Detection and generation of circularly polarized (CP) light is an essential operation in optical communication, quantum computing, molecular spectroscopy, magnetic recording and imaging applications. Increasing demand for subwavelength and high efficiency detectors has intrigued numerous research groups to approach this problem by designing twisted optical metamaterial and helical structures, spiral plasmonic lens and chiral organic transistors. More recently, owing to the notion of chirality, the detection of handedness of light has been made possible in the context of hot electron injection in plasmonic metamaterials, as well as in all-dielectric metasurface [1-2]. However, complicated fabrication procedure, bandwidth limitation and low values of transmission and circular polarization dichroism are still deemed as impeding factors for most practical applications.
Here we have experimentally demonstrated a hybrid metal-dielectric metasurface for CP light detection in transmission mode. First, based on the birefringence effect we design a quarter waveplate (QWP) by patterning a PECVD-grown silicon layer in the form of a periodic array with rectangular unit cells. This can be achieved by electron beam lithography, followed by inductively coupled plasma etching. The degree of anisotropy in phase accumulation between the fast and slow axes of QWP can be tailored by engineering the aspect ratio, occupation factor and silicon thickness. These degrees of freedom, on the other hand, provide a significant versatility to tune the operation wavelength (visible to NIR) and bandwidth broadening (few hundreds of nanometers). Moreover, due to low loss in dielectric materials in contrast to plasmonic structures, the measured transmission is as high as 95%, associated with a remarkable degree of circular polarization (DOCP>98%). Depending on the handedness of incident light, the QWP output will be linearly polarized +45 or -45 degrees with respect to the major or minor axes of QWP. Therefore, integration of a metallic grating separated by a fused silica spacer layer can almost completely block or pass the output, hence forming a binary detector. The extinction ratios measured in experiment are up to 13, while the overall transmission is close to 90%. Further design optimization can lead to even higher extinction up to 400.
The proposed structure exhibits various advantages including scalability and CMOS compatibility, compact footprint (few tens of micron) and superior DOCP, high extinction ratio and transmission. Moreover, it shows robustness against imperfections in fabrication process which is deemed desirable in comparison with other chiral metamaterial designs in literature. Therefore, it can be a great candidate for imaging, sensing applications and communication systems.
 Wei Li, et al., Nature Communications 6, 8379 (2015)
 Jingpei Hu, et al., Sci Rep. 7: 41893 (2017)