Molecular design of azo-carbazole monolithic dyes for updatable full-color holograms

For the use of updatable full-color holography in practical applications, azo-carbazole (ACzE) monolithic dyes dispersed in poly (methyl methacrylate) (PMMA) ﬁ lms with high diffraction ef ﬁ ciency, write/delete capabilities and transparency are of extreme importance. We synthesized seven types of novel ACzE monolithic dyes and examined the in ﬂ uence of substituents introduced in the para position of the phenyl ring and the distinct strength of the substituents (NO 2 : NACzE, CN: CACzE, CH 3 CO: AACzE, F: FACzE, CH 3 O: MACzE, OH: HACzE and NH 2 : AmACzE) on the phenyl ring of the ACzEs. Spectroscopic and holographic optical properties, including absorption coef ﬁ cient α , wavelength dependence of diffraction ef ﬁ ciency η , response time τ , the thickness of the recorded gratings d , the Bragg angle Δ W and the Q -factor, required for the optimization of updatable full-color holograms were investigated. On the basis of the spectroscopic and holographic optical results, the appropriate writing/reading beams were determined as follows: 561/ 4 600 nm for NACzE, 532/ 4 560 nm for CACzE (and AACzE), 491/ 4 530 nm for MACzE (and HACzE). According to the spectroscopic and holographic optical results, we propose here an insight designed to allow updatable full-color holography using ACzEs, and demonstrate a multi-color hologram in MACzE/PMMA.


INTRODUCTION
An updatable full-color hologram is the ultimate imaging technology. The term holography is taken from the Greek words, holos (whole) and graphe (writing or drawing). Unlike pseudo-three-dimensional (3D) images or virtual reality on a two-dimensional computer display, a hologram is a truly 3D and freestanding image that does not simulate spatial depth nor require a special viewing device. Theoretically, holograms could someday be transmitted electronically to a special display device in your home and business. Unfortunately, current holographic technology has not lived up to expectations, which were largely driven by the science fiction portrayed in the 'Star Wars' film in 1977. Today, the main applications of holograms are in the fields of security and authentication for branding, bank notes, and passports, among other applications. Holographic imaging is largely consigned to artistic exhibitions or specialized optical components. Practical applications of dynamic (updatable) full-color holograms have many issues that remain to be resolved; one of which is the extreme difficulty of designing materials that can accommodate updatable full-color holography.
Organic molecules have attracted particular attention for use in updatable holography. In addition, other photonic applications such as holographic memory, optical switches and waveguides have been proposed. 1-5 Examples of updatable holography include azobenzene molecules in polymer matrices, 6-8 liquid crystals, 9,10 polymeric liquid crystals [11][12][13][14][15] and photorefractive polymers. [16][17][18][19] In particular, azobenzene molecules are the most promising candidates for use in updatable holography because azobenzenes are well-known photosensitive chromophores that undergo trans-cis photoisomerization upon irradiation with light of an appropriate wavelength in a solution or in an appropriate host matrix. [20][21][22][23][24] The repetition of such photoisomerization within a host matrix leads to perpendicular alignment of the long axis of the azobenzene chromophore to the electric field vector of the linearly polarized light, which eventually induces anisotropy. Therefore, azobenzene-functionalized polymers have attracted considerable attention due to their large refractive index anisotropy induced by linearly polarized light irradiation below the glass transition temperature. 25,26 We previously developed updatable red-colored holograms using an NO 2 -substituted azo-carbazole (ACzE) monolithic dye, 3-[(4-nitrophenyl)azo]-9H-carbazole-9-ethanol (NACzE), dispersed in poly (methyl methacrylate) (PMMA), [27][28][29] and proposed a relationship between the holographic properties and isosbestic point. However, the spectroscopic and optical characteristics of the ACzE monolithic dyes are not yet thoroughly understood.
In this study, our motivation was to discover novel azobenzene dyes that contained ACzE as their basic molecular structure for use in updatable holography with the following intended colors: red, yellow, green, blue and even full color. For this purpose, we investigated the influence of introducing substituents in the para position of the phenyl ring as well as the distinct strength of the substituents (NO 2 , CN, CH 3 CO, F, CH 3 O, OH and NH 2 ) on the phenyl ring. Furthermore, we proposed a design that allows updatable full-color holography utilizing ACzE monolithic dyes on the basis of the spectroscopic and optical results.

