Dual-band tunable and strong circular dichroism in a metal-graphene hybrid zigzag metasurface.

Most active chiral metasurfaces operate in a single band and have an unidirectionally tunable circular dichroism (CD). Here, we propose a zigzag metasurface composed of a Z-shaped metallic strip and a L-shaped graphene strip to realize the dual-band tunable and strong CD. Two strong CD values of -0.88 and 0.88 are found at f= 0.86 THz and f= 1.23 THz, respectively. The strengths and resonant frequencies of these two CD signals can be tuned by varying graphene's Fermi energy (E). Strikingly, the CD value at 0.86 THz undergoes a continuous adjustment in a large range from 0.79 to -0.88 when E increases from 0.32 eV to 1.00 eV, implying that the proposed metasurface supports the switching of CD signal between on-, off- and reverse-states. Based on the strong CD signals, the capability of the metasurface as a biosensor to detect Avian Influenza viruses is demonstrated. This work will advance the development of broadband tunable chiral-optical devices.