Wavelength Dependence of a Vertically Coupled Resonator-Waveguide System

Coupling of light to and from a microdisk resonator is a crucial step for the integration of this photonic structure in a photonic integrated circuit. However, the most common lateral coupling scheme, based on a point contact with a coplanar bus waveguide, suffers from strong wavelength dependence. This is a limiting factor for exciting efficiently the resonant modes in a broad spectral region. In the present paper, we propose a solution based on a different configuration, known as vertical coupling configuration, where a bus waveguide is buried below the micro disk. We demonstrate theoretically and experimentally that the long interaction region provided by this geometry allows to extend the optimal coupling spectral range from IR to visible. This feature constitutes a remarkable advantage over the usual laterally coupled devices for many different applications, such as frequency conversion, allowing bandwidth limitations to be significantly diminished.