基于五能级结构手征原子媒质，对光学时域隐身在外场调控下的实现进行了研究。在该五能级手征原子系统下，推导了原子媒质与光场参数相关的等效电磁响应参数的广义表达式，进而根据群折射率计算输出光脉冲。数值计算了五能级原子系统的外场以及原子媒质相关参数对隐身时域窗口大小以及形成时域窗口的光脉冲强度的影响。结果表明，时域窗口大小随控制场Rabi频率改变但基本不依赖于控制场的相位变化；两个耦合场相位在一个周期内的变化对左右旋圆极化输出波特性影响相反；两个耦合场中，当其中一个耦合场的Rabi频率减小时，对应时域窗口增大，但同时也会使形成窗口的脉冲强度减弱，而另一个耦合场Rabi频率则对时域窗口的大小影响较小。因此改变控制场和两个耦合场的强度、相位和失谐等参量可有效地调控输出脉冲的加速和延迟，产生显著的隐身时域窗口，提供了一种可操控的光学时域隐身的方案。

Atomic arrays provide an important quantum optical platform with photon-mediated dipole–dipole interactions that can be engineered to realize key applications in quantum information processing. A major obstacle for such applications is the fast decay of the excited states. By controlling two-band Bloch oscillations of single excitation in an atomic array under an external magnetic field, here we show that exotic subradiance can be realized and maintained with orders of magnitude longer than the spontaneous decay time in atomic arrays with the finite size. The key finding is to show a way for preventing the wavepacket of excited states scattering into the dissipative zone inside the free space light cone, which therefore leads to the excitation staying at a subradiant state for an extremely long decay time. We show that such operation can be achieved by introducing a spatially linear potential from the external magnetic field in the atomic arrays and then manipulating interconnected two-band Bloch oscillations along opposite directions. Our results also point out the possibility of controllable switching between superradiant and subradiant states, which leads to potential applications in quantum storage.

We propose and experimentally realize a class of quasi-one-dimensional topological lattices whose unit cells are constructed by coupled multiple identical resonators, with uniform hopping and inversion symmetry. In the presence of coupling-path-induced effective zero hopping within the unit cells, the systems are characterized by complete multimerization with degenerate -1 energy edge states for open boundary condition. Su–Schrieffer–Heeger subspaces with fully dimerized limits corresponding to pairs of nontrivial flat bands are derived from the Hilbert spaces. In particular, topological bound states in the continuum (BICs) are inherently present in even multimer chains, manifested by embedding the topological bound states into a continuous band assured by bulk-boundary correspondence. Moreover, we experimentally demonstrate the degenerate topological edge states and topological BICs in radio-frequency circuits.

Metasurfaces with spin-selective transmission play an increasingly critical role in realizing optical chiral responses, especially for strong intrinsic chirality, which is limited to complex three-dimensional geometry. In this paper, we propose a planar metasurface capable of generating maximal intrinsic chirality and achieving dual-band spin-selective transmission utilizing dual quasi-bound states in the continuum (quasi-BICs) caused by the structural symmetry breaking. Interestingly, the value of circular dichroism (CD) and the transmittance of two kinds of circular polarization states can be arbitrarily controlled by tuning the asymmetry parameter. Remarkable CD approaching unity with the maximum transmittance up to 0.95 is experimentally achieved in the dual band. Furthermore, assisted by chiral BICs, the application in polarization multiplexed near-field image display is also exhibited. Our work provides a new avenue to flexibly control intrinsic chirality in planar structure and offers an alternative strategy to develop chiral sensing, multiband spin-selective transmission, and high-performance circularly polarized wave detection. The basic principle and design method of our experiments in the microwave regime can be extended to other bands, such as the terahertz and infrared wavelengths.

