Different laws of physics apply to bodies that are less than a hair's breadth apart than to objects that are further away - for example, in the transfer of heat. Oldenburg physicist Svend-Age Biehs explores these amazing phenomena. He was recently honoured with the Gustav Hertz Prize of the German Physical Society.
Hyperbolic metamaterials, magneto-plasmonics and thermotronics: when Dr Svend-Age Biehs is asked to describe his research in a few words, even many physicists are at a loss for words. This is normal, reassures the habilitated scientist from the Condensed Matter Theory working group. "I often feel the same way when I hear a lecture from a colleague who is working on a completely different topic," he admits.
The fact that the technical terms in Bieh's field of research sound particularly exotic may be due to the fact that much of it is still relatively new. The 41-year-old is dedicated to the question of how heat is transferred at the nanoscale, over distances of a millionth of a metre or less. Physicists refer to this as near-field heat transport. "This field of research didn't even exist 20 years ago," says Biehs. The scientist, currently a Heisenberg scholarship holder of the German Research Foundation, addressed the problem of heat transport over very small distances in his Diplom thesis back in 2004 - and has since made a decisive contribution to advancing the field. This was recognised by the German Physical Society in March with the Gustav Hertz Prize, its most important award for young scientists. Biehs has "made a wealth of pioneering contributions to the theoretical foundations and future applications of thermal near-field effects", according to the citation.
More heat than Planck allows
The fascinating thing about near-field heat transport is that theoretical physicists are breaking new ground in exploring it, and well-known laws are failing. For example, it has been shown that heat transport between two objects increases dramatically if they are less than around ten micrometres apart at room temperature. "In principle, any material can transfer much more heat in the near field than should be possible according to Planck's law of radiation," explains Biehs.
The fundamental law of blackbody radiation by physicist Max Planck describes how the radiation from the sun or a light bulb, for example, is distributed across the electromagnetic spectrum - and how this depends on the temperature of the respective emitter. According to this, there is a maximum amount of energy that an object can emit via radiation. However, this limit does not apply in the near field, as theory and experiments have shown in recent years. The excess is known as "super-Planckian radiation".
Svend-Age Biehs continued a theory developed back in the 1950s that explains this phenomenon in his habilitation thesis. According to this theory, super-Planckian radiation comes from electromagnetic waves that propagate on the surface of a body or are completely reflected there. The decisive factor: These waves do not end abruptly at the surface, but also exist in a tiny area beyond the boundary. If there is another object there, the waves can jump over the gap and heat the neighbouring body. Biehs modified the formula that describes the additional heat flow so that the contribution of the individual waves can be better understood.
Targeted control of heat flows
Another interesting aspect of near-field heat transport is its wide range of possible applications. For example, researchers in Oldenburg led by Prof Dr Achim Kittel have developed a globally unique microscope that can measure heat flows between a sample and a tip belonging to the microscope. "This scanning thermal microscope can be seen as a kind of thermal imaging camera for the nanoscale," says Biehs. The device makes it possible to measure previously inaccessible properties of solids.
Another idea is to develop technologies with which heat can be converted into electricity more effectively than before - a kind of thermo-photovoltaics on the nanoscale. Biehs is exploring the underlying theory together with Dr Philippe Ben-Abdallah from the French state research institution Institute d'Optique in Palaiseau near Paris. Among other things, the two are interested in the question of how much the super-Planckian heat flow can be increased. They are focussing, for example, on materials that are made up of alternating extremely thin layers of a metal and a non-conductive material. This class of materials, known as "hyperbolic metamaterials" due to their unusual properties, can be designed to radiate heat particularly efficiently in the near field, according to the calculations of the two researchers. This could be useful for the targeted control of heat flows at the nanoscale.
Biehs and Ben-Abdallah have also developed ideas for components that can fulfil similar functions to electronic switching elements. In reference to electronics, they speak of "thermotronics". A thermal diode, for example, would only allow the heat flow to pass in one direction. A thermal near-field transistor could be used to switch, amplify or modulate the heat flow between two bodies in a targeted manner. "In principle, you could use these elements for logical operations like in a computer, but this would be far too slow for practical applications," says Biehs. He sees potential applications in nanotechnology, where it could be necessary to heat or cool objects without contact, such as mirrors in high-precision laser experiments. The physicist is convinced that the well-known thermal radiation still has many more surprises in store.