Liquid Crystals as an Active Medium: Novel Possibilities in Plasmonics (original) (raw)

LCs show advantages compared to pure LCs because of their improved electro-optical characteristics due to enhanced photoluminescence [1], reduced driving voltage , improved order parameter and dielectric anisotropy , fast response time and spontaneous vertical LC alignment . On the other hand, if NPs are considered for their plasmonic properties, they can find in LCs a convenient host that can promote their nanoscale self-organization. Depending on the considered phase (nematic, cholesteric or smectic), LCs show short or longrange order. By accurately matching the capping material of the NPs, their size and shape with the right LC host it is possible to induce NPs to follow this order and hence achieve self-organization . Even if noticeable, this is not the only advantage of using LCs as a host for NPs. Being birefringent, LCs represent a possible way for realizing optical metamaterials with tunable functionalities: indeed, according to Mie's theory, the localized surface plasmon resonance (LSPR), typical of NPs, depends on the refractive index of the medium surrounding them . For LCs, this value can be finely controlled in a broad interval (typically from 1.5 to 1.7) by means of external stimuli (electric fields or by temperature changes). When combining plasmonic NPs with LCs, depending on the chosen LC mesophase, many configurations emerge. In the following, we will consider several of them in detail, providing evidence for advantages and potential drawbacks. Considering these configurations, the main distinction derives from the way plasmonic subunits are put in contact with the LC phase. So that, we identify systems where metal nanoparticles are immobilized on a substrate and layered with LCs or the case where NPs are homogeneously dispersed in the bulk of the mesophase. Even if extensive, this review is not conceived to be exhaustive: many configurations have already been exploited but many more are still to come, which explains the wide interest demonstrated by the scientific community to this topic.