With pad, pencil, and algorithms

A native of Ebikon, Switzerland, Dominik Sidler spent four years doing postdoctoral research in Hamburg at the Max Planck Institute for the Structure and Dynamics of Matter. Last year he returned to Switzerland with his wife and two children, who were born in Hamburg. Everything still feels a bit new, but Dominik Sidler is happy to be closer to his parents again. “And honestly, as an enthusiastic mountaineer, I was already struggling with withdrawal symptoms in the lowlands of northern Germany.”

What experiments can’t do

At PSI, Sidler works in the Laboratory for Material Simulations – exactly the right place for a physicist who has specialised in using computer models to develop theories and thus to predict the chemical properties of materials. “The simulations give me insights on the atomic level – this domain remains hidden in many experiments.” That then allows much more precise interpretation of measurements and observations from the experiments. 

Why didn’t he go into experimental research himself? For Sidler's taste, there are too many hard-to-control adjustment screws: “Theory and simulation suit me better. They’re cleaner.” Sidler particularly likes the fact that the physical mechanisms he uncovers with his theories and simulations are often astonishingly simple. The only difficulty lies in identifying them. “The search for this beautiful simplicity in the midst of complexity fascinates me.”

Enigmatic effects

Sidler also plans to combine the simple with the complex as part of his current project. It is called Unravelling the Mysteries of Vibrational Strong Coupling (UnMySt). It has been funded by the European Research Council (ERC) with ten million euros. Set to start in spring 2025, the project is scheduled to run for six years. But what mysteries are we talking about here? Those of polaritonic chemistry – a relatively young field of research that, despite its name, is strongly influenced by theoretical physics.

The mysterious thing is that the chemical properties of substances can be changed just by placing them in an optical resonator. This is a kind of experiment box with tiny mirrors arranged opposite each other. Inside the box, the interaction between matter and light is intensified, leading to what is known as a strong light-matter coupling. As has been observed in experiments, this coupling changes the reaction properties – that is, chemical reactions run slower or faster.

The strong interaction between light and matter has been known from laser physics for decades, but no one knows how it can change the chemical properties of a substance. Finding out is anything but simple, because the materials in the resonator do not change as a whole, but rather on the level of few individual molecules – a change that is local but nevertheless can lead to completely new chemical properties.

A new theory is needed!

The current standard theories say that such a local effect should not happen at all, which means these theories are inadequate. This is every theoretician's dream: It offers the opportunity to build a new explanatory foundation that comprehensively describes the physical mechanisms behind the phenomenon.

That is exactly what Sidler is working on within the framework of the project. He is developing reliable theories, simulation methods, and predictions for polaritonic chemistry. So far, the experimental results have only been hit and miss. A lot of experimental setups have been tried out – sometimes a chemical effect occurred, but mostly not. That should change as soon as the right theory is found. This then creates guiding principles for future experiments in which a desired chemical effect can be brought about in a targeted manner.

That could in turn pave the way for practical applications, for example in medicine. Here, polaritonic chemistry could one day help make it easier to separate right- and left-handed molecules in the synthesis of drugs. In this context, right- or left-handedness means that molecules are identical but have different configurations. One molecule is the mirror image of the other. The problem: While one molecule has a healing effect, its mirror image may be extremely harmful. Efficient separation is therefore essential so that unwanted side-effects can be avoided.

Interdisciplinarity makes it work better

Industry has already expressed interest, so Sidler's work is clearly seen as relevant to practice despite its focus on “dull” theory. And it is absolutely interdisciplinary, as the ERC project shows. Special knowledge, approaches, and ways of thinking from very different areas flow together: theoretical chemistry in Pennsylvania, experimental chemistry in Strasbourg, experimental physics in Tel Aviv, and theoretical physics in Hamburg and at PSI.

Sidler’s starting point for the project is the central question of why it is only in certain experiments that the chemical properties of materials change, while often nothing happens at all. A colleague in Hamburg will try to mathematically verify the equations he constructs for this purpose. These are then solved on powerful computers. Finally, the experimental colleagues get their turn: They have to see if an experiment will confirm the hypothesis. Their results in turn will form the basis for new theoretical models. “Only together can we reach our goal. The past ten years of research in the field of polaritonic chemistry have shown that isolated observations do not bring progress. Theory and experiment must complement each other,” says Sidler. 

A retreat for contemplation

Sidler feels well equipped for this tricky task – after all, he was a staff officer in the Swiss army for a long time. There he had training in endurance and perseverance. “Both are also essential in research, as first attempts often fail. The trick is to persevere – and to recognise in time that an approach is hopeless.” During his time in the military, Sidler also learned to take responsibility for others. When dealing with people, logic does not always get you anywhere. You need empathy and persuasiveness, as well as the ability to motivate and be a role model. These are insights that can easily be transferred to research. Sidler knows how to approach students and postdocs – supervising them is one of his tasks at PSI.

Sidler feels at home at PSI and says there’s nothing he lacks. The large research facilities can also be used for experiments as part of his project. But what a theoretician needs most of all is a place to retreat to, to close the door and program in peace or think deeply about an idea. Nevertheless, he still considers it ideal to have communal areas, with a blackboard and coffee, near the office. At such meeting points, “spontaneous encounters and discussions” occur – which often help to find solutions to troublesome problems. Sometimes this kind of exchange also gives rise to new research ideas, excellent matter for deep contemplation in the quiet of one's own room.