1. Introduction

Recent technological development for three-dimensional (3D) television has attracted many peoples, because the true 3D display provides us a realistic world just as it exists in front of us. Holography invented by Gabor [1] is a unique technique to display the three-dimensional (3D) objects recorded in the media. With this invention and the later development of lasers they have been widely used in the field of graphical arts and security etc.

In 1990s, digital holographic technology using a CCD device [2–4] and hologram imaging technique using a spatial light modulator (SLM) [5] have been introduced. Recent computer technologies with high processing speed and high capability allow us high-speed numerical holographic construction and reconstruction of 3D imaging [6] and the synthesis and display of dynamic holographic 3D scenes with real-world objects [7]. Using a holographic screen, large-scale 3D projection is reported [8]. Despite of these technological developments, however, spatial resolution is not surpassed those for the conventional holograms (5000 lines/mm) [7].

Conventional holograms are permanently recorded in the media of silver halides, photopolymers, or dichromated gelatin. These holograms are fixed after several processing, and are easily reproduced with coherent light such as laser or incoherent light sources of LEDs. These media lack the capability of image-updating, resulting in the limitation of use. Rewritable hologram has been presented using photorefractive inorganic crystals in the past. However, they have the disadvantage of difficultness of crystal growth to large scale, which prevents their application to large scale imaging and display. On the contrary, photorefractive polymers [9–12] are the promising media for providing updatable or rewritable holograms with large scale. Thus, using photorefractive polymer devices, updatable real-time three-dimensional holographic display would be constructed with capability of updatable recording and simultaneous replaying.

Peyghambarian’s group demonstrated updatable holographic display using a stereogram of hogel images recorded on photorefractive polymer based on PATPD polymer under high electric voltage [13,14]. Owing to assistance of high electric field, their system has the response speed of 2 seconds per frame [14,15]. Tsutsumi et al. have demonstrated two dimensional images were recorded in the photorefractive polymer based on poly(N-vinyl carbazole) (PVCz) composite and simultaneously reconstructed using a probe beam [16]. Applying high electric field gives the faster response rate, but the risk of dielectric breakdown. In this meaning, lower electric field or non-electric field driven is preferred. If the photorefractive polymer devices work at lower electric field or non-electric field, the possibilities of dielectric breakdown become zero. Thus, photorefractive response without applying electric field has several advantages of no tilting of device and no damage for dielectric breakdown. In the last decade, several studies photorefractive response polymer composites have been reported without applying electric field [17–23]. Diffraction efficiency of ca. 90% and optical gain of 224 cm⁻¹ were obtained in PVCz composites [19] and optical gain of 451 cm⁻¹ is reported in 3-[(4-nitrophenyl)azo]-9H-carbazole-9-ethanol (NACzE) (30 wt%) doped polymethylmethacrylate (PMMA) [23] without applying electric field. These polymer composites have potentials of holographic applications of signage which does not demand the restrict requirement of the video-rate response. However holographic nature of this monolithic compound dispersed polymer composite has not been reported yet.

In this paper, we present here a new updatable holographic 3D imaging based on monolithic compound dispersed polymer composite with capability of recording and displaying new images within a few seconds and viewing for a long time without applying electric field.

2. Experimental sections

Schematic apparatus for the updatable real time holographic imaging of the object is illustrated in Fig. 1 . Laser beam (Spectra Physics, 300mW @ 532 nm) is split off by a polarized beam splitter: One s-polarized beam is spread out with the combination of an objective lens (40x) and another lens, which is used for an object beam. Another s-polarizedbeam also spread out is used as a reference beam to interfere with the object beam from the object on the device film. Intensity ratio between object and reference beams is controlled with a half wavelength wave plate in front of a polarized beam splitter (PBS). Typical intensity of an object beam is 4.8 mW and that of a reference beam 14 mW. Recorded hologram is simultaneously reconstructed by a p-polarized probe laser beam @ 642 nm which is widely spread out using a pair of lenses. Laser source is an Omicron-Laserage semiconductor laser PhoxX 642 (140 mW @ 642 nm). Typical intensity of probe beam on a device is 10 - 50 mW. Laser Raman spectroscopy of sample device was performed using a Laser Raman Microscope RAMAN-11, Nanophoton, Japan.

