Probing the transition from an uncoupled to a strong near-field coupled regime between bright and dark mode resonators in metasurfaces

The coupling of multiple plasmonic resonators that sustain bright or dark modes provide intriguing spectral signatures. However, probing the onset of coupling effects while engaging the resonators with an increasing proximity has not yet been studied experimentally in detail. Nevertheless, this is of utmost importance to bridge the phenomenological understanding with the peculiarities of real-world-samples. Here, we take advantage of the ability to control spatial dimensions of THz metasurfaces deep in the sub-wavelength domain to study different regimes that occur while coupling split-ring-resonators that sustain a bright and a dark mode with increasing strength. We identify the length scales at which the resonators are uncoupled and then enter the regimes of weak, moderate, and strong coupling. It is shown that a strong coupling takes place only at distances smaller than one hundredth of the resonance wavelength. Understanding the features that emerge from such hybridization is important to take advantage of fundamental effects in metamaterials such as classical analogs of electromagnetically induced transparency, lasing spaser, near-field manipulation, and sensing with dark mode resonances.