1. Introduction

Updatable holography has great potential for next-generation holographic memory [1–3], true three-dimensional (3D) display [4,5] and biomedical imaging applications [6–8] due to its rewritability for the complete wave field of a 3D realistic object. These applications require a combination of critical holographic performances such as high diffraction efficiency, long data persistence, controllable erasure and reversibility [2–4]. Photopolymers are typical holographic recording materials possessing high diffraction efficiency and high photo-sensitivity, but they are irreversible and require post processing to fix holograms [9,10]. Well-studied photorefractive polymers are good candidates for reversible holographic recording. However, an electrical field with high voltage has to be applied across the photorefractive polymer for both recording and readout, to achieve high diffraction efficiency and data persistence [4,11–13].

Recently, holographic polymer-dispersed liquid crystals (H-PDLCs) have been attracted attention due to their promising applications in flat panel displays and switchable or tunable optical devices [14–18]. H-PDLC structures are normally fabricated by applying the coherent interference of laser radiation to a homogeneous mixture containing photoreactive monomer, initiator and liquid crystal. The periodic interference pattern induces locally different photo-polymerization rates, creating a holographic grating structure constituted by nano-scale phase-separated LC/polymer domains. The diffraction can be switched electrically or optically due to the re-alignment of LC molecules that cancels the refractive index profile of holographic grating. H-PDLCs exhibit large diffraction efficiency and fast switching speed. However, after complete holographic polymerization, it is impossible to change the as-formed polymer morphology by external influences. Therefore, the recorded holographic grating in the PDLC is not updatable although it can be temporally switched off by an external field.

2. Material and optical reinforcement concept

To explore reversible holographic recording with large diffraction efficiency, controllable erasure and long data persistence, we investigate a concept of optical reinforcement mechanism in nano-scale domains by developing a novel system consisting of a synthesized photo-reconfigurable co-polymer backbone network embraced by a soft LC material, as illustrated in Fig. 1 . The polymer backbone (“bone”) is reconfigurable and surrounding fluid LC molecules (“muscle”) are reoriented collectively, resulting in remarkable enhancement of the strength of holographic grating formation. In the presence of a recording field (Fig. 1(a)), the co-polymer chains are reconfigured to construct a framework of holographic grating, which is then strengthened dramatically by the surrounding fluid LC molecules due to their collective alignment (growing “muscle”, Fig. 1(b)) in bright fringes. The photoinduced reconfiguration is implemented through the reorientation of side-chain azobenzene chromophores of the synthesized co-polymer (Fig. 1(c)). The azobenzene chromophores possess highly-efficient reversible photo-isomerization property between two isomers, trans and cis [19–21]. The repeated trans-cis isomerization process induces the chromophore reoriented with its axis perpendicular to the polarization direction of excitation light field (Fig. 1(d)) [22,23]. The LC fluidity facilitates the ease of polymer chain reconfiguration during holographic recording. On the other hand, the polymer network stabilizes the LC collective orientation and the consequential optical enhancement after the recording.

 

Fig. 1 Holographic recording based on a concept of optical reinforcement. (a) Holographic interference pattern on a recording film. The polymer chains are reconfigured in the bright interference fringes while remaining randomly distributed in the dark fringes. (b) The bone-muscle-like concept. Holographic grating framework constructed by polymer backbones is strengthened by surrounding fluid LC molecules due to their collective alignment. (c),(d) The photoinduced reconfiguration implemented through the reorientation of side-chain azobenzene chromophore with its axis perpendicular to the polarization direction of recording light field.

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The bone-muscle-like strengthening concept also provides the feasibility of using small amount of photoactive absorbers in the system, which is important for fabrication of volume holographic media of high quality to record phase-based holograms. Many azobenzene-functionalized materials including azobenzene polymers and azobenzene LCs have been studied for holographic recording [23–26]. However, their data persistence is generally poor due to lack of recording stabilization mechanism. Although some photoaddressable polymers show large birefringence, their inherent high absorption coefficient prevents the use of thicker thickness for holographic applications [27,28]. Azobenzene-contained PDLCs are also studied for switchable holographic applications, but either the diffraction efficiency is low or the recording is not updatable [29,30]. There are also reports which exploit azo-copolymer films as the optically-switchable liquid crystal alignment layer [31–34]. Such methods can be used for reversible holographic recording but need complicate configurations.

