Sparse Multiwall Carbon Nanotube Electrode Arrays for Liquid-Crystal Photonic Devices

Multiwall carbon nanotubes (MWCNTs) are normally grown as tangled masses by laser ablation or arc discharge. However, using plasma enhanced chemical vapor deposition (PECVD) techniques it is possible to grow dense aligned mats of “grass-like” MWCNTs as well as individual nanotubes in sparse arrays through the use of e-beam patterning of the catalyst. The fact that they exhibit very high conductivity and aspect ratio means that we can use them as electron source, as has been demonstrated in field emission displays, and as microwave sources. Conducting MWCNTs can also be used as electrode structures in optically anisotropic media such as liquid crystals, as potential alignment layers, and making novel new micro-optical components possible. Their ability to appear as large (with respect to the size of the liquid-crystal molecules) structures within a liquid-crystal device means that there is a strong interaction between the nanotubes and the liquid-crystal material. This interaction can then be interpreted as an optical interaction through the optical anisotropy of the liquid crystal. Hence, nanostructures can be used to form defect centers in liquid-crystal materials, which can then be manipulated by applying an external electric field. On the other hand, considering the fact that the diameter of a MWNT is from tens of nanometers to a hundreds of nanometers, the interaction between the nanotubes and liquid crystal is restricted to the micrometer scale, which is much smaller compared with current liquid crystal devices. In this Communication, we demonstrate an electrically switchable micro-optical component based on a sparse array of MWCNTs grown on a silicon surface, which forms one of the electrodes in a liquid-crystal cell. The nanotubes act as individual electrode sites which spawn an electric field profile, dictating the refractive index profile with the liquid-crystal cell. The refractive index profile then acts as a series of graded index profiles which form a simple lens structure. By changing the electric field applied it is possible to tune the properties of this graded index structure and, hence, the optical structure. When individual nanotubes are subjected to an applied electrical field, they form a field profile from the tube tip to the ground plane which is approximately Gaussian in shape, as indicated by the dotted line in Figure 1a. If the nanotube electrodes are immersed in a planar aligned nematic liquid crystal material, as shown in Figure 1b, with no field applied, then the liquid crystal molecules will align parallel to the upper substrate surfaces owing to the planar alignment pro-