Ultrafast Transmission Electron Microscopy


generate a satisfactory image. However, due to space-charge effects in the TEM col- umn, the electron probe beam pulse is typ- ically no shorter than ∼1 ns for a 200 keV beam. If the sample is relatively immune to the high-charge pulses needed for the imaging, a short burst of several probe pulses can be used to create a “movie” that captures the sample’s full transition in a single process cycle. Tese techniques have been developed in dynamic TEM (DTEM) discussed in detail aſter the UTEM section below.


UTEM Until recently, UTEM was possible


Figure 1: Timescales for phenomena studied in materials science (red), life sciences (blue), semiconduc- tor (gray), and nanotechnology (green). Accessible timescales of complementary UTEM techniques are also superimposed.


only through laser-based methods. As there have been excellent review articles [3,5] on laser-UTEM across a wide range of reversible processes, only a brief summary will be provided here for completeness. In laser-UTEM, the standard electron emit- ter is replaced by a photocathode, oſten alongside major modifications of the elec- tron gun, and electron pulses are gener- ated aſter the photocathode is struck by an ultrafast laser pulse focused to a small spot size (typically 1–25 nJ and 10–25 μm, respectively). Te sample pump beam is typically a lower harmonic (longer wave- length) from the same laser system or can be a separate (phase-locked) laser, although a non-laser excitation source can also be employed. While technically challenging, the core technologies were well developed prior to laser-UTEM. Significant complexi- ties arose during laser-UTEM implementa- tion and image interpretation. For example, photocathode size and energy spread greatly reduced the electron beam bright- ness and coherence, prompting additional research in these areas [7,8]. Additionally, ultrafast laser development relies on non- linear amplification, where frequency mix- ing is more effective at shorter pulse lengths (into the fs regime) and limited in repetition rate (∼100 kHz, typical), resulting in image generation times of over 15 minutes at the 1 e-


/pulse limit used in some stroboscopic


studies. Long-term driſt issues in both the microscope and the laser performance can then affect the image quality. Laser-UTEM can regularly achieve spatio-temporal reso- lutions below 1 nanometer depending on the native TEM and 100 fs (10-22


ms). A


Figure 2: Laser-based TEM techniques (a) single-frame DTEM, (b) movie mode DTEM, and (c) laser-UTEM. Reprinted from [3], with permission from Elsevier.


48


comparison of the laser-based techniques is shown in Figure 2 with laser-UTEM repre- sented in Figure 2c.


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