# Perspectives in Liquid-Crystal-Aided Nanotechnology and Nanoscience
## 3 Photonic Applications  
Surface plasmon waves, which are the collective oscillation in electron density at the interface of a dielectric and a metallic material, have received great attention since the 1960s [233]. They offer the opportunity to merge electronics and photonics at nanoscale dimensions due to their particular capability to guide and localize electromagnetic waves in sub-wavelength metallic structures, and have been widely exploited in various research fields including transforming photo-conversion processes [234; 235], improving weak fluorescence signals for biological imaging [236; 237], enabling nano-lasers or spontaneous amplified irradiation [238; 239], realizing photo-thermal conversion [240]  
Figure 7: (**a**) Threshold voltage (**top left**), splay elastic constant (**top right**), and electro-optic response time (**bottom**) as functions of GO concentration in 5 CB, for small and large sizes. (**b**) Schematic illustration of the Fréedericksz transition of a device cell filled with a GO flakes dispersed nematic liquid crystal. Reproduced with permission from [225]. Copyright (2016) John Wiley and Sons. and so on. Moreover, a series of plasmonic devices, such as light sources, waveguides, lenses, filters, or dynamic colour displays, have already been demonstrated [241; 242; 243; 244]. In the past few decades, metal nanoparticles have already been widely utilized as functional building blocks for obtaining new generations of nanomaterials with tunable surface plasmonic resonance (SPR). Metal nanoparticles show SPR when stimulated by electromagnetic radiation of an appropriate wavelength due to the quantum confinement effect of free electrons. In particular, gold nanoparticles have received great attention and been found to be pivotal for SPR applications such as scanning near-field optical microscopy [245] and biosensors [246] for several reasons. First, the plasmonic resonance of most metals occurs in the ultraviolet (UV) region of the spectrum, while gold shows this resonance in the visible region. Secondly, gold is chemically inert and does not oxidize, which is a very important property in optical applications since oxide layers will drastically complicate the calculation of optical properties. Thirdly, gold nanoparticles are compatible with biological systems, which will be quite important for investigations and in vivo analysis. Last but not least, there are now many new physical and chemical methods for the synthesis of gold nanoparticles with a wide range of diameters and shapes [247]. The optical characteristics of gold nanoparticles in visible and near-infrared (NIR) wavelength ranges are mainly determined by the collective response of electrons. The strong interactions between the incident electro-magnetic field and free electron clouds at the gold nanoparticle's surface will displace the electron clouds from their equilibrium position, thus inducing surface polarization charges that act as a restoring force on the electron clouds. This collective oscillation of free electrons will enhance the electric field that is produced at the interfaces of nanoparticles, and also confine it to a scale shorter than the wavelength of the incident electromagnetic field. At a macroscopic scale, such resonant excitation will lead to both strong scattering and absorption of light [247; 248; 249]. As a result, SPR is highly dependent on the diameter, shape, and dielectric constant of gold nanoparticles as well as the surrounding medium [248; 250; 251; 252; 253], implying that the SPR of gold nanoparticles can be tuned by changing these parameters. Although recent progress in the fabrication of nanomaterials has enabled us to produce metal nanoparticles with arbitrary size and shape, it is still time-consuming and costly to design plasmonic architectures with complicated structures for different optical applications. New methods for externally tuning SPR are still in demand.