MATERIALS AND METHODS
The 3-(phenylazo)-9H-carbazole-9-ethanols (ACzEs) with different substituents on the phenyl group (NACzE, CACzE, AACzE, FACzE, MACzE, HACzE and AmACzE) were synthesized as shown in Scheme 1. The ACzEs with an NO 2 , CN and CH 3 CO substituent (NACzE, CACzE and AACzE) were synthesized by an azo-coupling reaction. The ACzE with an NH 2 substituent (AmACzE) was synthesized in an ammonium chloride solution containing Fe powder. 30 The HACzE was methoxylated to produce the ACzE with a CH 3 O substituent (MACzE). The ACzE with an F substituent (FACzE) was synthesized from the AmACzE according to ta Balz-Schiemann reaction. 31 The ACzE with an OH substituent (HACzE) was produced through an Ullmann reaction, 32 a Pd-catalyzed reaction that forms a carbazole structure, and a reduction reaction to add the amine substituent to the carbazole, followed by an azo-coupling reaction with a phenol. The HACzE was methoxylated to produce the ACzE with a CH 3 O substituent (MACzE). The chemical structures of the products were characterized by 1 H-nuclear magnetic resonance ( 1 H-NMR) (and 13 C-NMR; AV-600, Bruker BioSpin Inc., Billerica, MA, USA, 600 MHz, tetramethylsilane-TMS-as an internal standard), Fourier transform infrared (FT/IR-4700, Jasco Co., Tokyo, Japan) and timeof-flight mass spectrometry (MicroTOF, Bruker Daltonics Inc., Billerica, MA, USA). Melting points (mp) were determined using a melting-point apparatus (MP-500P, Yanaco, Kyoto, Japan) with a digital thermometer. Detailed synthesis procedures and the melting point for each ACzE, and assignment of each compound by NMR, mass and Fourier transform infrared spectroscopy are summarized in the Supplementary Information. Especially, 13 C-NMR spectra of ACzEs are shown in Supplementary Figure S1 PMMA with a molecular weight of ca. 100 000 g mol − 1 was used for the polymer matrix. Several approaches were used to incorporate azobenzene chromophores into polymer matrices: random incorporation (guest-host systems), binding to main or side chains or formation of thin-layer structures such as Langmuir-Blodgett films. In this study, we employed the guest-host system for the ACzE/PMMA films and used one of the better known ratios for an azo-carbazole/PMMA system (30/70 wt%). 33,34 The ACzE monolithic dyes with different substituents, NO 2 , CN, CH 3 CO, F, CH 3 O, OH and NH 2 , were mixed with PMMA in tetrahydrofuran. The mixtures were left stirring overnight and then cast on a hot plate at 70°C for 24 h. The obtained composites were sandwiched between two glass plates at 240°C. The thickness of the composite films was controlled to ca. 80 μm by inserting polyimide spacers. Ultraviolet − visible absorption spectra were obtained using a Lambda 1050 UV/Vis/NIR spectrophotometer (PerkinElmer, Waltham, MA, USA). The absorption coefficient α is given by α = Aln(10)/d, where A is the measured absorbance and d is the thickness of the sample. The isosbestic point was measured after the sample films were irradiated by a mercury lamp.
