Research Of Negative Refraction In Optical Wavelength Range

Negative refraction is a novel phenomenon of optical propagation showing thatrefractive light position at the same side of the normal with incident light，consequently，it provides new approach to manipulate optical behavior to realizesubwavelength superlens and invisible cloak devices and bring out many enormouspotential applications such as nano waveguide, steer devices, quantum photonics andnonlinear optical switch and so on. Negative refraction has become hot researchfiled.2000, D. R. Smith et al. firstly realized experimentally negative refraction inartificial structures composed of metallic wires arrays and slit resonant rings atmicrowave range, additionally, negative refraction has been realized experimentallyin the micro-or nano structures such as the representative of fishnet metamaterial inoptical wavelength range. However，These structures realizing negative refraction iscomplex and need to be prepared by way of complicated nano processing technology.Negative refraction in these structures exist strong energy loss, so that restrictsfurther practical applications. Combination with unique property of manipulatingoptical propagation of liquid crystals through the alignment of liquid crystalmolecules, the thesis explores that negative refraction at optical wavelength isrealized in different constitute structures.Firstly, negative refraction in nematic liquid crystals is investigatedtheoretically. Negative refraction angle can be influenced by birefringence Δn,molecule aligned direction and incident angles and so on. Negative refraction can beobtained when the angle φ between aligned direction of liquid crystal molecules andinterface are more than0°and less than90°. The maximal negative refraction angleis obtained when the angle between aligned direction of liquid crystal molecules andinterface is approximately equal to40°. Liquid crystal with larger Δn can realizelarger negation refraction angle, maximal-14°is obtained experimentally for Δn=0.42at λ=532nm. Calculation results demonstrate tunable negative refractioncan be achieved through the reorientation of liquid crystals molecules by voltagecontrol.For overcoming the confinement including certain incident direction and smallnegative refraction angles, depending on the principle of negative refraction ofhyperbolic metamaterial possessing dielectric tensors with opposite signs, thedielectric tensors of aligned liquid crystals doped metallic nanospheres areinvestigated and achieved hyperbolic dispersion relations. Negative refraction can berealized at approximately wavelength450~560nm range. For smaller radius andlarger volume fraction of silver nanospheres and larger Δn of liquid crystals,negative refraction can be obtained easier. Optical hyperbolic metamaterialcomposed porous alumina embedded silver nanowires is fabricated experimentallyby means of two-step anodize oxidize methods and alternating current depositionmethod. Making use of reflectivity, experiments demonstrate negative refraction atλ=780nm.Due to strong energy loss in silver nanospheres/liquid crystals and silvernanowire/Alumina, hyperbolic metamaterial composed of silver nanowiresembedding reverse hexagonal lyotropic liquid crystal (RHLLC) is proposed torealize wide-spectrum negative refraction and can reduce optical absorption becauseof small permittivity of reverse hexagonal lyotropic liquid crystal. Throughtheoretical analysis, silver nanowires/RHLLC can realize all-angle negativerefraction at approximately more than440nm optical wavelength range and negativerefraction angles increase as the wavelengths decrease. Energy loss is compensatedby adding conveniently gain dye molecules in to reverse hexagonal lyotropic liquidcrystal embedded silver nanowires. The simulated results demonstrate transmittancecan be improved remarkable in contrast to material with no gain.