Dynamically tunable slow light characteristics in graphene based terahertz metasurfaces

We propose a graphene based metasurface exhibiting dynamic slow light behavior via electromagnetically induced transparency (EIT) effect in the terahertz regime. The unit cell of the metasurface consists of bright and dark graphene rod resonators in a planar orthogonal configuration. The onset and tunability of the EIT effects are investigated through manipulating the near field electromagnetic coupling among the bright and dark resonators. An analytical method based on three level Lorentz oscillator model is employed to validate the simulated results. Further, we investigate the dynamic tunability of EIT effect by altering the Fermi energy of the graphene resonators. A blue shift of 0.59 THz in the EIT transparency peak is observed as the Fermi energy is increased from 0.3 eV to 1.0 eV. Similar dynamic tunability is investigated for the slow light characteristics (i.e., group delay, group index & group velocity) along with the delay bandwidth product. It is observed that an increase in the Fermi energy ultimately leads to an increase in group delay, group index & delay bandwidth product with their corresponding maximum values as 0.76 ps, 22.78 & 0.14 respectively for Fermi energy as 1.0 eV. For this case, the normalized group velocity reaches a minimum value of 0.04, thus, confirming the dynamic tunability of slow light behavior in the studied metasurfaces. Our findings can lead to the development of active slow light components like modulators, sensors, filters, buffers, ultrafast switches, and compact delay lines etc. in the terahertz domain.