New time-resolved photoemission microscopy setup pushes resolution limits

Time-resolved photoemission microscopy (TR-PEEM) is a technique that combines the excellent time resolution of ultrafast optical excitation with the high spatial resolution of photoemission electron microscopy. The method has the unique ability to investigate ultrafast dynamics on the nanoscale.

Brixner et al. present an experimental setup using TR-PEEM that enables an impressive 3 nm lateral resolution and optical excitation by a broadband tunable (215-970 nm) dual-output laser source that can generate sub-20 fs excitation pulses at a 1 MHz repetition rate.

The researchers were able to create phase-coherent pulse pairs in an actively phase-stabilized interferometer for ultrafast single-color or two-color pump–probe imaging experiments. They further implemented a liquid-crystal-based pulse shaper that was used to create phase-stable, multi-pulse sequences for coherent 2D spectroscopy of photoelectron emission at the nanoscale.

As an experimental validation of their setup, the authors successfully performed single-pulse spectroscopy, single-color pump–probe imaging, and coherent 2D spectroscopy on plasmonic gold nanostructures. By analyzing the emitted photoelectron yield, the TR-PEEM system revealed differences in the temporal dynamics on a 33 nm length scale. In addition, coherent 2D nanoscopy spectra were obtained from a 12 nm wide region of interest within one photoemission hot spot. A two-color pump–probe scheme applied to a monolayer 2D material (WSe₂) revealed local differences of the population dynamics.

The new TR-PEEM setup allows researchers to investigate the dominating mechanisms of energy transport with high temporal and spatial resolution, which has relevance for the efficiency of new functional materials. It also can be used to explore couplings in hybrid nanostructures and single quantum systems, and how these are influenced by external parameters like the substrate or the optical excitation field.

Source: “Space- and time-resolved UV-to-NIR surface spectroscopy and 2D nanoscopy at 1 MHz repetition rate,” by Bernhard Huber, Sebastian Pres, Emanuel Wittmann, Lysanne Dietrich, Julian Lüttig, Daniel Fersch, Enno Krauss, Daniel Friedrich, Johannes Kern, Victor Lisinetskii, Matthias Hensen, Bert Hecht, Rudolf Bratschitsch, Eberhard Riedle, and Tobias Brixner, Review of Scientific Instruments (2019). The article can be accessed at http://doi.org/10.1063/1.5115322 .