Effect of spatial mode coupling: S₁₂ parameter (log. scale (dB)) and absorption A (linear scale (lin.)) for several values of distance p₂ between two resonators. The side lengths of the resonators are a_m = 60 μm and a_e = 90 μm. (a) Out of frequency degeneracy regime (125 ≤ p₂ ≤ 200 μm); (b) in frequency degeneracy regime (100 ≤ p₂ ≤ 124 μm). The merged minima of the S₁₂ parameter attain very weak values (≲ − 51 dB) when p₂ = p_2c = 124 μm, which correspond to the minimum of the refractive index n_effmin = −2.2.