Space–time metasurfaces are promising candidates for breaking Lorentz reciprocity, which constrains light propagation in numerous practical applications. There is a substantial difference between carrier and modulation frequencies in space–time photonic metasurfaces that leads to negligible spatial pathway variation of light and weak nonreciprocal response. To surmount this obstacle, herein, the design principle of a high-quality-factor space–time gradient metasurface is demonstrated at the near-infrared regime that increases the lifetime of photons and allows for strong power isolation by lifting the adiabaticity of modulation. The all-dielectric metasurface consists of an array of silicon subwavelength gratings (SWGs) that are separated from distributed Bragg reflectors by a silica buffer. The resonant mode with ultrahigh quality-factor exceeding 10⁴ is excited within the SWG, which is characterized as magnetic octupole and features strong field localization. The SWGs are configured as multijunction p–n layers, whose multigate biasing with time-varying waveforms enables modulation of carriers in space and time. The proposed nonreciprocal metasurface is exploited for free-space optical power isolation by virtue of modulation-induced phase shift. It is shown that under time reversal and by interchanging the directions of incident and observation ports, power isolation of ≈35 dB can be maintained between the two ports in free space.

光的传播通常是互易的，传统的互易性光学器件面临光学衍射极限等问题，限制了其性能的进一步提升。非互易性光学器件能够实现光的单向可控传输，非互易拓扑光子态具有抗背向散射、免疫障碍物和缺陷等多种优异的物理性质，因而在光集成电路、非线性光学等多个领域具有潜在的应用价值。本文聚焦并回顾了非互易拓扑光子学的应用价值和研究进展，分别介绍了通过旋磁材料光子晶体、基于磁表面等离子激元、利用光学非线性效应和基于时间调制等不同的途径和手段实现非互易光子拓扑态的理论依据和研究进展，比较各自特点，并对其未来发展趋势和面临的关键问题进行了分析和展望。

It is experimentally verified that nonreciprocal photonic systems with continuous translation symmetry may have an ill-defined topology. The topological classification of such systems is only feasible when the material response is regularized with a spatial-frequency cutoff. We experimentally demonstrate that adjoining a small air layer to the relevant material interface may effectively imitate an idealized spatial cutoff that suppresses the nonreciprocal response for short wavelengths and regularizes the topology. Furthermore, it is experimentally verified that nonreciprocal systems with an ill-defined topology may be used to abruptly halt the energy flow in a unidirectional waveguide due to the violation of the bulk-edge correspondence. In particular, we report the formation of an energy sink that absorbs the incoming electromagnetic waves with a large field enhancement at the singularity.

Multi-band signal propagation and processing play an important role in quantum communications and quantum computing. In recent years, optical nonreciprocal devices such as an optical isolator and circulator are proposed via various configurations of atoms, metamaterials, nonlinear waveguides, etc. In this work, we investigate all-optical controlled nonreciprocity of multi-band optical signals in thermal atomic systems. Via introducing multiple strong coupling fields, nonreciprocal propagation of the probe field can happen at some separated frequency bands, which results from combination of the electromagnetically induced transparency (EIT) effect and atomic thermal motion. In the proposed configuration, the frequency shift resulting from atomic thermal motion takes converse effect on the probe field in the two opposite directions. In this way, the probe field can propagate almost transparently within some frequency bands of EIT windows in the opposite direction of the coupling fields. However, it is well blocked within the considered frequency region in the same direction of the coupling fields because of destruction of the EIT. Such selectable optical nonreciprocity and isolation for discrete signals may be greatly useful in controlling signal transmission and realizing selective optical isolation functions.

Synchronization is of importance in both fundamental and applied physics, but its demonstration at the micro/nanoscale is mainly limited to low-frequency oscillations such as mechanical resonators. We report the synchronization of two coupled optical microresonators, in which the high-frequency resonances in the optical domain are aligned with reduced noise. It is found that two types of synchronization regimes emerge with either the first- or second-order transition, both presenting a process of spontaneous symmetry breaking. In the second-order regime, the synchronization happens with an invariant topological character number and a larger detuning than that of the first-order case. Furthermore, an unconventional hysteresis behavior is revealed for a time-dependent coupling strength, breaking the static limitation and the temporal reciprocity. The synchronization of optical microresonators offers great potential in reconfigurable simulations of many-body physics and scalable photonic devices on a chip.