 

Fig. 1 Optical setup schematics to record the hologram of the object.

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3. Results and discussion

Polymer composite of NACzE (30 wt%) doped PMMA [22,23] was operated without applying electric field. NACzE consists of carbazole moiety and nitrophenyl azo unit in one molecule. Photorefractive quantities were measured using a conventional two-beam coupling method and a four-wave mixing method. The device was made by melting the final composite between two glass plates with spacers to fix the film thickness. Typical thickness is 50 μm. Obtained composite has long-term stability against crystallization. No applying field provides dielectric breakdown free which is the great advantage compared with the normal photorefractive polymer composite operated under applying high voltage of 4 to 8 kV.

Figure 2(a) shows the optical gain when NACzE content is changed in the PMMA composite. Optical gain was measured using a conventional two-beam coupling method. When content of NACzE is 30 wt%, optical gain of 451 cm⁻¹ is measured at 632.8 nm in PMMA composite. Diffraction efficiency of 40% was measured using a conventional degenerate four-wave mixing at 632.8 nm [21]. These parameters are the fundamentals of photorefractive responses. Especially diffraction efficiency directly relates to the brightness of hologram recorded in the photorefractive polymer device. Large optical gain due to the asymmetric energy transfer is a proof of phase shift of the refractive index modulation relative to the interfered illumination pattern. In the condition of applying electric field, space charge field produced between trapped hole charge carriers and localized electrons is responsible for the phase shift of refractive index modulation in the photorefractive media. However, the present holographic response should be considered in the condition of space charge field free. Thus other photorefractive mechanism should be involved. Recently, to explain the optical gain observed in sol-gel materials containing azobenzene dye under no bias voltage, new model considering the influence of Poynting vector on photoisomerization is proposed [24]. Laser Raman spectroscopy shows NACzE is in all trans conformation in PMMA before and after laser illumination. This result suggests no contribution of photoisomerization or photoinduced dichroism to present holographic response. As shown in Fig. 2(b), a broad absorption is measured in PMMA in the wavelength region around 550 nm in the absorption spectrum. In other matrix such as polyisoprene liquid gum, this broad peak was not appeared and no holographic response was measured. We think that PMMA matrix may associate with the broad absorption and the present holographic response. As discussed below, the response rate of diffraction at 532 nm is in the order of seconds, whereas that at 632.8 nm reported previously is in the order of tens minute or slower [22]. The difference may be due to the difference in absorption at these wavelengths. However, to fully understand the holographic mechanism in the present system, more detailed investigation is required.

 

Fig. 2 (a) Optical gain when the content of NACzE is varied. (b) Absorption spectra of NACzE/PMMA film and NACzE in DMF solution. (c) Photograph of a 7.5 × 5 cm² monolithic compound dispersed polymer composite device next to the typical test sample.

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We have demonstrated updatable holographic images using a monolithic compound dispersed polymer composite device with size of 7.5 × 5 cm² next to a typical laboratory test sample as shown in Fig. 2(c). Updatable holograms is recorded in the medium of polymer composite of NACzE doped PMMA without applying field. Figure 3 shows the sequential photographs of the device of polymer composite when the object and reference beams are illuminated. Hologram image was appeared at almost the same time of the onset of the illumination of the both beams. Typical intensity of object beam is 10 mW/cm² and that of reference beam is 6.6 mW/cm². In a photograph (a) just before the object and reference beams are illuminated, only object coin can be seen through the device. At 2 s after the illumination of both beams, as shown in (b), hologram image of coin recorded is clearly appeared in the device. At 5, 10 and 30 s passed, as shown in (c), (d), and (e) respectively, hologram image was brighter in the device. After further 1 min illumination and the object and reference beams are off, hologram image remains for a few hours. These dynamic recording and simultaneous replaying hologram are shown in Media 1. These holograms can be completely erased by over-recording or slight heating. Until now it has never been reported for the photorefractive polymer device without applying electric field with response rate of recording in the order of seconds. As shown in the Figure and Media 1, these holograms can be recorded and simultaneously replayed under an ordinary fluorescent room light. It has a long memory under an ambient condition with a room light without any assistance of electric field (electric power).

