Philipp Ambichl

Breaking of PT-symmetry in bounded and unbounded scattering systems

PT-symmetric scattering systems with balanced gain and loss can undergo a symmetry-breaking transition in which the eigenvalues of the non-unitary scattering matrix change their phase shifts from real to complex values. We relate the PT-symmetry breaking points of such an unbounded scattering system to those of underlying bounded systems. In particular, we show how the PT-thresholds in the scattering matrix of the unbounded system translate into analogous transitions in the Robin boundary conditions of the corresponding bounded systems. Based on this relation, we argue and then confirm that the PT-transitions in the scattering matrix are, under very general conditions, entirely insensitive to a variable coupling strength between the bounded region and the unbounded asymptotic region, a result that can be tested experimentally and visualized using the concept of Smith charts.

Spatiotemporal Control of Light Transmission through a Multimode Fiber with Strong Mode Coupling

We experimentally generate and characterize eigenstates of the Wigner-Smith time-delay matrix, called principal modes, in a multimode fiber with strong mode coupling. The unique spectral and temporal properties of principal modes enable global control of temporal dynamics of optical pulses transmitted through the fiber, despite random mode mixing. Our analysis reveals that well-defined delay times of the eigenstates are formed by multipath interference, which can be effectively manipulated by spatial degrees of freedom of input wave fronts. This study is essential to controlling dynamics of wave scattering, paving the way for coherent control of pulse propagation through complex media.

Generating Particlelike Scattering States in Wave Transport

We introduce a procedure to generate scattering states which display trajectorylike wave function patterns in wave transport through complex scatterers. These deterministic scattering states feature the dual property of being eigenstates to the Wigner-Smith time-delay matrix Q and to the transmission matrix t(†)t with classical (noiseless) transmission eigenvalues close to 0 or 1. Our procedure to create such beamlike states is based solely on the scattering matrix and successfully tested numerically for regular, chaotic, and disordered cavities. These results pave the way for the experimental realization of highly collimated wave fronts in transport through complex media with possible applications such as secure and low-power communication.

Wave propagation through disordered media without backscattering and intensity variations

A fundamental manifestation of wave scattering in a disordered medium is the highly complex intensity pattern the waves acquire due to multi-path interference. Here we show that these intensity variations can be entirely suppressed by adding disorder-specific gain and loss components to the medium. The resulting constant-intensity waves in such non-Hermitian scattering landscapes are free of any backscattering and feature perfect transmission through the disorder. An experimental demonstration of these unique wave states is envisioned based on spatially modulated pump beams that can flexibly control the gain and loss components in an active medium.

Invariance property of wave scattering through disordered media

Significance The diffusion of particles and waves through disordered media encompasses a large variety of phenomena, from the motion of insects to the scattering of electrons or light in complex environments. One of the core features of diffusive transport is that the mean length of trajectories traversing a system depends only on the size of the system and of its boundary, which are both independent of the microscopic structure of the underlying medium. Here we show, based on insights from wave-scattering theory, that this fundamental invariance property can be significantly extended beyond the diffusive random walk picture. Our result not only provides an interesting link between all the diverse fields in which wave scattering plays a role but also holds promise for a number of practical applications. A fundamental insight in the theory of diffusive random walks is that the mean length of trajectories traversing a finite open system is independent of the details of the diffusion process. Instead, the mean trajectory length depends only on the systems boundary geometry and is thus unaffected by the value of the mean free path. Here we show that this result is rooted on a much deeper level than that of a random walk, which allows us to extend the reach of this universal invariance property beyond the diffusion approximation. Specifically, we demonstrate that an equivalent invariance relation also holds for the scattering of waves in resonant structures as well as in ballistic, chaotic or in Anderson localized systems. Our work unifies a number of specific observations made in quite diverse fields of science ranging from the movement of ants to nuclear scattering theory. Potential experimental realizations using light fields in disordered media are discussed.

