Using a proprietary thin film with a super-fast holographic response and no applied electric field, the authors have produced a real-time dynamic holographic display-concept demonstration with holographic images that can be refreshed on the order of a millisecond without crosstalk. The film's combined properties make it suitable for a large-sized, real-time, dynamic, color, holographic 3-D display.
by Hongyue Gao, Xiao Li, Zhenghong He, Yikai Su, and Ting-Chung Poon
HOLOGRAPHY1-4 is a technique for displaying real objects or scenes in three dimensions. A holographic display provides realistic 3-D imagery, allowing an observer to perceive light with the naked eye as it would be if scattered by an actual object or scene. This technique has attracted considerable attention in recent years. Large-sized and static holographic displays, including full-parallax holographic stereograms created by using special holographic recording materials, and very small-sized and dynamic holographic displays based on current commercially available spatial light modulators (SLMs), have been realized by some companies and scientific research institutes. However, a large-sized, dynamic, full-color, holographic 3-D display has proved elusive, due to several technology limitations.
Recently, however, dynamic holography has progressed substantially due to the discovery of some new holographic materials. In 2010, a holographic 3-D telepresence, dynamic and near real time, was presented by Blanche et al. in Nature, in which a holographic display that can refresh images every 2 sec was demonstrated in a photorefractive polymer as a holographic material.4Recently, a quasi-real-time holographic display with a refresh rate of five holographic images per second was reported by Kinashi et al.5 However, these achievements do not yet represent real-time dynamic holographic displays.
In this article, we describe how we created a real-time, dynamic holographic display by using holographic recording thin film with a super-fast optical response. Holographic images can be refreshed on the order of a millisecond without crosstalk using this new film. Both the formation time and self-erasure time have been shown to be around 1 msec, with the film being completely self-erased in each cycle. Moreover, because there is no need to apply any external electric field onto the thin film, it is easy to fabricate it into a large size without the need for creating true pixels because there is no required matrix-addressing structure. We think this holographic film is potentially suitable for a large-sized holographic display because it will produce higher resolution and a much larger viewable area than existing commercially available spatial light modulators (SLMs).
As is known, human observers can see and process the scattered light from an object, which includes intensity information as well as wavefront/phase information. In conventional photographic two-dimensional displays, only intensity information is recorded and subsequently displayed. Therefore, the wavefront/phase information of the object cannot be reconstructed. In holography, coherent light is used to record interference between light scattered from the 3-D object and a reference beam to form a hologram in the recording medium [see Fig. 1(a)]. Hence, both the intensity and wavefront/phase information of the object are recorded. To reconstruct the original 3-D object, we simply illuminate the recorded holographic information, i.e., the hologram, with a coherent reference beam [see Fig 1(b)]. However, this does not present a perfect holographic technique. It is necessary to solve some practical problems in terms of the size of the hologram, real-time response, resolution of the recording medium, color issues, etc., to create a true 3-D holographic video display. We have studied holographic 3-D displays for several years and recently obtained some interesting results in terms of real-time versions of such displays.
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Fig. 1: (a) The holographic recording process and (b) the reconstruction process.
For this purpose, our team developed an experimental holographic film that is fabricated by sandwiching a mixture of a liquid crystal and a photosensitive material with two glass substrates. We used a sample of this experimental film in a thickness of about 50 μm in a 3 cm x 2 cm size. We measured the response time of the thin film and found that we could achieve performance suitable for a real-time holographic display. The experimental setup for this holographic display is shown in Fig. 2. A reference beam and an object beam, which are derived from a Nd:YAG laser (λ = 532.8 nm), are set to be p-polarization by a half-wave plate. A spatial light filter, an SLM, and a lens are placed in the object beam path, where the spatial information of an image is displayed on the SLM. A He–Ne laser beam (λ = 632.8 nm) is also set to be p-polarization to probe the writing region of the sample.
Fig. 2: An experimental setup for a holographic display uses a liquid-crystal thin film without an applied electric field as a sample. Ms are mirrors, BS is a beam splitter, λ/2 is a half-wave plate, L is a lens, SLF is a spatial light filter, SLM is a spatial light modulator, Io is the object beam, Ir is the reference beam, and Ip is the readout beam or reconstruction beam.
