Principal Investigator Karl Berggren
Recent progress in focused-ion-beam (FIB) technol- ogy has enabled the fabrication of electron optical elements such as zone-area plates, phase plates, and beamsplitters. These nanofabricated elements can be used to perform Zernike phase-contrast imaging, ho- lography and beam aberration correction in a conven- tional transmission electron microscope (TEM). We have fabricated a grating-Mach-Zehnder-electron-interferometer, using FIB milling of a single-crystalline silicon workpiece. Shown schematically, the interferometer uses two thin layers of silicon as diffraction gratings; the first to split the incident electron beam, and the second to recombine two of the diffract- ed beams. The gap between the gratings in our interfer- ometer was 20 μm. Fabrication of the gratings from a single crystalline silicon workpiece ensures alignment and precise positioning. We obtained a rotational align- ment of ~100 μrad and a grating positioning accuracy of 100 nm.
We inserted this interferometer in the sample holder of a 200 kV TEM (JEOL 2010F). We used an electron beam with a diameter of 240 nm on the first grating and convergence semi-angle of 4 mrad in our experiment. As shown in figure 2(b), when imaging the second grating (figure 2(a), sample z-height z1) at high- resolution (Ψ0), we obtained a lattice-resolved image of silicon. As we raised our sample holder z-height to move the imaging plane below the second grating (z2-z5), the first-order diffracted beam from this grating (Ψ0g) moved closer to the first-order diffracted beam from the first grating (Ψgg), and the two beams overlapped 20 μm below the second grating (z6). Figure 2(c) is a high- resolution image of the overlapping beams, showing interference fringes of period 0.32 nm, which was expected from the interference of first-order silicon diffracted beams.
This interferometer could be used to perform electron holography in any TEM, as well as interaction-free imaging using the Elitzur-Vaidman scheme.