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

#### **3.1.1 Measurement set up**

152 Holograms – Recording Materials and Applications

(a) (b) 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

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 photo-

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,

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

enough to consider just one exciting beam intensity condition for the analysis.

**M** 

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

**He-Ne, 633nm** *D* **<sup>1</sup>**

*I***W**

*I***T**

**A1**

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

**A2**

*D* **<sup>2</sup> S PBS**

*Sample*

*E C* and the photo-induced dichroism spectrum △*AD*(λ) = *A*⊥(λ)-*A*//(λ)=

λλλ

**-0.005 0.000 0.005 0.010 0.015 0.020**

Photoinduced dichroism Δ*AD*

**450 500 550 600 650 700 750 800 -0.010**

<sup>Δ</sup>*AD* = *A*<sup>⊥</sup> - *A*//

λ / nm

<sup>Δ</sup>*nB* = *n*<sup>⊥</sup> -*n*// **-1.0 -0.5 0.0 0.5 1.0 1.5 2.0**

*E C* and the photo-induced

Photoinduced birefringence

Δ*nB*(x10- 4)

**300 400 500 600 700 800**

 *T*// *T*<sup>⊥</sup>

λ / nm

induced refractive index changing spectrum () () () Δ= − *nn n*

Δ*n*(633nm)/Δ*A*(633nm)=0.00994 and Δ*n B*(633nm)/ Δ*AD*(633nm)=0.006115.

() () () Δ= − *AA A* λλλ

**sample** 

*T* / %

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

Holographic Image Storage with a 3-Indoly-Benzylfulgimide/PMMA Film 155

used to change the polarization states of the waves, here four different polarization recording: parallel linearly polarization recording, parallel circularly polarization recording, orthogonal linearly polarization recording and orthogonal circularly polarization recording were studied, everyone was constructed by horizontal, vertical, left circular and right

D

**IO**

**IR**

**ID**

θ

Q2

BS2

LD M5

S2

P Q3

Fig. 14. Schematic of the experimental setup for measuring real-time polarization hologram

In four kinds of polarization recording, and different polarization reading, the diffracted wave polarization states (DWPS) obtained in the experiments are shown in table 1 and the

Fig.15(a,c,d,e). From them the curves of the conditions when *I*C has same polarization state with *I*R were compared in Fig.16a. It can be seen that there exist an optimal exposure about

DE dynamic curves at 633nm of different kinds of holograms recorded in the sample can be calculated from the photochromic and photo-induced anisotropic properties of the sample written in section 2.5, by using the DE formulas written in Table 1 [7]. Where the (

indicate the photoinduced anisotropy of the sample under the irradiation of linearly polarized light at some exposure, (*n*e-*n*o) indicate the corresponding birefringence,

circularly polarized light at this time (at the area of light strips in the interference field),

indicates the amplitude transmission of the sample before the illumination of light (at the

transmission of the E-form and C-form sample respectively, *n*M, *nm*, *n*E and *n*C indicate the corresponding refractive index of the sample,*Ψi=k0·ni·d* (*i*=e, o, M, m, E, C), and it is defined

theoretically calculated diffraction efficiency kinetics curves of parallel linearly polarization hologram are shown in Fig.15b. In parallel circularly polarization recording hologram, the DE curves are same with each other for any kinds of polarized reconstruction light *I*C, which

o)/2 indicate the amplitude transmission of the sample for nonpolarized light or

τE and τ

j)/2, *Ψ0ij=*(*Ψi*+*Ψj*)/2, *Ψ1ij=*(*Ψi*-*Ψj*)/2 (*i*, *j* = e, o, M, m, E, C).The

Sample

η

M4

**IC**

*+1* ~ *t* are shown in

C indicate the amplitude

M3

τeτo)

> τm

O

**IA & IE**

circular polarized four kinds of lights like shown in table 1.

A1

Q1

measured kinetic first order diffraction efficiency (DE) curves

area of dark strips in the interference field, isotropy),

A2

A3

Q4

M2

M1

PBS

BS1

He-Ne

diffraction kinetics.

**3.1.2 Measurement results** 

**3.1.3 Theoretical analysis** 

τM=(τe+τ

that τ0ij=(τi+τj)/2, τ1ij=(τiτ

2×78.6mW/cm2×3.75s≈590 mJ/cm2.

