Dark modes, slow modes, and coupling in multimode systems

We present general aspects of coupling among two or more families of modes with a view to deepen the insight on so-called “dark modes.” We first review the relationship of dark modes, “coherent population trapping,” “rotating wave approximation,” “coupled-mode theory,” and a few related concepts. The approach we emphasize is related either to inhomogeneous light–matter strong coupling or to the variety of multimode coupled systems designed for slowing down light or for filtering light. Some semantic caveats are discussed, notably down to what can be termed “dark” and “bright” in as simple a system as a distributed Bragg reflector case. A generic “N–F” classification simply states that whatever the total number N of modes, the key point is the number NF of “prediagonal” families, since the number Nb of bright modes is simply NF leaving Nd=N−NF dark modes. © 2009 Optical Society of America OCIS codes: 130.0130, 270.0270, 130.5296, 140.3945, 260.2030.