New Kind Of Microscopy Developed

The duration of their snapshot is a second like a second to the age of the universe: Together with the Australian scientist Tim Davis and the working group of Harald Gießen (University of Stuttgart), CENIDE physicists have developed a possibility with ultrafast vector microscopy to detect electrical fields on surfaces to be determined in high resolution in terms of time and space. This method was used to track the dynamics of optical skyrmions in time for the first time. In its current issue, the renowned science magazine publishes this breakthrough in nano-optics.

Interactions between light and matter are the basis of nano-optics. Researchers working with it can observe and influence the properties and states of tiny structures and even individual molecules using spectroscopic and microscopic methods. As with the optical computers still in its infancy: Since their structures are sometimes much smaller than the wavelength of light, tricks such as nano-antennas are required to be able to couple them effectively. But it is very difficult to analyze the electrical fields around such structures in space and time.

With vector microscopy based on time-resolved 2-photon photoemission microscopy, a team of physicists led by UDE professor Frank-J. Meyer zu Heringdorf, the Australian expert for nano-optics Dr. Timothy J. Davis and Professor Harald Gießen have now achieved a pioneering achievement: They were able to determine the electric fields on a metal surface with pinpoint accuracy and time - down to 10 nanometers of local resolution and in the sub-femtosecond range.

Ultra-short laser pulses combined with vector calculation
To do this, they used gold microcrystallites, on the surface of which they generated a surface plasmon polariton after nanostructuring using an ultrashort laser pulse; an electron wave that propagates on the surface. A few femtoseconds after the excitation, a second laser pulse reads out the electric field of the wave. However, the interrogation pulse can only analyze that component which is polarized identically, ie in which the electric field of the interrogating laser pulse and that of the plasmon on the surface point in the same direction.

The scientists reconstructed the field vectors by determining two field components in two experiments with different query polarizations. The third could then be calculated from the first two using the Maxwell equations. "This is a real breakthrough," explains Meyer zu Heringdorf, a member of the UDE Collaborative Research Center "Non-equilibrium dynamics of condensed matter in the time domain". "This way, every point of an electric field on a surface can be observed at any time - in the smallest structures."

Vibrating skyrmions observed
Because of their promising properties in magnetic systems, research is currently concerned with the question of whether the magnetic properties of skyrmions can also be transferred into optics. The researchers therefore demonstrated the value of the method they developed by first tracking the dynamics of optical skyrmions over time.

For this purpose, the team created plasmons on the gold surface, the electric fields of which formed optical skyrmions (see picture). The researchers then systematically increased the time interval between exciting and detecting laser pulses by around 100 attoseconds. A film of the up and down swinging skyrmions resulted from the sequence of the reconstructed field images.

Since the methodology is universally applicable to electrical fields on surfaces, vector microscopy can be used to investigate field distributions in optical nanostructures with a precision that would have been unthinkable just a few years ago.