S1


Table 1. DWPS and DE of different kinds of polarization holograms

used to change the polarization states of the waves, here four different polarization recording: parallel linearly polarization recording, parallel circularly polarization recording, orthogonal linearly polarization recording and orthogonal circularly polarization recording were studied, everyone was constructed by horizontal, vertical, left circular and right circular polarized four kinds of lights like shown in table 1.

Fig. 14. Schematic of the experimental setup for measuring real-time polarization hologram diffraction kinetics.

### **3.1.2 Measurement results**

154 Holograms – Recording Materials and Applications

Table 1. DWPS and DE of different kinds of polarization holograms

In four kinds of polarization recording, and different polarization reading, the diffracted wave polarization states (DWPS) obtained in the experiments are shown in table 1 and the measured kinetic first order diffraction efficiency (DE) curves η*+1* ~ *t* are shown in Fig.15(a,c,d,e). From them the curves of the conditions when *I*C has same polarization state with *I*R were compared in Fig.16a. It can be seen that there exist an optimal exposure about 2×78.6mW/cm2×3.75s≈590 mJ/cm2.

#### **3.1.3 Theoretical analysis**

DE dynamic curves at 633nm of different kinds of holograms recorded in the sample can be calculated from the photochromic and photo-induced anisotropic properties of the sample written in section 2.5, by using the DE formulas written in Table 1 [7]. Where the (τeτo) indicate the photoinduced anisotropy of the sample under the irradiation of linearly polarized light at some exposure, (*n*e-*n*o) indicate the corresponding birefringence, τM=(τe+τo)/2 indicate the amplitude transmission of the sample for nonpolarized light or circularly polarized light at this time (at the area of light strips in the interference field), τm indicates the amplitude transmission of the sample before the illumination of light (at the area of dark strips in the interference field, isotropy), τE and τC indicate the amplitude transmission of the E-form and C-form sample respectively, *n*M, *nm*, *n*E and *n*C indicate the corresponding refractive index of the sample,*Ψi=k0·ni·d* (*i*=e, o, M, m, E, C), and it is defined that τ0ij=(τi+τj)/2, τ1ij=(τiτj)/2, *Ψ0ij=*(*Ψi*+*Ψj*)/2, *Ψ1ij=*(*Ψi*-*Ψj*)/2 (*i*, *j* = e, o, M, m, E, C).The theoretically calculated diffraction efficiency kinetics curves of parallel linearly polarization hologram are shown in Fig.15b. In parallel circularly polarization recording hologram, the DE curves are same with each other for any kinds of polarized reconstruction light *I*C, which

Holographic Image Storage with a 3-Indoly-Benzylfulgimide/PMMA Film 157

(a) (b)

It can be seen that the maximum values' ratio of the measured values is basically coincide with the theoretically analyzed one. Only for parallel linearly polarization hologram, in the

efficiency is lower than parallel linearly polarization reconstruction condition

can be proved to be very small when the *I*C=*I*O/100=*I*R/100 comparing to the affection of non-linear absorption of the film, whose detail calculation progress will not be given here,

And the theoretical results are larger than the experimental results, and the reaction is quicker (optimal exposures are about 590 mJ/cm2 and 430mJ/cm2 respectively in experimental results and theoretical results), this may be caused by: (1) the sample is not homogeneous, the density is different at different area; (2) the incidence angles of beams

the experiment are about 8.2º, so the intensities of them on the sample plane will be cos

0.9898 times of the values used in calculation; (3) the photoreaction rate constants used in

And it can be seen that no matter during the ordinary holograph recording process in photochromic media, or during the polarization holograph recording process in photoinduced anisotropy media, there exits an overshooting peak in the diffraction efficiency, which then decays to a lower permanent level or also to zero. From the theoretical analyses, it can be deduced that is caused by the diminishing of fringe contrast mainly caused by the nonlinear saturation effects of photoisomerization process and photo-induced anisotropy process. In experiment, there also exit the diminishing of fringe contrast caused by a photochemically active readout beam and unequal intensities of object and reference waves. It can be theoretically calculated that the effects of them show very smaller than that of the

Suppose that the holographic recording is a linearity recording, from the spectra of Δ*A*, Δ*n*, Δ*A*D and Δ*n*B, shown in Fig.3b and Fig.4b, using the DE formulas of different kinds

polarization recording holograms in Fulgide/PMMA film written by Gaussian beams: (a)

Fig. 16. The diffraction efficiency kinetics curves comparison of different kinds of

). It is because that, in calculation the affection of *C*

where it also can be deduced that the reading beam affection is larger in *C*

condition.

