Optical activity has many potential applications in different scientific fields including molecular biology and analytical chemistry. The optical activity comprises of circular dichroism and circular birefringence. The former generally refers to different direct transmissions levels of both left and right circularly polarized waves while the latter denotes the rotational ability of polarized electromagnetic waves. Even though structural chirality can be applied using both two-dimensional and three-dimensional materials, optical activity is limited to a specific spectral range in most of the natural materials.
This has led to the fabrication of artificial three-dimensional metamaterials based on an array of chiral nanostructures with enhanced optical activities. However, fabrication of artificial metamaterials has not been fully achieved due to the complexity and difficulty in fabricating three-dimensional chiral metamaterials at optical wavelengths.
Nonchiral anisotropic planar metamaterials have been explored previously for optical activity. The materials, however, needs to be placed at oblique incidence to enable extrinsic chirality promoted by mutual interaction of the incident electromagnetic wave and the metamaterial structure. Extrinsic chirality can be realized by designing planar structure with resonant responses and in-plane anisotropy. For example, two-dimensional phosphorous has exhibited exemplary intrinsic anisotropy. The property has henceforth been explored for numerous potential applications.
Recently, National University of Defense Technology research team led by Professor Jianfa Zhang looked at the possibility of obtaining optical activity in unpatterned monolayer black phosphorus. The new fabrication technique does not require the use of resonant structures and other sophisticated nanofabrication technology. This is because it can be tuned with the changes in the doping concentration level. Consequently, they investigated the role of extrinsic three-dimensional chirality in initiating the optical activity, that is, circular dichroism and circular birefringence. Their research work is currently published in the research journal, Optics Letters.
The authors managed to obtain strong optical activity in the unpatterned monolayer black phosphorous without the use of the resonant nanostructures. The extrinsic chirality highly contributed to the optical activity. This was attributed to the mutual orientation of the black phosphorous film and the incident light just like that observed in the previous metamaterials. On the other hand, by controlling the Fermi level of black phosphorous through electrostatic or chemical doping, it was possible to tune the optical activity. This is because the rise in the Fermi level resulted in a corresponding increase in the conduction which further enhanced the structural absorption and reflection.
Indeed National University of Defense Technology-based scientists successfully demonstrated the strong optical activity in unpatterned monolayer black phosphorus. Generally, the technique is relatively cost-effective as compared to other sophisticated nanofabrication methods as it does not require the use of resonant structures. According to the authors, the optical activity can be improved through the addition of a layer of black phosphorous and two-dimensional materials. Therefore, the study provides insights that will advance research on optical activity and other-polarization dependent optical effects for different application.
Hong, Q., Xu, W., Zhang, J., Zhu, Z., Yuan, X., & Qin, S. (2019). Optical activity in monolayer black phosphorus due to extrinsic chirality. Optics Letters, 44(7), 1774.Go To Optics Letters