Omnidirectional reflection by multilayer dielectric mirrors

Periodic structures reflect strongly at frequencies and angles of incidence corresponding to photonic bandgaps. It has recently been shown that for a suitable choice of high and low refractive indices n h and n ℓ , a high-low dielectric multilayer can reflect strongly at all angles of incidence. An accurate and simple analytic approximation for the location of the band edges for the s and p polarizations is given. The region in the ( n ℓ , n h ) plane where omnidirectional reflection occurs is determined. This region lies above a curve which has its minimum value of n h (≈2.247 n 1 ) at n ℓ ≈1.492 n 1 , where n 1 is the refractive index of the medium of incidence. The minimum occurs when the optical paths in the high and low refractive index layers are in the ratio 1.362 to 1.