Dynamic control of reflective chiral terahertz metasurface with a new application developing in full grayscale near field imaging.

The chiral metasurface possesses wide applications in terahertz (THz) band. It is still an important issue to control the chiral properties of metasuface by a simple and effective method. We design a reflective chiral THz metasurface that reflection medium in bottom is based on monolayer graphene on low resistance Si substrate. This structure allows a more reasonable approach to tune Fermi level (E F) of graphene compared with previous report. THz reflection change in graphene as varying E F can control in a large dynamic range the chiral properties of metasurface, inducing the state switch between 'On' and 'Off'. The dynamic control is also effective for various further applications such as vortex focusing and near field imaging. Significantly, we report for the first time the variation rule of reflection amplitude for reflecting suppressed circularly polarized light, with the change in rotation angle of top metal ring of metasurface. Based on this, we develop an innovation application, called full grayscale near field imaging (FGNFI) that can show full grayscale details compared with traditional near field imaging that can only show two grayscale states. Image 1 We propose a new structure that using monolayer graphene on surface of low resistivity silicon substrate acts as a reflection medium in reflective chiral terahertz metasurface. When bias voltage is applied between graphene and silicon, electrons and holes in silicon can be injected into graphene to tune the chirality between 'On' and 'Off' states. An innovation near field imaging application is developed by using the regular amplitude change of reflection suppressed circular polarization with metal ring rotation. The new application we developed can theoretically achieve the imaging with a series of grayscale values between the brightest to the dimmest states, which give more color details and thus is called full grayscale near field imaging (FGNFI). The tunable chirality 'On' and 'Off' can be used not only for traditional applications, but also for our newly developed FGNFI application. • A new graphene tuned recflective chiral terahertz metasurface is proposed. • Monolayer graphene on surface of low resistivity Si acts as a reflection medium. • Appling voltage between graphene and Si obtains the chirality On and Off states. • New application called full grayscale near field imaging (FGNFI) is developed. • FGNFI can achieve full grayscale details. [ABSTRACT FROM AUTHOR]