分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： Useful in the enhancement of light-matter interaction, localization of light is at the heart of photonics studies. Different approaches have been proposed to localize light, including those based on dynamical localization, topological trivial or nontrivial defects in the band gap of photonic crystals, and bound states in the continuum. Recent studies on non-Hermitian skin effect have provided us new means to localize waves. In this work, we propose a new method towards localized light, called photonic corner skin modes arising from second-order non-Hermitian skin effect and gain-loss symmetry on a lattice. Specifically, we propose to make use of small pseudo-inversion symmetric gain/loss, which does not close the band gap, to realize a photonic Chern insulator with chiral edge states. The chiral edge states then accumulate at certain corners of the system. Intriguing phenomena such as corner skin modes arising from an underlying bipolar second-order non-Hermitian skin effect and multiple-corner skin modes are predicted in continuous systems.
分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： We present a one-dimensional coupled ring resonator lattice exhibiting a variant of the non- Hermitian skin effect (NHSE) that we call the anomalous Floquet NHSE. Unlike existing approaches to achieving the NHSE by engineering gain and loss on different ring segments, our design uses fixed on-site gain or loss in each ring. The anomalous Floquet NHSE is marked by the existence of skin modes at every value of the Floquet quasienergy, allowing for broadband asymmetric transmission. Varying the gain/loss induces a non-Hermitian topological phase transition, reversing the localization direction of the skin modes. An experimental implementation in an acoustic lattice yields good agreement with theoretical predictions, with a very broad relative bandwidth of around 40%.
分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： Non-Hermitian skin effect (NHSE) in non-Hermitian lattice systems, associated with a point gap on the complex energy plane, has attracted great theoretical and experimental interest. Much less is studied on the so-called second-order non-Hermitian skin effect, where the bulk does not support a point gap but localization at the corner still occurs. This work discovers a class of hybrid skin-topological modes as the second-order non-Hermitian skin effect without asymmetric couplings. Specifically, by only adding gain/loss to two-dimensional Chern insulators and so long as the gain/loss strength does not close the line gap, all the topological edge states are localized at one corner under the open boundary condition, with the bulk states extended. The resultant non-Hermitian Chern bands can be still topologically characterized by Chern numbers, whereas the hybrid skin-topological modes are understood via some auxiliary Hermitian systems that belong to either intrinsic or extrinsic second-order topological insulator phases. By proposing an innovative construction of auxiliary Hamiltonian, our generic route to hybrid skin-topological modes is further successfully extended to nonequilibrium topological systems with gain and loss, where the anomalous Floquet band topology is no longer captured by band Chern numbers. The extension thus leads to the intriguing finding of nonequilibrium hybrid skin-topological modes. In addition to offering a straightforward route to experimental realization of hybrid topological-skin effects, this study also opens up a promising perspective for the understanding of corner localization by revealing the synergy of three important concepts, namely, non-Hermitian topological insulator, second-order non-Hermitian skin effect, and second-order topological insulator.
分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： Two-dimensional (2D) laser arrays are shown to be achievable at a large scale by exploiting the interplay of higher-order topological insulator (HOTI) physics and the so-called non-Hermitian skin effect (NHSE). The higher-order topology allows for the amplification and hence lasing of a single-mode protected by a band gap; whereas the NHSE, widely known to accumulate population in a biased direction in non-Hermitian systems, is introduced to compete with the topological localization of corner modes. By tuning the system parameters appropriately and pumping at one site only, a single topologically protected lasing mode delocalized across over two dimensions emerges, with its power widely tunable by adjusting the pump strength. Computational studies clearly indicate that the lasing mode thus engineered is stable, and the phase difference between nearest lasing sites is locked at zero, even after the disorder is accounted for. The total power of the lasing mode forming a 2D topological laser array is proportional to the area of the 2D lattice accommodating a HOTI phase. Based on existing experiments, we further propose to use coupled optical ring resonators as a promising platform to realize large-scale 2D laser arrays.
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