Unidirectional flow of light with high autofocus in a two-dimensional hybrid Si-ZnO photonic crystal

Abstract

A photonic structure built on a Si-ZnO substrate described by a rectangular region with a square lattice of circular cylindrical air-holes presenting a unidirectional transmission of a light beam was studied. The unidirectional transmission was obtained introducing a triangular spatial asymmetry region without air-holes adjacent to a photonic structure using as a starting point the total internal reflection effect. The optical properties of the photonic structure were optimized by numerical calculations and used to build a photonic structure by the FIB technique. Both, the calculated and the experimentally measured value of the forward transmittance of the photonic diode were 94% at the design working wavelength equal to the average backward transmittance of the photonic structure was around 10%. The forward and backward transmittances gave a one-way contrast ratio equal to 0.808. It was found an extraordinary effect of autofocus and confinement of the optical electric field associated with the incident light beam along the central region for forward propagation direction. In the case of backward propagation direction, the amplitude of the optical electric field transmitted through the photonic structure was almost negligible, showing only an appreciable magnitude close to the triangular region.