**Azobenzene-Containing Materials for Hologram**

Haifeng Yu and Takaomi Kobayashi

*Top Runner Incubation Center for Academia-Industry Fusion and Department of Materials Science and Technology, Nagaoka University of Technology, Japan* 

#### **1. Introduction**

94 Holograms – Recording Materials and Applications

Kohler C., Schwab X., (2006). Optimally tured sptial light modulators for digital holography,

Gerbreders V., Teteris J., Sledevskis E. (2007). Photoinduced changes of optical reflectivity in

Applied Optics, Vol.45, No.5, p. 960-967

As2S3-Al system, JOAM, Vol.9, No.10, 3153.

With the development of information technology, quick rate of data transfer and high capacity of data storage are expected for advanced recording media. Although data storage media and technique have been developed from soft discs, hard disk, CD-ROM and DVD-ROM to multi-layered blue-ray discs, undoubtedly, holography is one of the most fascinating and attractive techniques. As shown in Figure 1, which illustrates comparison of media in data storage and holographic ones. The optical holography is based on a threedimensional storage method (Bieringer, 2000; Kogelnik, 1969). This provides unique opportunities for the next-generation storage technique by a simple recording and reading process.

Fig. 1. Comparison of several data storage media (above) and scheme of holographic storage (below).

Azobenzene-Containing Materials for Hologram 97

pattern is recorded as a density variation in recording media. The other is a phase–type hologram, in which fringe patterns are recorded as a change in surface structure or refractive index. Theoretically, the diffraction efficiency of phase–type holograms is always higher than that of amplitude–type ones. Accordingly, the phase–type holograms for data recording are superior to the latter and most studies on holography are related to the phase– type ones. On the other hand, as shown in the bottom illustration of Figure 2, according to the thickness of recording films, the holograms can be categorized into Raman-Nath type (thin films) and Bragg type (thick films). The Raman-Nath hologram recorded in a thin film causes multiple diffraction of an incident beam, leading to low diffraction efficiency of the first diffraction beam with the maximum of about 34%. The Bragg hologram shows a single

Generally, storage media materials are among the most important for holographic applications (Smith, 1977). In the past two decades, much attention has been focused on the inorganic crystals as erasable holographic media. While organic materials offer incomparable advantages, such as simple processing, low cost and versatility. Desirable materials for holograms should exhibit high diffraction efficiency, fast response, high resolution, stable and reversible storage, low-energy consuming in the recording and reading processes as well as easy mass production. Therefore, introduction of organic materials is needed. Until now, none of them meets entirely the above-mentioned requirement, and absence of novel materials with

As one of the best known chromophores, azobenzene (AZ) with two benzene rings connected with an azo (–N=N–) bridge in its chemical structures (Figure 3) has attracted much attention of materials chemists for their potential applications in holography. AZ– containing materials with photoresponsive functions can be easily modulated into hierarchical patterns by adjusting the input light properties – wavelength, intensity, polarization, phase, interference pattern, etc. As a result, holograms can be recorded in AZ– containing materials by photoinducing an orientation change of molecules in a periodic pattern obtained from interference of two coherent light beams, the object and the reference beams. Since photo-modulation of refractive index can be obtained in AZ–containing materials, the recorded hologram is phase–type. In this chapter, recent progress in AZcontaining liquid crystalline (LC) or amorphous polymers, polymer-dispersed low-

An AZ moiety is well–known for its reversible photoisomerization, which may act as both a mesogen and a photoresponsive moiety when it is attached with soft substituents. Both photoisomerization and photoinduced LC–to–isotropic phase transition are involved in AZcontaining LC materials. A myriad of AZ derivatives can be tailored by modifying the substituents in the two benzene rings, resulting in their maximum absorption from the ultraviolet to visible regions. According to the electron-absorption properties, three kinds of AZ chromophores have been summarized as shown in Figure 3 (Natansohn & Rochon, 2002; Kumar & Neckers, 1989). The left side "AZ" has relatively poor π-π\* and n-π\* absorbance overlap and the lifetime of the cis-isomer is relatively long. The middle one is "amino-AZ", and there is significant overlap of the two bands and the cis-isomer lifetime is shorter. The right side AZ is "pseudostilbene", where the AZ is usually substituted with electron-donor

diffraction, which enables 100% diffraction efficiency.

high performance has become bottleneck of the holographic technique.

molecular weight compounds or glassy oligomers will be discussed.

and electron-acceptor substituents in both ends of the framework.

