Triple-coupled normal mode splitting in Fabry-Pérot microcavity contained rectangular hole magnetic metamaterials in THz region

The interaction between light and matter inside a microcavity has been intensively studied for a long time, but most of the research has focused on the coupling between the electric dipole of the material and the electric field inside the cavity. We replaced the material with a metamaterial, focused on its characteristic magnetic response, and studied its interaction with the magnetic field inside a Fabry-Pérot (FP) microcavity. In this study, we utilized the fact that a rectangular hole metamaterial (RH), known as a magnetic current antenna, behaves as a magnetic dipole. This RH also has a high reflectance, so it also functions as a mirror. Taking advantage of this property, we investigated the optical properties of three different FP cavity structures containing RH metamaterials in the THz region. First, we investigated the transmission properties and dispersion relationship by transmission line theory analysis. Next, to fabricate the actual sample, we designed it using the finite differential time domain (FDTD) method and investigated the magnetic field distribution inside the sample. We then fabricated a sample by photolithography and lift-off processes and measured its transmission spectra using THz time-domain spectroscopy. As a result, we found that it was possible to observe triple-coupled normal mode splitting caused by the strong coupling between the magnetic field and the magnetic dipole. This phenomenon does not appear in a strongly coupled system of two coupled oscillators, such as the well-known cavity polariton, which consists of an ordinary electric field and an electric dipole.