In this paper, we proposed an all-optical flip-flop device based on the multimode interference (MMI) with photonic crystal (PC) waveguides. The proposed all-optical logic gate was numerically studied by the finite-difference time-domain method (FDTD). In recent years, the photonic crystal is a very popular topic due to photonic band gaps (PBGs) which were known to exist in the periodically modulated dielectric materials. We try to combine the MMI principle and photonic crystal waveguide to design a whole new all-optical RS flip-flop device based on the MMI PC waveguide structures. By introducing a point defect rod both in the control ports, we can make outgoing field propagating in the output waveguide or not. The intensity profiles show snapshots optioned at different times. This numerical results show that the proposed PC waveguide structure could really function as an all-optical RS flip-flop logic gate. Note that if we have an RS flip-flop, we can realize various much complex logic processing based on it.
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