**2. AZ-containing materials** 

In 1948, Gabor first reported the holography method (Gabor, 1948), but this was not widely studied until the discovery of laser technique in 1960s (Kogelnik, 1969). The laser technology can easily supply two light beams with the same frequency and a stable phase difference for interference, which is necessary for recording information of one object (wavefronts, in Figure 2). Therefore, holography becomes to be a unique technique that enables simultaneous recording of both phases and amplitudes of light waves.

Fig. 2. Recording of one object (wavefronts) with interference of two coherent laser beams (above) and classification of holograms.

One of the most fascinating features of holography is that it is capable of recording and displaying a complete three-dimensional image of an object (a lot of wavefronts in Figure 2). In holography, recording the phase and amplitude of light waves are performed by periodic alternation of physical properties of materials. According to the manner of recording of interference patterns, holograms are mainly classified into two types (Collier, 1971; Smith, 1977). As shown in Figure 2, one is an amplitude–type hologram, in which the interference

In 1948, Gabor first reported the holography method (Gabor, 1948), but this was not widely studied until the discovery of laser technique in 1960s (Kogelnik, 1969). The laser technology can easily supply two light beams with the same frequency and a stable phase difference for interference, which is necessary for recording information of one object (wavefronts, in Figure 2). Therefore, holography becomes to be a unique technique that enables

Fig. 2. Recording of one object (wavefronts) with interference of two coherent laser beams

One of the most fascinating features of holography is that it is capable of recording and displaying a complete three-dimensional image of an object (a lot of wavefronts in Figure 2). In holography, recording the phase and amplitude of light waves are performed by periodic alternation of physical properties of materials. According to the manner of recording of interference patterns, holograms are mainly classified into two types (Collier, 1971; Smith, 1977). As shown in Figure 2, one is an amplitude–type hologram, in which the interference

(above) and classification of holograms.

simultaneous recording of both phases and amplitudes of light waves.

pattern is recorded as a density variation in recording media. The other is a phase–type hologram, in which fringe patterns are recorded as a change in surface structure or refractive index. Theoretically, the diffraction efficiency of phase–type holograms is always higher than that of amplitude–type ones. Accordingly, the phase–type holograms for data recording are superior to the latter and most studies on holography are related to the phase– type ones. On the other hand, as shown in the bottom illustration of Figure 2, according to the thickness of recording films, the holograms can be categorized into Raman-Nath type (thin films) and Bragg type (thick films). The Raman-Nath hologram recorded in a thin film causes multiple diffraction of an incident beam, leading to low diffraction efficiency of the first diffraction beam with the maximum of about 34%. The Bragg hologram shows a single diffraction, which enables 100% diffraction efficiency.

Generally, storage media materials are among the most important for holographic applications (Smith, 1977). In the past two decades, much attention has been focused on the inorganic crystals as erasable holographic media. While organic materials offer incomparable advantages, such as simple processing, low cost and versatility. Desirable materials for holograms should exhibit high diffraction efficiency, fast response, high resolution, stable and reversible storage, low-energy consuming in the recording and reading processes as well as easy mass production. Therefore, introduction of organic materials is needed. Until now, none of them meets entirely the above-mentioned requirement, and absence of novel materials with high performance has become bottleneck of the holographic technique.