The diffraction efficiency was measured using a non-degenerate four-wave mixing technique with six different writing lasers at 632.8, 594, 561, 532, 491 and 473 nm. Another laser at 632.8 nm was used as a reading (probe) beam. The two writing laser beams of p-polarization were intersected on the sample surface at angles of − 7.5°and 7.5°. The weak intensity of the p-polarized reading beam probed the refractive index gratings in the sample from the direction opposite to one side of the writing beams. We used the intensities of a transmitted beam (I t ) and diffracted beam (I d ) to calculate the internal diffraction efficiency η with equation (1): The angular selectivity of the recorded gratings for different writing beam wavelengths was recorded using the geometry shown in Supplementary Figure S2. Laser sources for writing were 25 mW diode-pumped solid-state lasers: Blues at 473 nm, Calypso at 491 nm, Samba at 532 nm, Jive at 561 nm and Mambo at 594 nm (Cobolt, Solna, Sweden), and a 10 mW He-Ne laser at 632.8 nm (CVI Melles Griot, Albuquerque, NM, USA). The probe laser for reading was a 10 mW He-Ne laser (LASOS Lasertechnik GmbH, Jena, Germany). The sample was fixed on a rotation stage at 0°, so that the normal angle crossed the axial bisector of the two writing beams. The recording process was the same as for the non-degenerate four-wave mixing setup. The intensity of the reading beam at 632.8 nm was monitored rotationally on a maximum center angle θ max that corresponded to the Bragg-matched diffraction efficiency (ξ = 0). Calculations based on the semi-empirical molecular orbital theory AM1 for 3D-optimized structures and their highest occupied molecular orbital (HOMO) π-electron clouds were performed using WinMOPAC 3.0 software (Fujitsu, Tokyo, Japan). A multi-color hologram was reproduced using an updatable multi-color holographic display system. The original image was divided into red and green color images using a HOLOEYE DVI video signal splitter, and each image on a spatial light modulator (Pluto, 1920 × 1080 pixel resolution and 8.0 μm pixel size, HOLOEYE Photonics AG, Berlin-Adlershof, Germany) was recorded in a MACzE/PMMA film device using a single longitudinal mode blue laser (Genesis CX-488-2000, 488 nm, 2 W, Coherent Inc., Santa Clara, CA, USA). The color hologram was reproduced using a green laser (OBIS 532 nm, 100 mW, Coherent Inc.) and a red laser (diode-pumped solid-state laser series 640 nm, 100 mW, LASOS Lasertechnik GmbH).

Optical properties
The structure of the ACzEs and the ultraviolet-visible absorption spectra of the ACzEs in PMMA are shown in Figure 1. The spectra of the ACzE/PMMA films contain a broad absorption region that extends from 350 to 600 nm. NACzE has a maximum at 440 nm, CACzE has a maximum at 426 nm, AACzE has a maximum at 415 nm, MACzE has a maximum at 394 nm, HACzE has a maximum at 395 nm, FACzE has a maximum at 395 nm and AmACzE has a maximum at 410 nm, which are attributed to π − π* transitions of the trans-form of the azobenzene chromophore. The maximum absorption wavelength (λ max ) shifted to a shorter wavelength (hypsochromic shift) with an increase in the electron-donating ability of the substituent. The broad absorption bands due to the n − π* transitions of the cis-isomer of the azobenzene chromophore are located at~500 nm at room temperature, irrespective of the substituent, except for the amino substituent. The cis-form of AmCzE clearly appears at 443 nm, which is a specific characteristic of aminoazobenzene-type molecules in which the π − π* and n-π* transitions nearly overlap. 35 The spectral features of the transand cis-forms were assigned by a spectral simulation of a firstprinciple molecular dynamics method (B3LYP/6-31G(d)). For the ACzE/PMMA films, the position of the isosbestic point, in which the spectra of the trans-to-cis isomerization overlap, was also estimated by the spectral simulation. Azobenzenes can be classified into three spectroscopic classes: azobenzene-type molecules, aminoazobenzenetype molecules and pseudo-stilbenes. 36 The features of pseudo-stilbene absorption spectra are significantly bathochromic-shifted, and the cis-isomer has an extremely short half-life. Accordingly, ACzEs should be classified as pseudo-stilbenes if they exhibit a bathochromic shift, their cis-form has a short half-life, and the changes in their spectra representing the trans-cis photoisomerization are not clearly probed.
The 3D-optimized electronic structure of the trans-isomer of each of the ACzEs in the ground state computed with the AM1 semiempirical method are also shown in Figure 1. The negative phase of the wave function is illustrated in blue, and the positive phase is in red. All calculations referred to isolated molecules (gas phase). The geometry optimization was performed by minimizing the binding energy of the molecule. The HOMO (frontier molecular orbitals) as an electron donor reflects the ability of the molecule to donate an electron. The π-electron cloud of the HOMO is primarily spread over the carbazole ring and azobenzene chromophore. Similar distributions of HOMOs were obtained for all of the ACzEs. The frontier molecular orbitals are shown on the 3D-optimized structures of the trans-isomers to provide a better understanding of the effect of the substituent on the energy levels and absorption spectra of the ACzEs.  The HOMOs are delocalized over the carbazole and N = N chromogen, whereas the LUMOs (lowest unoccupied orbitals) are predominantly localized over the substituent phenyl ring. The addition of electron-withdrawing substituents, such as NO 2 , CN, COCH 3 and F, lowers the energies of the unoccupied levels, which leads to the bathochromic shift. Conversely, the addition of electron-donating groups, such as OCH 3 , OH and NH 2 , decreases the ground-state dipole moment, which lowers the energies of the HOMOs. Therefore, the addition of stronger electron-donating substituents generally leads to a larger HOMO-LUMO gap (hypsochromic shift in the absorption spectra). Photographs of the ACzE/PMMA films are also shown in Figure 1.
