Simulation of resonant behavior and negative refraction in metal nanowire composites

摘要

A metal-in-dielectric metamaterial structure different from that composed of split-ring-resonators and wire units was proposed. The metamaterial layer is composed of randomly distributed parallel pairs of nanowires of subwavelength size that form electromagnetically active units. It was predicted that the metamaterial should exhibit macroscopic negative refraction. In a recent paper fabrication of the metamaterial in the form of periodic array of parallel golden nanorods with trapezoidal cross section was reported and a negative refractive index of n = -0.3 was observed at a wavelength 1.5 μm (200 THz). In this paper we simulate response of a single pair of nanowires to near-infrared illumination and observe surface plasmon resonances using FDTD method. We simulate light propagation through the metamaterial slab made of one, two and three layers. In each layer the nanowires cover 10% of the surface. In simulations made for a single layer medium, negative refraction is observed for wavelengths from 1.55 to 2.1 μm, with Δλ/λ ≈ 0.3. When the number of layers increases, the range of negatively refracted wavelengths becomes narrower. For a narrow range of wavelengths that are close to the resonant frequency the intensity transmission of three layers reaches -7dB for the angle of incidence of 10°. Then layers with two orientations of nanowires are considered. In the first stack of layers all nanowires are oriented in parallel. This configuration assures plasmon resonances for both the electric and magnetic components of electromagnetic wave in all layers. In the second stack, nanowires in two subsequent layers are oriented perpendicularly. In the second layer, the plasmon resonance for the electric component of light is due to the oblique incidence of light. For a small angle of incidence of a near infrared narrow Gaussian beam we calculate two characteristics: the attenuation vs. wavelength and the lateral shift of the beam on the plane-parallel slab vs. wavelength. For a narrow range of wavelengths simulations show negative refraction of a beam incident the plane of the nanowires and a corresponding shift in the far field.