Fig. 1. Schematic diagram of HCG

Fig. 2. Reflectance of different grating parameters versus wavelength in TE mode (a) grating period, (b) fill factor, (c) grating thickness, (d) stress buffer layer thickness

Fig. 3. Reflectivity as function of low refractive index sub-layer thickness and wavelength in TE mode

Fig. 4. （a）Reflectivity spectrum of TE-HCG（Λ= 0. 781 μm，f = 0.254，Tg= 0. 166 μm，T_L=0. 189 μm，T_A=0. 17 μm），（b）Reflectivity spectrum of TM-HCG（Λ= 0. 416 μm，f = 0.618，Tg= 0. 242 μm，T_L=0. 109μm，T_A=0. 175μm）

Fig. 5. The reflectivity spectrum of TE-HCG and the fill factor difference between the upper and lower gratings is 5%，10% and 15%

Fig. 6. The reflectivity spectrum of TM-HCG and the fill factor difference between the upper and lower gratings is 5%，10% and 15%