Experimentally measured results; (b) Theoretically calculated results

orthogonal linearly polarization reconstruction condition ( *C*

**0.0**

**0.5**

**1.0**

4

2

3

1

η+1 / %

**1.5**

**0 20 40 60 80**

*t* / s

), the diffraction

is not considered, which

 ⊥ *O* // *R* 

> θin

> > θ ≈

 ⊥ *O* // *R* 

**2.0** 1 parallel linearlly

2 orthogonal linearly 3 parallel circularly 4 orthogonal circularly

**0 20 40 60 80**

*t* / s

1 parallel linearlly 2 orthogonal linearly 3 parallel circularly 4 orthogonal circularly

**0.0 0.2 0.4 0.6 0.8 1.0**

condition than in *C*

 // *O* // *R* 

calculation is a little bit larger than real ones.

nonlinear saturation effects, which will not be given here.

**3.2 The DE spectra of different kinds of holograms in fulgide film** 

η +1 / %

(*C* // *O* // *R* 

4

2

3

1

is also same with that of parallel linearly polarization hologram for circularly polarized *I*C. In orthogonal linearly polarization recording hologram, the DE curves are also same with each other for any kinds of polarized *I*C, which is shown in Fig.16b. For orthogonal circularly polarization recording hologram, when *I*C has same polarization state with *I*R ( *O* ⊥ *R* //*C* ), the DE curve is shown in Fig.16b; when *I*C has orthogonal polarization state with *I*R, the DE is zero; when *I*C is linearly polarized, the DE is half of that in *O* ⊥ *R* //*C* condition. The theoretical DE kinetics curves of different kinds of polarization recording holograms for *I*<sup>C</sup> has same polarization state with *I*R, are compared in Fig.16b.

Fig. 15. The diffraction efficiency kinetics curves comparison of different kinds of polarization recording and different kinds of reading in Fulgide film: (a) Experimental results (ER) of parallel linearly polarization recording (b) theoretically calculated curves of parallel linearly polarization hologram (c) ER of Parallel circularly polarization recording; (d) ER of Orthogonal linearly polarization recording; (e) ER of Orthogonal circularly polarization recording

is also same with that of parallel linearly polarization hologram for circularly polarized *I*C. In orthogonal linearly polarization recording hologram, the DE curves are also same with each other for any kinds of polarized *I*C, which is shown in Fig.16b. For orthogonal circularly

the DE curve is shown in Fig.16b; when *I*C has orthogonal polarization state with *I*R, the DE

theoretical DE kinetics curves of different kinds of polarization recording holograms for *I*<sup>C</sup>

**<sup>0</sup> 10000 20000 30000 40000 0.0**

 parallel linearly reading orthoganal linearly reading circularly reading

Exposure / mJcm-2

**0.0 0.1 0.2 0.3 0.4 0.5**

η+1 / %

(d) (e)

Fig. 15. The diffraction efficiency kinetics curves comparison of different kinds of polarization recording and different kinds of reading in Fulgide film: (a) Experimental results (ER) of parallel linearly polarization recording (b) theoretically calculated curves of parallel linearly polarization hologram (c) ER of Parallel circularly polarization recording; (d) ER of Orthogonal linearly polarization recording; (e) ER of Orthogonal circularly

4 3 1

2

 ⊥ *R* //*C* ),

1 vertical linearly reading 2 horizontal linearly reading 3 right circularly reading 4 left circularly reading

**0 20 40 60 80**

*t* / s

condition. The

 ⊥ *R* //*C* 

**0.0 0.2 0.4 0.6 0.8 1.0** 4 3 2 1

**0 20 40 60 80**

1 right circularly reading 2 left circularly reading 3 vertical linearly reading 4 horizontal linearly reading

*t* / s

η +1 / %

polarization recording hologram, when *I*C has same polarization state with *I*R ( *O*

is zero; when *I*C is linearly polarized, the DE is half of that in *O*

**0.2 0.4 0.6 0.8 1.0 1.2 1.4**

(a) (b) (c)

η A1 / %

**0 20 40 60 80**

*t* / s

<sup>2</sup><sup>1</sup> 1 vertical linearly reading 2 horizontal linearly reading 3 right circularly reading 4 left circularly reading

has same polarization state with *I*R, are compared in Fig.16b.

