Omnidirectional narrow bandpass filter based on metal-dielectric thin films

We show that a metal-dielectric Fabry-Perot (FP) structure can exhibit an omnidirectional transmission for p-polarized light, which means a passband is independent of the incidence angle of light. The omnidirectional passband occurs when the sum of the reflection phase shift at the metal-spacer interface and the propagation shift in the spacer region is almost 2pi for every incidence angle. We numerically and experimentally demonstrate such an omnidirectional narrow bandpass filter in an air/Ag/ZnS/Ag/glass structure. Moreover, we introduce an antireflection coating on both sides of the metal-dielectric FP structure. The transmittance will increase obviously, while the omnidirectional property remains the same.