Broadband optical reflector based on Si/SiO2 one-dimensional graded photonic crystal structure

We propose the design of broadband optical reflector using a one-dimensional graded photonic crystal (GPC) structure. The proposed structure consists of a periodic array of two alternate layers of SiO2 and Si with low and high values of refractive indices (n1 and n2) and thicknesses a and b, respectively. The GPC structure considered here has a linear change in the thickness of the constituent layers for a certain number of layers and has a periodic repetition of this unit. In order to calculate the reflection properties a transfer matrix method (TMM) has been used. This investigation shows that GPC structures have wider reflection bands in comparison to a conventional PC structure and the width of the reflection bands can be enlarged by increasing the number of graded periods and also by increasing the thickness of periods. We also observe that in a GPC structure, omnidirectional reflection (ODR) bands get enhanced without increasing the number of layers of the structure. Hence, a GPC structure can be used as a broadband optical reflector and the range of reflection can be tuned to any wavelength region by varying the thickness of the graded periods.