Strong coupling of two photons with a metamaterial plasmon in a terahertz cavity

Realization of strong interaction between photons and the plasmons of metamaterials (MMs) in a cavity is of practical interest because it has great potential for new ways of control of MMs, used for purposes such as sensing and information processing. Here, we investigate a complementary metamaterial (CMM) in a 1D photonic crystal cavity, and observe strong coupling of CMM plasmons with cavity photons. According to Barbinet's principle, such an interaction can be understood as the coupling of the magnetic field of the radiation with an effective in-plane magnetic dipole moment of the CMM. Moreover, we observe strong interaction among two photons and a plasmon for a CMM in a dual cavity, which represents a model system of the most simple, strongly coupled many-body system.