image: Figure 1 | Non-Hermitian photonic waveguides with skin effects. a, Schematic diagram of a non-Hermitian photonic system consisting of curved waveguides A and straight waveguides B. b, Trajectory of the curved waveguides. c The phase of coupling with respect to the change of bending amplitude and coupling distance. d, f The eigenfunction distributions of the system, where non-Hermitian losses are introduced into the sublattices A (d) and B (f) e, g The eigenfunction distributions of the system when the on-site losses are introduced into the sublattices A (e) and B (g).
Credit: Guo-Huai Wang et al.
Non-Hermitian systems exhibit many novel physical phenomena absent in conventional Hermitian systems. A key hallmark is the EPs, where eigenvalues and eigenstates coalesce simultaneously, leading to unique physical effects. Another intriguing phenomenon is the NHSE, characterized by the accumulation of an extensive number of eigenstates at boundaries, which breaks the conventional bulk-boundary correspondence. Recent research has increasingly focused on their interplay. However, when focusing on the framework of interaction within multiple non-Hermitian systems, the interplay between these two non-Hermitian hallmarks remains a considerably unexplored territory both theoretically and experimentally.
In a new paper published in Light: Science & Applications, a team of researchers, led by Professor Qi-Dai Chen from Jilin University, China, and co-workers have revealed the interplay between skin modes and EPs by coupling two non-Hermitian systems, each exhibiting independent NHSE. The research team implemented the NHSE in curved dissipative waveguide arrays, where periodic bending-induced artificial gauge fields and non-Hermitian loss distributions serve as versatile degrees of freedom to manipulate the localization direction of the skin modes (Fig. 1). They demonstrated that introducing losses individually into waveguide A or B leads to wave-function localization at opposite boundary. (Fig. 1d–g).
Their further investigation revealed that by coupling two systems with skin modes induced by different loss distributions, multiple pairs of EPs can emerge in the corresponding momentum-space band structure, indicative of an underlying parity-time symmetry (Fig. 2). Increasing inter-system coupling drives a transition from broken phase to exact phase, widening the separation of EPs and leading to the spectral collapse under periodic boundary conditions (Fig. 2b). This trend induces a phase transition in the majority of skin modes, causing the localization feature to progressively weaken and transition towards delocalization, thereby suppressing the coupled skin effect (Fig. 2d). Interestingly, once the loss distributions of the two subsystems become mismatched, the EPs vanish (Fig. 2c), and the attenuation of the skin effect is significantly alleviated (Fig. 2e). The research team experimentally demonstrated the concept of the interplay between EP and coupled NHSE by measuring the light propagation dynamics in photonic waveguide arrays fabricated by femtosecond laser direct writing techniques.
Journal
Light Science & Applications
Article Title
Coupled non-Hermitian skin effect with exceptional points