Planar metamaterials entirely made up of sub-wavelength micro/nano-units (e.g. “meta-atoms”) with tailored electromagnetic properties are basically termed as metasurfaces. Abundant outstanding wave-manipulation effects have already been discovered based on carefully designed periodic or inhomogeneous gradient metasurfaces. Generally, the resulting functional devices derived from manipulation of these fascinating effects possess excellent integration –optics applications since they are generally flat, thin and of high efficiency. During design of these metasurfaces, the most common approach employed entails utilization of electromagnetic properties. However, during such expedites, a phenomenon termed as angular dispersion is eminent, owing to the variance of the excitation angle. In a bid to evade this defective phenomenon, scientists have often opted to utilize meta-atoms of weak angular dispersion. Such evasive techniques have been adopted since the intricate physics underlying the intriguing behavior is yet to be comprehended. Furthermore, little knowledge on how to manipulate the angular dispersion of metasurfaces limits full realization of their potential.
Recently, Lei Zhou et al. from Fudan University established a theory to quantitatively analyze the behaviors in periodic metasurfaces by extending the photonic tight-binding method developed previously for few-resonator systems to periodic systems. In line with this, they also showed that the angular dispersion of a metasurface is dictated by the plasmonic couplings among meta-atoms inside the meta-system. Their work is currently published in the research journal, Physical Review Applied.
Briefly, the research method employed commenced with the experimental characterization of the rich angular-dispersion behaviors of a typical terahertz metasurface, consisting of a periodic array of subwavelength metallic split-ring resonators on a dielectric substrate. Next, they engaged in a theoretical analysis of angular dispersion in metasurfaces in order to fully comprehend the physical origin of the intriguing angular dispersion behaviors they had initially experimentally analyzed. Lastly, they proposed an alternative strategy to design meta-devices exhibiting impinging-angle-dependent multi-functionalities. As an illustration, they designed a polarization control meta-device that could behave as a half- or quarter-wave plate under different excitation angles.
The authors observed that plasmonic near-field coupling played a vital role in dictating the angular dispersions in metasurfaces, which could be utilized as an additional degree of freedom to design multifunctional metadevices. In addition, they noted that by employing their technique, they were able to validate their proposed theory with terahertz experiments on a metasurface exhibiting opposing angular dispersions for the two polarizations.
In summary, Fudan University scientists successfully presented the establishment of a theoretical framework to study the angular dispersions in periodic metasurfaces. In general, their findings significantly expand the capabilities of the metasurface in manipulating electromagnetic waves and can stimulate high-performance multifunctional metadevices for practical applications. Altogether, extending this concept to the design of inhomogeneous gradient metasurfaces with impinging-angle-dependent multi-functionalities would be of interest for future projects, particularly at high frequencies.
Meng Qiu, Min Jia, Shaojie Ma, Shulin Sun, Qiong He, Lei Zhou. Angular Dispersions in Terahertz Metasurfaces Physics and Applications. Physical Review Applied 9, 054050 (2018)Go To Physical Review Applied