As one of the best known chromophores, azobenzene (AZ) with two benzene rings connected with an azo (–N=N–) bridge in its chemical structures (Figure 3) has attracted much attention of materials chemists for their potential applications in holography. AZ– containing materials with photoresponsive functions can be easily modulated into hierarchical patterns by adjusting the input light properties – wavelength, intensity, polarization, phase, interference pattern, etc. As a result, holograms can be recorded in AZ– containing materials by photoinducing an orientation change of molecules in a periodic pattern obtained from interference of two coherent light beams, the object and the reference beams. Since photo-modulation of refractive index can be obtained in AZ–containing materials, the recorded hologram is phase–type. In this chapter, recent progress in AZcontaining liquid crystalline (LC) or amorphous polymers, polymer-dispersed lowmolecular weight compounds or glassy oligomers will be discussed.

#### **2. AZ-containing materials**

An AZ moiety is well–known for its reversible photoisomerization, which may act as both a mesogen and a photoresponsive moiety when it is attached with soft substituents. Both photoisomerization and photoinduced LC–to–isotropic phase transition are involved in AZcontaining LC materials. A myriad of AZ derivatives can be tailored by modifying the substituents in the two benzene rings, resulting in their maximum absorption from the ultraviolet to visible regions. According to the electron-absorption properties, three kinds of AZ chromophores have been summarized as shown in Figure 3 (Natansohn & Rochon, 2002; Kumar & Neckers, 1989). The left side "AZ" has relatively poor π-π\* and n-π\* absorbance overlap and the lifetime of the cis-isomer is relatively long. The middle one is "amino-AZ", and there is significant overlap of the two bands and the cis-isomer lifetime is shorter. The right side AZ is "pseudostilbene", where the AZ is usually substituted with electron-donor and electron-acceptor substituents in both ends of the framework.

Azobenzene-Containing Materials for Hologram 99

When AZ-containing materials possess an LC phase, photoinduced phase transition can be caused in the LC phase since the trans-AZ can be a mesogen because its molecular shape is rod-like, whereas the cis-AZ never shows any LC phase due to its bent shape. When photoinert mesogens (like cyanobiphenyl groups) exist with dopant AZs, photoinduced molecular cooperative motion can be observed (Ikeda, 2003). If a small proportion of AZ molecules change their alignment in response to an external light stimulus, the other LC molecules also alter their alignment, coinciding with the ordered AZs pre-aligned. Actually, only a small amount of energy as to induce an alignment change of about 1 mole% of LC molecules is enough to bring about the alignment change of the whole system of AZ and LC mixtures. In other words, a huge amplification of light signals is possible in LC photonic systems owing to the molecular cooperative motion. As shown in Figure 4, both photoalignment and photochemical phase transition can be isothermally caused although the photoinert mesogens do not absorb the actinic light. These properties have been widely

utilized in display, optical devices and holograms.

Fig. 4. Summarized properties of AZ-containing materials.

Fig. 3. Examples of three kinds of AZs classified by Natansohn and Kumar.

Generally, AZ-containing materials show several photoresponsive features like trans-to-cis photoisomerization, polarization-selective photoalignment, photochemical phase transition and photoinduced cooperative motions, as shown in Figure 4 (Yu & Kobayashi, 2010; Chen et al., 2010a). Upon UV irradiation, trans-to-cis photoisomerization often occurs for AZcontaining materials in solutions or solid states, leading to a large change in molecular shape and polarizability. The cis-isomers can return to their trans-isomers by thermal treatment or visible light irradiation. When a linearly polarized light is used, AZs selectively absorb light with the polarization direction parallel to their transition moments. The probability of the absorption is proportional to the cos2 θ (Figure 4), where θ is the angle between the transition moment of an AZ and the light polarization direction. Combining the polarization-selective trans-to-cis isomerization and un-selective back cis-to-trans isomerization, the number of AZ moieties with their transition moments normal to the light polarization direction gradually increases, resulting in the light-selective alignment, with transition moments of AZs almost perpendicular to the polarization direction of the actinic light. This is well known as the Weigert effect (Weigert, 1919). Such a photoalignment process is reversible, which means that AZs can be re-orientated to any controlled directions by choosing appropriate polarization direction of light (Yu & Ikeda, 2011).

