Light in the smallest of all possible worlds

Text: Bettina Mittelstraß

Vahid Sandoghdar makes things visible that no-one else has seen before. The internationally-renowned experimental physicist de-velops methods for observing and investigating matter on the nanometre scale and pushes back the boundaries of the discern-able. His name is directly linked to the development of the young research f eld of nano-optics, and his groundbreaking research will ensure that the Erlangen-Nürnberg region becomes a focus of in-ternational attention.

When a pilot looks down at the earth from a plane it is so small that he cannot always identify houses, let alone the reflector on a rotating bicycle wheel. The pilot’s perspective is comparable to that of the scientist examining the minuscule world on the borderline of solid matter. But, unlike the pilot, physicist Vahid Sandoghdar can perceive the very slightest fluorescence and observe individual building blocks in the nano-world which are more than 1000 times smaller than the diameter of a hair.

If you want to see the particles on the borderlines of the nanometre scale you need to receive signals from there – for example, light. Vahid Sandoghdar’s ambition focuses on the individual molecule.

“I’m determined to take a pure gedankenexperiment and implement it exactly in the lab,” says the inventive physicist. Where others give up, he refines the set-up of the experiment in the minutest detail. At the University of Konstanz in 2000, he managed to make a single molecule at the tip of a glass fibre glow by applying laser irradiation. This technique can be used for so-called near-field microscopy, especially as structures that are smaller than the wave-length of the light used cannot be seen using standard optical microscopes. This is why scientists direct the very smallest probes right up close to the object under scrutiny. The interaction between the light from the probe and the matter is evaluated by raster scanning – the combined data produces an image. Sandoghdar’s ‘nanoscopic probe’ is the tiniest possible and consequently achieves the greatest precision in the optical analysis of structure on the nanometre scale.

At ETH Zurich Sandoghdar demonstrated that a single molecule can block a laser beam or intensify a light beam. Again, using just a single molecule he successfully managed to create the first, smallest possible optical transistor. The results of his pure research are recognised worldwide and are generating, amongst other things, efforts to process signals optically rather than electronically.

Against the backdrop of his international career Vahid Sandoghdar has brought exceptionally broadly-based expertise to this area of southern Germany, where he has not only become a Humboldt Professor at the University of Erlangen-Nürnberg but also director of the Max Planck Institute for the Science of Light. Even in his habilitation Sandoghdar combined his experience of laser spectroscopy in the USA and his research into quantum optics in Paris with the latest findings in near-field microscopy and the first successful experiments to locate single molecules. The researcher is just as unimpressed by the boundaries between scientific fields as he is by the boundaries of the visible on the nanometre scale. Sandoghdar draws on the diversity of his knowledge and combines different experimental techniques in order to make the tiniest structures and particles visible. Today, ‘nano-optics’ describes the targeted combination of this expertise.

Observing single viruses

“It’s wonderful if you can combine your own pure research with applications,” says Vahid Sandoghdar. This is one of the things motivating him to concentrate more on using the interaction of light and matter on the nanometre scale in biophysics. “When it comes to interaction on the surface of cells there’s still a lot of research to be done in biochemistry, molecular biology and, indeed, bio-physics,” comments the Humboldt Professor. One of his most recent projects in the field of biophysics, for example, is a method for tracking down individual viruses and monitoring their path on an artificial cell membrane. “Basically, the virus disperses light like the reflector on a bicycle. If you work very carefully, you can discover it,” Sandoghdar explains. He presents the first films on a virus that appears as a luminous dot wandering around agitatedly on the surface of the membrane and getting stuck every so often. This revolutionary view of a complex biological system has led to new questions for researchers to address: Why does the virus get stuck? Is this the gate to the cell? How does the virus communicate?

In the field of nano-optics Vahid Sandoghdar has new ideas nearly every day. In order to continue actively shaping the dynamics of the subject, he thinks it is now even more important to take care selecting the best ideas. In his dual role as Humboldt Professor and institute director in Erlangen he wants to conduct high level optical research focusing on biophotonics and nano-quantum-optics. He also wants to bring together students, researchers, developers and users across the disciplines in an Optical Imaging Centre where tailored methods for cutting-edge research and new experiments can be developed. “There’s so much to learn, and I learn the best things when I’m doing the experiments myself.”