**2.1 Photochroism**

"Photochromism"is simply defined as a light-induced reversible change of color. "Photochromism is a reversible transformation of a chemical species induced in one or both directions by absorption of electromagnetic radiation between two forms, A and B, having different absorption spectra".The thermodynamically stable form A is transformed by irradiation into form B. The back reaction can occur thermally or photochemically (like shown in Fig.1) [2,3].

Fig. 1. A simple sketch map of photochromic

Fig. 2. A simple sketch map of the mechanism of photo-induced anisotropy

### **2.2 Photo-induced anisotropy**

In transparent materials with anisotropic dielectric permittivity, optical anisotropy can be observed, in which include the pleochroism (anisotropy of the material's absorption coefficient) and birefringence (anisotropy of the material's refractive index). **Pleochroism** means that the absorption of the material to the light depends not only on the wavelength of light, but also on the polarization state of light: for uniaxial crystals, it is called as **dichroism**; for biaxial crystals, it is called as **trichroism**. **Birefringence** means that the refractive index of the material depends not only on the wavelength of light, but also on the polarization state of light.

Many crystals have crystal lattice structure themselves, so they are **natural optical anisotropic media**. However, in some amorphous material, under the action of certain external field (such as electromagnetic fields, mechanical forces, etc.), their atoms or molecules will be orientated in certain rules, thus the material will change from isotropic into anisotropic macroscopically, which is called as the **artificial optical anisotropy**. In which, under the irradiation of polarized light, some isotropic materials will turn to be anisotropic, or the degree of anisotropic properties in some materials will change, this phenomenon is called as the **photo-induced anisotropy** (like shown in Fig.2). Which include the photo-induced dichroism (usually photo-induced anisotropic materials show the properties of uniaxial crystal) and photo-induced birefringence.

#### **2.3 Fulgide**

146 Holograms – Recording Materials and Applications

"Photochromism"is simply defined as a light-induced reversible change of color. "Photochromism is a reversible transformation of a chemical species induced in one or both directions by absorption of electromagnetic radiation between two forms, A and B, having different absorption spectra".The thermodynamically stable form A is transformed by irradiation into form B. The back reaction can occur thermally or photochemically (like

λ**<sup>B</sup>** λ**<sup>A</sup>**

**A**

 **Wavelength**

Isotropy (usually) Anitropy (after irradiated

In transparent materials with anisotropic dielectric permittivity, optical anisotropy can be observed, in which include the pleochroism (anisotropy of the material's absorption coefficient) and birefringence (anisotropy of the material's refractive index). **Pleochroism** means that the absorption of the material to the light depends not only on the wavelength of light, but also on the polarization state of light: for uniaxial crystals, it is called as **dichroism**; for biaxial crystals, it is called as **trichroism**. **Birefringence** means that the refractive index of the material depends not only on the wavelength of light, but also on the polarization state

Many crystals have crystal lattice structure themselves, so they are **natural optical anisotropic media**. However, in some amorphous material, under the action of certain external field (such as electromagnetic fields, mechanical forces, etc.), their atoms or molecules will be orientated in certain rules, thus the material will change from isotropic into anisotropic macroscopically, which is called as the **artificial optical anisotropy**. In

Fig. 2. A simple sketch map of the mechanism of photo-induced anisotropy

by linearly polarized light)

*hυ***<sup>2</sup> A**

**B** *hυ***<sup>1</sup>**

**B**

**2. Definition of fulgide, photochroism and photo-induced anisotropy** 

**Asorption / a.u.**

Fig. 1. A simple sketch map of photochromic

**2.2 Photo-induced anisotropy** 

of light.

**2.1 Photochroism** 

shown in Fig.1) [2,3].

#### **2.3.1 Structures of fulgides**

Early in 20th century, Stobbe and Eckert, (Leipzig University, Germany) [1,2] found that the condensation products of succinate and aromatic group of aldehydes, ketones have photochromic property. They stated in their article in 1905 that they named the derivatives of 1,3-butadiene-2,3-dicarboxylic acid and its acid anhydride as "fulgenic acid " and "fulgide", respectively (like shown in Fig.3), after the Latin word "fulgere" (glitter or shine), because some derivatives exhibited a variety of characteristic colors by light, and they usually formed shiny crystals.

