Refreshable Holographic 3-D Displays

In order to achieve holographic displays that can be refreshed, several properties must be combined, including efficient recording, erasing capability, and persistent storage (memory) of holograms. Here, the authors describe the development of the first updatable holographic 3-D display with memory, based on photorefractive polymers.

by Savas Tay and Nasser Peyghambarian

HOLOGRAPHY occupies a special place among the various approaches to three-dimensional (3-D) visualization, and since the day they were introduced, people have been fascinated by them. Holograms provide auto-stereoscopic images that are viewable directly without the need for special eyewear (Fig 1). They help reduce the unwanted side effects of using special goggles, such as motion sickness and eye fatigue.¹ They are capable of very high spatial resolutions, full color, and a high degree of realism. Such characteristics make them very valuable tools for a wide range of applications, including medical, industrial, military, and entertainment imaging.

Although mostly known throughout science-fiction and entertainment circles, holography is a powerful technique with a solid theoretical foundation, and early pioneers such as Gabor, Leith, Upatnieks, and Benton have come up with an amazing array of applications that use holographic optical recording and processing.^2,3 Established appli-cations of holography include non-destructive testing and evaluation, holographic data storage, nano-fabrication (interference lithography), and fabrication of diffractive optical elements used in spectroscopy, beam combining, and telecommunications.

Unfortunately, the lack of suitable recording materials has limited to date the realization of many other exciting applications, especially those that require the dynamic changing of recorded images. Most of the successful implementations of holography mentioned above involve static images, and the recording media used are write-once, read-many type of materials. Dynamic or semi-dynamic (re-freshable) recording materials could significantly improve the performance and extend the uses of holographic systems, and 3-D displays are one of the applications that could greatly benefit from such a capability.

Fig. 1: Illustration of the future holographic three-dimensional display.

To be suitable for refreshable 3-D displays, holographic recording material needs to have efficient writing and reconstruction (high sensitivity and diffraction efficiency), fast writing time, long image persistence (memory), capability of rapid erasure, and the potential for large display area – a combination of properties that has not been realized in a single material until recently. Previously, holographic recording materials included photo-polymers, silver halide films, or dichromated gelatin. Images on these materials are permanently written and therefore cannot be erased or refreshed. Reversable recording media such as inorganic PR crystals are extremely difficult to grow to larger than a few square centimeters in size.

On the other hand, PR polymers – organic-based dynamic holographic recording materials – are capable of significantly larger sizes, very high diffraction efficiency, and sensitivity, which make them a good candidate for use in refreshable and dynamic holographic displays.

Here, we report the details of the development of the first updatable holographic 3-D display based on PR polymers.⁴ With a 4 x 4-in. size, this is the largest holographic 3-D display to date and is capable of recording and displaying new images every few minutes. The main difference between our PR-polymer-based refreshable display and previously developed dynamic 3-D display systems based on opto-electronic systems is the memory capability of the PR polymers. The recording media employed in real-time dynamic systems such as acousto-optic or MEMS devices lack memory (storage of images) and require the continuous scanning of the image at video rate (30 Hz), which severely limits the achievable image size and resolution using these devices.

The holograms in our PR-polymer-based 3-D display can be viewed in a 45^o viewing zone without the need for scanning. They achieved resolution comparable to that of NTSC TV, they persist for several hours without the need for refreshing, and can be completely erased and updated with new images whenever desired. With a few minutes of a turnaround rate, they occupy a special region between real-time and static displays that could be named semi-dynamic or near real time. Such near-real-time display of information has significant uses in applications where the data is not generated or processed instantaneously, yet still require the updating of images. Such applications include medical and military imaging, entertainment, and advertisement.

How It Works

In PR polymers, the interference light pattern created by two intersecting, coherent laser beams is recorded as a refractive-index modulation or a phase hologram.⁵ This is achieved by electrical-charge generation in the illuminated areas through absorption of light, followed by transport and trapping in the dark regions. The spatial-charge distribution creates local electrical fields which in turn lead to a macroscopic refractive-index change through an electro-optic effect. The hologram can be erased via uniform illumination by a laser beam because the distribution of charges can be randomized by absorption of light. The record/erase cycle does not cause aging and degradation of the material, and new holograms can be recorded in the same location.

