Transmission properties of one-dimensional graded photonic crystals and enlargement of omnidirectional negligible transmission gap

We investigate numerically the transmission properties of one-dimensional (1D) graded photonic crystals (PCs) in detail. Under the condition of linearly graded structures, the gap broadening of three cases is calculated: the case when the thickness of low index layer is increased, the case when the thickness of high index layer is increased and the case when the thickness values of both layers are increased simultaneously. Then the gap broadening of another graded case with the thickness exponentially increased is presented. At last the enlargement of omnidirectional negligible transmission band gap is obtained.