Fast encirclement of an exceptional point for highly efficient and compact chiral mode converters

Abstract

Exceptional points (EPs) are degeneracies at which two or more eigenvalues and eigenstates of a physical system coalesce. Dynamically encircling EPs by varying the parameters of a non-Hermitian system enables chiral mode switching, that is, the final state of the system upon a closed loop in parameter space depends on the encircling handedness. In conventional schemes, the parametric evolution during the encircling process has to be sufficiently slow to ensure adiabaticity. Here, we show that fast parametric evolution along the parameter space boundary of the system Hamiltonian can relax this constraint. The proposed scheme enables highly efficient transmission and more compact footprint for asymmetric mode converters. We experimentally demonstrate these principles in a 57 μm-long double-coupled silicon waveguide system, enabling chiral mode switching with near-unity transmission efficiency at 1550 nm. This demonstration paves the way towards high-efficiency and highly integrated chiral mode switching for a wide range of practical applications.

Topological Photonic States in Waveguide Arrays

“…Some works have revealed that dynamically encircling EPs by adiabatically varying the parameters could lead to asymmetric mode transfer in a lossy waveguides system, thus realizing asymmetric mode/polarization converters. [134][135][136][137][138] Although the adiabaticity condition is still required for this system, the adiabatic approximation can be achieved via a fast parametric evolution along the boundary of the non-Hermitian Hamiltonian parameter space. Therefore, the mode-switching devices, requiring a small footprint, can be developed in the non-Hermitian regime.…”

Proximity‐Encirclement of Multiple Exceptional Points in Non‐Hermitian Photonic Waveguides

“…The study of non-Hermitian systems breaks the conventional scope of Hermitian Hamiltonians in closed systems, and also enriches the understanding about the quantum realm. [1] Once the gain or loss is introduced, exceptional points (EPs) are ubiquitous in non-Hermitian systems, such as those in optical microresonators, [2,3] coupled optical waveguides or cavities, [4][5][6] and fiber-based systems. [7,8] EPs are singularities where eigenvalues and the corresponding eigenvectors simultaneously coalesce, [9] and as one of the quintessential features in non-Hermitian physics, exhibit many topological properties that are not analogous to Hermitian systems.…”

“…[6,19,20] One of the most appealing phenomena is the state conversion when dynamically encircling an EP in a parameter space, exhibiting a chiral behavior. [21] This unique phenomenon has been demonstrated in myriad experiments of waveguides, [4,5,23,[25][26][27][28][29][30] evidencing the robustness to system perturbations including the form of the encircling loop and the device length. [22,23] The chiral switching is intriguing because the final state of system is solely determined by the encircling direction, regardless of the initial state.…”

“…[21,22,31] The overall chiral mechanism of dynamically encircling an EP is unraveled and analyzed theoretically through confluent hypergeometric functions in a two-level non-Hermitian system, [4,33] and successfully demonstrated in various experiments by encircling EPs. [4,5,[25][26][27][28][29][30]34] One might think that enclosing the EP is a prerequisite for the chiral dynamics. However, a recent experiment makes a breakthrough that the chiral feature is still preserved when encircling the EP's proximity, namely, without encircling an EP.…”

“…[ 21,22,31 ] The overall chiral mechanism of dynamically encircling an EP is unraveled and analyzed theoretically through confluent hypergeometric functions in a two‐level non‐Hermitian system, [ 4,33 ] and successfully demonstrated in various experiments by encircling EPs. [ 4,5,25–30,34 ]…”