3. Experiments and results

In our material system construction, a synthesized azobenzene side-chained methacrylate monomer (x) is polymerized with an aliphatic unsaturated monomer (y) to form the reconfigurable co-polymer backbone (x : y = 1: 9 wt, Fig. 1(c)). Before the polymerization, the monomers are mixed with a LC moiety (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 (~100 µm in diameter) as inter-plate spacers. The samples are then placed in a 75 °C oven overnight for polymerization reaction. Since no ITO electrodes are involved and the reaction process is facile, large-area quality films can be prepared. Thanks to the in situ polymerization characteristics and the self-supporting structures, the proposed material system can also be formed on curved or flexible substrates – an important feature for display applications. The as-prepared sample films exhibit slight opaque due to the formation of polymer network and resulting nanoscale phase-separated domains in the system, as shown in Fig. 2(a) . We found that samples synthesized with nematic LCs (such as E63 from Merck) have much higher diffraction efficiencies as compared to cholesteric type LCs (such as benzyl cholesterate). The nematic LC has a much stronger collective molecular alignment effect than the cholesteric type when anchored parallel to elongated trans azobenzene chromophores. This results in a significant enhancement of a holographic grating framed by photo-reconfigurable polymer chains.

 

Fig. 2 Holographic experiments and results. (a) Microscopic images of sample film. The upper image shows nanoscale domains after sample preparation. The lower image shows transformation to its isotropic phase at phase transition temperature. (b) Holographic recording and reading arrangement. Object and reference beams are obtained from a CW 532-nm diode-pumped solid state (DPSS) laser. Reading beam is from a 659-nm DPSS laser. (c) A typical experimental result of holographic recording and reading by using a synthesized new material system. Inset, holographic recordings by using a sample film containing only the azobenzene copolymer (left inset) and a LC sample doped with azobenzene chromophore monomer (right inset). Both samples exhibit fast storage volatility after turning off recording beams.

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Figure 2(b) shows the experimental setup of holographic recording and reading. In the experiment, two recording beams (object and reference, 10 mW each) intersecting in the film at an angle of 12 degrees are obtained from a compact CW 532-nm diode-pumped solid state (DPSS) laser. Another DPSS laser emitting at 659 nm merged into the reference beam acts as a reading beam incident to the sample with 5 mW. All the beams have the same diameter of about 2 mm overlapped in the sample film. Before recording, the sample film is heated to its phase transition temperature (T_p, about 70 °C) where the sample is in its isotropic phase (Fig. 2(a)) and the polymer chains become flexible. During cooling down, the sample film is shined by the recording beams and the diffracted signal emerging from the film is detected by a photodetector (the reading beam is on constantly). When the diffracted intensity reaches its maximum, the recording beams are turned off. After slightly decaying due to polymer chains relaxation and stabilization, the diffraction becomes stable as the material is insensitive to the reading beam. Figure 2(c) shows a typical result of holographic recording and reading using a sample film of copolymer (R₁: H and R₂: benzene) synthesized with E63 LC. The recording angle can reach to about 30 degrees, thus the resolution is near 1500 lines/mm according to $1/d=2n\sin (\theta /2)/\lambda$, here d grating spacing, n refractive index, θ recording angle and λ wavelength. The stored grating can maintain for about one year in ambient conditions. Instead of pre-heating, the local temperature on the recording area can quickly rise to T_p by using a high recording intensity (about 30 mW) at the very beginning of the recording. As soon as the sample reaches its phase transition state (within 1-2 seconds), holographic recording can be conducted by decreasing the recording intensity to its normal level. This is related with the elastic property of the LC and the flexibility of the polymer backbone. The minimum holographic recording intensity can be as low as about 1 mW. The diffraction efficiency depends on the recording intensity. To achieve the highest diffraction efficiency, the recording intensity should be 10 mW or higher. Although the minimum recording intensity can be used to record holograms, a long exposure will be experienced with relatively lower diffraction efficiency. After recording, however, the stabilities of the recorded holograms are not dependent on the intensities used for the recording.