Principal modes in multimode fibers: exploring the crossover from weak to strong mode coupling

We present experimental and numerical studies on principal modes in a multimode fiber with mode coupling. By applying external stress to the fiber and gradually adjusting the stress, we have realized a transition from weak to strong mode coupling, which corresponds to the transition from single scattering to multiple scattering in mode space. Our experiments show that principal modes have distinct spatial and spectral characteristic in the weak and strong mode coupling regimes. We also investigate the bandwidth of the principal modes, in particular, the dependence of the bandwidth on the delay time, and the effects of the mode-dependent loss. By analyzing the path-length distributions, we discover two distinct mechanisms that are responsible for the bandwidth of principal modes in weak and strong mode coupling regimes. Their interplay leads to a non-monotonic transition of the average principal mode bandwidth from weak to strong mode coupling. Taking into account the mode-dependent loss in the fiber, our numerical results are in qualitative agreement with our experimental observations. Our study paves the way for exploring potential applications of principal modes in communication, imaging and spectroscopy.

Focusing inside Disordered Media with the Generalized Wigner-Smith Operator

We introduce a wave front shaping protocol for focusing inside disordered media based on a generalization of the established Wigner-Smith time-delay operator. The key ingredient for our approach is the scattering (or transmission) matrix of the medium and its derivative with respect to the position of the target one aims to focus on. A specific experimental realization in the microwave regime is presented showing that the eigenstates of a corresponding operator are sorted by their focusing strength-ranging from strongly focusing on the designated target to completely bypassing it. Our protocol works without optimization or phase conjugation and we expect it to be particularly attractive for optical imaging in disordered media.

Spatiotemporal control of light transmission through a multimode fiber (Conference Presentation)

Optical pulses propagating through a multimode fiber with random mode mixing experience temporal broadening and distortion. Principal modes have been proposed to overcome modal dispersion. They are the eigenstates of the time delay operator and the associated eigenvalues are the delay times. Principal modes retain the spatial profiles of output fields to the first order of frequency variation. In the weak mode coupling regime, principal modes are superpositions of fiber eigenmodes with similar propagation constants. In the strong mode coupling regime, a principal mode is composed of all fiber modes with very different propagation constant, yet it has a well-defined delay time due to multipath interference, which can be controlled by adjusting the spatial profile of incident field. The spectral bandwidth of principal modes determines the temporal width of optical pulses that can be transmitted through the multimode fiber without distortion. In the weak mode coupling regime, principal modes with short and long delay times have broader bandwidths, while in the strong mode coupling regime, the principal modes with intermediate delay times have the broadest bandwidths. The opposite behaviors reveal two distinct mechanisms that are responsible for the principal mode bandwidth in the weak and strong mode coupling regimes. We further investigate how the mode-dependent loss modifies the principal modes. Our study provides physical understanding of spatiotemporal dynamics in a multimode fiber with varying degree of mode mixing, which is important for controlling pulse propagation through a multimode fiber.

Particle-like wave packets in complex scattering systems

A wave packet undergoes a strong spatial and temporal dispersion while propagating through a complex medium. This wave scattering is often seen as a nightmare in wave physics whether it be for focusing, imaging or communication purposes. Controlling wave propagation through complex systems is thus of fundamental interest in many areas, ranging from optics or acoustics to medical imaging or telecommunications. Here, we study the propagation of elastic waves in a disordered waveguide by means of laser interferometry. We demonstrate how the direct experimental access to the information stored in the scattering matrix of these systems allows us to selectively excite scattering states and wave packets that travel along individual classical trajectories. Due to their limited dispersion, these particle-like scattering states will be crucially relevant for all applications involving selective wave focusing and efficient information transfer through complex media.

Principal modes of a multimode fiber with strong mode coupling

We experimentally generate principal modes in a multimode fiber with strong mode coupling. They possess unique spectral and temporal properties. We reveal that principal modes are formed by multi-path interference effect, which is manipulated by the spatial degrees of freedom at the input.