Coupled-Mode Analysis of Vertically Coupled AlGaAs/AlOx Microdisk Resonators

This paper reports the experimental and theoretical assessments of the optical characteristics of recently introduced vertically coupled microdisk resonators made by the selective oxidation of AlGaAs multilayer structures. Experimental measurements show that the Q-factors are in the 10
³
-10
⁴
range for diameters ranging from 75 to 300 μm. To establish the origins of this limited performance, a coupled-mode-theory-based model of the single-access-waveguide-coupled resonator system was developed. It includes features which are specific to oxide-based vertically coupled resonators, namely, losses toward the slab waveguide lying under the resonator and a coupling region with an asymmetric and multilayer structure. Setting this simulation tool required the proposal and validation of a general criterion to select an appropriate set of decomposition permittivity profiles to be able to accurately model the characteristics of these more complex couplers using the coupled-mode-theory approach. This theoretical development is generic and can be now deployed to simulate any device which includes multiwaveguide couplers with arbitrary piece-wise-constant profile of the dielectric permittivity. Exploiting this particular development and experimental measurements of the disk sidewall roughness and of the coupling lengths, the calculated and experimental Q-factors are found to be in good agreement and allow establishing that the current performance is limited by the scattering losses and the slab leakage losses for small- and large-diameter devices, respectively.