Save time using maths: analytical tool designs corkscrew-shaped nano-antennae
The nano-antennae werde produced in an electron microscope by direct electron-beam writing. © HZB
For the first time, an HZB team has derived analytically how corkscrew-shaped nano-antennas interact with light. The mathematical tool can be used to calculate the geometry that a nano-antenna must have for specific applications in sensor technology or information technology.
The nanostructures from Katja Höflich's HZB team are shaped like corkscrews and made of silver. Mathematically, such a nano antenna can be regarded as an one-dimensional line that forms a helix, characterized by parameters such as diameter, length, number of turns per unit length, and handedness.
The nano corkscrews are highly sensitive to light: depending on frequency and polarisation, they can strongly enhance it. Because helical antennas have a handedness, they can select light quanta according to their handedness, i.e. their spin. This results in novel applications in information technology based on the spin quantum number of light. Another application may lay in sensor technology in detecting chiral molecular species down to the single molecule level.
Usually, the interaction of such nano-antennas with an electromagnetic field is determined using numerical methods. Each helix geometry, however, requires a new numerically expensive calculation.
For the first time, Höflich and her team have now derived an analytically exact solution of the problem. “We now have a formula that tells us how a nano-antenna with specific parameters responds to light”, says Höflich. This analytical description can be used as a design tool, as it specifies the required geometrical parameters of a nano-helix to amplify electromagnetic fields of desired frequencies or polarisation.
The HZB researchers were able to fabricate nano-antennae in an electron microscope at the CCMS corelab of HZB by using direct electron-beam writing. The electron beam first writes a helix-shaped carbon structure one point at a time. This structure is subsequently coated with silver. The actual measurements of the optical properties for these silver nano-antennae are in good agreement with the calculated properties predicted by the analytical model.
Optica (2019, Vol. 6, Issue 9): “Resonant behavior of a single plasmonic helix”; Katja Höflich, Thorsten Feichtner, Enno Hansjürgen, Caspar Haverkamp, Heiko Kollmann, Christoph Lienau, Martin Siles.
- Green hydrogen: A cage structured material transforms into a performant catalystClathrates are characterised by a complex cage structure that provides space for guest ions too. Now, for the first time, a team has investigated the suitability of clathrates as catalysts for electrolytic hydrogen production with impressive results: the clathrate sample was even more efficient and robust than currently used nickel-based catalysts. They also found a reason for this enhanced performance. Measurements at BESSY II showed that the clathrates undergo structural changes during the catalytic reaction: the three-dimensional cage structure decays into ultra-thin nanosheets that allow maximum contact with active catalytic centres. The study has been published in the journal ‘Angewandte Chemie’.
- An elegant method for the detection of single spins using photovoltageDiamonds with certain optically active defects can be used as highly sensitive sensors or qubits for quantum computers, where the quantum information is stored in the electron spin state of these colour centres. However, the spin states have to be read out optically, which is often experimentally complex. Now, a team at HZB has developed an elegant method using a photo voltage to detect the individual and local spin states of these defects. This could lead to a much more compact design of quantum sensors.
- Accelerator Physics: First electron beam in SEALabThe SEALab team at HZB has achieved a world first by generating an electron beam from a multi-alkali (Na-K-Sb) photocathode and accelerating it to relativistic energies in a superconducting radiofrequency accelerator (SRF photoinjector). This is a real breakthrough and opens up new options for accelerator physics.