For comparison, we also studied holographic recording by using a sample film containing only the azobenzene side-chain copolymer and an E63 LC sample doped with azobenzene chromophore monomer. Although both samples have relevant components used in the new material system, their diffraction efficiencies are much lower, as shown in the inset of Fig. 2(c). In addition, after turning off recording beams, both samples show extremely fast storage volatility. In the sample of bulk azobenzene copolymer, the dense molecular chains are difficult to move around. Thus, only a volatile trans-cis isomerization grating of the azobenzene chromophores is formed rather than a stable grating of the polymer chain reconfiguration. On the other hand, the doped chromophores in the fluid LC sample move too easily. As such, the molecular reorientation grating disappears as soon as the recording beams are turned off. The transient initial peak in the recording originates from a saturable absorption effect of azobzene chromophore. Unlike those conventional recording materials, our proposed composite system is unique with important holographic features of both persistence and reversibility. The ease of photo-reconfiguration in fluid state and the consequent stabilization of the optical enhancement supplement each other and are ideal for updatable holographic applications, allowing for further optimization of holographic performance.

As compared to other reversible holographic materials such as the photorefractive polymers which persist recorded data for several hours in the dark (and also require several kilovolts voltage across the polymer [4,11], the data persistence of our recording system is quite long. In fact, the nonvolatile reading can also be achieved even using the same wavelength as the recording but with a relatively weak intensity. As shown in Fig. 3(a) , the high-intensity 532-nm reading beam erases the recorded grating, as occurs in most reversible holographic materials. However, with a reading intensity lower than 1 mW, the grating becomes stable. This is because the 532-nm reading beam does not have enough energy to alter the polymer reconfiguration which becomes inflexible after the recording. Alternatively, the stored hologram can be easily erased by heating the sample to T_p. Figure 3(b) shows a thermal erasure process which takes only a few seconds to completely erase the grating. Considering temperature rising affected by the thermal resistance of substrates, the actual erasure time should be shorter. After erasure, a fresh hologram can be recorded again at the same location in the sample film. The sample shows no degradation for extended periods of usage (over a half year) in holographic recording experiments.

 

Fig. 3 Holographic nonvolatile reading and controllable erasure. (a) Holographic reading at 659-nm and 532-nm wavelengths. The recorded hologram is very stable under 659-nm reading. Nonvolatile reading can also be achieved at the same wavelength as the recording but with a relatively weak intensity. (b) Thermal erasure process which takes only about 4 seconds to completely erase the grating. Fresh holograms can be recorded again at the same location.

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With high diffraction efficiency and rewritability, the new hybrid material system is appropriate for updatable holographic image recording. To demonstrate this, we modify the holographic experimental setup. The 532-nm object recording beam is expended by a spatial filter and then collimated by an optical lens. The uniform expended 532-nm beam passing through a target is focused onto the sample film by a Fourier lens. Another 532-nm beam (the reference) is directly incident to the sample overlapping the region illuminated by the object beam. The Fourier plane of the object beam is located in front of the recording film so that the spot on the film has the diameter equal to that of the reference beam. A collimated 659-nm reading beam is incident from the recording side. The reconstructed light field is collected by an imaging lens. A CCD camera behind the lens captures both real-time and steady holographic reconstructions.

Figure 4(a) shows the experimental results of a transient holographic recording process. The red and green color images originate from concurrent reconstruction by the 659-nm reading beam and the 532-nm reference beam, respectively. Due to the long data persistence as demonstrated in Figs. 2(c) and 3(a), the reconstructed image is stable after turning off the recording beams (Fig. 4(b)). It should be noted that the constructed images are cropped from the target center (USAF Resolution Test Chart). The recorded hologram can be erased thermally in seconds and new holograms can be recording at the same sample location. Figure 4(c) shows updatable hologram recording with multiple write/rewrite cycles. The sample shows no degradation in optical properties and holographic performance after numerous cycles of hologram recording and erasure. This is important for updatable holographic displays and 3D imaging applications such as in-vivo biomedical imaging.

 

Fig. 4 Holographic image recording and reconstruction. (a) Transient holographic recording. From top to bottom, holographic recording at 10 seconds, 30 seconds and 50 seconds. (b) Holographic real-time reconstruction (top) and long-term persistence (bottom). (c) Multiple write/rewrite cycles of hologram recording. Fresh holograms can be recorded at the same sample location after erasure.

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4. Conclusion

With the photo-reconfiguration in fluid state and the stabilization of the consequential optical enhancement, the system naturally suits updatable holographic applications allowing for further engineering and optimization of holographic performance. Compared to conventional recording systems, the optically strengthening concept enables the enhancement of holographic performance with high diffraction efficiency, facilitating implementation of 3D display, 3D imaging, and ultrahigh-density data storage for a variety of applications. Our study will also benefit the next-generation high-density data storage. The reconfigurable bone-muscle-like enhancement concept could be extended to other kinds of optical or physical properties, leading to a variety of new applications and emerging technologies.