Fig. 3. Chemical molecular formula of the fulgenic acid (a) and fulgide (b)

To be photochromic, fulgides should have at least one aromatic ring or heteroaromatic ring (Ar) on the exo-methylene carbon atom, so that they form a 1,3,5-hexatriene structure that may undergo 6π-electrocyclization.

When three of the four substituents are same, the fulgide compounds have two isomers: cisisomer and trans-isomer. According to geometrical shape of the double bond connecting the aromatic ring and succinic acid, they are called as "E-form" And "Z-form" respectively (for example, the isomers of phenyl fulgide are shown in Fig.4). In which, the E-form has an "all cis-hexatriene" structural unit, so it can be photocyclized. When two of the four substituents are same, the fulgide compounds have three isomers: (E,E), (E,Z) and (Z,Z). When the four substituents are different with each other, the fulgide compounds have four isomers: (E,E), (E,Z), (Z,E) and (Z,Z).

Fig. 4. The two isomers of phenyl-fulgide: (a) Z-form; (b) E-form

Holographic Image Storage with a 3-Indoly-Benzylfulgimide/PMMA Film 149

isotropic characteristic at the initial state; when the sample is irradiated by linearly polarized light photo-selection of molecules take place. Molecules with long axes parallel to the exiting light polarization direction absorb the light strongly and turn to the other form very quickly, whereas those with long axes perpendicularly orientated to exiting light polarization have low absorption and stay at the initial form. As a result special orientation of two form molecules is induced and the sample shows optical anisotropy properties macroscopically. The angular distributions of molecules in anisotropy inducing progress are shown in Fig.7. So, under the irradiation of circularly polarized light, the sample shows also optical

**C C**

Photochromic organic compounds show great potential in the field of rewriteable holographic storage owing to its benefits like lower price, higher signal noise ratio (SNR), higher spatial resolution, higher sensitivity, less toxic, no need to special fixing information after recording, stable in the air or moisture, and easy to be modulated [1,2]. Their disadvantages are lower diffraction efficiency and difficulty to realize non-destructive readout. In which, fulgides are famous as thermally irreversible organic photochromic

The photochromic and photo-induced anisotropic properties of materials can be used in ordinary and polarization holographic recording respectively. It is known that fulgidedoped polymeric films are photochromic and photo-induced anisotropic, which can be used

have components with same polarization state, there the intensity grating exists and the holograms can be recorded, called as **ordinary hologram**. But for photo-induced anisotropic

be recorded, because that at this time although the intensity of the superposed light is a

Photo-induced anisotropic materials can record the polarization state of exciting beam, so the holographic gratings can be recorded, called as **polarization hologram**, in which not

Here we just consider four kinds of polarization holographic storage, including the parallel linearly, parallel circularly, orthogonal linearly and orthogonal circularly polarized holograms. Fig.8 is a simple sketch map showing the space modulation of the distributions and orientations (optical axis) of C-form and E-form molecules of fulgide films in these four kinds of polarization holographic storage under the linearity recording condition when *O=R=A*. The arrows and ellipses on the horizontal line indicate the periodical distributions

constant, but its polarization state changes with the phase difference between *O*

depending on the exposure), but also its polarization state can be stored.

have orthogonal polarization states, the holograms also can

can be stored (the photo-induced anisotropy is

Fig. 7. Angular distributions of molecules in anisotropy inducing progress

For isotropy photochromic materials, only when the object light *O*

and *<sup>R</sup>*

only the intensity and phase signals of *O*

**2.4 Holographic storage application of fulgide films** 

**E**

and reference light *<sup>R</sup>*

 and *<sup>R</sup>* .

isotropy.

compounds.

for both.

materials, even if the *O*

#### **2.3.2 The photochroism of fulgides**

The photochroism of fulgide occurs between one of the colorless open forms "E-form" and the photocyclized colored form (abbreviated as the C-form), like shown in Fig.5. The coloration mechanism of fulgide is the photochemical 6π-electrocyclization of the hexatriene moiety, is a cyclic reaction consistent with Woodward-Hoffmann selection rules. However, there exits an additional photochemical E-to-Z isomerization, the Z-form is not considered as an important component in the photochromic system. There has been no report that the Z-form cyclizes directly by absorbing on photon to give the C-form. Therefore, E-to-Z isomerization, competing with the photochromic E-to-C isomerization, is an energy-wasting as well as system-complicating process in terms of the "photochroism of fulgides". Using Improvement of the structure, this additional reaction can be suppressed. Usually E- and Zforms have maximum absorption in the UV region; C-form has maximum absorption in the visible region .

