Graphene/Carbon Nanotube Hybrid-Based Transparent 2D Optical Array
Holography is an ideal technology to realize 3D displays. Photorefractive material, in which the refractive index changes when exposed to light, has been used to realize active-type dynamic hologram images that require high external voltage to operate.To avoid such difficulties, photochromic materials such as azobenzene and diarylethene have been introduced but still have the drawbacks of long write/erase times. Another approach was the use of liquid crystals to modulate refractive index by controlling external bias. However, the pixel size is limited to larger than 5 μm, which is significant drawback to realize holograms.
An array of multiwalled carbon nanotube (MWCNT) pixels was fabricated to modulate refractive index in a desired location. A strong local field was generated by geometrically sharp carbon nanotubes that have high a field enhancement factor, which eventually locally modulates the refractive index of the liquid crystal medium at low operation voltages. Recently, carbon nanofiber arrays were synthesized on TiN/quartz substrate with 28 % optical loss. This is suitable for transmissive mode for optical devices, while reflective mode is limited due to the possibility of interference between reflective and diffractive lights. This method demonstrated relatively reasonable transmittance for this purpose, which could be used in a prototype design for holograms. Nevertheless, some of the light was absorbed in the cathode electrode due to the use of opaque metal.On the other hand, graphene, a 2D hexagonal lattice of carbon atoms, is highly conductive with 97.5% transmittance per layer,and has been used for several transparent and flexible electrodes.The purpose of this paper is to develop transparent an active-type 2D optical array that operates at low voltage. Instead of using opaque metals, a graphene electrode was used. An array of carbon nanotube pixels was synthesized on patterned catalysts using chemical vapor deposition. CNTs were uniformly distributed on each pixel with a typical height of 1 ▒ 0.2 Ám, which was confirmed using an optical surface profiler. The fabricated 2D grating array shows good diffraction efficiency with an absolute transmittance of 97 % at a relatively low operation voltage within ~ 0.05 V/Ám in a liquid crystal medium.