Slow-light regime and critical coupling in highly multimode corrugated waveguides

Abstract

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.

Graphene nanoribbons: Photonic crystal waveguide analogy and minigap stripes

“…We have shown in Ref. 21 that, when the coupling constant between modes of the uncoupled basis reaches a sufficiently high value, these anticrossings merge. They form a stripe of minigaps, also seen experimentally 22 in PhC waveguides.…”

“…We note that, even on the optics side, these phenomena are not specific of PhC: the same occurrence of mode anticrossing also arises for simply corrugated waveguides, when guided waves explore the Littrow regime. 21 In canonical PhC systems, symmetries naturally appear so that modes couple in pairs of branches as seen here in Fig. 1͑b͒.…”

“…20 We furthered this concept by a theoretical study 21 and by an experimental test of the heavily modulated photonic DOS. 22 The core of this effect is the ability of diffraction at the guide edge to localize light along the waveguide, notably if diffraction scatters waves back,.…”

“…In the optical regime, we have recently been able to visualize a strong DOS modulation associated with these stripes thanks to a specially designed experiment. 22 We elaborate on ways to exploit this further below, going to a "critical coupling regime" 21 ͑already apparent in Fig. 6 but this case is not so analog to Fig.…”

“…They form a stripe of minigaps, also seen experimentally 22 in PhC waveguides. Additionally, around a precise critical value of the coupling, 21 the branches delimiting these stripes of minigap loose most of their wiggles and adopt a much smoother curvature. Such an effect arises ͑unintentionally͒ in the middle of Fig.…”