Study of Far-Field Radiation Pattern in Nano-Photonic Devices and Its Application in Single Small Particle Detection

The optical properties of zero-mode waveguides (ZMW) in metal films has been studied extensively. Here, we investigate the far-field radiation pattern of two zero-mode wavegiudes surrounded by circular corrugations. We monitor the direction and intensity variation of the transmitted light of the sub-wavelength apertures in the back-focal plane of the microscope objective. Any phase contrast between two holes results in a deflection in the direction or intensity distribution of light in far-field pattern. We introduce a refractive index contrast between two ZMWs by anchoring a quantum dot within of one of them, which results in a phase difference between holes. Introduction of a phase difference can also be achieved by fabricating two asymmetric ZMWs. We show that in both cases the induced phase variation between two holes lead to a deflection in the far-field diffraction pattern. We find that the complex pattern of the transmitted light can be quantified through a scalar measure of asymmetry along the symmetry axis of the aperture pair. We also demonstrate that asymmetry parameters increase can be controlled by the number of circular rings surrounding ZMWs. Finally, we perform a series of Finite- Difference Time-Domain (FDTD) computational study to propose a new design of double-ZMWs sensors that can be used for small particle detection with higher sensitivity and also applicable to control the direction of the light.