Device physics

Update on 3D displays

Static three-dimensional images are easy to make using holographic techniques. Moving pictures are more of a problem. A palm-sized, updatable display using a specially designed polymer could be a breakthrough.

Moviegoers who crave that feeling of being 'inside' the action will welcome the news from Tay et al. on page 694 of this issue1. The authors report a development that brings this dream a step closer to reality — an updatable, three-dimensional display based on cheap, easy-to-process photorefractive polymer materials. The technology still has some way to go to maturity, but ultimately it's not just the cinéastes who could benefit: displays that can provide realistic three-dimensional images with a wide angular viewing range might also be used in military or medical contexts, such as the simulation of field situations or the guidance of keyhole surgery.

Three-dimensional (3D) visualization technologies have a long history2. Early 'stereoscopic' approaches — familiar from the 3D movie craze of the early 1950s — used 2D displays and special eye-wear with polarizing lenses or lenses of different colours, so that separate images were seen in each eye. Alternatively, two images were switched rapidly on and off, with viewing glasses shuttered in synchrony to make the display appear continuous.

That kind of eye-wear is cumbersome, and colour filters in particular can cause headaches if used over a long period. As a result, polarizing stereoscopic displays have not spread much beyond scientific applications. The development in the past decade of improved 'autostereoscopic' displays that are viewable with the naked eye, in which a 2D display equipped with a lens array brings different images to each of the viewer's eyes, creating an illusion of depth, has renewed interest in 3D imaging3. But these displays require the viewer to be situated at the correct distance from the screen to obtain the stereo effect. Multi-view displays, or head-tracking displays that correct the image for the viewer's location, are some help, but increase the cost of making, processing and projecting the images.

Holography is another well-known 3D display technology, and provides high-resolution views over wide angles4. Holographic displays are autostereoscopic, in that the 3D image is 'stored' in a material and can be viewed when illuminated with no need for glasses, lens arrays or other devices. They work by recording both laser light scattered from the object to be imaged and a plane-wave laser reference beam to form an interference pattern that is stored in 3D in, for example, a photopolymer. In such polymers, light in the interference pattern activates a chemical reaction that locally modifies the material's refractive index. This process stores all the optical amplitude and phase information needed for 3D image projection, but cannot be repeated: the image on the display is static.

Tay and colleagues' breakthrough1 is to demonstrate a reasonably sized display, measuring 10 cm × 10 cm, whose base medium is what is known as a photorefractive material. Photorefractive materials also capture image (amplitude and phase) information in 3D. But unlike photopolymers, they are dynamic storage media: information can be stored, erased and rewritten (Fig. 1).

Figure 1: Handy display.
figure1

a, The base medium of Tay and colleagues' updatable, three-dimensional holographic display1 is a photorefractive polymer onto which the 3D interference pattern of light scattered by an object and a reference laser beam can be etched volumetrically. In brighter regions of the interference pattern, mobile charge carriers — electrons (−) and 'holes' (+) left by departed electrons — are generated and the more mobile holes drift into the darker regions. The electric-field distribution, and so the local refractive index, of the medium are thus altered in a way that maps the amplitude and phase information of the light from the imaged object. b, When light from a second, reading laser beam illuminates the polymer, it is scattered in the same way as light on the original object, and a 3D image results. As the writing mechanism depends solely on the dynamics of charge carriers in the polymer, which in turn (for a given polymer and applied voltage) depends only on the incident interference pattern, the display is in principle fully updatable.

This kind of photorefractivity has been studied extensively in crystals of inorganic oxides such as lithium niobium oxide (LiNbO3; ref. 5). In these materials, the absorption of a holographic interference pattern leads to the generation of mobile charge carriers in bright regions of the interference pattern. Under an applied field (or, in a polar crystal, the internal field), the mobile charges drift and accumulate in the dark regions. The result is a change in the electric-field distribution, and resultant, proportional changes in the crystal's local refractive index through a phenomenon known as the electro-optic effect. Inorganic crystals are known to be extremely photosensitive, and images of high resolution can be produced in this way. Unfortunately, growing crystals of a large enough area for practical displays is both difficult and costly.

This is where photorefractive polymers6 become interesting: they possess all the capabilities of the inorganic crystals, but can be processed to produce large-area displays using low-cost, solution- or melt-based methods. The photorefractive polymer developed by Tay et al.1 contains a copolymer combining a molecular group that transports positive charge with a dye whose polarization changes nonlinearly in response to light. The dye rotates and aligns under the influence of an applied electric field, modifying the material's refractive index. The copolymer also acts as a photosensitizer, absorbing light and generating mobile charges at the 'writing' wavelength of 532 nanometres.

The material thus developed possesses a remarkable combination of the properties crucial to a photorefractive display. Its optical quality and homogeneity are improved by use of the copolymer to minimize phase separation (the separation of dissimilar components of the material, forming regions that scatter light, degrading the image quality). High photosensitivity, allowing the use of only moderate laser powers for writing and erasing, is assured by adding small amounts of both a fluorinated molecular group with a nonlinear optical response and a plasticizer to promote molecular motion. A diffraction efficiency of close to 90% leads to good display brightness and low read-out power.

Operating their display under an applied voltage of 9 kV during writing, to speed up charge transport, but 4 kV for read-out, the authors could write across the full area of the display in about 3 minutes and hold an image on it for up to 3 hours. A fully automated optical system processed the object beam into strips that were recorded sequentially in adjacent parts of the display, using a laser intensity of around 100 milliwatts per square centimetre and a good writing speed. The principle of operation is entirely scalable: with higher-power lasers or more sensitive photorefractive polymers, larger areas could be written at the same time, or a small area faster.

Future advances in the size and speed of updatable holographic 3D displays would create a powerful and high-resolution visualization technology. But there is fierce competition, driven by the size of the potential military, medical and entertainment markets, with large-area, flat-panel 3D displays and alternative real-time 3D displays that are coming on in leaps and bounds. For film fans and gamers itching to be in the midst of the action, the wait might not be too long.

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Perry, J. Update on 3D displays. Nature 451, 636–637 (2008). https://doi.org/10.1038/451636a

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