Abstract
Unidirectional radiation is of particular interest in high‐power lasing and optics. Commonly, however, it is difficult to achieve a unidirectional profile in such a system without breaking reciprocity. Recently, assisted by metamaterials without structural symmetry, antennas that radiate asymmetrically have been developed, hence providing the possibility of achieving unidirectionality. Nevertheless, it has been challenging to achieve extremely high radiation asymmetry in such antennas. Here, it is demonstrated that this radiation asymmetry is further enhanced when magnetic plasmons are present in the metamaterials. Experimentally, it is shown that a thin metamaterial with a thickness of ≈λ0/8 can exhibit a forward‐to‐backward emission asymmetry of up to 1:32 without any optimization. The work paves the way for manipulating asymmetric radiation by means of metamaterials and may have a variety of promising applications, such as directional optical and quantum emitters, lasers, and absorbers.
Highlights
We show that plane waves propagating in opposite directions may have dramatically different field profiles in the interiors of bianisotropic MMs, due to which systematic asymmetry is induced in the MMs' EM responses
It is evident that the occurrence of magnetic plasmons is crucial for maximizing the radiation asymmetry of such a system in practice
It is revealed that the radiation asymmetry is subject to internal bounds and may not exceed unity for a given bianisotropy
Summary
The well-known metasurfaces could have the ability of manipulating the profiles of electromagnetic (EM) waves, including the control of wave front, polarizations, holograms, angular momentums, etc.[1,2,3,4,5,6,7,8] Commonly, metamaterials (MMs) with asymmetric structures are known to exhibit asymmetric (or bianisotropic) EM responses, such as single-sided radiation and absorption patterns and unbalanced forward-backward emission.[9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26] In particular, asymmetric forward-backward radiation is of great interest in many photonic applications, such as high-power lasers and light detection.[27,28,29,30,31,32,33,34] To enhance this asymmetry, MM structures are often optimized through numerical approaches. The sole option for achieving both asymmetry and reciprocity is for waves coming from opposite directions to interact with the MM in an isolated manner.[36] This isolation causes the MM to totally reflect, which is a sign of bound breaking.[35,36] Interestingly, this isolation begins at k=0, i.e., where η+ and η− are purely imaginary and equal, in which case the polarization/magnetization of the MM cannot distinguish among waves coming from different directions
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