Coherence of a pulsed electron beam extracted from a semiconductor photocathode in transmission electron microscope

Abstract

Dynamic observations of nanoscale materials are important for investigations of the time evolution of optical couplings, distractions, and energy relaxations in a local site. To suppress electron‐beam damage of biological specimens or organic material in transmission electron microscopes (TEMs), a pulsed electron beam is expected to be applied for the probe beam. Therefore, we have begun developing a spin‐polarized pulse‐TEM (SPTEM), which comprises a photocathode‐type electron source (PES) and a low‐voltage TEM [1–3]. Several beam parameters of the PES are greatly superior to those of conventional thermal electron beams. In addition, PES has the ability to generate a sub‐picosecond pulse‐beam [4]. In continuous beam emissions, we have previously demonstrated that the SPTEM can provide both TEM images and diffraction patterns [2]. The TEM images were obtained at a spatial resolution of 1 nm with a 30‐kV acceleration voltage. The apparatus has an electron beam energy width below 114‐meV in the TEM without any monochrometors [6]. The energy width indicates that the temporal coherence is approximately 34 fs at 30‐eV beam energy. The brightness is measured by taking a spot size and a convergent angle on an image plane. The measured brightness is approximately 4 × 10 7 A cm −2 sr −1 at 30‐keV beam energy with a polarization of 82% and a drive‐laser power of 800 kW/cm 2 on the photocathode [6]. The brightness for 200‐kV beam energy is estimated to be 3 × 10 8 A cm −2 sr −1 , which is converted using a Lorentz factor. The order of the brightness is sufficient for an interference experiment. Figure 2 demonstrates interference fringes of a spin‐polarized electron beam using a newly installed biprism. The resulting electron beam exhibits a long coherence length owing to its low initial emittance of 2.6 nm rad, which can generate interference fringes representative of a first‐order correlation using an electron biprism. These results indicate that the SPTEM can provide enough coherence in both the lateral and longitudinal directions even if the semiconductor photocathode is used for an electron emitter. Pulse beam emission in the SPTEM was also performed using a combination of the semiconductor photocathode and an ultra‐short pulse laser, which can realize a time‐resolved measurement with the stroboscopic technique or the single‐shot technique. The photocathode has high quantum efficiency on the order of 10 −3 compared with other metal‐type photocathodes, which can realized not only a continuous emission but also a pico‐second pulse emission. The picosecond pulse duration was realized using a newly developed ultra‐short pulse laser system, which comprises a mode‐lock Ti‐Sapphire laser, a compensator for group velocity dispersion, and a pulse‐duration converter. Figure 2 shows a typical beam current measured using a Faraday‐cup type current monitor. The repetition rate of the pulse beam is synchronized with a drive laser system. Time‐resolved TEM imaging and pulsed interference fringes were also successfully conducted using a stroboscopic acquisition technique [7]. Figures 3a and b show the interference fringes using a continuous electron wave and a 20‐ps pulsed electron wave under the same condition of the electron optics, respectively. In the continuous–mode, a 1‐mA source current was used for the interference experiment. In contrast, the pulsed beam had a high charge of 150 fC/pulse with a repetition rate of 80 MHz, which is comparable with a 12‐mA average current. Consequently, despite its high current density, the pulsed electron beam emitted from the photocathode has sufficient coherence to realize a time‐resolved holography that can analyze phase information in a temporal space.