One of the easier ways to achieve "slow light"--the propagation of light at a group velocity much lower than is usual in an optical material--is to use a properly configured photonic-crystal waveguide (PCW). This is fortunate, because slow light is useful in modulators, optical signal processing, optical switches, and sensing, which all benefit from the small size of PC-based devices. But light is not easy to couple into a slow-light PCW; the mismatch in group velocity outside and inside the PCW makes coupling very inefficient. Attempts have been made to overcome this problem, but they all suffer from one or more problems, including low coupling efficiency near the band edge, narrowband performance, large physical size, and so on.
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