In this experiment, we used an SLM to form the test images because this is an easy way to carry real-time dynamic image information onto the signal beam. However, the SLM is not used as a holographic display. Here, our experiment is basic research used to demonstrate that a real-time dynamic holographic display can be realized in this film, and the SLM is a practical way to modulate the signal light to obtain dynamic incident images.
The hologram formation time and self-erasable time in the film are both about 1 msec measured by an oscilloscope,6 as demonstrated in Fig. 3. The graph in Fig. 3(a) shows that the hologram can be formed immediately in the sample once the recording light is turned on. The film is actually recording the hologram, which is carrying all the information of a 3-D image, not just combining multiple coherent light paths. In Fig. 3(b), it is shown that when the writing or recording light is turned off, the recorded hologram can be self-erased completely. There is no need to use any light or electrical field to erase it.
Fig. 3: (a) Hologram formation and (b) the self-erasure process appear with "ON" and "OFF" denoting that the writing light is turned on and off, respectively, illustrating a response time of the order of 1 msec.
If the response of the holographic material, especially the erasure of the hologram, is not fast enough, the recorded hologram cannot be erased completely, and then the diffraction efficiency of the following hologram will be affected and multiple images will be reconstructed from the holograms at the same time. This crosstalk is a potential problem in a holographic display. We demonstrated a holographic video display without crosstalk between holograms with this film. Figure 4 shows a real-time display of a rotating letter "B".
Fig. 4: These seven snap shots are from a real-time holographic display video.
The next step of our research is to study the color holographic display in an RGB model. As we know, an original color image includes red, green, and blue components. They should be recorded in three holograms. During the display process, the RGB images should be read out from these holograms, and then combined into a color image, as shown in Fig. 5. We believe that with the development of our work, further improvements could bring this film into practical holographic color display applications. Although we have not demonstrated this technology yet, we have studied RGB model color holographic displays and are improving the results.
Fig. 5: The above color holographic display is based on an RGB model.
Looking Forward
The film that we have created and are studying has much faster response times and higher resolutions and is more easily fabricated into a large-sized holographic display than current commercial SLMs. We hope to develop this film into a holographic TV platform in the future. Such a holographic TV would include a writing system, a display screen, and a readout system. The hologram would be recorded in the display screen by using the writing system, and then be read out from the display screen by the readout system to reconstruct the image. The response of this film is super-fast; therefore, we think a large-sized hologram cannot be written in the film by mechanical scans, as has been previously been demonstrated.
We have demonstrated angular multiplexing, peristrophic multiplexing, and other multiplexing modes in this film. Holographic multiplexings, i.e., multiple holograms synchronously displayed at a single location of the thin film, have also been demonstrated6 by our research, which shows the feasibility of an RGB-model color holographic display.
We have also created concept designs for a holographic TV system. If this system can be made, it will be large and heavy at first, much like early projection systems. But we think it is important to create a holographic TV. The writing system, display screen, and readout system are difficult but not impossible to imple-ment. With proper funding, we think we could make this holographic TV in 5–10 years.
In terms of future research, we also would like to slow down the response of the material, especially the hologram erasure, which is too fast for the requirements of this application. If the perfect holographic material can be achieved, mechanical scans could be used as a writing method, and the entire system would be greatly simplified.
Acknowledgments
This work is supported in part by the NSFC (11004037), the China Postdoctoral Science Foundation (20110490732), the Science and Technology Commission of Shanghai Municipality (09ZR1414800/11JC1405300), and the Shanghai Shuguang program (09SG13). The work is also supported by the Chinese Academy of Sciences Visiting Professorships for Senior International scientists Program under Grant Number 2010T2G17.
References
1S. A. Benton, Selected Papers on Three-Dimensional Displays (SPIE Optical Engineering Press, Bellingham, Washington, 2001).
2S. A. Benton and V. M. Bove, Holographic Imaging (Wiley-InterScience, 2008).
3T-C. Poon, ed., Digital Holography and Three-Dimensional Display (Springer, 2006).
4P.-A. Blanche et al., "Holographic three-dimensional telepresence using large-area photorefractive polymer," Nature 468, 80-83 (2010).
5K. Kinashi et al., "Dynamic holographic images using photorefractive composites," Topical Meeting on Biomedical Optics and 3D Imaging , OSA, JM3A.58 (2012).
6H. Gao et al., "Multiplexed holographic display based on a fast response liquid crystal film," in Digital Holography and Three-Dimensional Imaging, OSA Technical Digest , DM2C, 4 (2012). •