Vertically coupled microresonators and oscillatory mode splitting in photonic molecules

We report the formation of a photonic molecule by vertically coupling two microdisk resonators. We show that mode splitting monotonously increases when one resonator vertically approaches the other. However, when the vertical distance is kept fixed and one resonator is moved horizontally with respect to the other, the strength of coupling and hence the mode splitting demonstrate an oscillatory behavior. This is attributed to the interference of light coupled into the resonators through multiple coupling regions as confirmed by a theoretical model based on coupled mode theory.