The effect of substituents on the physicochemical properties of a series of related compounds with different substituents is quantitatively related to Hammett constants σ. 37 The absorption maximum (λ max ) depends on the nature of the substituent. The isosbestic point (λ iso ) and λ max of the series of ACzE compounds with different substituents are plotted as a function of the σ of the substituents in Figure 2. Due to electron-accepting groups, a positive relation between σ and λ max can be observed in the region of positive σ.
The bathochromic shift was measured with increasing σ values at σ40: λ max = 394 nm at σ = 0.006 for the F substituent and λ max = 440 nm at σ = 0.778 for the NO 2 substituent. A negative relation between λ max and σ can be observed in the negative σ region due to electron-donating groups. A V-shaped Hammett relation was then obtained, which has also been experimentally reported for substituted azobenzene. 38,39 Substituents with positive σ accelerate the chemical reaction, whereas substituents with negative σ decelerate the reaction.
When the electron donating ability of the substituent is stronger in the region of negative Hammett constants, a bathochromic shift of λ max is also observed: λ max = 388 nm at σ = − 0.170 for the CH 3 substituent and λ max = 410 nm at σ = − 0.660 for the NH 2 substituent. The positive and negative relations between λ max and σ indicate that the charge transfer between the two rings of the carbazole and benzene moieties occurs in both the positive direction and inverse direction, depending on the substituent. Namely, with electron-accepting substituents, electrons are intensely withdrawn from the carbazole to benzene moieties. With electron-donating substituents, weak withdrawal occurs from the benzene to carbazole moieties. Accordingly, the ACzEs on the same slopes have the same electronic transition mechanism.
Similar relations between the λ iso and Hammett constants σ are presented for the ACzEs in the PMMA films. The excitation at the λ iso intensely promotes the Weigert effect 40 because the excitation at λ iso can efficiently switch between the two isomer states, trans and cis. When ACzE is irradiated by a linearly or circularly polarized light, the  Through these processes, the photoalignment and subsequent reorientation of ACzE occurs quickly, which changes the refractive index. Thus, the selective excitation at the λ iso provides the specified holographic features that can be used to optimize the performance of updatable holograms.
Wavelength dependence of the writing beam As discussed above, the λ iso is an appropriate wavelength for hologram writing. To ensure the veracity of the above discussion, the dependence of the writing beam wavelength on the rewritable holographic properties was investigated. The time dependence of the diffraction efficiencies as a function of the writing beam wavelength for the ACzE/PMMA films was investigated. The response time of the diffraction is commonly used to evaluate the time required for the grating formation. To estimate the response time (grating formation time) τ for the respective writing beam wavelengths, the diffraction efficiencies as a function of time were fitted by the Kohlrausch-Williams-Watts-stretched exponential function shown in equation (2).
For the ACzE/PMMA films, the diffraction efficiencies and response times are shown in Table 1. Unfortunately, the holographic properties of FACzE/PMMA could not be evaluated because of a material shortage. For comparison, the holographic properties of the related materials are also listed at the bottom of Table 1. The rate of trans-cis photoisomerization depends on the free volume around the dyes as well as the interaction between the host matrix and the dye molecules, but it is also substantially affected by the writing beam wavelength. Thus, the writing beam wavelength is highly related to the rate- Table 1 Holographic and film properties of the ACzE/PMMA films relative to the writing beam wavelength  determining process for the grating formation in ACzE/PMMA films. For pseudo-stilbenes, the rate of the grating formation should be controlled by the quantum efficiency of the trans-cis photoisomerization and the half-life of the cis-form. Therefore, for the ACzEs, excitation at an isosbestic point is in good agreement with the faster response time for the holographic diffraction. The maximum holographic response obviously occurs at a wavelength near the λ iso . Total holographic performance is commonly evaluated using sensitivity S, which is defined as follows: where η' is an effective diffraction efficiency (η' = exp(− α 632.8 L/cosθ) η, where α 632.8 is the absorption coefficient at 632.8 nm, L is the sample thickness and θ is the internal angle (here θ = 0°)) considering the optical losses due to the absorption at the wavelength of the reading beam, τ is the response time and I is the writing beam intensity. The obtained sensitivities, S, gave the appropriate writing beam wavelengths for the respective ACzE/PMMA films as follows: 561 nm for NACzE, 532 nm or 561 nm for CACzE and AACzE, and 491 nm for MACzE and HACzE.