Z-form(colorless) E-form(colorless) C-form(colored)

Fig. 5. The photochromic reaction of fulgides

#### **2.3.3 The derivatives of fulgide**

The photochromic mechanism of fulgide is the photochemical 6π-electrocyclization process, so the carbonyl group and the aromatic ring are both not indispensable. For example, anhydride part can be instead of other functional groups, like succinimides (fulgimide), butanolides (fulgenolide), dieters (fulgenate) and compounds having an anhydride ring with modified carbonyl group (as shown in Fig.6) [1,2].

Fig. 6. Chemical molecular formula of the fulgide derivatives

#### **2.3.4 The photo-induced anisotropy of fulgide films**

We found that there exists photo-induced anisotropy in fulgide-doped polymeric films [4], which can be used in polarization holography application. When the colored states are irradiated by a linearly polarized 650 nm laser, the films returns to the bleached states and photo-induced anisotropy is produced during this process. The mechanism of photoinduced anisotropy in the fulgide films is the following [4]: anisotropic absorbing molecules of fulgide are immobilized randomly in the PMMA polymeric matrix, which shows

The photochroism of fulgide occurs between one of the colorless open forms "E-form" and the photocyclized colored form (abbreviated as the C-form), like shown in Fig.5. The coloration mechanism of fulgide is the photochemical 6π-electrocyclization of the hexatriene moiety, is a cyclic reaction consistent with Woodward-Hoffmann selection rules. However, there exits an additional photochemical E-to-Z isomerization, the Z-form is not considered as an important component in the photochromic system. There has been no report that the Z-form cyclizes directly by absorbing on photon to give the C-form. Therefore, E-to-Z isomerization, competing with the photochromic E-to-C isomerization, is an energy-wasting as well as system-complicating process in terms of the "photochroism of fulgides". Using Improvement of the structure, this additional reaction can be suppressed. Usually E- and Zforms have maximum absorption in the UV region; C-form has maximum absorption in the

> <sup>O</sup> R3 R

R1

O

Ar <sup>O</sup>

O

Z-form(colorless) E-form(colorless) C-form(colored)

The photochromic mechanism of fulgide is the photochemical 6π-electrocyclization process, so the carbonyl group and the aromatic ring are both not indispensable. For example, anhydride part can be instead of other functional groups, like succinimides (fulgimide), butanolides (fulgenolide), dieters (fulgenate) and compounds having an anhydride ring

R4 Ar <sup>O</sup>

UV VIS

R3 R4

R1 <sup>O</sup>

O

**2.3.2 The photochroism of fulgides** 

Ar

R1

**2.3.3 The derivatives of fulgide** 

O

UV UV

O

with modified carbonyl group (as shown in Fig.6) [1,2].

R3

R2

R 4

fulgenolide

Fig. 6. Chemical molecular formula of the fulgide derivatives

**2.3.4 The photo-induced anisotropy of fulgide films** 

O

O

R 3

R2

R4

We found that there exists photo-induced anisotropy in fulgide-doped polymeric films [4], which can be used in polarization holography application. When the colored states are irradiated by a linearly polarized 650 nm laser, the films returns to the bleached states and photo-induced anisotropy is produced during this process. The mechanism of photoinduced anisotropy in the fulgide films is the following [4]: anisotropic absorbing molecules of fulgide are immobilized randomly in the PMMA polymeric matrix, which shows

fulgenate

R1

CO2R5

CO2 R6

R3

R 4

O

R1 X

<sup>O</sup> R2

thioahydride

R6

R1 R 5

R

Fig. 5. The photochromic reaction of fulgides

R4 R3

visible region .

