A system of N two-level atoms cooperatively interacting with a photonic field can be described as a single giant atom coupled to the field with interaction strength . This enhancement, known as Dicke cooperativity in quantum optics, has recently become an indispensable element in quantum information technology. Here, we extend the coupling beyond the standard light-matter interaction paradigm, enhancing Dicke cooperativity in a terahertz metasurface with N meta-atoms. The cooperative enhancement is manifested through the hybridization of the localized surface plasmon resonance in individual meta-atoms and surface lattice resonance due to the periodic array. Furthermore, through engineering of the capacitive split-gap in the meta-atoms, we were able to enhance the coupling rate into the ultrastrong coupling regime by a factor of . Our strategy can serve as a new platform for demonstrating effective control of fermionic systems by weak pumping, superradiant emission, and ultrasensitive sensing of molecules.
Keywords: Dicke cooperativity; metamaterials; plasmon; terahertz; ultrastrong coupling.