The reading beam wavelength is an important deciding factor of a hologram's color. Theoretically, use of the same wavelength for writing and reading is preferred. However, from the standpoint of the absorption spectra of the chromophores, the color of the hologram is restricted by longer wavelengths than the writing beam wavelength. An absorption coefficient of o300 cm −1 should be selected for the reading beam wavelength to avoid strong absorption of the reading beam. Thus, it is preferable to select a red reading beam for NACzE; red or yellow reading beams for CACzE and AACzE; and red, yellow, and green reading beams for MACzE and HACzE.
To reproduce high-quality holograms for holographic display devices independently of the magnification factor, high transparency and no scattering are required. Therefore, stability, fatigue resistance and durability of the sample films become important factors. For stability, we used all of the sample films without observing any deterioration because a wavelength in the region of visible light, primarily the isosbestic point wavelength, (not ultraviolet light) was adopted as the writing beam. For this reason, the present samples are not deteriorated by light, unlike a typical photochromic system, which degrades the holographic properties by light illumination. The fatigue resistance, which is related to the stability, of the samples was excellent. When the hologram was recorded and erased and recorded again, the diffraction efficiency was the same and the response time was not changed. We repeated these procedures several times and holographic performances did not deteriorate. We confirm that the ACzEs/PMMA exhibit excellent durability of 2-3 years or more. Haze value (%), defined by the ratio of diffuse transmittance (%)/total light transmittance (%) × 100, was measured for ACzEs/PMMA films. The obtained average haze value of the ACzEs/PMMA films was 4% except for the AmACzE/PMMA film (haze% = 67%), which can be maintained for 2-3 years. These results indicate that dye aggregation or phase separation did not occur. We propose here that the haze value of updatable full-color holographic displays is preferably 4% or below.
Angle dependence of the writing beam Volume holograms are categorized as transmission and reflection holograms. In transmission holograms, the grating planes are normal to the surface of the holographic medium. For the writing of such holograms, two writing beams (object and reference beams) enter from the same side of the sample surface. These holograms can be reproduced by transmitting a beam of light through the holographic medium. Transmission holograms have a high angular selectivity. 41,42 In reflection holograms, the grating planes are parallel to the surface of the hologram medium. For the recording of such holograms, the object and reference beams enter from opposite sides of the holographic recording medium. These holograms can be reproduced by reflecting a beam from the grating planes in the holographic recording medium. Such holograms are known to have a high spectral selectivity. 43 To optimize the performance of the writing beam wavelength for rewritable holograms, the angular selectivity of recorded holograms should be evaluated relative to the writing beam wavelength. A coupled-wave analysis proposed by Kogelnik 44 was introduced to predict the relationship between the incident angle and diffraction efficiency in thick gratings. The angular selectivities of recorded gratings for different writing beam wavelengths are shown in Figure 3. The diffraction efficiency η for the symmetric gratings and the lossless transmission gratings is defined 44 as where sinc(x) = sin(x)⧸x. The variables ν and ξ are as defined in Kogelnik et al. 44 where ν is the grating strength, The grating thicknesses d in the ACzE/PMMA films were obtained from curve fitting the angular selectivity measurements of the recorded gratings shown in Figure 3. The obtained angular dependences for the ACzE/PMMA films are also summarized in Table 1. A shorter writing beam wavelength, an absorption coefficient α much 48000 cm − 1 or a grating thickness o10 μm is better for each sample film, considering a wide viewing angle. However, from the point of view of higher sensitivity S and a diffraction efficiency of 420%, the appropriate grating thickness is ca. 20 μm. Furthermore, when the writing beam wavelength is near the isosbestic point, the absorption coefficients α of the ACzE/PMMA films at the writing beam wavelength are in the range of 286-2476 cm − 1 , which is more appropriate for holographic recordings.
