Tunable Surface Gap Plasmon Devices with a Liquid Crystal Elastomer Gap

A liquid crystal (LC) consists of nanometer-sized birefringent molecules with a high aspect ratio. In the nematic phase, the polarization of the LC can be easily tuned by applying an electrical bias and this is widely used in modern display technology. In recent years, a solid format of liquid crystal, namely, the liquid crystal elastomers (LCE) has captured notable attention in the field of liquid crystals due to its unique elastomeric properties. In a microscopic view, a LCE is a rubber-like material, in which nematic phase liquid crystal molecules are embedded and cross-linked in a prescribed deformable network. The network is usually made up by a polymer that has a two level energy system. When the electrons are excited (optically, electrically or thermally) from the ground state to the excited state, the network can be made to stretch. Attached to the deforming network, the embedded liquid crystal molecules also rotate, leading to not only dimensional changes, but also a change in the state of polarization for the light that is transmitted through the LCE film.<br/><br/>Previous work has studied the mechanical tunability at macro- or meso- scales, where reversible deformation of LCE films can be observed through optical and thermal excitation. In our work, we take full advantage of such tunability (dimensional and polarization change) of the LCE material and use the material to tune the optical resonances of nanophotonic devices. In one concrete application, we sandwich a 5 nm thick LCE film between 100 nm diameter gold nanoparticles and a gold substrate to form a Metal-Insulator-Metal (MIM) surface gap plasmonic devices. Through UV light excitation, the thickness of the LCE gaps shows 5%-10% change which leads to an easily observable (~20 nm) resonance wavelength shift in the visible range. We envision that the LCE, as a material platform, could be applied to a broad range of metasurface and photonic devices to achieve reversible optical tuning.