Dicke-Cooperativity-Assisted Ultrastrong Coupling Enhancement in Terahertz Metasurfaces.

Abstract

A system of <i>N</i> two-level atoms cooperatively interacting with a photonic field can be described as a single giant atom coupled to the field with interaction strength <math xmlns="http://www.w3.org/1998/Math/MathML"><mo>∝</mo><msqrt><mi>N</mi></msqrt></math>. 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 <i>N</i> 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 <math xmlns="http://www.w3.org/1998/Math/MathML"><msqrt><mi>N</mi></msqrt></math>. 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.