Photonics with loss and disorder

Abstract

Progress in photonics has traditionally been linked to technological advances infabricating ever more complex optical devices. In this effort to create arbitrary dielectric structures, two components that have remained largely unexploited are loss and disorder. This is because of the prevailing view that the absorption of lightby the loss and the seemingly random scattering induced by a disordered material are disadvantageous and thus of no practical interest. Recent theoretical insights and the emerging experimental possibilities to shape and detect very complicated light fields are currently, however, changing this traditional way of thinking.In this thesis we try to accelerate this paradigm shift by exploring promising theoretical concepts for controlling the influences of loss and disorder and for turning them to an advantage. Based on our recent work, where we showed how adding a judiciously designed pattern of loss and gain on a given photonic structure leads to light fields that are immune to scattering in a disordered medium, we show here several novel phenomena with interesting features. We demonstrate how to make a (disordered) structure unidirectionally invisible by adding a tailored loss and gain distribution to it. Light propagating through such a structure cannot be distinguished from light that travels through a uniform structure, such that it can be considered as invisible although in homogeneities are still present, but compensated by gain and loss. Moreover, we present the first experimental implementation of our concept in a disordered acoustic waveguide where we demonstrate that by absorbing and amplifying sound waves in a well-defined way, we can make the waveguide one-way transparent for an incoming sound wave that would otherwise get perfectly reflected in the absence of gain and loss. By adding the right loss and gain distribution to a given structure, we can not only suppress scattering in the entire structure, but also force the wave to have a predetermined intensity distribution. This concept can be used to create a strong intensity focus at a certain position even inside disordered media, which is of great interest in many fields of physics. A conceptually different approach to cope with the presence of disorder without introducing loss and gain into the structure builds on the recent experimental breakthrough to characterize a highly complex medium by measuring its scattering matrix. Using these newly available data sets, we first show in a simple system how so-called “particle-like scattering states” – states that have a beam-like wave function – can be generated. Building up on this knowledge, we study the propagation of light waves in a smoothly correlated disorder which gives rise to the formation of a network of branches. We develop a strategy that allows us to address individual branches using particle-like states such that we can steer light through a disordered system on only a single branch rather than on multiple of them. Using a similar concept to the one used to find particle-like states, we devise strategies for micro-manipulating targets inside disordered materials in an optimal manner. Specifically, we study how wavefronts directed from the outside onto the medium have to be shaped to manipulate a target buried inside a disorder by applying a well-defined momentum, pressure or torque as well as to achieve a focus inside the target. In addition to the introduction of our novel theoretical approach we also show a corresponding implementation in a microwave setup that demonstrates our predictions in a convincing way. Ultimately, we show that perfect absorption of a wave’s intensity can be achieved by a small absorptive element embedded inside a disordered structure by carefully shaping the incident wavefront, the wave’s frequency as well as the amount of absorption. This effect of “coherent perfect absorption” is, for the first time, demonstrated experimentally in a disordered medium in a microwave setup. With this work we hope to trigger further experiments that demonstrate the enormous potential of loss and disorder for innovation in photonics.