R3

2

R4

fulgimide

O

N-R5 <sup>R</sup>

R <sup>1</sup> O isotropic characteristic at the initial state; when the sample is irradiated by linearly polarized light photo-selection of molecules take place. Molecules with long axes parallel to the exiting light polarization direction absorb the light strongly and turn to the other form very quickly, whereas those with long axes perpendicularly orientated to exiting light polarization have low absorption and stay at the initial form. As a result special orientation of two form molecules is induced and the sample shows optical anisotropy properties macroscopically. The angular distributions of molecules in anisotropy inducing progress are shown in Fig.7. So, under the irradiation of circularly polarized light, the sample shows also optical isotropy.

Fig. 7. Angular distributions of molecules in anisotropy inducing progress

#### **2.4 Holographic storage application of fulgide films**

Photochromic organic compounds show great potential in the field of rewriteable holographic storage owing to its benefits like lower price, higher signal noise ratio (SNR), higher spatial resolution, higher sensitivity, less toxic, no need to special fixing information after recording, stable in the air or moisture, and easy to be modulated [1,2]. Their disadvantages are lower diffraction efficiency and difficulty to realize non-destructive readout. In which, fulgides are famous as thermally irreversible organic photochromic compounds.

The photochromic and photo-induced anisotropic properties of materials can be used in ordinary and polarization holographic recording respectively. It is known that fulgidedoped polymeric films are photochromic and photo-induced anisotropic, which can be used for both. 

For isotropy photochromic materials, only when the object light *O* and reference light *<sup>R</sup>* have components with same polarization state, there the intensity grating exists and the holograms can be recorded, called as **ordinary hologram**. But for photo-induced anisotropic materials, even if the *O* and *<sup>R</sup>* have orthogonal polarization states, the holograms also can be recorded, because that at this time although the intensity of the superposed light is a constant, but its polarization state changes with the phase difference between *O* and *<sup>R</sup>* . Photo-induced anisotropic materials can record the polarization state of exciting beam, so the holographic gratings can be recorded, called as **polarization hologram**, in which not only the intensity and phase signals of *O* can be stored (the photo-induced anisotropy is depending on the exposure), but also its polarization state can be stored.

Here we just consider four kinds of polarization holographic storage, including the parallel linearly, parallel circularly, orthogonal linearly and orthogonal circularly polarized holograms. Fig.8 is a simple sketch map showing the space modulation of the distributions and orientations (optical axis) of C-form and E-form molecules of fulgide films in these four kinds of polarization holographic storage under the linearity recording condition when *O=R=A*. The arrows and ellipses on the horizontal line indicate the periodical distributions

Holographic Image Storage with a 3-Indoly-Benzylfulgimide/PMMA Film 151

photochromic forms, whose molecular formulas are shown in Fig.9. In this film we also

**VIS UV**

E-form C-form Fig. 9. Molecular formula of the Indolyfulgimide and the photochromic reaction. Left, E-

**2.5.2 Spectra of photochromic and photo-induced anisotropy properties of the sample**  The absorption of the C-form (AC(λ)) and E-form film (AE(λ)) were measured using a UV-VIS-IR spectrophotometer (UV-3101PC, Shimadzu Inc., Japan), which were shown in Fig.10a. And the measurement of the photo-induced dichroism is performed by measuring the transmission spectra of the film for testing light polarized parallel (*T*//(λ)) and perpendicular (*T*⊥(λ)) to the polarization direction of the exciting beam after the C-form film

**N CH3** **CH3**

**N**

**CH2**

**O**

**O**

**CH3**

**CH3 CH3**

**300 400 500 600 700 800**

**366 573**

λ / nm

**432 750**

**-0.02**

**-0.01**

**0.00**

Δ *n*

**0.01**

**0.02**

ΔA

Δ*n*

found the photo-induced anisotropy property [4].

**O**

**O**

**300 400 500 600 700 800**

λ / nm

573nm, 2.78

A

**N**

**CH2**

is excited by the linearly polarized 650 nm laser (shown in Fig.11a).

