Analysis of Cavity–Waveguide Coupling in Two-Dimensional Photonic Crystals

We investigate theoretically cavity-waveguide coupling in 2-D photonic crystals. In using photonic crystal cavities in a device such as an optical circuit, the cavities must be coupled to a waveguide. For optimum performance, careful design of the device structure is necessary. In order to find out the rule for the design, we evaluate coupling strength by carrying out finite-difference time-domain (FDTD) calculations with systematic variation of structural parameters. Results are interpreted in terms of the density of the states of waveguide bands and symmetry of the cavity and waveguide modes. We found that the selection rule based on parity of the modes breaks down because of linewidth broadening, which arises from transition in finite time.