The holograms will gradually fade (decay) over time because the trapped charges are randomized by thermodynamic processes inherently present in materials. PR polymers that are designed to have fast recording time usually also have high hologram decay rates. However, for an updatable 3-D display application, a material with rapid recording and slow decay (long persistence) is required. Such a small ratio of persistence time to recording time, combined with other shortcomings such as small area, low diffraction efficiency, and susceptibility to electrical and optical damage has previously prevented the development of updateable holographic displays based on PR polymer composites.

Fig. 2: 4-in. photorefractive polymer devices shown side by side with a smaller (5 mm) sample.

Recently, we have developed new PR-polymer devices with favorable dynamic properties which make them suitable for use in updatable 3-D displays.⁴ The new polymer system consists of a copolymer with a hole-transporting moiety and a carbaldehyde aniline group (CAAN) attached through an alkoxy linker. A copolymer approach is used, which helped eliminate the phase separation between the functional components commonly seen in homopolymer PR composites while allowing larger non-linear chromophore doping. A copolymer with a polyacrylic backbone was used to attach pendant groups, tetraphenyldiaminobiphenyltype (TPD), and CAAN in the ratio 10:1 by the synthetic modification of the polyacrylate TPD (PATPD) polymer. The host PATPD-CAAN copolymer provides optical absorption and charge generation/transport at the writing wavelength (532 nm). A plasticizer, 9-ethyl carbazole (ECZ) was added to the composite to reduce the glass-transition temperature. A large refractive-index change was achieved by adding 30-wt.% fluorinated dicyanostyrene (FDCST) chromophore.

Device Fabrication

A composite of PATPD-CAAN:FDCST:ECZ (50:30:20 wt.%) was formed into thin-film devices by melting the composite between two transparent indium-tin-oxide-coated glass electrodes. The active layer thickness was set to 100 μm by using glass spacer beads. Figure 2 shows a 4 x 4-in. active-area thin-film device made from this composite next to a typical laboratory test sample. The device showed no dielectric breakdown for extended periods of use (several months) in our display setup, with hundreds of write/erase cycles experienced at high applied voltages (9 kV) and writing optical intensities around 100 mW/cm².

The PR thin-film devices show a diffraction efficiency of nearly 100% at an applied voltage of 5 kV in steady-state four-wave mixing measurements. We have developed a new recording technique to modify both rise and decay times by changing the applied voltage. Starting at a higher than usual applied voltage (9 kV), we have achieved a fast recording time of less than a second. We have then reduced the voltage to its optimal value of 5 kV, which ensured a long persistence time with high diffraction efficiency. The temporarily increased voltage during writing facilitates efficient separation of electron-hole pairs and improves the drift characteristics, forcing the charges to travel faster, while also increasing the orientational order parameter and rotational speed of the chromophores.

The display hologram is generated by holographic stereography. This technique is based on optical multiplexing of a limited number of viewpoints of the same object (2-D perspectives at different angles) onto different parts of a recording medium to recreate 3-D perception along with parallax. This powerful technique does not require the actual object to be present for recording. It can use data from any device capable of providing 2-D perspectives of an object of interest. Imaging methods such as magnetic resonance imaging, computer-assisted tomography, aerial and satellite 3-D imaging, synthetic aperture radar, integral photography, or computer-assisted modeling can be integrated with this technique. For an actual object, the simplest way to generate the perspective data would be to capture video of the object from different angles using a camera moving on circular tracks. Once the 3-D perspective views are created, they are sliced into multiple view zones, and the view zone for each perspective is combined into a single image. This composite image, called a hogel, is what is actually recorded in a single slice or pixel in the holographic recording material.

Fig. 3: Optical layout of the 3-D display.

