Holographic storage is expected to become one of the main optical storage techniques because of its significant advantages, such as large capacity, high density, fast data transmission, and because it is spatially addressable parallel.^[1–3] This technique relates to many significant areas, such as holographic three-dimensional (3D) displays, holographic disks, volume holographic correlators, and holographic prints,^[4–7] making it an important research area. Recently, some achievements have been reported. One of the good results is the news that the GE Global Research announced 500 GB data could be recorded on media in an ordinary DVD size in their holographic data storage system.^[8] Furthermore, Horimai et al. developed a collinear holographic system to write and read data for holographic disks, greatly simplifying the original optical systems.^[4] Chen et al. proposed a dually modulated multiplexed holographic storage technique for holographic disks, providing a novel idea to improve storage capacity.^[5] Cao et al. combined holographic storage and optical pattern recognition. They have made great progress in volume holographic correlators.^[6] Blanche et al. used a holographic printing technique to time-sequentially write each “hogel” of computer-generated holograms into recording media, and obtained 3D reconstructed images.^[7]

Recording media is one of the key factors in holographic storage. Holographic storage materials mainly include silver halide,^[9] inorganic crystals,^[10–13] and polymers.^[14,15] Among these media,^[16–34] photopolymer is considered to be a good candidate for big data storage and other commercial applications because it has some good properties such as high sensitivity, large optical nonlinearity, simple production process, and low cost.

The difference between the polymer and polymer dispersed liquid crystal (PDLC) for holographic storage has not been discussed before. In addition, in-line holographic storage using TMPTA-based polymer has also been reported yet in previous works. In this paper, we investigate holographic storage of 3D images and data in photopolymer films without an applied electric field. The diffraction efficiency in the film can exceed 80% and the definition of hologram is better than those in the PDLC film. This work would be useful to study holographic recording materials.

Figure 1 shows a schematic diagram of hologram formation in a photopolymer sample. The object beam and reference beam are set as the collinear, and they are used to generate interference pattern inside the sample. In the bright areas, the excited initiators generate radicals, and the monomers are polymerized by the radical polymerization reaction. The monomer density in the bright areas decreases, therefore, density difference is formed, and non-polymerized monomers diffuse from the dark areas into the bright areas. The polymerization reaction terminates until the monomer density regains balance. A difference in refractive index between the bright and dark areas is built in this process. Therefore, the recording beam profile can be stored as the difference in refractive index, and the hologram is formed.

Fig. 1.

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	Fig. 1. Formation of hologram in photopolymer sample.

In our experiment, the photopolymer is a mixture consisting of TMPTA monomer, Rose Bengal, N-phenylglycine and N-vinylpyrrollidone, whose weight ratio is 66:2.0:1.8:25.2. Then, the mixture was made uniform by ultrasonic mixing for about 30 min. A cell was formed by sandwiching two glass substrates with the thickness controlled by Mylar spacers. The uniform mixture was poured into the cell by a capillary action, and then the thin polymer film was fabricated. The preparation process requires the reduction of interference from visible light in case of unnecessary polymerization, and air bladders in the cell should be eliminated as soon as possible. We made a sample of size, 40 mm×40 mm, and a thickness of 100 μm. The absorption spectrum of this sample is illustrated in Fig. 2. The spectral curve shows that this material is sensitive to light at wavelengths of 450 nm–600 nm in visible wavelengths.

Fig. 2.

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	Fig. 2. Absorption spectrum of the polymer film.

The diffraction efficiency was investigated by using two-beam coupling setup, as shown in Fig. 3. An Nd:YAG laser (λ = 532 nm) was used as the recording light, and an He–Ne laser was used as the probe light. Recording angle θ, the angle between the two coupling beams, was set to be 26°, and the probe angle φ, the angle between the probe beam and the normal perpendicular to the sample plane, was 15°. There was no electric field applied to this film.

Fig. 3.

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	Fig. 3. Two-beam coupling setup for diffraction efficiency measurement. M1, M2, and M3 are mirrors, PBS is a polarizing beam splitter, λ/2 is a half-wave plate. Recording angle θ is the angle between the two coupling beams, and the probe angle φ is the angle between the probe beam and the normal perpendicular to the sample plane.

The diffraction efficiency as a function of recording intensities was measured, as illustrated in Fig. 4. In Fig. 4(a), it shows that the diffraction efficiency slowly increases under lower exposure intensity, and it takes about 20 s to reach the maximum, 35%, at the intensity of 1 mW/mm². The diffraction efficiency reaches the maximum value of 84% and does not change at the intensity of 5 mW/mm², when the recording light is turned off in less than 2 s. However, the diffraction of efficiency would immediately reduce and stay at a low value if the film is over exposed, as shown by the red curve in Fig. 4(a), and exposure time is more than 60 s. This is due to the absorption of this material. The sample absorbs more recording light at higher exposure intensity and too much absorbed energy erases the recorded holograms in the film, then, the diffraction efficiency decreases. Therefore, there is optimal recording time at different exposure intensities. If the recording laser is turned off when the diffraction efficiency reaches its highest level, the diffraction efficiency can maintain the maximum. Furthermore, there is also optimal intensity for the film, i.e., the diffraction efficiency would not increase with the increase of the recording intensity above the optimal intensity. Figure 4(b) shows that the diffraction efficiency of the recorded hologram in film is more than 80% at the recording intensity, 5 mW/mm²–7 mW/mm². Therefore, the recording beam intensity and exposure time should be strictly controlled when the material is used as a holographic storage medium.

Fig. 4.

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	Fig. 4. (a) Hologram formation process and (b) diffraction efficiency as a function of the recording beam intensity.

For applications of static holographic 3D displays, holographic disks, we investigated holographic property of photopolymer and PDLC films. Absorption measurement of the polymer showed that it is sensitive to the visible wavelengths, range of 470 nm–600 nm. Therefore, blue and green lasers can be used as a recording light. The diffraction efficiency which is a key parameter of the refractive index hologram recorded in materials was studied, and hologram formation process was illustrated in polymer films. Furthermore, the static analog hologram carrying 3D image, i.e., the wavefront of the surface of real object was recorded in the films, and the 3D reconstruction with good quality was achieved. Compared to PDLC films, the polymer in holographic applications has more advantages, such as higher holographic definition and no need of any electric field in hologram recording and reconstruction. For big data storage in this polymer, we chose an in-line holographic writing system as a holographic disk system, which is simpler than an off-axis holographic system. The whole page of information was recorded at a small film point by one exposure, which needed no postprocessing, and the information was permanently stored. This paper focuses on 3D image and data storage by holography in photopolymer films. The work demonstrates that the applications of these films in analog and digital hologram storage may be helpful for static 3D display and big data permanent storage research.