Band gaps and leaky-wave effects in resonant photonic-crystal waveguides

The paper addresses the leaky stop bands associated with resonant photonic crystal slabs and periodic waveguides. We apply a semianalytical model pertinent to the second band to compute the dispersion curves describing the leaky stop band and verify its correctness by rigorous band computations. This approximate model provides clear insights into the physical properties of the leaky stop band in terms of explicit analytical expressions found. In particular, it enables comparison of the structure of the bands computed in complex propagation constant, implying spatially decaying leaky modes, with the bands computed in complex frequency, implying temporally decaying modes. It is shown that coexisting Braggcoupling and energy-leakage mechanisms perturb the bands in complex propagation constant whereas these mechanisms are decoupled in complex frequency. As a result, the bands in complex frequency are well defined exhibiting a clear gap. These conclusions are verified by numerical diffraction computations for both weak and strong grating modulations where the resonance peaks induced by external illumination are shown to closely track the band profile computed in complex frequency. Thus, in general, phase matching to a resonant leaky mode occurs via real propagation constant that is found by dispersion computations employing complex frequency.