Fig. 3. Examples of three kinds of AZs classified by Natansohn and Kumar.

by choosing appropriate polarization direction of light (Yu & Ikeda, 2011).

Generally, AZ-containing materials show several photoresponsive features like trans-to-cis photoisomerization, polarization-selective photoalignment, photochemical phase transition and photoinduced cooperative motions, as shown in Figure 4 (Yu & Kobayashi, 2010; Chen et al., 2010a). Upon UV irradiation, trans-to-cis photoisomerization often occurs for AZcontaining materials in solutions or solid states, leading to a large change in molecular shape and polarizability. The cis-isomers can return to their trans-isomers by thermal treatment or visible light irradiation. When a linearly polarized light is used, AZs selectively absorb light with the polarization direction parallel to their transition moments. The probability of the absorption is proportional to the cos2 θ (Figure 4), where θ is the angle between the transition moment of an AZ and the light polarization direction. Combining the polarization-selective trans-to-cis isomerization and un-selective back cis-to-trans isomerization, the number of AZ moieties with their transition moments normal to the light polarization direction gradually increases, resulting in the light-selective alignment, with transition moments of AZs almost perpendicular to the polarization direction of the actinic light. This is well known as the Weigert effect (Weigert, 1919). Such a photoalignment process is reversible, which means that AZs can be re-orientated to any controlled directions When AZ-containing materials possess an LC phase, photoinduced phase transition can be caused in the LC phase since the trans-AZ can be a mesogen because its molecular shape is rod-like, whereas the cis-AZ never shows any LC phase due to its bent shape. When photoinert mesogens (like cyanobiphenyl groups) exist with dopant AZs, photoinduced molecular cooperative motion can be observed (Ikeda, 2003). If a small proportion of AZ molecules change their alignment in response to an external light stimulus, the other LC molecules also alter their alignment, coinciding with the ordered AZs pre-aligned. Actually, only a small amount of energy as to induce an alignment change of about 1 mole% of LC molecules is enough to bring about the alignment change of the whole system of AZ and LC mixtures. In other words, a huge amplification of light signals is possible in LC photonic systems owing to the molecular cooperative motion. As shown in Figure 4, both photoalignment and photochemical phase transition can be isothermally caused although the photoinert mesogens do not absorb the actinic light. These properties have been widely utilized in display, optical devices and holograms.

Fig. 4. Summarized properties of AZ-containing materials.

Azobenzene-Containing Materials for Hologram 101

In films of poly[4'-( 2-acryloxy)ethylamino-4-nitroazobenzene] (pDR1A, Figure 5a), surfacerelief gratings with a sinusoidal shape was recorded with an interference pattern of two light beams at 514 nm. The obtained grating structures were stable but could be erased by heating the polymer above its glass transition temperature. In addition, no permanent damage of the lm was observed. Multiple gratings can be simultaneously written and gratings can be overwritten (Rochon et al., 1995). Using an epoxy-based amorphous polymer containing AZ side groups (Figure 5b), surface-relief gratings with relatively large amplitude was successfully inscribed with two laser beams at 488 nm. Furthermore, recording perpendicular gratings on the same lm was also achieved. Such surface-relief gratings in amorphous polymer films showed uniform and controllable morphologies, like the depth of relief, the grating periodicity, and so son. Moreover, more complicated topological surfaces were tailored by superimposing several surface-relief gratings. Recently, the recorded surface-relief structures were studied for potential applications as LC alignment (Li et al., 1999), polarization discriminator, waveguide couplers (Viswanathan et

Fig. 5. Surface-relief gratings recorded in AZ-containing amorphous polymer materials.

al., 1999) and antireflective coatings (Natansohn & Rochon, 2002).
