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Optical Near-field Electron Microscopy

Periodic Reporting for period 1 - ONEM (Optical Near-field Electron Microscopy)

Reporting period: 2021-01-01 to 2021-12-31

Ever since the discovery of bacteria by van Leeuwenhoek in the 17th century, advances in microscopy have led to ground-breaking discoveries, that revolutionized science, technology, and medicine. Our consortium aims at introducing Optical Near-field Electron Microscopy (ONEM), a new concept for label-free imaging of interfaces with nanometric spatial resolution, and without dose-induced specimen damage.

This is achieved via a unique combination of light and electron optics: First, the specimen is illuminated with light, which is non-invasive. Very close to the sample, the resulting optical fields are converted into an electron beam using the photo-electric effect. Since the local beam current is proportional to the local light intensity, imaging the electrons yields information about the sample under study.

Our consortium sets out to build the world’s first ONEM. We aim at proof-of principle demonstrations of our new technology in electrochemistry, plasmonics, and membrane biology. A more detailed description of ONEM can be found in the following publication:
Raphaël Marchand, Radek Šachl, Martin Kalbáč, Martin Hof, Rudolf Tromp, Mariana Amaro, Sense J. van der Molen, and Thomas Juffmann, Optical Near-Field Electron Microscopy, Phys. Rev. Applied 16, 014008 (2021).
In the first period of the project we have focused on realizing the key components that are required for realizing ONEM:

1. A ONEM microscope: we decided to base our first ONEM on an existing aberration-corrected low energy electron microscope (LEEM). We designed a light optics scheme that can be retrofitted onto the LEEM system for illumination. It will enable illumination at various wavelength and with controllable polarization.

2. A sample holder: ONEM requires an ultrathin membrane, which separates the specimen from the photocathode and the adjacent vacuum. We have developed methods for creating, and characterizing graphene and SiN membranes, and have investigated several approaches for their subsequent functionalization.

3. An ultrathin photocathode that converts the optical near-fields into a spatially varying electron beam. We have learned how to grow efficient ultrathin alkali antimonide photocathodes using pulsed laser-deposition and thermal evaporation of the constituents. We have also implemented the necessary technologies within our ONEM apparatus.
Once our prototype is up and running we aim at proof-of principle demonstrations in three scientific fields with large socio-economic impact:

1) Plasmonics: It is often challenging to characterize light-material interactions and plasmonic devices on the nanometer scale - especially in a liquid environment. ONEM can do that and will facilitate the design and implementation of such devices that have both scientific and clinical applications.

2) Electrochemistry: Corrosion, electro-plating, battery charging – all these electrochemical processes are of vital importance to our society. And yet, it is difficult to study them on the nanoscale. Uniquely, ONEM could study them in-situ, with nanometric resolution, large field of view, and without beam-induced artefacts.

3) Membrane biology: Superresolution fluorescence microscopy has led to significant advances in our understanding of molecular biology. ONEM promises to offer similar spatial resolution, but for unlabelled specimens. We will use ONEM to study the dynamics of supramolecular protein complexes in tethered bilayers, which can be formed close to the photocathode. Such studies could promote ONEM as an innovative tool for biology, the life sciences, and medicine.
The Low Energy Electron Microscope at Leiden University that will form the basis for ONEM.