Tunable and nonlinear metamaterials for controlling circular polarization

Controlling polarization using metamaterials has been one of the research areas that attract immense attention. In particular, the symmetry of the structure plays an important role in controlling polarization-sensitive optical phenomena. Circular polarization control, which is used for important applications such as circular dichroism spectroscopy, requires designing the symmetry of a metamaterial with circular polarization eigenstates. In the linear response, a giant optical activity was observed in chiral metamaterials. It is possible to actively tune the magnitude and sign of polarization by external stimuli or deforming the chiral metamaterial. Furthermore, in the nonlinear optical response, the metamaterial having the rotational symmetry enables wavelength conversion of circularly polarized light and also controls the phase thereof. This article provides an overview of these previous achievements on the metamaterials for controlling circular polarization with isotropic linear response. The article also discusses the prospects of these technologies that will enable polarization control, not only in the visible region but also in the long-wavelength (terahertz) and extremely short-wavelength (vacuum ultraviolet and extreme ultraviolet) regions in combination with the two advanced technologies: high-order harmonic generation and microelectromechanical systems.