The optical behavior of coupled systems, in which the breaking of parity and time-reversal symmetry occurs, is drawing increasing attention to address the physics of the exceptional point singularity, i.e., when the real and imaginary parts of the normal-mode eigenfrequencies coincide. At this stage, fascinating phenomena are predicted, including electromagnetic-induced transparency and phase transitions. To experimentally observe the exceptional points, the near-field coupling to waveguide proposed so far was proved to work only in peculiar cases. Here, we extend the interference detection scheme, which lies at the heart of the Fano lineshape, by introducing generalized Fano lineshapes as a signature of the exceptional point occurrence in resonant-scattering experiments. We investigate photonic molecules and necklace states in disordered media by means of a near-field hyperspectral mapping. Generalized Fano profiles in material science could extend the characterization of composite nanoresonators, semiconductor nanostructures, and plasmonic and metamaterial devices.
Modeling of waveguide and near-field resonant transmission in a photonic molecule close to the exceptional point. a Schematics of two coupled-photonic cavities tested by a waveguide, where the e.m. field is propagating from left to right. The waveguide is coupled (red arrows) to the resonator with losses γ1, which is also coupled with coupling strength Κ to the upper resonator with losses γ2. b, c Calculated waveguide transmission change due to the presence of the photonic molecule for different values of κ to reproduce both the Autler–Townes splitting (ATS) case, for κ=1.2Κ0, and the electromagnetic-induced transparency (EIT) case, for κ=0.8Κ0, with γ1/γ2=10 and γ1/γ2=1.1, respectively. The resonant frequencies of the single uncoupled cavities are ω1=ω2=Κ0. d Schematics of the photonic molecule tested by a near-field probe (blue cone) with incoming and outgoing e.m. waves. e, f Calculated transmission change through the near-field probe for the same cases reported in b and c, respectively. All spectra are calculated by means of the coupled-mode theory (Supplementary Note 4). Horizontal dashed lines represent the zero value of each lineshape