1. Introduction

Updatable holography has a long-lasting issue of volatility in that the reading process erases the recorded information when using the same wavelength as the writing [1–6]. The two features, non-volatility and reversibility, conflict with each other but are solicited together by many potential holographic applications, especially in three-dimensional (3D) display [7–9], imaging [10–13], and data storage [14–16]. To precisely retrieve the recorded information without color distortion and dimension infidelity, the reading wavelength has to be the same as the writing. In conventional methods of updatable holography without post-fixing process, however, the wavelengths for recording and reading have to be well separated to avoid hologram erasure [17–19]. Photochromic doubly-doped LiNbO3 showing two different energy levels of traps has been studied for non-volatile holographic recording [20]. This unique method enables non-volatile readout but only lasts for several weeks and optical erasure occurs in the beginning of reading process. In addition, the crystal is expensive and the growth of large area is not practical.

2. Holographic recording mechanisms

To achieve an updatable and non-volatile holographic recording, we exploit a completely new recording mechanism based on a composite polymer system containing photosensitive Azo-chromophores. Figure 1 illustrates the recording mechanism based on the unique features of molecular transformations of the Azo-chromophores [21–23]. The chromophores are initially in their stable trans state (an elongated molecular form) with molecular orientation randomly-distributed as shown in Fig. 1(a). The trans chromophores are sensitive only to the light of short wavelengths such as blue or green color light which is not required for coherence and narrow band. When irradiated by the short-wavelength light, the Azo-chromophores are isomerized to their cis state (a bent molecular form) as shown in Fig. 1(b), while the absorption shifts to the long-wavelength spectral region [23, 24]. The unstable cis-state Azo-chromophores are sensitive to the long-wavelength (such as red color light) of the coherent optical recording field. The chromophores are reoriented by the recording polarization upon being photo-isomerized back to the trans state [25]. The molecular reorientation takes place only in the bright interference fringes as shown in the Fig. 1(c). The molecules located in the dark fringes will stay in their cis state. Turning off all light beams, the unstable cis-state molecules in the dark fringes go back spontaneously to their trans form but with a random molecular reorientation. This process forms a nonvolatile grating (Fig. 1(d)) in the Azo-chromophores-contained recording film. This grating can be reconstructed without volatility by using a laser beam at the same wavelength as the recording, because the grating is formed with the stable trans state which is insensitive to the coherent recording and reading wavelength. The azobenzene chromophores are in nanometer scale – the cis azobenzen is only 0.56 nanometer while the trans is about 1 nanometer in length [24].

 

Fig. 1 Mechanism of recording a non-volatile and updatable hologram based on unique photoisomerization and molecular reorientation features. The Azo-chromophore has two states (shown on the top), elongated trans form with a molecular axis and bent cis form with no axis. (a) The chromophores are initially in their stable trans state (blue strips) with randomly-distributed reorientation. (b) Upon short-wavelength light irradiation, isomerized to their cis state (red circles). (c) Molecular reorientation occurs only in the bright interference fringes. (d) A non-volatile grating is formed after turning off all light beams.

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3. Experiments and results

To enhance the holographic recording, we introduced a nematic LC into the above mentioned Azo-chromophores system to form a hybrid LC-enhanced composite. In the material system, a synthesized azobenzene side-chained methacrylate monomer is polymerized with an aliphatic unsaturated monomer to form the reconfigurable co-polymer backbone [26]. Before the polymerization, the monomers are mixed with a LC moiety (E63, about 60 wt%) and small quantity of a crosslinking agent and a sensitive initiator. The system does not contain any solvent and is sandwiched between two glass substrates with micro-spheres as spacers. The polymer chain is reconfigurable due to the molecular reorientation of the attached Azo-chromophores as illustrated in Fig. 2(a) . The surrounding fluid LC molecules are reoriented collectively anchoring toward the reconfigured polymer chains, resulting in a remarkable enhancement of holographic grating formation. The LC fluidity facilitates the ease of polymer chain reconfiguration during holographic recording, while the polymer network stabilizes the LC collective orientation after the recording. Therefore, the interplay of the components in the newly designed hybrid material system offers both updatable and non-volatile characteristics for the holographic recording. The diffraction efficiency observed is at least 10% by using a 50-µm thick film of the new hybrid polymer system. In comparison, the doubly-doped LiNbO3 crystal requires about 1-mm thickness to achieve the same diffraction efficiency of non-volatile holographic recording [20].

 

Fig. 2 (a) Optical enhancement due to the collective alignment of LC molecules nearby polymer chains. (b) Experimental results of non-volatile holography using the same wavelength for both recording and reading. Inlaid: The recorded information can be erased thermally (within a few seconds).

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Figure 2(b) shows the experimental results of non-volatile and updatable holographic recording by using the new material system. In the experiments, a 659-nm diode-pumped solid-state laser is divided into two beams with equal intensity of about 10 mW. The two coherent beams are s-polarized and intersected into the sample film with an angle of about 10 degrees. The spot size on the sample is about 2 mm in diameter and is overlapped by another s-polarized 532-nm (or any color in green or blue spectral region) beam which can be incoherent and broadband. By increasing the green beam intensity to about 30 mW, the local temperature rises to the polymer glass transition temperature Tg of about 70°C, which can be observed in the beam illuminating area on the sample film that changes to a clear liquid-like state from a little opaque appearance. Since the polymer chains move much easier above Tg, this facilitates photo-induced molecular isomerization and reorientation processes. Alternatively, the sample films can be heated above the Tg using a thermal controller just before the recording and the same results can be obtained by using a green beam of lower intensity.

