Nonlinear Propagation And Coupling Of Surface Plasmon Polaritons At The Interface Between Metal And Liquid Crystal Thin Films

A Surface Plasmon Polariton (SPP) is an electromagnetic wave confined at theinterface between media with opposite sign permittivities, whose effective index ofrefraction is very sensitive to the change of optical properties near the interface. So,effective index of refraction of the SPP could be manipulated by changing the structuralor optical parameters of the system, which can be achieved in the metal-liquid crystalhybrid system. When voltage is applied to this system, reorientation of the director ofliquid crystals will occur; the optical properties of the system will change accordingly.Hence, the manipulation of the effective refraction index of SPP is realized. When ametal-liquid crystal hybrid system consists of a photoconducting layer, an optical fieldis also able to induce the manipulation of the SPP. Based on the typical metal-liquidcrystal hybrid structure, we mainly study the optical and electrical manipulation of theSPP using the extra-large anisotropic dielectric, and the interference between a lightwave in free space and the SPP is also explored.The dependence of the effective refraction index of a SPP in the metal-liquid crystalhybrid system on the variation of the liquid crystal alignment was given using theperturbation method. And the validity of this method was verified under sufficientlylarge electric fields. Then, we used the perturbation method to numerically study theelectrical and optical dependent responses of a SPP at the metal-liquid crystal interfaceunder small electric fields, after introducing a special photoconducting PVK: C60film asthe aligning layer. Here, we first fabricated nematic liquid crystal cells with the PVK:C60film as the aligning layer, and experimentally characterized its influence onpotential drop across the liquid crystal cells as well as its photoconducting properties atdifferent doped concentrations of C60. Then, we analyzed the dependence of themanipulation of a SPP by the nematic director reorientation on the PVK: C60film withdifferent thickness as well as the doped concentration of C60.When the SPP is excited by a laser source, the nonuniform intensity of the SPP willchange the voltage drop across the liquid crystal layer via the photoconducting effect ofthe aligning layer. So, the liquid crystal’s alignment changes when the dropped voltageabove Fréederickz threshold, which then retroacts on the SPP and induces the change ofthe SPP. This is the so-called self-modulation of a SPP. On the basis of the knownoptoelectric properties of PVK: C60materials, we first numerically investigated thedependence of the self-modulation of the SPP on the thickness of the photoconductingaligning layer and the applied voltage. Then, using an alkanethiol self-assembledmonolayer as thin as2nm acted as the special functional aligning layer, the couplingprocess during the self-modulation of a SPP was investigated by experiment. And the steady-state strength of the self-modulation was measured under the application ofdifferent external dc voltages and incident light intensities.In the end, we discussed the interference of a light in free space and a SPP。Thisinterference can induce the modulation of the nematic director orientation under thecooperation of an external electric field across the liquid crystal cell. In order to produceorientational gratings and observable diffraction effect in nematic liquid crystal, thespatial distribution of the interfering field under different experimental conditions wasanalyzed, which will pave the way for the further study.