electron Microscopy Research Articles

Spatial-temporal characterization of photoemission in a streak-mode dynamic transmission electron microscope.

A long-standing motivation driving high-speed electron microscopy development is to capture phase transformations and material dynamics in real time with high spatial and temporal resolution. Current dynamic transmission electron microscopes (DTEMs) are limited to nanosecond temporal resolution and the ability to capture only a few frames of a transient event. With the motivation to overcome these limitations, we present our progress in developing a streak-mode DTEM (SM-DTEM) and demonstrate the recovery of picosecond images with high frame sequence depth. We first demonstrate that a zero-dimensional (0D) SM-DTEM can provide temporal information on any local region of interest with a 0.37 μm diameter, a 20-GHz sampling rate, and 1200 data points in the recorded trace. We use this method to characterize the temporal profile of the photoemitted electron pulse, finding that it deviates from the incident ultraviolet laser pulse and contains an unexpected peak near its onset. Then, we demonstrate a two-dimensional (2D) SM-DTEM, which uses compressed-sensing-based tomographic imaging to recover a full spatiotemporal photoemission profile over a 1.85-μm-diameter field of view with nanoscale spatial resolution, 370-ps inter-frame interval, and 140-frame sequence depth in a 50-ns time window. Finally, a perspective is given on the instrumental modifications necessary to further develop this promising technique with the goal of decreasing the time to capture a 2D SM-DTEM dataset.

Stress Effects in Laser-Pulsed Chromium Films Tracked by Nanosecond Transmission Electron Microscopy

The application of laser treatment is rapidly expanding, and understanding the decisive processes is a must. Laser-patterned chromium films show a damage very much different from that in other metals. High-speed electron microscopy and selected area diffraction disclosed a new ablation mechanism based on travelling stress waves, and launched by a fast first order solid state transition during heat-up and cool-down. A brittle-to-ductile phase transition, thermal expansion and thermocapillarity, the currently assumed mechanisms for laser-induced damage in chromium films, actually are not decisive.

Nanosecond electron microscopes

Combining electron optics, fast electronics and pulsed lasers, a transmission and a photoelectron emission microscope were built, which visualize events in thin films and on surfaces with a time resolution of several nanoseconds. The high-speed electron microscopy is capable to track fast laser-induced processes in metals below the ablation threshold, which are difficult to detect by other imaging techniques. The material response to nano- and femtosecond laser pulses was found to be very different. It was dominated by thermo/chemocapillary flow and chemical reactions in the case of nanosecond pulses, and by mechanical deformations and non-thermal electron emission after a femtosecond pulse.

Dynamics of laser pulse-induced melts in Ni–P visualized by high-speed transmission electron microscopy

Patterns produced in laser-molten amorphous Ni–P films are of interest for applications in cover layers of magnetic disks, but their formation is not yet understood. This work reports first images of the actual motion in solidifying laser-produced Ni–P melts by high-speed electron microscopy and X-ray microanalysis of the evolving structures. The findings suggest, that existing thermo/chemocapillary models of flow in impure liquid metals may be applied to dilute Ni–P alloys.

High-speed electron microscopy of nanocrystallization in AlNi films by nanosecond laser pulses

Nanocrystallization of 3 Al: 1 Ni and Al3Ni films is realized with short laser pulses (7 and 20 ns), and traced by high-speed transmission electron microscopy on the nanosecond time scale. Composite films melt within the laser pulse above a threshold fluence, and solidify to a mixture of nanocrystalline Al, Al3Ni, and Al3Ni2 at cooling rates of ≈ 108 K/s. Al3Ni crystals are amorphized by a laser pulse within 5 ns above a threshold temperature and with negligible absorbed latent heat. A nanophase composed of Al, Al3Ni, Al3Ni2 is quenched, as in composites. The reported effects should occur in all intermetallic compounds having a complex lattice like Al3Ni, and composites of equal overall composition. 3 Al: 1 Ni and Al3Ni-Schichten wurden mit Laserpulsen (7 und 20 ns) erstmalig nanokristallisiert und die Umwandlungen mit der Hochgeschwindigkeits-Transmissionselektronenmikroskopie verfolgt. Misch-Schichten werden durch einen Laserpuls oberhalb einer Grenzfluenz wahrend des Pulses geschmolzen. Sie erstarren zu einem Gefuge aus nanokristallinem Al, Al3Ni, Al3Ni2 bei Abkuhlraten von ≈ 108 K/s. Al3Ni Kristalle amorphisieren innerhalb von 5 ns oberhalb einer Grenztemperatur und mit vernachlassigbarer latenter Warme. Auch bei Al3Ni wird ein Gemisch aus Al-, Al3Ni-, Al3Ni2-Nanokristallen eingefroren. Diese Effekte sollten bei allen intermetallischen Verbindungen mit komplizierten Gittern wie Al3Ni auftreten sowie bei Elementgemischen der gleichen Zusammensetzung.

Ablation of metal films by picosecond laser pulses imaged with high-speed electron microscopy

The ablation of free-standing metal films by picosecond laser pulses (50 ps, 0.6–8 J/cm2, 532 nm) was visualized by electron microscopy. Triple-frame high-speed transmission electron microscopy was applied, with exposure times below 5 ns and frame repetition times ≳20 ns. Ablation was observed to proceed by hole opening within 5–30 ns, speeding up with increasing laser fluence, and punching out of most of the melt. Once opened, the holes expanded by capillary forces with a velocity of ≊100 m/s for some 10 ns. At large pulse fluences the evaporation was collimated along the film axis. This and the absence of patterned flow of the melt are conspicuous differences to the ablation of films by nanosecond laser pulses. The effects typical for picosecond pulses are attributed to an evaporation that is one-sided because of a thermal gradient along the film axis, and that exerts a large recoil.

High-speed electron microscopy of laser-induced vaporization of thin films

Laser-driven explosive vaporization of free germanium films was investigated with a high-speed transmission electron microscope. Double frame and streak imaging with nanoseconds exposure and sweep times were exploited to visualize the fast processes. Using focused 30-ns laser pulses of about 1 μJ and 10 MW/cm2, the films were shattered within 100–300 ns by three cooperating mechanisms: Marangoni flow, jerky boiling, and capillary forces. These effects are very different from those ablating supported films. They suggest that large temperature gradients of 1000 K/μm centered near 4000 K are generated within the laser spot, and the homogeneous nucleation theory of boiling is not applicable.

Performance of A Laser-Pulsed Thermal Electron Gun

High-speed electron microscopy strongly demands a high-brightness electron gun in order to increase the number of image forming electrons. A few years ago, a laser-pulsed high-brightness electron gun was introduced. Fig.1 shows the experimental set-up, A standard triode system was supplemented with a Nd:YAG laser, focussing optics and a modified anode, which incorporates the laser deflection mirror. The frequency doubled laser pulse (τ =5 ns, λ = 532 nm) is focused through a window onto the tip of the tungsten hairpin emitter. The laser treated area (≈ 100 μm diameter) is heated well above the melting point, which results in a strong thermal electron emission. Due to rapid heat-up and fast cool-down of the tungsten surface short electron pulses with a duration of 20 ns and a maximum current of 20 mA at 80 kV are emitted. A destruction of the tungsten wire is avoided, too. Laser energies used for the generation of electron pulses are in the range of 100 μJ. Due to these minor modification, the DC operation of the electron gun is not disabled, which allows a convenient adjustment in the DC mode and then switching into the pulsed operation mode. Fig.2 shows a typical electron pulse emitted by the gun. Shorter electron pulses up to 5 ns can be generated by a beam blanking unit.