In-plane emission manipulation of random optical modes by using a zero-index material.

ABSTRACT

In this work, we have proposed to implement a zero-index material (ZIM) to control the in-plane emission of planar random optical modes while maintaining the intrinsic disordered features. Light propagating through a medium with near-zero effective refractive index accumulates little phase change and is guided to the direction determined by the conservation law of momentum. By enclosing a disordered structure with a ZIM based on all-dielectric photonic crystal (PhC), broadband emission directionality improvement can be obtained. We find the maximum output directionality enhancement factor reaches 30, around 6-fold increase compared to that of the random mode without ZIM. The minimum divergence angle is ∼6° for single random optical mode and can be further reduced to ∼3.5° for incoherent multimode superposition in the far field. Despite the significant directionality enhancement, the random properties are well preserved, and the Q factors are even slightly improved. The method is robust and can be effectively applied to the disordered medium with different structural parameters, e.g., the filling fraction of scatterers, and different disordered structure designs with extended or strongly localized modes. The output direction of random optical modes can also be altered by further tailoring the boundary of ZIM. This work provides a novel and universal method to manipulate the in-plane emission direction as well as the directionality of disordered medium like random lasers, which might enable its on-chip integration with other functional devices.