 E-form C-form

**-3 -2 -1 0 1 2 3**

(a) (b)

Fig. 10. Spectra of photochromic properties of Indolyfulgimide /PMMA film:(a) Absorption spectra of two forms; (b) Absorption difference spectrum and the corresponding refractive

Δ

A

**N CH3**

form; right, C-form

366nm, 7.15

index changing spectrum

**CH3**

**CH3 CH3**

**CH3**

of intensity magnitudes and polarization states of the recording field corresponding to the different phase differences Δϕ along the *x*-axis. The rectangle frames under them indicate the distribution and orientation of the molecules under the corresponding light irradiations, where the black and white color indicate the C-form and E-form molecules respectively, and the arrows indicate the direction of induced optical axes, and the star flower ( ) means the sample is isotropic at the place.

Fig. 8. The distributions of the interference fields in four kinds of polarization holography storage and the corresponding molecular distributions and orientations

From Fig.8, it can be seen that in the parallel circularly polarized hologram just ordinary hologram exits, and in the orthogonal polarized condition, just polarization hologram exits, but in the parallel linearly polarized holography, the ordinary hologram and polarization hologram will be recorded together.

#### **2.5 Photochroism and photo-induced anisotropy of 3-indoly-benzylfulgimide/PMMA film**

#### **2.5.1 Material**

The fulgide material studied here is 3-indoly-benzylfulgimide, which was synthesized by the Stobbe condensation routine [2]. The target compound of 3mg was dissolved in a 0.lml 10% (by weight) PMMA-cyclohexanone solution. Then the solution was coated on a 1-mm thick K9 glass plane (∅ 25 mm × 1.5 mm) with a spin coater and dried in air. The thickness of the film is determined to be about 10µm by microscopy of the cross section. The photochromic or photo-induced anisotropic properties of fulgides are due to a reversible photochromic (photoisomerization) reaction that occurs between one of the colorless E-form (bleached state) and the C-form (colored state). These are the two spectrally separated

of intensity magnitudes and polarization states of the recording field corresponding to the

the distribution and orientation of the molecules under the corresponding light irradiations, where the black and white color indicate the C-form and E-form molecules respectively, and the arrows indicate the direction of induced optical axes, and the star flower ( ) means the

Δϕ **=** ϕ**O -** ϕ**<sup>R</sup> 0** π**/2** π 3π**/2 2**π 5π**/2 3**π 7π**/2 4**π

Fig. 8. The distributions of the interference fields in four kinds of polarization holography

From Fig.8, it can be seen that in the parallel circularly polarized hologram just ordinary hologram exits, and in the orthogonal polarized condition, just polarization hologram exits, but in the parallel linearly polarized holography, the ordinary hologram and polarization

**2.5 Photochroism and photo-induced anisotropy of 3-indoly-benzylfulgimide/PMMA** 

The fulgide material studied here is 3-indoly-benzylfulgimide, which was synthesized by the Stobbe condensation routine [2]. The target compound of 3mg was dissolved in a 0.lml 10% (by weight) PMMA-cyclohexanone solution. Then the solution was coated on a 1-mm thick K9 glass plane (∅ 25 mm × 1.5 mm) with a spin coater and dried in air. The thickness of the film is determined to be about 10µm by microscopy of the cross section. The photochromic or photo-induced anisotropic properties of fulgides are due to a reversible photochromic (photoisomerization) reaction that occurs between one of the colorless E-form (bleached state) and the C-form (colored state). These are the two spectrally separated

storage and the corresponding molecular distributions and orientations

along the *x*-axis. The rectangle frames under them indicate

different phase differences Δ

sample is isotropic at the place.

**(a) Parallel linearly** 

**(b) Orthogonal linearly** 

**(c) Parallel circularly** 

**(d) Orthogonal circularly**

hologram will be recorded together.

**film** 

**2.5.1 Material** 

ϕ

photochromic forms, whose molecular formulas are shown in Fig.9. In this film we also found the photo-induced anisotropy property [4].

