Chiral transmission by an open evolution trajectory in a non-Hermitian system

Exceptional points(EPs),at which two or more eigenvalues and eigenstates of a resonant system coalesce,are associated with non-Hermitian Hamiltonians with gain and/or loss elements.Dynamic encircling of EPs has received significant interest in recent years,as it has been shown to lead to highly nontrivial phenomena,such as chiral transmission in which the final state of the system depends on the encircling handedness.Previously,chiral transmission for a pair of eigenmodes has been realized by establishing a closed dynamical trajectory in parity-time-(PT-)or anti-PT-symmetric systems.Although chiral transmission of symmetry-broken modes,more accessible in practical photonic integrated circuits,has been realized by establishing a closed trajectory encircling EPs in anti-PTsymmetric systems,the demonstrated transmission efficiency is very low due to path-dependent losses.Here,we demonstrate chiral dynamics in a coupled waveguide system that does not require a closed trajectory.Specifically,we explore an open trajectory linking two infinite points having the same asymptotic eigenmodes(not modes in PT-and anti-PT-symmetric systems),demonstrating that this platform enables high-efficiency chiral transmission,with each eigenmode localized in a single waveguide.This concept is experimentally implemented in a coupled silicon waveguide system at telecommunication wavelengths.Our work provides a new evolution strategy for chiral dynamics with superior performance,laying the foundation for the development of practical chiral-transmission devices.

Complex skin modes in non-Hermitian coupled laser arrays

From biological ecosystems to spin glasses,connectivity plays a crucial role in determining the function,dynamics,and resiliency of a network.In the realm of non-Hermitian physics,the possibility of complex and asymmetric exchange interactions(|κ_(ij)|≠|κ_(ji)|)between a network of oscillators has been theoretically shown to lead to novel behaviors like delocalization,skin effect,and bulk-boundary correspondence.An archetypical lattice exhibiting the aforementioned properties is that proposed by Hatano and Nelson in a series of papers in late 1990s.While the ramifications of these theoretical works in optics have been recently pursued in synthetic dimensions,the Hatano-Nelson model has yet to be realized in real space.What makes the implementation of these lattices challenging is the difficulty in establishing the required asymmetric exchange interactions in optical platforms.In this work,by using active optical oscillators featuring non-Hermiticity and nonlinearity,we introduce an anisotropic exchange between the resonant elements in a lattice,an aspect that enables us to observe the non-Hermitian skin effect,phase locking,and near-field beam steering in a Hatano-Nelson laser array.Our work opens up new regimes of phase-locking in lasers while shedding light on the fundamental physics of non-Hermitian systems.