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Large and periodically corrugated optical waveguide structures are shown to possess specific modal regimes of slow-light propagation that are easily attainable. The very multimode nature of the coupling is studied by employing coupled-mode theory and the plane-wave expansion method. Given a large enough light cone, associated with a surrounding medium with low enough refractive index, we notably identify a critical slowdown regime with an interesting bandwidth-slowdown product. Essential features of these original systems are further explored: the nature of the coupled modes, the role of gain, symmetry effects, polarization, and relation with photonic-crystal systems. Practical systems are introduced using finite-difference time-domain methods, which provides first-order rules for the use of the above phenomena and their implementation in devices.

We present a general recipe for tailoring flat dispersion curves in photonic crystal waveguides. Our approach is based on the critical coupling criterion that equates the coupling strength of guided modes with their frequency spacing and results in a significant number of the modes lying collectively in the slow-light regime. We first describe the critical coupling scheme in photonic crystal waveguides using a simple coupled mode theory model. We also determine that canonical photonic crystal waveguides natively correspond to strongly coupled modes. Based on these analyses, our design recipe is as follows: Tune the profile of the first Fourier component of the waveguide periodic dielectric boundary to lower the coupling strength of the guided modes down to its critical value. We check that this generalized tuning may be accomplished by adjusting any desired optogeometric parameter such as hole size, position, index etc. We explore the validity of this general approach down to the narrow two-missing rows waveguides. The interest of this method is to circumvent most of the common trial-and-error procedures for flatband engineering.

Broad photonic crystal waveguides forming open resonators are shown to support an hitherto unnoticed lasing pattern. The feedback for lasing originates in Littrow-type reflections of higher-order modes from the waveguide boundaries. The authors employ plane wave and finite-difference time-domain simulations of bulk crystal and waveguide to substantiate the concept of a distributed Littrow reflector. Experimental results are reported for a 10-m-wide photonic crystal waveguide deeply etched into InP substrate. In-plane lasing and low modal threshold gain due to longer path lengths are the key features of this resonator.

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International audienceWe display a photonic embodiment of the Demkov-Ostrovsky solution to the crossing of two manifolds made of equidistant modes thanks to broad periodic waveguides. We find clearly narrowing resonances that signal the singular, flat dispersion case that we had termed "critical coupling." The reconciliation of band-edge confinement and cavity confinement, two pillars of photonics, appear from the guide length dependence of spectra. We suggest the generality of the Demkov-Ostrovsky or critical coupling flat dispersion across all kinds of waves, e. g., electronic and acoustic

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