Emulating exceptional-point encirclements using imperfect (leaky) photonic components: asymmetric mode-switching and omni-polarizer action

Abstract Dynamically encircling an exceptional point (EP) in parity-time (PT) symmetric waveguide systems exhibits interesting chiral dynamics that can be applied to asymmetric mode switching for symmetric and anti-symmetric modes. The counterpart symmetry-broken modes (i.e., each eigenmode is localized in one waveguide only), which are more useful for applications such as on-chip optical signal processing, exhibit only non-chiral dynamics and therefore cannot be used for asymmetric mode switching. Here, we solve this problem by resorting to anti-parity-time (anti-PT) symmetric systems and utilizing their unique topological structure, which is very different from that of PT-symmetric systems. We find that the dynamical encircling of an EP in anti-PT-symmetric systems with the starting point in the PT-broken phase results in chiral dynamics. As a result, symmetry-broken modes can be used for asymmetric mode switching, which is a phenomenon and application unique to anti-PT-symmetric systems. We perform experiments to demonstrate the new wave-manipulation scheme, which may pave the way towards designing on-chip optical systems with novel functionalities.

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Exceptional Points and Asymmetric Mode Switching in Plasmonic Waveguides

Journal of Lightwave Technology ◽

10.1109/jlt.2016.2609899 ◽

2016 ◽

Vol 34 (22) ◽

pp. 5258-5262 ◽

Cited By ~ 53

Author(s):

Shaolin Ke ◽

Bing Wang ◽

Chengzhi Qin ◽

Hua Long ◽

Kai Wang ◽

...

Keyword(s):

Mode Switching ◽

Plasmonic Waveguides ◽

Asymmetric Mode ◽

Exceptional Points

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Topological mode switching in modulated structures with dynamic encircling of an exceptional point

Proceedings of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rspa.2020.0766 ◽

2021 ◽

Vol 477 (2245) ◽

pp. 20200766

Author(s):

Linlin Geng ◽

Weixuan Zhang ◽

Xiangdong Zhang ◽

Xiaoming Zhou

Keyword(s):

Energy Input ◽

Normal Modes ◽

Exceptional Point ◽

Mode Switching ◽

Input State ◽

Dynamic Modulation ◽

Exceptional Points ◽

Modulated Structures ◽

Starting Point ◽

Open Question

Exceptional points are special degeneracies occurring in non-Hermitian systems at which both eigenfrequencies and eigenmodes coalesce simultaneously. Fascinating phenomena, including topological, non-reciprocal and chiral energy transfer between normal modes, have been envisioned in optical and photonic systems with the exceptional point dynamically encircled in the parameter space. However, it has remained an open question of whether and how topological mode switching relying on exceptional points could be achieved in mechanical systems. The present paper studies a two-mode mechanical system with an exceptional point and implements the dynamic encircling of such a point using dynamic modulation mechanisms with time-driven elasticity and viscosity. Topological mode switching with robustness against the input state and loop trajectories has been demonstrated numerically. It is found that the dynamical encircling of an exceptional point with the starting point near the symmetric phase leads to chiral mode transfer controlled mainly by the encircling direction, while non-chiral dynamics is observed for the starting point near the broken phase. Analyses also show that minor energy input is required in the process of encircling the exceptional point, demonstrating the intrinsically motivated behaviour of topological mode switching.

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Chiral mode transfer of symmetry-broken states in anti-parity-time-symmetric mechanical system

Proceedings of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rspa.2021.0641 ◽

2021 ◽

Vol 477 (2256) ◽

Author(s):

Linlin Geng ◽

Weixuan Zhang ◽

Xiangdong Zhang ◽

Xiaoming Zhou

Keyword(s):

Mechanical Systems ◽

Pt Symmetry ◽

Exceptional Point ◽

Mode Switching ◽

Symmetric System ◽

Starting Point ◽

Spring System ◽

Mass Spring ◽

Open Question ◽

Symmetric Systems

Non-Hermitian systems with parity-time (PT) symmetry reveal rich physics beyond the Hermitian regime. As the counterpart of conventional PT symmetry, anti-parity-time (APT) symmetry may lead to new insights and applications. Complementary to PT-symmetric systems, non-reciprocal and chiral mode switching for symmetry-broken modes have been reported in optics with an exceptional point dynamically encircled in the parameter space of an APT-symmetric system. However, it has remained an open question whether and how the APT-symmetry-induced chiral mode transfer could be realized in mechanical systems. This paper investigates the implementation of APT symmetry in a three-element mass–spring system. The dynamic encircling of an APT-symmetric exceptional point has been implemented using dynamic-modulation mechanisms with time-driven stiffness. It is found that the dynamic encircling of an exceptional point in an APT-symmetric system with the starting point near the symmetry-broken phase leads to chiral mode switching. These findings may provide new opportunities for unprecedented wave manipulation in mechanical systems.

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