10.1117/2.1200910.1833 Holographic 3D displays using nanotubes Tim Wilkinson A new type of liquid-crystal device has been fabricated to generate a tunable, graded refractive index across the depth of each pixel. Liquid crystals offer exciting potential for creating real-time high-resolution 3D display systems. A number of attempts have already been made using lenticular flat-panel displays, auto- stereoscopy, and volumetric systems. However, none of these devices have been able to achieve a satisfactory image, either in depth or resolution. For the reproduction of a true 3D image, a fully complex hologram is the ultimate solution but is very difficult to display using current technology, requiring pixel res- olutions beyond what current systems are capable of. Ferroelec- tric liquid-crystal-over-silicon (LCOS) microdisplays have been used to project simple 3D images (see Figure 1). But both the quality and viewing angle of the image are limited by the bi- nary phase modulation of the LCOS technology. There has also been some success at generating full-color 3D projections using amplitude holograms. 1 However, the system was bulky, expen- sive, and ultimately limited by difficulties inherent in modulat- ing only the amplitude. A purely phase-only hologram (or kinoform) offers the best solution for building 3D displays. But again, there are limits to our ability to produce true holographic images using current technologies. One of the main limitations is actually the pixel geometry of LCD devices. Most use a traditional rectangular 2D shape with a uniform electric field applied between top and bot- tom electrodes. Our solution to the problem, rather than con- structing elaborate architectures, is to address this rudimentary electric field. As a result, we have developed a new liquid-crystal pixel structure using a vertically grown multiwall carbon nano- tube (MWCNT) as conducting element within the pixel. Conducting MWCNTs can be used to build 3D electrode structures in optically anisotropic media such as liquid crys- tals, making novel new micro-optical components possible: see Figure 2(a). 2 The electric field generated within the pixel when it contains the MWCNT is no longer uniform but rather is now almost Gaussian in shape: see Figure 2(b). The appearance of Figure 1. 3D images projected from a ferroelectric liquid-crystal-over- silicon microdisplay. Figure 2. (a) Nanotube electrodes grown on a silicon substrate. (b) Electric-field profile generated by having the 5μm nanotube grown within a 10μm pixel. the electrodes as large structures (with respect to the size of the liquid-crystal molecules) within the device indicates a strong in- teraction between the nanotubes and the pixel material. We can reinterpret the field as an optical interaction by looking at the anisotropy (birefringence) of the liquid crystal. The variation in refractive index across the depth of the pixel then maps as a graded index which ultimately forms a phase profile for that pixel. By changing the electric field, it is possible to tune the properties of this graded-index structure and hence modulate the light into an ideal kinoform. Using this new technology, the next generation of LCOS de- vices can be fabricated to include a vertically grown MWCNT Continued on next page