What is and what is not electromagnetically induced transparency in whispering-gallery microcavities

There has been an increasing interest in all-optical analogues of electromagnetically induced transparency and Autler–Townes splitting. Despite the differences in their underlying physics, both electromagnetically induced transparency and Autler–Townes splitting are quantified by a transparency window in the absorption or transmission spectrum, which often leads to a confusion about its origin. While the transparency window in electromagnetically induced transparency is a result of Fano interference among different transition pathways, in Autler–Townes splitting it is the result of strong field-driven interactions leading to the splitting of energy levels. Being able to tell objectively whether an observed transparency window is because of electromagnetically induced transparency or Autler–Townes splitting is crucial for applications and for clarifying the physics involved. Here we demonstrate the pathways leading to electromagnetically induced transparency, Fano resonances and Autler–Townes splitting in coupled whispering-gallery-mode resonators. Moreover, we report the application of the Akaike Information Criterion discerning between all-optical analogues of electromagnetically induced transparency and Autler–Townes splitting and clarifying the transition between them. Optical analogues of electromagnetically induced transparency and Autler–Townes splitting originate from different mechanisms but both are quantified by a transparency window. Here, Peng et al. use the Akaike information criterion to discriminate between the two regimes in coupled whispering gallery mode microresonators.