The contradirectional coupling between a pair of planar dielectric waveguides, whose guiding layers are respectively a negative index medium and a positive index medium (PIM, i.e., the conventional medium), is theoretically investigated using the coupled mode theory. It is shown that, by tapering the middle segment of the PIM guiding layer to introduce a certain phase shift, the energy flow circulation can be established based on the contradirectional coupling effect, and, thus, the electromagnetic energy is localized in the waveguide system. This phenomenon is further verified numerically with the finite-difference time domain method. The quality factor of this open waveguide cavity is also discussed.
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