High-Q cavities without a complete photonic band gap

Dr. Steven G. Johnson

Department of Physics
Massachusetts Institute of Technology
Cambridge, MA 02139

Although the most ideal system for optical cavities is a 3d crystal with an omnidirectional band gap, the challenge of their fabrication has led many to consider simpler 1d and 2d periodic systems lacking a complete gap. Such a hybrid system is the photonic-crystal slab, employing a combination of vertical index guiding and a two-dimensional band gap in the horizontal plane. For high-contrast systems, operating below the light line, the in-plane periodic structure itself does not cause out-of-plane scattering, even for strong periodicity. However, any time the periodicity is broken, e.g. by a resonant cavity or waveguide bend, vertical scattering is possible. In this talk, we discuss mechanisms for minimizing such scattering losses, focusing in particular on the case of high-Q resonant cavities, which are most sensitive to any losses. We show how one can trade off field localization for Q (cavity lifetime), and alternatively demonstrate how one can cancel low-order multipole moments of the radiated field in order to maximize Q for a fixed degree of localization. Moreover, we also present a simple new structure, a 1d-periodic stack of cylindrical waveguides, in which arbitrary Q can be achieved for finite modal volume, if only a single analytical condition on the dielectric contrasts is satisfied. Such high-Q cavities with strong confinement are an integral component of many potential devices, from lasers to channel-drop filters to coupled-cavity slow-light waveguides.