Fig. 1. Formation process of grass shaped alumina

Fig. 2. Schematic of gradient refractive index structure

Fig. 3. Structural schematic of the layered platform calculation method

Fig. 4. Structural schematic of column calculation method after layering

Fig. 5. Simulation diagram

Fig. 6. Average reflectivity of different substrate temperatures under different boiling processes

Fig. 7. Cross sections of grass shaped alumina at different heights captured

Fig. 8. Schematic of center point position and binarized images of top view images at different heights

Fig. 9. Refractive index distribution and spectral curves of different compartments

Fig. 10. Fitting spectra and measured spectra

Fig. 11. Scattering curve and fitting spectra with different initial refractive indices