What happens between atoms and molecules when they react with each other cannot yet be observed directly. A team led by Jürgen Popp and Jer-Shing Huang has developed a strategy to amplify the weak signals of their interaction with light – a crucial step toward being able to see right into the middle of a chemical reaction in the future

We bring knowledge „to light“. When we gain insight, „a light comes on“. There are many metaphors about the insight-giving effect of light. And not without reason: „If we want to learn something about our environment, about matter and molecules that surround us and the fundamental processes that take place in them, we need light,“ says Jürgen Popp. „From the simplest microscopes to state-of-the-art imaging techniques based on the interaction of laser light with matter, light is the key to what we see.“ Popp and his teams at Leibniz IPHT and the Institute of Physical Chemistry at the University of Jena are researching spectroscopic methods that make it possible to see right into the middle of a chemical reaction.

To this end, they are optimizing Raman spectroscopy – a spectroscopic-analytical method that already has many applications, for example in the analysis of drinking water, foodstuffs, or clinical diagnostics for the detection of pathogens. Using Raman spectroscopy, any material and any molecule can be clearly identified without contact. „The spectrum of a sample is like a chemical fingerprint,“ illustrates Michael Schmitt from Popp’s team. However: the effect of inelastic light scattering underlying the Raman signal is a weak process; if the researchers want to look at individual molecules, they have to amplify the signals to be able to measure them at all.

This is precisely what Jürgen Popp and Michael Schmitt are working on together with Jer-Shing Huang, head of the research department Nanooptics at Leibniz IPHT, as part of the Collaborative Research Center „Nonlinear optics down to atomic scales“ at the University of Jena. At the end of 2020, they took a first decisive step: with partners from Taiwan, the Jena researchers combined two methods to effectively amplify the Raman signal.

Weak Raman signal is ­amplified plasmonically

On the one hand, they use plasmonic nanostructures for this purpose. Such optical antennas can be used to illuminate even nanometer-sized areas and thus increase the resolution of imaging methods. Here, however, the researchers use plasmonic structures to amplify the weak Raman signal itself: Electrons in the nanostructures are excited with a laser to form so-called surface plasmons. This creates a strong electric field with which molecules are absorbed and can interact at the nanostructure surfaces. This „surface-enhanced Raman scattering“ (SERS) enhances the interaction between Raman excitation light and the molecules under investigation and thus also the intensity of the Raman scattering.

Nanostructures of gold

For the experiments, the research team uses nanostructures made of gold. These are milled into mirror-smooth individual crystals, known as gold flakes, that are only hundreds of micrometers wide and about 300 nanometers thin. „We use different sizes and shapes of the nanostructures and want to find out how the design affects the plasmonic effect,“ explains Jer-Shing Huang. In doing so, the researchers are proceeding in a highly planned manner: Theoretical groups of the SFB led by Stefanie Gräfe and Ulf Peschel from the University of Jena first model the interaction of the structures with light on the computer in order to ­derive the optimal design parameters for the desired effect.

In addition to the SERS method, the researchers use another way to amplify the Raman signal: Through nonlinear interactions between light and material, the Raman-scattered light excited via intense short-pulse lasers is coherently focused. As a result, „Coherent anti-Stokes Raman Scattering“ (CARS) also leads to amplified Raman signals.

The researchers conclude that this know-how can significantly improve the detection limit of Raman spectroscopy. „In addition to the existing advantages of the method – such as the fact that sample molecules can be used directly without dyes – there is now a high sensitivity,“ Jürgen Popp makes clear. The goal, adds Michael Schmitt, is to refine the methodology to the point where it can be used to directly observe chemical reactions on single molecules, „the dream of every chemist.“


Plasmonic Nanostructures

These are tiny optical antennas. As with radio or television antennas, optical antennas can be used to concentrate electromagnetic waves in one place and convert the wave into an electric current or, conversely, to radiate an electric signal in the form of a wave. The length of the antenna is adapted to the wavelength of the electromagnetic radiation. Unlike radio waves, which have a wavelength of several meters, visible light has wavelengths of only about 380 to 780 nm. Optical antennas must therefore be extremely small. Only the development of nanotechnology in the 2nd half of the 20th century made it possible to produce such small structures.

When the nanostructures are illuminated, the electromagnetic light wave and the mobile conduction electrons in the metal interact. The electrons are set into collective oscillations called surface plasmon polaritons, or plasmons. This allows much smaller structures to be illuminated and detected than with an ­ordinary light microscope.