A sketch of the 3-D display system we developed is presented in Fig. 3. The writing light source is a doubled YAG laser at 532 nm, a wavelength that is located within the absorption band of the PR material. The hogel information is loaded to the object beam using a spatial light modulator (SLM) controlled by a computer. The object beam interferes at the sample position with a reference beam to create a holographic representation of the hogel.

Writing is performed sequentially: the first hogel is recorded in the sample at the first location; next, the writing beams are turned off and the sample is moved to the second location where the second hogel is recorded. Once all the hogels have been recorded, the sample is moved to the reading position where the hologram can be viewed.

We have used red (633 nm) light from a low-power He-Ne laser in an LED to illuminate the holographic display. LEDs possess the advantage of negligible speckle, owing to their low coherence, but using a rotating diffuser in conjunction with the laser can also be used to reduce coherence-related speckle. Hologram erasure is accomplished by illuminating the sample with a homogeneous beam at a wavelength that is within the absorption band of the material.

The current system (Fig. 4) supports horizontal-parallax-only (HPO) holograms. In many applications, HPO imaging is an effective approximation of 3-D representation because humans perceive depth using the horizontally offset eyes. HPO reduces the number of hogels required to write the full hologram, thereby reducing the overall recording time. We note that our technique is compatible with full parallax imaging as well. We have recorded HPO holograms 4 x 4 in. in size with complex and high-quality images (see Fig. 5) using the system described above within 2 minutes. These images can be viewed for up to 3 hours without significant decay in image brightness and contrast, and can be erased anytime within a few minutes, thus comprising the first updateable holographic 3-D display with memory.

Our 3-D display features a total horizontal viewing angle of ±45° with uniform brightness. The images are viewable directly on the PR thin-film device without the need for intermediate projection tools or magnification between the recorded image and the viewer. New images can be recorded when desired. The snapshots of the holograms presented in Fig. 5, which were captured using a CCD camera, are only a modest reproduction of the effect actually experienced upon direct viewing. This is principally due to the astigmatism introduced by the HPO recording technique and electronic artifacts such as saturation, to which the human visual system is relatively insensitive.

Conclusion

We do not anticipate any practical limit on the achievable display size using PR poymers: large devices can be fabricated and/or tiled together and the plastics industry has already shown the ability to laminate extremely large multi-layered thin films. Moreover, the persistence and diffraction efficiency of the material make it a leading candidate for future full-parallax displays, which typically require two orders of magnitude more information content than HPO displays. For larger full-parallax displays, a combination of short-pulse recording and thermal fixing can be used, which are future areas of research for holographic 3-D display development. Full-color imaging can also be implemented by fine tuning the absorption characteristics of the polymer composite.

In summary, we have developed PR polymer devices that combine exceptional properties such as large size, high efficiency, fast recording, image persistence, long lifetime, and resistance to optical and electrical damage, satisfying many of the major requirements for use in holographic 3-D displays. These advances have allowed us to demonstrate the largest PR holographic 3-D display to date. Holographic image-updating capability can significantly extend the applications of 3-D displays in the fields of entertainment, education, medical, and technical imaging.

Fig. 4: Shown is the author and the 3-D display setup. The 3-D display can be viewed directly on the PR thin-film device.

References

¹N. A. Dodgson, "Autostereoscopic 3-D Displays," Computer 38, 31-36 (2005).

²M. R. Chatterjee and S. Chen, "Digital Holography and Three-Dimensional Display: Principles and Applications," T. Poon, ed.(Springer, New York, NY, 2006), Chapter 13, pp. 379-425.

³S. A. Benton and V. M. Bove, Jr., Holographic Imaging (Wiley Inter-Science, (2008).

⁴S. Tay et al., "An updateable holographic three-dimensional display," Nature 451, 694 (2008).

⁵O. Ostroverkhova and W. E. Moerner, "Organic photorefractives: Mechanism, materials and applications," Chem. Rev. 104, 3267-3314 (2004). •

Fig. 5: Images from the photorefractive-based 3-D updateable display.