A real-time dynamic holographic material using a fast photochromic molecule

2012 
Recently, 3D display technology has attracted much attention due to the spread of 3D televisions. A real-time, dynamic, 3D display can be achieved using holograms, which can reconstruct vast amounts of information and can produce 3D images that appear similar to how humans see the original objects. Hence, holography has attracted much attention as a next-generation 3D display technology that requires no special eyewear1,2,3,4,5,6,7,8. However, no commercial use of holographic 3D display technology has been developed to date because of serious issues that have not yet been solved. The major problem to be solved in the production of a practical holographic 3D display is the development of large size photoresponsive materials that can update holographic images in real time. Photorefractive polymeric materials are one member of a class of candidates that make it possible to refresh images every few seconds7,8,9. However, the photorefractive effect requires the application of a high voltage to record light-based information. In contrast, photochromic materials can store light-based information without applying a voltage. Photochromic materials are a well-known class of molecules that change their colour upon irradiation with light; the photogenerated species can be reversed to the initial species either thermally or by subsequent irradiation with a specific wavelength of light10. In addition, large size photochromic polymer films can be easily prepared from a polymer solution containing photochromic molecules by a simple solution-casting method. In this study, we prepared and analysed a real-time dynamic holographic material using a fast photochromic polymer film with unique photoresponsive characteristics. Holograms, which are recordings of the interference patterns in two- or three-dimensional media, are a key technology for the development of future 3D displays1. The interference pattern can be formed when the reference beam, a point source of light of fixed wavelength, encounters light of the same fixed wavelength arriving from the object. When the hologram is illuminated by the reference beam alone, the diffraction pattern recreates the wavefronts of light from the original object, and an image indistinguishable from the original object is recreated. True real-time updatable holographic displays will become feasible after the development of an ideal photoresponsive material. The refractive index or transmittance of such a material can be modulated only when it is irradiated with light, and they recover rapidly to the initial values after the cessation of irradiation. The use of photochromic2,3,4,5,11,12 and photorefractive7,13 materials in the development of real-time dynamic holographic recording media have been reported, but no ideal photoresponsive materials have been developed to date. Real-time dynamic holography should be able to update 3D images without a time lag between the movement of the objects and the movement of the recreated 3D images. Recently, Peyghambarian et al. successfully developed a quasi-real-time dynamic holographic display that can refresh images every two seconds8. They employed photorefractive polymeric materials in an updatable holographic 3D display. However, the refresh rate is not sufficient to display 3D images at a rate that is comfortable for the human eye. The improvement of the refresh rate using a revolutionary new photoresponsive material has been desired. We recently developed a unique series of photochromic [2.2]PC-bridged imidazole dimers with a [2.2]paracyclophane ([2.2]PC) moiety that couples two diphenylimidazole groups. The dimers exhibit instantaneous colouration upon exposure to UV light and rapid fading in the dark14,15,16. Upon UV light irradiation, the C–N bond between the two imidazole rings of the [2.2]PC-bridged imidazole dimer is homolytically cleaved to give a pair of imidazolyl radicals, and the colour of the solution changes from colourless to blue. In contrast to any other currently available photochromic molecules, the [2.2]PC-bridged imidazole dimers have high quantum yields close to unity for the bond-cleavage reactions, enabling the visible inspection of the colouration upon UV light irradiation, even with their fast thermal bleaching rate. The coloured species of the [2.2]PC-bridged imidazole dimer has a first-order decay with a half-life of tens of milliseconds, which is favourable for detection by the human eye. Thermally reversible photochromic molecules offer the opportunity to change and reset the molecular properties by simply turning a light source on and off. Photochromic molecules exhibiting such intense photocolouration and fast thermal bleaching performance could be promising materials for fast light modulator applications. Hence, we expected that the use of the fast photochromic polymer film as a dynamically updatable holographic recording medium would enable the real-time updating of object information. Here, we describe the development of a fast photochromic polymer for use in a real-time dynamic holographic material that can be easily prepared by a simple casting method from a solution of the photochromic polymer. The real-time control of 2D holographic images using the photochromic polymer film yields a speed equivalent to the time resolution of the human eye.
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