We study the Lorentz nonreciprocal transmission and nonreciprocal complex band structures supported in a $\mathcal{P}\mathcal{T}$-symmetric waveguide sandwiched by magnetized layers. We found that, near the emergency and coalescence of exceptional points, nonreciprocal transmission can be significantly enhanced by the non-Hermitian properties. With the help of gain-loss balance, one-way light transmission could be achieved even in the presence of a weak magnetic field. The results may offer us insight into the interplay between Lorentz nonreciprocity and non-Hermiticity.

The Pancharatnam–Berry (PB) phase is generally utilized to realize a single wavelength spin-dependent function or dual-wavelength functions but operating only in one spin state. A dual-wavelength multifunctional metasurface relying on both spins has been rarely designed due to the rather complicated degrees of freedom to be considered. In this Letter, both dynamic and PB phases are adopted, instead of a pure PB phase, to propose a multiplexing metasurface that can independently and simultaneously manipulate left- and right-handed circularly polarized incidences at dual wavelengths. It is demonstrated experimentally as well as numerically that such spin-dependent dual-wavelength metalenses can make circularly polarized incidences of different wavelengths split into and focus at multi-dimensional positions. Our work demonstrates a new avenue in designing spin-dependent dual-wavelength multifunctional optical devices.

Dual-focusing effect with a cylindrical vector-light characteristic (i.e., radial and azimuthal polarizations) is theoretically proposed and numerically demonstrated by spin-decoupled phase control with all-silicon metalens. Attributed to the polarization dependence, the pair of focusing cylindrical vector beams can be interchanged by orthogonally switching the polarization of incident light. We demonstrate the unique contributions of focused radial and azimuthal vector beams to longitudinal and transverse optical forces on glass spheres, respectively, by calculations based on the Maxwell stress tensor. This paper presents the use of all-silicon metalens with highly-compact vector beams, promising for applications such as multidimensional optical trapping.

Asymmetric radiation in upward and downward directions supported by leaky modes can be generally exhibited in the reciprocal but geometrical up-down mirror-symmetry-breaking structures, i.e., photonic crystal slabs. Here, we theoretically propose and numerically demonstrate a scheme to break the reciprocity, and to generate and manipulate the unidirectional guided resonances (UGRs) in a one-dimensional magneto-optical (MO) grating, which radiate solely toward one side with no emission in the opposite direction. Under an external magnetic field, it is found that genuine UGRs (or quasi-UGRs) in topological nature, depicted by a phase winding number of far-field radiation, can be created by hybridizing the different orthogonal bands in such a nonreciprocal grating. We further investigate the evolution, annihilation, and band transition for residential position of UGRs in three-dimensional parameter space, and we also notice that the emission direction may be switched from one side to the other due to the tuning of the asymmetric coupling. Finally, the excitation of UGRs is examined in a lossless system related to directional coupling of external waves, and it can also be utilized to realize perfect absorption in a lossy MO grating structure under single-sided illumination. The general principles presented here may lead to an efficient and active approach for the dynamic manipulation of topological radiation asymmetry, and they may serve as new inspiration to control flexibly the resonant and topological properties of light-matter interactions.

It is important to arbitrarily manipulate optical intensity, an important degree of freedom to light, on microscales, which is a fundamental requirement for integration and miniaturization of optical devices. Metasurfaces have shown unprecedented capabilities for manipulating light in terms of phase, intensity, and polarization. Here, an ultrathin metasurface composed of silver nanorods illuminated by linearly polarized light is demonstrated to manipulate optical intensity in subwavelength scales. By modulating rod orientations, gradient reflectance of light can be achieved on dual-wavelength regimes with contrast reflection intensities. Further, a nanorod metasurface, embedded with a picture of a panda profile, is experimentally designed for grayscale imaging, and the measurements demonstrate that two complementary grayscale images can be displayed at 633 and 900 nm. The grayscale imaging by a proposed ultrathin metasurface with dual-wavelength, complementary, and subwavelength-resolution characteristics provides a simple but efficient way for tailoring optical intensity on subwavelength scales, which is promising for a variety of applications such as encryption and decryption, display, information security, and optical communication.

We show that Maxwell's demon-like nonreciprocity can be supported in a class of non-Hermitian gyrotropic metasurfaces in the linear regime. The proposed metasurface functions as a transmission-only Maxwell's demon operating at a pair of photon energies. Based on multiple scattering theory, we construct a dual-dipole model to explain the underlying mechanism that leads to the antisymmetric nonreciprocal transmission. The results may inspire new designs of compact nonreciprocal devices for photonics.