Investigation of Vertical Leakage Loss for Whispering-Gallery Modes in Microcylinder Resonators

The vertical leakage characteristics are investigated for whispering-gallery modes (WGMs) in the microcylinder resonators with vertical semiconductor waveguiding. The mode coupling between HE and EH modes at the boundaries between the core and cladding layers results in a vertical leakage loss for the WGMs, if the transformed modes can propagate in the cladding layer. The core-layer-thickness-dependent vertical leakage loss caused by the internal interference is demonstrated and numerical simulated for the WGMs by the finite-difference time-domain (FDTD) technique. An oscillation of mode Q factor is found with the increase of the thickness of core layer due to the internal interference between the transmitted and reflected leakage modes, and the vertical leakage loss can be canceled at some thicknesses of the core layer because of the destructive interference. Furthermore, the vertical leakage loss can also be controlled by the external interference in the microcylinder resonator with two vertical coupled core layers. The enhancements of Q factors for both symmetric and antisymmetric modes are observed by varying the gap between the two core layers.