Optical resonators with high quality (Q) factors are paramount for the enhancement of light–matter interactions in engineered photonic structures, but their performance always suffers from the scattering loss caused by fabrication imperfections. Merging bound states in the continuum (BICs) provide us with a nontrivial physical mechanism to overcome this challenge, as they can significantly improve the Q factors of quasi-BICs. However, most of the reported merging BICs are found at Γ point (the center of the Brillouin zone), which intensively limits many potential applications based on angular selectivity. To date, studies on manipulating merging BICs at off-Γ point are always accompanied by the breaking of structural symmetry that inevitably increases process difficulty and structural defects to a certain extent. Here, we propose a scheme to construct merging BICs at almost an arbitrary point in momentum space without breaking symmetry. Enabled by the topological features of BICs, we merge four accidental BICs with one symmetry-protected BIC at the Γ point and merge two accidental BICs with opposite topological charges at the off-Γ point only by changing the periodic constant of a photonic crystal slab. Furthermore, the position of off-Γ merging BICs can be flexibly tuned by the periodic constant and height of the structure simultaneously. Interestingly, it is observed that the movement of BICs occurs in a quasi-flatband with ultra-narrow bandwidth. Therefore, merging BICs in a tiny band provide a mechanism to realize more robust ultrahigh-Q resonances that further improve the optical performance, which is limited by wide-angle illuminations. Finally, as an example of application, effective angle-insensitive second-harmonic generation assisted by different quasi-BICs is numerically demonstrated. Our findings demonstrate momentum-steerable merging BICs in a quasi-flatband, which may expand the application of BICs to the enhancement of frequency-sensitive light–matter interaction with angular selectivity.

Recently, the concepts of parity–time (PT) symmetry and band topology have inspired many novel ideas for light manipulation in their respective directions. Here we propose and demonstrate a perfect light absorber with a PT phase transition via coupled topological interface states (TISs), which combines the two concepts in a one-dimensional photonic crystal heterostructure. By fine tuning the coupling between TISs, the PT phase transition is revealed by the evolution of absorption spectra in both ideal and non-ideal PT symmetry cases. Especially, in the ideal case, a perfect light absorber at an exceptional point with unidirectional invisibility is numerically obtained. In the non-ideal case, a perfect light absorber in a broken phase is experimentally realized, which verifies the possibility of tailoring non-Hermiticity by engineering the coupling. Our work paves the way for novel effects and functional devices from the exceptional point of coupled TISs, such as a unidirectional light absorber and exceptional-point sensor.

氧还原(ORR)反应是燃料电池等清洁能源阴极的关键反应, 其反应动力学复杂, 阴极需使用Pt等贵金属催化剂。然而Pt价格昂贵, 且载体炭黑在高电位环境下稳定性欠佳, 导致电池部件成本高且寿命短。二维过渡金属硫属化合物(2D TMDs)具有高比表面积与可调节的电学性能, 且稳定性强, 有望在维持活性的同时提高燃料电池阴极的耐久性。本文梳理了近年来2D TMDs在ORR催化剂领域的最新研究进展: 首先概述了2D TMDs的结构、性质及ORR反应机理; 其次分析了调控2D TMDs的ORR性能策略, 包括异质元素掺杂、相转变、缺陷工程与应力工程等, 介绍了2D TMDs基异质结构对ORR性能的提升作用; 最后, 针对该领域目前存在的挑战进行展望与总结。

对二能级原子与饱和铁氧体材料板之间的Casimir-Polder量子真空扭矩进行了研究。利用格林张量推导求得二能级原子与饱和铁氧体之间Casimir-Polder扭矩的表达式。结合不同的原子偶极矩和铁氧体特殊的电磁特性，从原子频率、原子位置、外磁场和圆极化偶极矩极化平面所受微扰等方面对Casimir-Polder扭矩进行分析。结果表明，原子位置和跃迁频率会影响Casimir-Polder扭矩的强度，而扭矩随铁氧体所处外磁场强度的变化特性在不同的原子位置和频率下会呈现非单调的拐点。此外，原子圆极化偶极矩在受到一定的角度微扰下，其扭矩具有回归原旋转平面趋势的平衡稳定性。因此，通过改变铁氧体所处的外磁场环境，可以调控原子的Casimir-Polder扭矩，这为二能级系统旋转态的加速或冷却至量子基态的控制提供一种新方法。