Figure 1. Experiment overview.

(a) Schematic view of the experiment. The sub-wavelength metamaterial resonator is fabricated on top of a semiconductor substrate. Light illuminates the sample at normal incidence and is polarized along the y axis. The matter dipoles (ISTs) and the metamaterial resonators show a single resonance. When coupled, the resonance splits into two polariton branches. The plane wave shows the relative length scale of incoming light to metamaterial resonator. (b) Scanning electron microscopy (SEM)-image of a single dogbone-metamaterial resonator with a resonance at 31.6 THz and fabricated using 100 nm thick gold. The scale bar corresponds to 500 nm. (c) Self-consistent band-structure calculation of sample A (λ_IST: 10 μm). The quantum-well is homogenously doped to a density of 1.25 × 10¹² cm⁻². The optically active transition happens between the ground state (black) and the first excited state (red). The Fermi level is set between these two levels.