 

Fig. 3 Photographs of hologram image of object and object coin seen through a device. (a) Photograph just before the object and reference beams are illuminated. No hologram image. (b) Hologram image of object recorded at the time of 2 s after both beams are illuminated. (c) Hologram image at 5 s after illumination. (d) Hologram image became bright at 10 s after illumination. (e) Hologram image became brighter at 30 s after illumination. Hologram image could be preserved for a few hours without both beams (Media 1).

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Figure 4 shows the series of hologram image when different coin object is over-recorded on a same position of the polymer composite device after one coin object image was recorded. No extra laser illumination is required to erase previous image. Namely when new image is over-recorded, previous gratings is erased by the newly recorded gratings. Update or sequentially writable holograms of coins are shown. Figure 4(a) is the photograph before recording. Figure 4(b) shows the photograph of hologram image of heads of coin after recording. Figure 4(c) is the photograph of updated hologram image of tails of coin after over-recording. Figure 4(d) shows the photograph of further updated hologram image of heads of coin after over-recording again. These dynamic over-recording and replaying hologram are shown in Media 2. Holographic image in device is easily updated by another holographic image. The present device has the feature that updatable images can be recorded, erased and over-recorded at the same position.

 

Fig. 4 Photographs of updated hologram images reconstructed. (a) Before recording. (b) Photograph of hologram image of heads of coin after recording. (c) Photograph of updated hologram image of tails of coin after over-recording. (d) Photograph of further updated hologram image of heads of coin after over-recording again (Media 2).

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Figure 5(a) shows the photograph of the hologram image of cat in the device and object cat when recording under fluorescent room light. Hologram reconstructed by probe laser can be clearly seen under fluorescent room light. The hologram image of cat appeared in the back of the device. Figure 5(b) shows the same hologram reconstructed from the device rotated around 180 degrees. The hologram was reconstructed by probe laser in the dark. The hologram image appeared in front of the device. It is seen larger than that appeared in the back. Unfortunately, because of the two-dimensional nature of the photograph taken by CCD camera, it lacks the information of depth. With naked eyes, it is clearly seen in the back or in the front of device. Figure 5(c) shows the same hologram in the device held by K. K under fluorescent room light. This is the proof that this device works without applying high voltage. As the nature of hologram, hologram images recorded in the polymer composite device is completely full parallax.

 

Fig. 5 Hologram image reconstructed. (a) Hologram image of cat in the sample device and object cat when recording under fluorescent room light. (b) The same hologram image reconstructed by red probe beam in the dark. (c) The same hologram image in the device held by K. K.. Image of K. K. (c) is used with permission.

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Present polymer composites can be a candidate for the updatable holographic materials to record a set of hogel data and subsequent reconstruction of 3D images of cars, aircraft and steam locomotives proposed by Peyghambarian’s group [13–15].

The present hologram is recorded in 7.5 × 5 cm² device. It is easy to scale-up the area of the present device. It means that this new holographic material has a promising feature of large scale display such as a signage application etc. Non-electric field driven device has the advantage of no risk of dielectric breakdown on an operation. The response rate of hologram display in the present polymer composite device does not satisfy the video-rate display. But the response rate can be improved by adjusting the glass transition temperature of polymer composite and new design of materials as well as matrices. Furthermore, we continuously progress the advanced research to clarify the uncertainty with respect to the mechanism leading to the observed behavior.

4. Conclusions

We have successfully demonstrated updatable real-time holographic imaging using a monolithic compound dispersed polymer composite device with high diffraction efficiency without applying electric field. Holograms of the objects are clearly recorded in the polymer composite and simultaneously reconstructed by a probe beam. The present 3D holographic display technology will have a potential use of 3D digital signage and 3D digital show window etc.