**0 20 40 60 80**

1 vertical linearly reading 2 horizontal linearly reading 3 right circularly reading 4 left circularly reading

*t* / s

**0.00**

polarization recording

**0.05**

**0.10**

η+1 / %

**0.15** 4 3

**0.0 0.2 0.4 0.6 0.8 1.0**

η+1 / %

4 3 2

1

Fig. 16. The diffraction efficiency kinetics curves comparison of different kinds of polarization recording holograms in Fulgide/PMMA film written by Gaussian beams: (a) Experimentally measured results; (b) Theoretically calculated results

It can be seen that the maximum values' ratio of the measured values is basically coincide with the theoretically analyzed one. Only for parallel linearly polarization hologram, in the orthogonal linearly polarization reconstruction condition ( *C* ⊥ *O* // *R* ), the diffraction efficiency is lower than parallel linearly polarization reconstruction condition (*C* // *O* // *R* ). It is because that, in calculation the affection of *C* is not considered, which can be proved to be very small when the *I*C=*I*O/100=*I*R/100 comparing to the affection of non-linear absorption of the film, whose detail calculation progress will not be given here, where it also can be deduced that the reading beam affection is larger in *C* ⊥ *O* // *R* condition than in *C* // *O* // *R* condition.

And the theoretical results are larger than the experimental results, and the reaction is quicker (optimal exposures are about 590 mJ/cm2 and 430mJ/cm2 respectively in experimental results and theoretical results), this may be caused by: (1) the sample is not homogeneous, the density is different at different area; (2) the incidence angles of beams θ in the experiment are about 8.2º, so the intensities of them on the sample plane will be cosθ ≈ 0.9898 times of the values used in calculation; (3) the photoreaction rate constants used in calculation is a little bit larger than real ones.

And it can be seen that no matter during the ordinary holograph recording process in photochromic media, or during the polarization holograph recording process in photoinduced anisotropy media, there exits an overshooting peak in the diffraction efficiency, which then decays to a lower permanent level or also to zero. From the theoretical analyses, it can be deduced that is caused by the diminishing of fringe contrast mainly caused by the nonlinear saturation effects of photoisomerization process and photo-induced anisotropy process. In experiment, there also exit the diminishing of fringe contrast caused by a photochemically active readout beam and unequal intensities of object and reference waves. It can be theoretically calculated that the effects of them show very smaller than that of the nonlinear saturation effects, which will not be given here.

#### **3.2 The DE spectra of different kinds of holograms in fulgide film**

Suppose that the holographic recording is a linearity recording, from the spectra of Δ*A*, Δ*n*, Δ*A*D and Δ*n*B, shown in Fig.3b and Fig.4b, using the DE formulas of different kinds

Holographic Image Storage with a 3-Indoly-Benzylfulgimide/PMMA Film 159

effect of the polarization of the auxiliary light is very small, the regular is: in the parallel linearly polarized recording the diffraction efficiency is slightly higher when the auxiliary light is parallel to the polarization state of the recording lights than orthogonally polarized; in the orthogonal circularly polarized recording the polarization direction of the auxiliary light nearly have no effects. So we choose the vertical polarization for the LD laser in the below experiments.

