Dynamically encircling exceptional points in a waveguide: asymmetric mode switching from the breakdown of adiabaticity

TL;DR

This paper demonstrates the first experimental realization of dynamically encircling an exceptional point in a waveguide, revealing asymmetric mode switching and breakdown of adiabaticity, with potential applications in robust wave control.

Contribution

It introduces a novel method to map exceptional point encircling onto waveguide scattering, enabling direct observation of associated phenomena for the first time.

Findings

Successful experimental mapping of exceptional point encircling in a waveguide

Observation of asymmetric mode switching during transmission

Breakdown of adiabaticity effects in the waveguide system

Abstract

Physical systems with loss or gain feature resonant modes that are decaying or growing exponentially with time. Whenever two such modes coalesce both in their resonant frequency and their rate of decay or growth, a so-called "exceptional point" occurs, around which many fascinating phenomena have recently been reported to arise. Particularly intriguing behavior is predicted to appear when encircling an exceptional point sufficiently slowly, like a state-flip or the accumulation of a geometric phase. Experiments dedicated to this issue could already successfully explore the topological structure of exceptional points, but a full dynamical encircling and the breakdown of adiabaticity inevitably associated with it remained out of reach of any measurement so far. Here we demonstrate that a dynamical encircling of an exceptional point can be mapped onto the problem of scattering through a two-mode waveguide, which allows us for the first time to access the elusive effects occurring in this context. Specifically, we present experimental results from a waveguide structure that steers incoming waves around an exceptional point during the transmission process. In this way mode transitions are induced that make this device perfectly suited as a robust and asymmetric switch between different waveguide modes. Our work opens up new and exciting avenues to explore exceptional point physics at the crossroads between fundamental research and practical applications.