Abnormal optical characteristics of the waveguide-grating structures

In this paper we investigate the optical property of the waveguide-grating structures (WGS) with the grating period in the scale of micrometer, which is larger than the wavelength of the incident light. In such structures, the higher-order diffracted beams can also couple into the waveguide, while in the structure that the period of grating is much smaller than the wavelength of incident light, only the ± 1 orders of diffraction can excite the propagation modes of the waveguide. Numerical calculations of extinction of the higher-order structure show that except for the extinction peaks, enhanced transmission can also be obtained, which is also observed in experiment. Further simulations indicate that the enhancement can be tuned by changing the structural and the geometric parameters of the WGS device. In addition, the wavelength that extraordinary transmit is decided by the duty cycles of the gratings, so, the enhancement might be attributed to the phenomena of missing order in gratings with large period.