Tunable dual-band linear-to-circular polarization conversion based on the electromagnetically induced transparency utilizing the graphene metamaterial

A tunable dual-band linear-to-circular polarization conversion (LTCPC) related to electromagnetically induced transparency (EIT) is theoretically proposed by employing the graphene metamaterial in the terahertz (THz) regime. Since the electric resonance (bright mode) and the magnetic resonance (quasi-dark mode) for the x- and y-polarized (TM and TE) waves are mutually coupled, the EIT behavior is realized due to the destructive interference. Two transparent EIT windows have emerged in 1.452-1.661 THz and 1.348-1.683 THz when the TM and TE waves are incidents. The corresponding values of the maximum group delay and group index respectively are 225 ps, 162 ps, 1636, and 1177. The LTCPC is achieved at 1.432 THz and 1.676 THz when the graphene does not exist. While the graphene is introduced, the LTCPC is dynamically adjustable by controlling the Fermi energy (E-f) of the graphene. The optimal ARs can dynamically change between 1.438 (1.689) THz and 1.452 (1.696) THz while the E-f changes from 0.1 eV to 0.9 eV. A theoretical investigation of two-oscillator model further confirms the effectiveness and consistency of the simulation results. The presented metamaterial with a thickness of subwavelength and high transparency for the electromagnetic waves opens a new path to the applications in beam steering and polarization controls.