A grating can be classified as a volume (or thick) and planar (or thin) hologram. A hologram is considered thick if the thickness of the recording medium is greater than the spacing between the interference fringes. Otherwise, the hologram is considered to be thin. A dimensionless analytic factor, the Q-factor, helps to distinguish between the two types of holograms. 45 The Q-factor is defined as where d is the thickness of the recording grating, n is the refractive index of the recording grating, λ is the writing beam wavelength and Λ is the spacing between the grating period. The obtained Q-factors are also shown in Table 1. If Q410, only one strong diffraction order is observed, and the Bragg condition is satisfied. If Qo1, multiple diffraction orders are observed, and this type of diffraction is called the Raman-Nath condition. In addition to the above two critical conditions, an intermediate Q-factor region, 1 ⩽ Q ⩽ 10, is assumed to be an intermingled regime of the Bragg condition and the Raman-Nath condition. Consequently, the appropriate holographic properties appear in an intermingled Bragg and Raman-Nath regime with a Q-factor of 1 ⩽ Q ⩽ 10 and a Bragg condition of Q410.
On the basis of a comprehensive assessment of these spectroscopic and holographic optical results, a schematic representation of the appropriate writing and reading beams is shown in Figure 4 for NACzE, CACzE and MACzE: the appropriate writing beam wavelengths are 561 nm for NACzE, 532 nm for CACzE and 491 nm for MACzE; the appropriate reading beams are a red reading beam with a wavelength longer than 600 nm for NACzE; red or yellow reading beams with wavelengths longer than 560 nm for CACzE; and Figure 4 Schematic representations of beam colors for recording and reading each of the ACzE/PMMA films. NACzE/PMMA: yellow color recording and red color reading. CACzE/PMMA: green color recording and red or yellow color reading. MACzE/PMMA: blue color recording and red, yellow and green color reading. Future sample: violet color recording and red, yellow, green and blue color reading. The NACzE/PMMA and CACzE/PMMA films are designed for an updatable mono-color hologram. The MACzE/PMMA film is designed for an updatable multi-color hologram using red and green laser beams. The future sample may be effective for an updatable full-color hologram using red, green and blue laser beams. A practical updatable multi-color hologram in MACzE/ PMMA film is shown in Figure 5.  (7). For all samples, the viewing angle becomes wider when the recording wavelength becomes shorter because of shallower grating depth due to the large absorption coefficient.
Molecular design of azo-carbazole monolithic dyes K Kinashi et al red, yellow, and green reading beams with wavelengths longer than 530 nm for MACzE. The simultaneous illumination of red and green reading beams displays the multi-color hologram corresponding to the respective reading beam. Multi-color hologram reproduction using a MACzE/PMMA is shown in Figure 5. The updatable holographic display system for a multi-color hologram is shown in Figure 5a: the original image is divided into red and green color images through a video signal splitter. Each image on a spatial light modulator is individually recorded in the MACzE/PMMA film device using a single longitudinal mode blue laser. Each hologram is reproduced using a green laser and a red laser to synthesize the color hologram. A clear multi-color hologram of red ('1') yellow ('2') and green ('3') numerals can be reproduced with red and green reading laser beams as shown in Figure 5b. The CIE1931 chromaticity diagram at the maximum brightness is also shown in Figure 5c. Circles in the figure correspond to colors in the hologram. A recent study determined that 2,4,6trichloro (Cl)-substituted ACzE presents an absorption peak at 384 nm. 46 A green-colored hologram was produced using a PMMA film containing the trichloro-substituted ACzE. Using the present approach, we can predict that holograms using unsubstituted ACzE can be written by a violet light and read by a blue light (future in Figure 4). The synthesis of unsubstituted ACzE is challenging and a target of future research.

CONCLUSIONS
Seven types of azo-carbazole (ACzE) monolithic dyes were synthesized. The influence of substituents introduced in the para position of the phenyl ring and the distinct strength of the substituents (NO 2 , CN, CH 3 CO, F, CH 3 O, OH and NH 2 ) on the phenyl ring of the ACzEs were investigated on the basis of spectroscopic and holographic optical aspects. The experimental evidence strongly suggests that the appropriate writing beam wavelengths are 561 nm for NACzE, 532 nm for CACzE (and AACzE) and 491 nm for MACzE (and HACzE). The appropriate wavelengths for the reading beams are longer than 600 nm for NACzE, longer than 560 nm for CACzE (and AACzE) and longer than 530 nm for MACzE (and HACzE).
Material design that is suitable for updatable full-color holography is highly challenging, but very attractive and advantageous for many practical applications such as digital signage.