Linear‐dielectric‐based in‐plane optical design is attractive yet challenging in nano‐optical technologies when combined with the compatibility with complementary metal‐oxide‐semiconductor technology. Here, by exciting an electric symmetric dipole mode (ESDM), exploiting the interplay between an electric quadrupole mode and a magnetic dipole mode in a homogeneous isotropic linear dielectric rod, an in‐plane retroreflector is successfully realized in the case of no dependence on asymmetry arising from anisotropic/bianisotropic materials, complex unit cell, or asymmetric environments. The ESDM is analogous to an electric dipole (ED) in the sense that its radiation is overwhelmingly dominant in the forward and backward directions, while its phase symmetry is in stark contrast to the antisymmetry of ED modes. The studies herein show symmetric dipole modes are one of general mechanisms for retroreflection. Such a retroreflector combines many other outstanding advantages such as low loss, unitary efficiency, and simple configuration with low fabrication demands. The in‐plane manipulation for waves makes it an appealing platform for manipulating particles and developing on‐chip optical cavities, light sources in integrated optical circuits, optical parametric oscillators, and interferometers.
We theoretically demonstrate a Dirac fermion metagrating which is an artificially engineered material in graphene. Although its physics mechanism is different from that of optical metagrating, both of them can deliver waves to one desired diffraction order. Here we design the metagrating as a linear array of bias-tunable quantum dots to engineer electron beams to travel along the -1st-order transmission direction with unity efficiency. Equivalently, electron waves are deflected by an arbitrary large-angle ranging from 90° to 180° by controlling the bias. The propagation direction changes abruptly without the necessity of a large transition distance. This effect is irrelevant to complete band gaps and thus the advantages of graphene with high mobility are not destroyed. This can be attributed to the whispering-gallery modes, which evolve with the angle of incidence to completely suppress the other diffraction orders supported by the metagrating and produce unity-efficiency beam deflection by enhancing the -1st transmitted diffraction order. The concept of Dirac fermion metagratings opens up a new paradigm in electron beam steering and could be applied to achieve two-dimensional electronic holography.
We demonstrate a new electromagnetic mode which is formed by the dynamic interaction between a magnetic quadrupole mode and an electric monopole mode in a two-dimensional electromagnetic Helmholtz cavity. It is termed a magnetic symmetric dipole mode since it shares similarity with a magnetic dipole mode in the sense that their radiation is both overwhelmingly dominant in the forward and backward directions with respect to the incident wave. However, the phase distribution in the two radiation directions is symmetric, in stark contrast to the antisymmetry of magnetic dipole modes. When the Helmholtz cavities are arranged in a line, the incident wave will be reflected back to the source, in other words, retroreflection occurs because of the peculiar properties of magnetic symmetric dipole modes. We show that the retroreflection is quite robust against the disorder of the orientation angle of Helmholtz cavities and there exists a wide tolerance for wavelength and the outer radius of the cavity. With low fabrication demands, this might offer a feasible solution for the design of ultrathin retroreflectors towards device miniaturization and the realization of multiplexing holography.
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