Under the irradiation of different intensity auxiliary lights, the diffraction efficiencies of two kinds of holograms were measured in real time. The kinetics curves of the diffraction efficiencies of parallel linearly polarized recording and the maximum values and stable values contrasts are shown in Fig.18(a,b), and the results in orthogonal circularly polarized

(c) (d) Fig. 18. (a,b): Diffraction efficiencies of parallel linearly polarization holograph under the irradiation of different intensity auxiliary lights: (a) kinetics curves; (b) maximum values

and stable values compare. **(c,d):** Diffraction efficiencies of orthogonal circularly polarization holograph under the irradiation of different intensity auxiliary lights: (c)

**0.00**

**0.04**

**0.08**

η **/ %**

**0.12**

(a) (b)

**0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40**

η **/ %**

**0 50 100 150 200 250 300**

**405nm / mW/cm2**

**0.16** Orthogonal circularly recording

**0 50 100 150 200 250 300**

*<sup>I</sup>***405nm / mW/cm2**

*I*

Parallel linearly recording

 DE max DE fix

 DE max DE fix

**3.3.2 Measurement of the effect of the intensity of the auxiliary light** 

 *<sup>I</sup>***405= 0 mW/cm<sup>2</sup>** *<sup>I</sup>***405= 66.7 mW/cm<sup>2</sup>** *<sup>I</sup>***405= 133.3 mW/cm<sup>2</sup>** *<sup>I</sup>***405= 233.3 mW/cm<sup>2</sup>** *<sup>I</sup>***405= 300 mW/cm<sup>2</sup>**

 *<sup>I</sup>***405= 0 mW/cm<sup>2</sup>** *<sup>I</sup>***405= 66.7 mW/cm<sup>2</sup>** *<sup>I</sup>***405= 133.3 mW/cm<sup>2</sup>** *<sup>I</sup>***405= 233.3 mW/cm<sup>2</sup>** *<sup>I</sup>***405= 600.3 mW/cm<sup>2</sup>**

recording are shown in Fig.18(c,d).

Parallel linearly +UV recording

**0.0**

**0.00**

**0.05**

5 4 3

2 1

**0.10**

η **/ %**

**0.15**

**0.1**

**0.2**

η **/ %**

**0.3**

1

**0.4**

**0 20 40 60 80**

**t / s**

Orthogonal circularly + UV recording

**0 20 40 60 80**

**t / s**

kinetics curves; (d) maximum values and stable values compare

of polarization holographies, shown in table 1, the DE spectra of the ordinary holograms (i.e. parallel circularly polarization recording) and polarization holograms (orthogonal linearly and orthogonal circularly polarization recording) can be calculated, which are shown in Fig.17, where the solid line, dot dash lines and dash lines indicate the ηtotal, ηA and ηP respectively. It can be seen that at 450nm and 700nm the diffraction efficiencies are higher, while the absorption is very small, where the recorded information can be read out without any photochromic reaction, i.e. the non-destructive reconstruction can be realized.

Fig. 17. Diffraction efficiency spectra of the (a) Ordinary holography; (b) Orthogonal linearly polarization holography; (c) Orthogonal circularly polarization holography

#### **3.3 Effects of auxiliary light and object reference ratio to DE of Fulgide film**

No matter during the ordinary holograph recording process in photochromic media, or during the polarization holograph recording process in photo-induced anisotropy media, there exits an overshooting peak in the diffraction efficiency, which then decays to a lower permanent level or also to zero, because of the diminishing of fringe contrast caused by a photochemically active readout beam, unequal intensities of object and reference waves and the nonlinear saturation effects of photoisomerization process and photo-induced anisotropy process. It is known that in ordinary holographic recording, this decreasing process can be eliminated by illuminating the hologram with a uniform control beam that has the effect of molecular backconversion photochrome [8]. It was found that in polarization holographic recording, this method also can be used [8]. In this section experiments done with an ordinary hologram and a polarization hologram recorded in a 3-indoly-benzylfulgimide/PMMA film at 633 nm have shown that a control beam at 405 nm can increase the stable-state diffraction efficiency, thus, allowing to decrease the rigorous requirements on the recording time, the object reference ratio and the reading beam intensity in the holographic recording. The affections of object reference ratio (ORR) to DE of different holograms recorded in 3-indoly-benzylfulgimide/PMMA film were also measured.

The optical set up shown in Fig.14 is also used in this experiment. Here recording with linearly polarized beams with identical states of polarization (scalar hologram) and recording with circularly polarized beams with orthogonal states of polarization (polarization hologram) were studied. The holograms were reconstructed by light with the same polarization as the reference light.