Fig. 9. Molecular formula of the Indolyfulgimide and the photochromic reaction. Left, Eform; right, C-form

#### **2.5.2 Spectra of photochromic and photo-induced anisotropy properties of the sample**

The absorption of the C-form (AC(λ)) and E-form film (AE(λ)) were measured using a UV-VIS-IR spectrophotometer (UV-3101PC, Shimadzu Inc., Japan), which were shown in Fig.10a. And the measurement of the photo-induced dichroism is performed by measuring the transmission spectra of the film for testing light polarized parallel (*T*//(λ)) and perpendicular (*T*⊥(λ)) to the polarization direction of the exciting beam after the C-form film is excited by the linearly polarized 650 nm laser (shown in Fig.11a).

Fig. 10. Spectra of photochromic properties of Indolyfulgimide /PMMA film:(a) Absorption spectra of two forms; (b) Absorption difference spectrum and the corresponding refractive index changing spectrum

Holographic Image Storage with a 3-Indoly-Benzylfulgimide/PMMA Film 153

**0 20 40 60 80 100**

4 Fitting curve of *T*<sup>⊥</sup>

Fig. 13. Photo-induced anisotropic transmission curves of Indolyfulgimide/PMMA film on the directions parallel or perpendicular to exciting beam polarization depending on the exposure or erasing time: (a) Experiment curves measured at different exiting beam intensity; (b) Simulation of experimental results; (c) Calculated curves of uniformity light For the analysis, we consider a fully bistable Fulgide system, neglecting side reactions like E-Z isomerization or aging effects. Using numerical calculation method [6], where the intensity Gaussian beam distribution of the He-Ne laser beam has been considered, the experimental curves were simulated (shown as the dash lines in Fig.13b) and the best fitting values γ//=0.00289cm2/mJ, γ⊥=0.0012cm2/mJ were obtained.. Then the photo-induced anisotropic

The system configuration for measuring the real-time hologram first order diffraction kinetics of the Fulgide film is schematically illustrated in Fig.14. A He-Ne laser (Melles Griot Inc., USA, 25-LHP-928, 632.8nm, 35mW, vertical linear polarized) is used to generate recording beams (object beam *I*O and reference beam *I*R) and readout beam (reconstruction beam *I*C), and a laser diode LD (Power Technology Inc., USA, IQ2A18, 405nm, 10mW, vertical linear polarized) is used as the auxiliary light source (*I*A) and erasing light source (*I*E). The He-Ne laser beam is split into the *I*O, *I*R and *I*C after beam splitter BS1 and polarization beam splitter PBS, in which *I*R and *I*C are phase conjugated (counterpropagated) beams. The diffracted light *I*D of *I*C, diffracted by the dynamic holographic grating established by the interference between the *I*O and *I*R, will be phase conjugated with the *I*O, whose power was real time detected by a digital power meter '*D*' (United Detector Technology company, USA,11A Photometer / Radiometer, 254~1100nm, *I*max=10mW, resolution is 0.01nW) and a digital oscilloscope '*O*' (Tektronix company, USA, TDS3032, 300MHz, 2.5GS/s, 1mV) after reflected by the BS2 (R47%). The *I*O and *I*R are symmetrically incident on the sample (Fulgide film), whose intersection angle 2θ=16.5°, so the recorded grating is a non-incline grating. Shutter S1 and S2 controls the exposure time of red and purple beams. The continuously adjustable attenuators A1~A3 are used to adjust the intensities of the waves, in this experiment *IO*=*IR*=78.6mW/cm2 and *IC*=0.786mW/cm2 (i.e. *IO*:*IR*:*IC*=100:100:1). This insures that the sub-reflection gratings formed by *IO* and *IC* as well as *IR* and *IC* can be ignored. The Quarter-wave plates Q1~Q4 and the polarizer P are

*I i* =157mW/cm2

2 4

1 3

(a) (b) (c)

transmission curves of uniformity light are calculated like shown in the Fig.13c.

