TY - JOUR A1 - Meng, Fanqi A1 - Thomson, Mark David A1 - Klug, Bernhard A1 - Čibiraitė, Dovilė A1 - Ul-Islam, Qamar A1 - Roskos, Hartmut T1 - Nonlocal collective ultrastrong interaction of plasmonic metamaterials and photons in a terahertz photonic crystal cavity T2 - Optics express N2 - Light-matter interaction in the strong coupling regime is of profound interest for fundamental quantum optics, information processing and the realization of ultrahigh-resolution sensors. Here, we report a new way to realize strong light-matter interaction, by coupling metamaterial plasmonic "quasi-particles" with photons in a photonic cavity, in the terahertz frequency range. The resultant cavity polaritons exhibit a splitting which can reach the ultra-strong coupling regime, even with the comparatively low density of quasi-particles, and inherit the high Q-factor of the cavity despite the relatively broad resonances of the Swiss-cross and split-ring-resonator metamaterials used. We also demonstrate nonlocal collective interaction of spatially separated metamaterial layers mediated by the cavity photons. By applying the quantum electrodynamic formalism to the density dependence of the polariton splitting, we can deduce the intrinsic transition dipole moment for single-quantum excitation of the metamaterial quasi-particles, which is orders of magnitude larger than those of natural atoms. These findings are of interest for the investigation of fundamental strong-coupling phenomena, but also for applications such as ultra-low-threshold terahertz polariton lasing, voltage-controlled modulators and frequency filters, and ultra-sensitive chemical and biological sensing. Y1 - 2019 UR - http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/51270 UR - https://nbn-resolving.org/urn:nbn:de:hebis:30:3-512705 SN - 1094-4087 N1 - © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement VL - 27 IS - 17 SP - 24455 EP - 24468 PB - Soc. CY - Washington, DC ER -