  Metamaterials and plasmonics are powerful tools for unconventional
manipulation and harnessing of light. Metamaterials can be engineered to
possess intriguing properties lacking in natural materials, such as negative
refractive index. Plasmonics offers capabilities to confine light in
subwavelength dimensions and to enhance light-matter interactions.
Recently,graphene-based plasmonics has revealed emerging technological
potential as it features large tunability, higher field-confinement and lower
loss compared to metal-based plasmonics. Here,we introduce hybrid structures
comprising graphene plasmonic resonators efficiently coupled to conventional
split-ring resonators, thus demonstrating a type of highly tunable
metamaterial, where the interaction between the two resonances reaches the
strong-coupling regime. Such hybrid metamaterials are employed as high-speed
THz modulators, exhibiting over 60% transmission modulation and operating speed
in excess of 40 MHz. This device concept also provides a platform for exploring
cavity-enhanced light-matter interactions and optical processes in graphene
plasmonic structures for applications including sensing, photo-detection and
nonlinear frequency generation.