#### **3.3.1 Measurement of the effect of polarization of the auxiliary light**

First the effect of the state of polarization of the auxiliary light was studied. The diffraction efficiencies of two different kinds of holographs were measured when the 405nm auxiliary beam is vertical and horizontal linearly polarized respectively. And the results show that the

of polarization holographies, shown in table 1, the DE spectra of the ordinary holograms (i.e. parallel circularly polarization recording) and polarization holograms (orthogonal linearly and orthogonal circularly polarization recording) can be calculated, which are shown in Fig.17,

can be seen that at 450nm and 700nm the diffraction efficiencies are higher, while the absorption is very small, where the recorded information can be read out without any

**500 600 700 800**

λ / nm

Fig. 17. Diffraction efficiency spectra of the (a) Ordinary holography; (b) Orthogonal linearly

No matter during the ordinary holograph recording process in photochromic media, or during the polarization holograph recording process in photo-induced anisotropy media, there exits an overshooting peak in the diffraction efficiency, which then decays to a lower permanent level or also to zero, because of the diminishing of fringe contrast caused by a photochemically active readout beam, unequal intensities of object and reference waves and the nonlinear saturation effects of photoisomerization process and photo-induced anisotropy process. It is known that in ordinary holographic recording, this decreasing process can be eliminated by illuminating the hologram with a uniform control beam that has the effect of molecular backconversion photochrome [8]. It was found that in polarization holographic recording, this method also can be used [8]. In this section experiments done with an ordinary hologram and a polarization hologram recorded in a 3-indoly-benzylfulgimide/PMMA film at 633 nm have shown that a control beam at 405 nm can increase the stable-state diffraction efficiency, thus, allowing to decrease the rigorous requirements on the recording time, the object reference ratio and the reading beam intensity in the holographic recording. The affections of object reference ratio (ORR) to DE of different holograms recorded in 3-indoly-benzylfulgimide/PMMA film

The optical set up shown in Fig.14 is also used in this experiment. Here recording with linearly polarized beams with identical states of polarization (scalar hologram) and recording with circularly polarized beams with orthogonal states of polarization (polarization hologram) were studied. The holograms were reconstructed by light with the

First the effect of the state of polarization of the auxiliary light was studied. The diffraction efficiencies of two different kinds of holographs were measured when the 405nm auxiliary beam is vertical and horizontal linearly polarized respectively. And the results show that the

**3.3.1 Measurement of the effect of polarization of the auxiliary light** 

(a) (b) (c)

**3.3 Effects of auxiliary light and object reference ratio to DE of Fulgide film** 

polarization holography; (c) Orthogonal circularly polarization holography

ηtotal, ηA and η

> **0.000 0.001 0.002 0.003 0.004 0.005**

η+1 / %

η A η R ηtotal

633nm

P respectively. It

633nm

η A η R ηtotal

**500 600 700 800**

λ / nm

where the solid line, dot dash lines and dash lines indicate the

**0.0000**

**0.0004**

η+1 / %

**0.0008**

**0.0012**

633nm

**300 400 500 600 700 800**

λ / nm

η A ηP ηtotal

were also measured.

same polarization as the reference light.

η+1 / %

photochromic reaction, i.e. the non-destructive reconstruction can be realized.

effect of the polarization of the auxiliary light is very small, the regular is: in the parallel linearly polarized recording the diffraction efficiency is slightly higher when the auxiliary light is parallel to the polarization state of the recording lights than orthogonally polarized; in the orthogonal circularly polarized recording the polarization direction of the auxiliary light nearly have no effects. So we choose the vertical polarization for the LD laser in the below experiments.

#### **3.3.2 Measurement of the effect of the intensity of the auxiliary light**

Under the irradiation of different intensity auxiliary lights, the diffraction efficiencies of two kinds of holograms were measured in real time. The kinetics curves of the diffraction efficiencies of parallel linearly polarized recording and the maximum values and stable values contrasts are shown in Fig.18(a,b), and the results in orthogonal circularly polarized recording are shown in Fig.18(c,d).

