Research On Nagetive Refraction Metamaterials In Visible Wavelength Based On Nano-Metals And Liquid Crystals

Electromagnetic propagation of negative refraction is an amazing phenomenon, manifesting that the incident light and refraction of light on the normal side in the interface between two media. In 1968, Veselago theoretically predicted that the material with a negative index of refraction can realize negative refraction at the interface, when the permittivity and permeability of a material are negative at the same time. Negative refraction materials can be employed to realize a variety of applications beyond reality: negative refraction slab lens material can make super-lens, sub-wavelength resolution imaging can break the diffraction limit, it has great applications in quantum photonics, Nano-waveguides, cloaking, and optical detection, and it is a research hotspot in recent years.In 2001, D.R.Smith et al. firstly demonstrated negative refraction with the micro-structure of wire and resonator ring arrays in microwave band. In 2008, Dolling G. realized negative refraction in visible range by fishnet metamaterials, such negative refractive materials of micro-and nano-structure is very complicated, the fabrication requires extremely high Nano-fabrication techniques such as electron beam lithography, physical vapor deposition, reactive ion etching etc. The large size sample can not be fabricated by these methods. The loss of incident energy is up to 50% at visible light. These issues restrict the practical process of negative refraction materials.According to the theory of hyperbolic dispersion metamaterials, when the motion of free electrons be constrained in one or two spatial directions. the dielectric constant is negative, so that you can achieve negative refraction. So silver nanowire array structure is chosen as the main negative refraction structure, because of its simple structure and the not complicated fabrication. The fill rate, the skin-depth, and the incident wavelength are theoretically studied for smaller loss material, fill rate of 0.4 per cent and about 10 nm diameter of the nanowires is the best size dimensions via simulations and calculations. This structure of materials realized negative refraction at a wavelength of 410 nm in visible area, and the loss is not high.In 2008, Zhang Xiang et al. fabricated silver Nanowire array with the diameter of 60 nm and line spacing of 110 nm, by using chemical method of alumina membrane template, which obtained negative refraction at wavelength of 660 nm in the optical range. But the aperture of alumina templates in chemical process is difficult to be controlled in size of 10 nm and the template cannot be removed. In this paper we employ a novel method to fabricate the negative refraction metamaterials with extremely small unit cells. The reverse hexagonal lyotropic crystalline is used as a soft template to fabricate silver nanowire arrays with 10 nm diameter and 15 nm center-to-center distance. The loss is accepted and the template is removable. The reverse hexagonal lyotropic liquid crystals is composed of the AOT(C20H37NaO7S), p-xylene and water, Aerosol OT contains one hydrophilic head and two hydrophobic tails. The hydrophilic heads of Aerosol OT arrange inwardly contact with H2 O, and the hydrophobic tails attach to the outside p-xylene. The aqueous solution of silver nitrate is isolated in the channels which are arranged in hexagon due to the self-assembly effect. AgNO3 solution is used instead of water in this study. The reverse hexagonal lyotropic liquid crystalline phases formed under conditions of constant temperature 5°C and without direct light exposure. The reverse hexagonal liquid crystalline phase could be aligned parallel to the electric field of 3 V. As a result, the high-density nanowire arrays grew perpendicular to the electrode. After the electrodeposition, the negative electrode ITO glass was softly washed by ethanol and ultrasonic cleaning. The fabricated sample was characterized by scanning electron microscopy(SEM),it shows that the nanowire diameter is 10 nm and line spacing is 15 nm. 92.31% elements of the substrate is Ag in the spectrum test EDX. It confirms the successful growth of Ag Nanowire arrays.As nanowire array film is too thin to be determined by measurement of negative refractive with refraction angle, we apply an efficient method to verify the hyperbolic dispersion of the fabricated material by measuring the angle dependent reflectance of S and P polarized beam. The s-and p-polarized reflectance of several wavelengths(730nm, 632 nm, 532 nm and 410nm) which cover the visible range, are experimentally measured. The fabricated sample satisfies hyperbolic material properties, which means that the fabricated silver Nanowire array achieve negative refraction throughout the visible band.Negative refractive materials will have more applications in optical switches, cloaking, a series of control and so on, when negative refraction materials can be switched between positive and negative refraction in visible range. We proposed a voltage-tunable hyperbolic dispersion metamaterial(HDMs) consisting of silver nanowires and nematic liquid crystals(NLCs). The metamaterial shows a novel electro-optic effect: external electric field driven switch between elliptic and hyperbolic dispersion. The anisotropic molecular shape and alignment of liquid crystal are extremely sensitive to external electric field. In the voltage-free case, the liquid crystal shows isotropic dielectric constant. The liquid crystal molecules are arranged along the electric field as the electric-field is applied to the liquid crystal. The dielectric of constant the material changes when the liquid crystal changes. Tunable range between positive and negative refraction is 20 nm when the Δn is 0.2. For further investigation, calculations between Δn and dispersion tunability are carried out, different Δn values(0.2, 0.3, 0.4, 0.5, 0.6) of NLCs are employed. The tunable range increases as the Δn increases.In this paper, we study the fabrication methods of super fine silver Nanowire array systematically by combining the theoretical and experimental methods. We believe that the results of this study will push ahead with the application of negative refraction materials, allowing potential applications in photonic devices.