We found that the recorded hologram is non-volatile after turning off recording beams. No obvious erasure effect occurs, even in the beginning of the reading process unlike the LiNbO3 crystal [20]. The recorded information can be maintained for months at room temperature and in ambient lighting conditions (estimated at least one year in a dark condition). The recording is also updatable with the short-wavelength light acting as an erasure field to wipe out the previously-recorded hologram during the new recording. The recorded information can also be erased quickly (within a few seconds – faster than optical erasure) when heating polymer sample above the Tg. Then, new holograms can be recorded again at the same location. It should be mentioned that our material system is different from holographic polymer-dispersed liquid crystals (H-PDLCs) in that the recorded grating is not updatable although it can be temporally switched off by an external electric or optical field [27–31].

For practical holographic recording, the 3D object can be a 2D sequence data (i.e., hogel data) transformed from a computer graphics file or a camera-acquired file containing 2D clips viewed from different angles [8]. In the recording process, the hogel data will be displayed using a spatial light modulator (SLM), which is illuminated by an expanded and collimated laser beam. The emerging beam (i.e., object beam) bearing the information is then focused by an optic lens onto the holographic recording polymer film. The Fourier transform components of the data on the recording film are interfered with a reference beam of the same wavelength to form a hologram. For large-area updatable and non-volatile holographic recording, we proposed to use a scheme as shown in Fig. 3 . Instead of sample film movement and transition, a 2-D beam steering device can be used to scan the object beam. The reference beam overlaps the object beam (or illuminates the whole film). The initial state of the recording material is in their trans state which is stable and not sensitive to the reference beam. The object beam bearing the recording information combines with an incoherent light beam of short-wavelengths. The combined beam will be projected by the beam steering device to the recording film. The trans state of the recording material are then photo-sensitive and isomerized to their cis state. As such, a non-volatile hologram can be formed in the sample area illuminated by the combined beams. The advantage of this design is that large-area holograms can be recorded by low power lasers and the recording film remains still during the recording. Steering light beams can be much quicker than the movement and transition of the recording film.

 

Fig. 3 Schematic of proposed large-area updatable and non-volatile holographic recording. (a) A non-volatile hologram is formed in the area illuminated by the object beam combined with an incoherent light beam. Instead of sample movement, a 2-D beam steering device can be used to scan the combined beams. (b) The reference beam can be used to reconstruct the stored holograms by turning off the coherent object beam along with the incoherent light.

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Our holographic recording technique is unique unlike conventional holographic methods of which the expanded reference beam will erase the stored information. After recording, the coherent object beam along with the incoherent short wavelength light will be turned off. The reference beam may be used as the reading beam to reconstruct the stored holograms, as shown in Fig. 3(b). The reading beam illuminates the recording film and a complete 3D image (such as 3D mapping data) will be reconstructed. Since the recorded holograms are formed by the stable trans state of material system, the stored data will not be erased by the reading beam even though it is operated at the same wavelength as the coherent recording. Figure 4 shows an experimental 3D recording result reconstructed from a hologram that is stored in one spot of the sample film (not whole sample area) using a transparent topographic image which has both spatial and phase information. Note that the dense black fine dots on the letters “r” and outer sparse large spots are not noises but the surface roughness. The experiment demonstrates the feasibility of recording 3D mapping data by using our updatable and non-volatile holographic technique.

 

Fig. 4 Experimental result of a hologram reconstruction. The recorded surface-relief image contains both spatial and phase information.

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In our recording concept, the non-volatile and updatable hologram is recoded as a volume phase grating with diffraction properties well predicted using Kogelnik model [32]. However, the diffraction efficiency may be limited by the quantum yields of the trans-cis photoisomerization and molecular reorientation. Theoretically, the diffraction efficiency could reach 100% by using high-efficient photo-isomerization Azo-chromophores LC systems. The time response can also be improved by designing new Azo-polymer chains and LC molecules. Recently, S. Xu et al [33] reported a new photo-sensitive liquid crystal polymer with time response in milliseconds. It may be possible to record a non-volatile and updatable hologram in this type of photo-polymer by using our unique recording approach.

4. Conclusion

Our technique of non-volatile and updatable holography is all-optical and thus is convenient for practical applications. With the same wavelength for both recording and reading, the reconstructed image will be the true copy of the original object with no color distortion. This also facilitates the implementation of an updatable full color 3D display through recording and reconstructing multi-color holograms of the same object. Since the recording mechanism is based on molecular reorientation, the recording media show dimensional stability with no shrinkage effect unlike other polymer systems. In addition, there is no requirement of using a high-voltage electrical field across the film during the recording and reading [8]. The use of a polymer system reduces the cost, allowing for scaling the recording film to very large sizes or using curved substrates for 3D display and imaging applications, which is very challenging for inorganic crystals. Finally, the new hybrid composite system comprised of organic polymer, chromophore and LC materials can be easily tailored to further improve the holographic performances.