Fig. 18. (a,b): Diffraction efficiencies of parallel linearly polarization holograph under the irradiation of different intensity auxiliary lights: (a) kinetics curves; (b) maximum values and stable values compare. **(c,d):** Diffraction efficiencies of orthogonal circularly polarization holograph under the irradiation of different intensity auxiliary lights: (c) kinetics curves; (d) maximum values and stable values compare

Holographic Image Storage with a 3-Indoly-Benzylfulgimide/PMMA Film 161

40° with a uniform speed. The diffracted light was real time detected by a digital power meter and a digital oscilloscope. Measurement result is shown in Fig.22 as solid line, and the

1 1:1 2 1:0.9 3 1:0.3 4 1:0.05

(a) (b)

Fig. 19. The diffraction efficiency of parallel linearly polarization holograph recording at different ORR: (a) kinetic curves (*without auxiliary light*); (b) maximum values compare (*without auxiliary light*); (c) kinetic curves(*with auxiliary light*); (b) maximum values and stable

(c) (d)

**0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16**

η **/ %**

**0.0**

**0.1**

**0.2**

η **/ %**

**0.3**

**0.4**

**012345**

Parallel linearly + UV recording

 DE max DE fix

*I* ref / *I*obj

**<sup>012345</sup> 0.00**

*I* ref / *I*obj

Parallel linearly recording

dashed line is its theoretical fitting curve by the formula *y*=*y*0⋅sinc2(a*x*).

**0 20 40 60 80**

**0 20 40 60 80**

**t / s**

1 1:1 2 1:0.9 3 1:0.3 4 1:0.05

**t / s**

Parallel linearly + UV recording

**0.4** Parallel linearly recording

**0.0**

**0.00**

4

values compare (*with auxiliary light*)

3 2 1

**0.05**

**0.10**

η **/ %**

**0.15**

4

3

2 1

**0.1**

**0.2**

η **/ %**

**0.3**

These results show that, under the irradiation of auxiliary light, the diffraction efficiency stable-state values can be increased no matter in ordinary hologram or in polarization hologram, and there exits an optimal intensity, under which the maximum stable-state diffraction efficiency can be obtained. When *I*O=*I*R=78.6mW/cm2 and *I*C=0.786mW/cm2, in the parallel linearly recording, the optimal intensity of the purple light, is larger than 300mW/cm2; in the orthogonal circularly recording, the optimal intensity of the purple light is about 233mW/cm2. And it can be seen that, when there is no auxiliary light, the diffraction efficiency is depending on the exposure too much, there exits rigorous requirement on the recording time. But if turn on the auxiliary light, the requirement is decreased.

#### **3.3.3 Measurement of the effect of the auxiliary light on the ORR requirement**

In this experiment *I*O=78.6mW/cm2, *I*C=0.786mW/cm2 and *I*A=267mW/cm2 were used. In the conditions of with and without auxiliary light, the diffraction efficiencies of two kinds of holograms were measured at different intensities of reference light. The experimental results of parallel linearly polarization hologram and of orthogonal circularly polarization hologram are shown in Fig.19 and Fig.20 respectively. It can be seen that in all kinds of polarization hologram, no matter turn on the auxiliary light or not, the ORR has great effect on the diffraction efficiency. But in ordinary holography, with auxiliary light irradiation, the diffraction efficiency changing in the area near ORR=1:1 is much slower than without auxiliary light irradiation, so the rigorous requirement on ORR can be decreased. In the polarization holography, when the reference beam intensity is a little bit higher than that of object beam, the diffraction efficiency will be maximum, the reason is currently unclear and is a subject of further investigation.

#### **3.4 Dependence of DE on the reading beam incidence angle**

The angular selectivity of the sample (*i.e.* the dependence of DE on the reading beam incidence angle) was measured, and the experimental set-up is shown in Fig.21. A 650nm LD is used as the recording and reading light source, and a 405nm LD is used as the erasing light source. Adjustment of coherence was made by creating a Michelson interferometer. The 650nm laser was first adjusted perpendicularly to the face of the beam-splitter cube, and then the transmitted and reflected beams intersect with each other on the sample after reflected by the mirrors. The interference fringes were tuned for optimum contrast by moving one of the mirrors (M2) with a translation stage. The two writing beams are symmetrically incident on the sample (Fulgide film), whose intersection angle 2θB=10° corresponding to a grating spacing Λ = 3.73 µm, so the recorded grating is a non-incline grating, and the incidence angle of object light (signal light) and reference light are θs=- 5°and θr=5°respectively. The sample is placed on the precision rotary platform (M-062 model, PI company, Germany), and the recording point of the hologram in the sample is on the shaft of platform. So when the platform is rotated, the recording point does not move. The continuously adjustable attenuators A1~A2 are used to adjust the intensities of the waves, in this experiment *I*O= *I*R= 75mW/cm2 and *I*C=0.1875mW/cm2 (*i.e. I*O:*I*R:*I*<sup>C</sup> =400:400:1). By observing the spots on the screen behind the sample, the best exposure time can be determined (see the next Section). Before reconstruction, the sample was rotated to one direction 20°. In the reconstruction time, the platform is rotated to the opposite direction

