Realization of “trapped rainbow” in 1D slab waveguide with surface dispersion engineering

We present a design of a one dimensional dielectric waveguide that can trap a broadband light pulse with different frequency component stored at different positions, effectively forming a “trapped rainbow” [Nature 450, 397 (2007)]. The spectrum of the rainbow covers the whole visible range. To do this, we first show that the dispersion of a SiO2 waveguide with a Si grating placed on top can be engineered by the design parameter of the grating. Specifically, guided modes with zero group velocity(frozen modes) can be realized. Negative Goos-H¨anchen shift along the surface of the grating is responsible for such a dispersion control. The frequency of the frozen mode is tuned by changing the lateral feature parameters (period and duty cycle) of the grating. By tuning the grating feature point by point along the waveguide, a light pulse can be trapped with different frequency components frozen at different positions, so that a “rainbow” is formed. The device is expected to have extremely low ohmic loss because only dielectric materials are used. A planar geometry also promises much reduced fabrication difficulty.