Chiral Metasurfaces for Asymmetric Transmission

Abstract

The coupling of electromagnetic (EM) fields can yield interesting applications through polarization manipulation. One means for achieving it is through chirality. A chiral structure, by inducing a magnetic moment parallel to incident electric field, results in cross-coupling of EM fields. Chiral devices exhibit a number of functionalities such as negative refractive index, optical activity and most importantly asymmetric transmission (AT). The phenomenon of asymmetric transmission manifests a direction-sensitive property through manipulation of wave polarization. Such a property can serve fruitful in applications where diode-like control is required for EM waves. The current trend in research is to manifest AT in a broad frequency range along with high efficiency. Most of the designs published have employed multi-layered structures to yield broadband response. However, increasing the number of layers increases overall thickness which is inconsistent with developing trend towards miniaturized profiles. Therefore, achieving an efficient operation at wide frequency ranges in a thin form factor is challenging. Furthermore, the designs reported till date are mostly operational at normal incidences only which makes them yet unsuitable for practical applications and for that reason to engineer angularly robust AT operation is highly essential. This thesis aims to overcome the two short comings in current research by firstly introducing a chirality enhancement technique, namely angle-induced chirality (AIC), which by introducing a slight angle to splits of a spilt ring resonator enables an efficient and broadband AT operation in a thin bi-layered configuration. Secondly, a detailed study revealing the dependencies of AT with respect to angular stability is presented on account of three critical aspects; surface impedance mismatch at oblique incidences, formation of grating lobes at higher incident angles and LC modeling of a miniaturized unit cell. As a result, an angularly stable AT operation with broadband transmission above 80% up to 60◦ is realized in a bi-layered configuration. In addition, a multilayered variant of the design is also demonstrated that by benefiting from Fabry Perot-like cavity depicts an enhanced operational bandwidth of 64% while maintaining an angularly robust performance. All the designs as a re- sult of the study entailed in this thesis are fabricated and validated through measurements.