**3. Holographic recording properties of one kind of indoly-**

γ=0.00289)

*T* / %

**0 2000 4000 6000 8000 10000 <sup>0</sup>**

*Exposure* / mJ cm-2

*T*⊥ *T*//

 (γ=0.0012)

1 Experimental result of *T*// 2 Experimental result of *T*<sup>⊥</sup> 3 Fitting curve of *T*// (

*t* / s

**0 5000 10000 15000 20000 25000 <sup>0</sup>**

*Exposure* / mJ cm-2

**benzylfulgimide/PMMA film** 

**3.1.1 Measurement set up** 

**3.1 Kinetics of diffraction efficiency (DE)** 

exciting=314.4 mW/cm2

exciting=157.2 mW/cm2

exciting=314.4 mW/cm2

exciting=157.2 mW/cm2

)

)

*T* / %

)

)

1 *T*// (*I*

2 *T*// (*I*

3 *T*<sup>⊥</sup> (*I*

4 *T*<sup>⊥</sup> (*I*

*T* / %

Fig. 11. Spectra of photo-induced anisotropic properties of Indolyfulgimide /PMMA film excited by linearly polarized light: (a) Transmission spectra on directions parallel and perpendicular to exciting beam polarization; (b) Dichroism and birefringence spectra

From Fig.10a and Fig.11a, the photo-induced absorption changing spectrum () () () Δ= − *AA A* λλλ *E C* and the photo-induced dichroism spectrum △*AD*(λ) = *A*⊥(λ)-*A*//(λ)= lg(*T*//(λ)/*T*⊥(λ)) were obtained, which are shown as solid lines in Fig.10b and Fig.11b respectively. Assuming that △A and △*AD* are zero outside of the band 300~800nm, the photoinduced refractive index changing spectrum () () () Δ= − *nn n* λλλ *E C* and the photo-induced birefringence spectrum △*nB*(λ) =*n*⊥(λ)-*n*//(λ) can be calculated according to the Kramers-Kronig relation [5], where *n*E *, n*C*, n*// and *n*⊥ are the refractive indexes of E-form, C-form and of the film excited by linearly polarized light along the photo-induced extraordinary and ordinary axes, respectively. The calculated Δ*n*, Δ*nB* are plotted as dot lines in Fig.10b and Fig.11b respectively. Kramers-Kronig relation can be satisfied during all the photochromic reaction progress, so at one wavelength λ, Δ*n*(λ) is proportional to Δ*A*(λ) and Δ*nB*(λ) is proportional to ΔAD(λ) at different exciting time. From the Fig.10b and Fig.11b, it can be seen that at 633nm in this sample, Δ*n*(633nm)/Δ*A*(633nm)=0.00994 and Δ*n B*(633nm)/ Δ*AD*(633nm)=0.006115.

#### **2.5.3 Dynamics of photochromic and photo-induced anisotropy properties of the sample**

The transmission growing up kinetics of the sample at 633nm were measured on the parallel and perpendicular directions to exciting beam polarization, when the C-form sample was being excited with 314mW/cm2 and 157mW/cm2 intensity linearly polarized 633nm He-Ne lasers (*I*W) respectively, and an 1mW/cm2 633nm laser beam is used as the testing beam (*I*T), the optical setup and the results are shown in Fig.12 and Fig.13a. From Fig.13a, it can be seen that the photochromic reaction and photo-induced anisotropy progress of fulgide material is an optical cumulating progress, which just depending on the Exposure, so it is enough to consider just one exciting beam intensity condition for the analysis.

Fig. 12. Schematic of the experimental setup for measuring transmission kinetics.

Fig. 13. Photo-induced anisotropic transmission curves of Indolyfulgimide/PMMA film on the directions parallel or perpendicular to exciting beam polarization depending on the exposure or erasing time: (a) Experiment curves measured at different exiting beam intensity; (b) Simulation of experimental results; (c) Calculated curves of uniformity light

For the analysis, we consider a fully bistable Fulgide system, neglecting side reactions like E-Z isomerization or aging effects. Using numerical calculation method [6], where the intensity Gaussian beam distribution of the He-Ne laser beam has been considered, the experimental curves were simulated (shown as the dash lines in Fig.13b) and the best fitting values γ//=0.00289cm2/mJ, γ⊥=0.0012cm2/mJ were obtained.. Then the photo-induced anisotropic transmission curves of uniformity light are calculated like shown in the Fig.13c.

#### **3. Holographic recording properties of one kind of indolybenzylfulgimide/PMMA film**
