Forces on dipoles in optical fields with time-dependency

The fascinating mechanical interaction between light and matter is one of the foundations of modern quantum-optics. The last 40 years saw the development of numerous methods to trap and manipulate atoms, nano- and micro-particles in the light fields of one or several lasers. Tools such as optical lattices, optical tweezers and optical cavities have reached such a high level of precision, that particles cooled to the lowest temperatures now can be used to solve fundamental questions of quantum mechanics. Optical trapping and manipulation of larger objects such as biological cells has nowadays become an important cross-disciplinary effort in life-sciences. The rapid progress in this field of so called radiation forces requires also a detailed analysis of small effects. And it turns out that small additional radiation forces arise if the amplitude, phase or other characteristics of a laser-field are changed in the course of time. These additional effects can behave very differently as compared to what we are used from the typical, dominant forces. It can happen that atoms which are usually trapped at maximal values of the local light intensity are repelled from these maxima, if exposed to a time-varying laser field. The project f.dot - forces on dipoles in optical fields with time-dependency will target these peculiar additional forces from time-dependent electromagnetic fields. In collaboration with Prof. Steve Barnett at the University of Glasgow we will study how these presumed tiny forces can accumulate to significant effects over the course of time. A special focus will lie on designing favoured forces by a specific time-dependent manipulation of the involved light-fields. Finally we will study the effects of these additional forces on the interaction between several particles moving in the same light field. The knowledge about these unusual radiation forces gained in Glasgow shall be consolidated and spread within the Austrian scientific community in a final return phase at the Institute for Theoretical Physics at the University of Innsbruck. In view of the rapid progress in the area of controlled matter-light interaction, small but unexpected effects gain importance. This project shall help to reach the ambitious targets of quantum-optical research.

How do light and matter interact? Can a beam of light exert a force on a particle? Already Johannes Keppler asked these questions and suggested that radiation pressure from the sun helps to shape a comets tail. Today laser beams are routinely used in laboratories to trap, cool and study individual atoms. But this exciting progress in technology and experiment puts the burden on theorists to use extra care in their calculations. The interaction between light and matter is full of surprises and subtle details. One of these subtleties is the so called Röntgen interaction, a small correction in the theoretical description of light-matter interaction. Usually it is assumed that atoms only interact with the electric part of an electromagnetic wave (light). The Röntgen interaction however shows that there is also a weak coupling to the magnetic component of a radiation field. The Schrödinger fellowship f.dot forces on dipoles in optical fields with time dependence studied this Röntgen interaction with a special focus on radiation forces resulting from this subtle interaction. During a two year stay at the University of Glasgow and the return phase at the Leopold-Franzens-Universität Innsbruck we tried to probe the nature of this interaction. What happens if one modifies the intensity of a light beam? What forces will appear if atoms ate irradiated by two beams of different frequency? Are these effects big enough to be measured in current experiments? As so often, the route of research took unexpected turns. We could show that the Röntgen interaction opens the door for a second relativistic effect to appear in light matter interaction: Einsteins famous E = mc2 , the equivalence between mass and energy. When atoms absorb light, they take up energy. That means they also get a tiny bit heavier. This change in mass then appears well hidden in the equations describing the radiation forces on atoms and surprisingly this has been overlooked so far in this context. Although the aspect of mass and energy in atomic motion is mostly a theoretical curiosity, the results on Röntgen forces might be noticeable in experiments. Today the best clocks and most accurate measurements on fundamental constants are based on atoms trapped, cooled and controlled by light. Increased theoretical understanding of mechanical light-matter interaction will thus help us to optimise these experiments.