Shining a Light on Metamaterials

Illustration of ultrafast electron diffraction of optically-excited metamaterials. (Source: K. Mohler, LMU)

Physicists from the University of Konstanz, Ludwig-Maximilians-Univer­sität München (LMU Munich) and the University of Regensburg have success­fully demonstrated that ultrashort electron pulses experience a quantum mechanical phase shift through their interaction with light waves in nano­photonic materials, which can uncover the nanomaterials’ func­tionality. So, nanostructured meta­materials are used to generate novel optical effects for the purpose of developing particularly efficient solar cells, cloaking devices or catalysts. These materials achieve their extra­ordinary properties through a grid-like arrangement of smallest building blocks on length scales well below the wavelength of the excitation.

The characterization and development of such meta­materials requires a deep under­standing of how the incident light waves behave when they hit these tiny structures and how they interact with them. Consequently, the optically-excited nano­structures and their electromagnetic near fields must be measured at spatial resolutions in the range of nanometers and, at the same time, at temporal resolutions below the duration of the excitation cycle (~10-15 s). However, this cannot be achieved with conven­tional light microscopy alone. In contrast to light, electrons have a rest mass and therefore offer 100,000 times better spatial resolution than photons. In addition, electrons can be used to probe electro­magnetic fields and potentials due to their charges. A team led by Peter Baum has now succeeded in applying extremely short electron pulses to achieve such a measurement. To that end, the duration of the electron pulses was compressed in time by means of terahertz radiation to such an extent that the researchers were able to resolve the optical oscillations of the electro­magnetic near fields at the nanostructures in detail.

“The challenge involved with this experiment lies in making sure that the reso­lution is sufficiently high both in space and in time. To avoid space charge effects, we only use single electrons per pulse and acce­lerate these electrons to energies of 75 kilo­electron volts”, explains Baum, head of the working group for light and matter at the University of Konstanz’s Department of Physics. When being scattered by the nano­structures, these extremely short electron pulses inter­fere with themselves due to their quantum mechanical properties and generate a diffraction image of the sample.

The investigation of the optical-excited nano­structures is based on pump-probe experiments. After the optical excitation of the near fields, the ultrashort electron pulse arrives at a defined point in time and measures the time-frozen fields in space and time. “Accor­ding to the predictions of Aharonov and Bohm, the electrons experience a quantum mechanical phase shift of their wave function when travelling through electro­magnetic potentials”, explains Kathrin Mohler, a doctoral researcher at LMU Munich. These optically-induced phase shifts provide information about the ultrafast dynamics of light at the nanostructures, ultimately delivering a movie-like sequence of images that reveals the inter­action of light with the nano­structures.

These experiments illustrate how electron holography and diffraction can be harnessed in the future to improve our under­standing of fundamental light-matter interactions underlying nano­photonic materials and meta­materials. In the long term, this may even lead to the development and optimi­zation of compact optics, novel solar cells or efficient catalysts. (Source: U. Konstanz)

Reference: K. J. Mohler et al.: Ultrafast electron diffraction from nanophotonic waveforms via dynamical Aharonov-Bohm phases, Sci. Adv. 6, eabc8804 (2020); DOI: 10.1126/sciadv.abc8804

Link: Ultrafast Electron Imaging Research Group, Ludwig-Maximilians Universität München, Munich, Germany

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