Non-Hermitian Physics and Complex Scattering
The propagation of waves is a topic spanning many spatial and temporal scales, from the fundamental quantum aspects of light-matter interaction to the scattering of radio waves in complex media like the earth’s atmosphere. On all these levels the input from theoretical physics is essential for understanding and controlling such wave phenomena. At the institute, considerable effort is being dedicated to two specific topics in the vast field of wave physics: (i) the non-Hermitian physics associated with waves in systems that are subject to both amplification (gain) and dissipation (loss) as well as (ii) the scattering of waves in disordered media. In both of these research areas, we collaborate strongly with various experimental teams; to come as closely as possible to the situation encountered in the laboratory, we employ numerical techniques and run simulations on the Vienna Scientific Cluster.
Control of scattering properties
In the field of non-Hermitian physics (i), we are especially interested in controlling the scattering properties of systems by tailoring the spatial distribution of gain and loss in them. Using such an approach, we found, e.g., that a highly disordered system can be made completely transparent and even invisible by adding a tailored gain/loss distribution to it [see Fig. 7(a) for the case of a Gaussian laser beam]. Special attention also receives a particular non-Hermitian singularity, called an exceptional point, that leads to quite a number of fascinating phenomena that we could recently demonstrate in collaboration with different experimental groups in nano-photonics and in laser physics.
(a) A Gaussian laser beam entering a disordered medium from the left gets scattered and builds up a highly complicated interference pattern (left panel). By adding a tailored distribution of gain and loss to this medium, the beam can propagate like in free space (right panel). (b) A specially designed laser beam that applies a well-defined torque onto the quadratic target in the middle, turning it in clockwise direction.
Control of light propagation
In the field of complex scattering (ii) we are pursuing the challenging goal of controlling how light propagates through disordered media. (Think here of the speckle patterns arising when directing a laser beam at a piece of paper.) How to deal with the complex interferences arising in this context is a challenging question arising in many fields of physics—from biomedical optics to observational astronomy. What comes to our advantage here is the fact that scattering is a deterministic process—at least for classical waves—such that the shape of an incident wave front determines how the wave will propagate through a medium. This insight forms the basis for a series of modern experiments that use spatial light modulators to characterize and to control light fields even in strongly disordered media. Our contributions to this newly emerging field of wave front shaping include, e.g., a concept to generate waves that follow a specific path across a disordered medium or that focus onto a designated point inside of it. Moreover, we also showed how to design waves in order to micro-manipulate a target embedded inside a disordered environment. By tuning the incident wave front we could recently achieve the first realization of a random anti-laser, i.e. the time-reverse of a random laser in the sense that a random medium was shown to perfectly absorb a suitably engineered incoming wave front.