Dark and bright solitons in resonantly absorbing gratings

We consider an optical medium consisting of a periodic refractive-index gra ting and a periodic set of thin layers of two-level systems resonantly inte racting with the electromagnetic field. Recently, it has been shown that su ch a system gives rise to a vast variety of stable bright solitons. In this work, we demonstrate that the system has another very unusual property: st able bright solitons can coexist with stable continuous-wave (cw) states an d stable dark solitons (DS's). Depending on the parameters' values, a DS fr equency band coexists (without overlap) with one or two bright-soliton band s. Quiescent (standing) DS's are found in an analytical form, and moving on es are obtained numerically. Simulations show that a considerable part of t he DS solutions are completely stable against arbitrary small perturbations . The fact that this system supports both stable bright and dark solitons f or the same parameters values may find interesting applications in photonic s. [S1063-651X(99)13311-7].