Electron microscopy enables researchers to visualize tiny objects such as viruses, the fine structures of semi­conductor devices, and even atoms arranged on a material surface. Focusing down the electron beam to the size of an atom is vital for achieving such high spatial resolution. However, when the electron beam passes through an electro­static or magnetic lens, the rays of electrons exhibit different focal positions depending on the focusing angle and the beam spreads out at the focus. Correcting this spherical aberration is costly and complex, meaning that only a select few scientists and companies possess electron microscopes with atomic resolution.

Researchers from Tohoku University have proposed a new method to form an electron lens that uses a light field instead of the electro­static and magnetic fields employed in conventional electron lenses. A pondero­motive force causes the electrons traveling in the light field to be repelled from regions of high optical intensity. Using this pheno­menon, a doughnut-shaped light beam placed coaxially with an electron beam is expected to produce a lensing effect on the electron beam.

The researches theo­retically assessed the charac­teristics of the light-field electron lens formed using a typical doughnut-shaped light beam – a Bessel or Laguerre-Gaussian beam. From there, they obtained a simple formula for focal length and spherical aberration coefficients which allowed them to determine rapidly the guiding parameters necessary for the actual electron lens design. The formulas demons­trated that the light-field electron lens generates a negative spherical aberration which opposes the aberration of electro­static and magnetic electron lenses. The combination of the conventional electron lens with a positive spherical aberration and a light-field electron lens that offset the aberra­tion reduced the electron beams size to the atomic scale. This means that the light-field electron lens could be used as a spherical aberration corrector.

“The light-field electron lens has unique charac­teristics not seen in conven­tional electro­static and magnetic electron lenses,” says Yuuki Uesugi, assistant professor at the Institute of Multidisciplinary Research for Advanced Materials at Tohoku University. “The reali­zation of light-based aberration corrector will signi­ficantly reduce installation costs for electron micro­scopes with atomic resolution, leading to their widespread use in diverse scientific and industrial fields,” adds Uesugi. Looking ahead, Uesugi and colleagues are exploring ways for the practical appli­cation of next-generation electron micro­scopes using the light-field electron lens. (Source: Tohoku U.)

Reference: Y. Uesugi et al.: Properties of electron lenses produced by ponderomotive potential with Bessel and Laguerre–Gaussian beams, J. Opt. 24, 054013 (2022); DOI: 10.1088/2040-8986/ac6524

Link: Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Miyagi, Japan