  A split-ring resonator is a prototype of meta-atom in metamaterials. Though
noble metal-based split-ring resonators have been extensively studied, up to
date, there is no experimental demonstration of split-ring resonators made from
graphene, an emerging intriguing plasmonic material. Here, we experimentally
demonstrate graphene split-ring resonators with deep subwavelength (about one
hundredth of the excitation wavelength) magnetic dipole response in the
terahertz regime. Meanwhile, the quadrupole and electric dipole are
observed,depending on the incident light polarization. All modes can be tuned
via chemical doping or stacking multiple graphene layers. The strong
interaction with surface polar phonons of the SiO2 substrate also significantly
modifies the response. Finite-element frequency domain simulations nicely
reproduce experimental results. Our study moves one stride forward toward the
multi-functional graphene metamaterials, beyond simple graphene ribbon or disk
arrays with electrical dipole resonances only.