These results show that, under the irradiation of auxiliary light, the diffraction efficiency stable-state values can be increased no matter in ordinary hologram or in polarization hologram, and there exits an optimal intensity, under which the maximum stable-state diffraction efficiency can be obtained. When *I*O=*I*R=78.6mW/cm2 and *I*C=0.786mW/cm2, in the parallel linearly recording, the optimal intensity of the purple light, is larger than 300mW/cm2; in the orthogonal circularly recording, the optimal intensity of the purple light is about 233mW/cm2. And it can be seen that, when there is no auxiliary light, the diffraction efficiency is depending on the exposure too much, there exits rigorous requirement on the recording time. But if turn on the auxiliary light, the requirement is

**3.3.3 Measurement of the effect of the auxiliary light on the ORR requirement** 

In this experiment *I*O=78.6mW/cm2, *I*C=0.786mW/cm2 and *I*A=267mW/cm2 were used. In the conditions of with and without auxiliary light, the diffraction efficiencies of two kinds of holograms were measured at different intensities of reference light. The experimental results of parallel linearly polarization hologram and of orthogonal circularly polarization hologram are shown in Fig.19 and Fig.20 respectively. It can be seen that in all kinds of polarization hologram, no matter turn on the auxiliary light or not, the ORR has great effect on the diffraction efficiency. But in ordinary holography, with auxiliary light irradiation, the diffraction efficiency changing in the area near ORR=1:1 is much slower than without auxiliary light irradiation, so the rigorous requirement on ORR can be decreased. In the polarization holography, when the reference beam intensity is a little bit higher than that of object beam, the diffraction efficiency will be maximum, the reason is currently unclear and

The angular selectivity of the sample (*i.e.* the dependence of DE on the reading beam incidence angle) was measured, and the experimental set-up is shown in Fig.21. A 650nm LD is used as the recording and reading light source, and a 405nm LD is used as the erasing light source. Adjustment of coherence was made by creating a Michelson interferometer. The 650nm laser was first adjusted perpendicularly to the face of the beam-splitter cube, and then the transmitted and reflected beams intersect with each other on the sample after reflected by the mirrors. The interference fringes were tuned for optimum contrast by moving one of the mirrors (M2) with a translation stage. The two writing beams are symmetrically incident on the sample (Fulgide film), whose intersection angle 2θB=10° corresponding to a grating spacing Λ = 3.73 µm, so the recorded grating is a non-incline grating, and the incidence angle of object light (signal light) and reference light are

r=5°respectively. The sample is placed on the precision rotary platform (M-062

model, PI company, Germany), and the recording point of the hologram in the sample is on the shaft of platform. So when the platform is rotated, the recording point does not move. The continuously adjustable attenuators A1~A2 are used to adjust the intensities of the waves, in this experiment *I*O= *I*R= 75mW/cm2 and *I*C=0.1875mW/cm2 (*i.e. I*O:*I*R:*I*<sup>C</sup> =400:400:1). By observing the spots on the screen behind the sample, the best exposure time can be determined (see the next Section). Before reconstruction, the sample was rotated to one direction 20°. In the reconstruction time, the platform is rotated to the opposite direction

θs=-

decreased.

5°and θ

is a subject of further investigation.

**3.4 Dependence of DE on the reading beam incidence angle** 

40° with a uniform speed. The diffracted light was real time detected by a digital power meter and a digital oscilloscope. Measurement result is shown in Fig.22 as solid line, and the dashed line is its theoretical fitting curve by the formula *y*=*y*0⋅sinc2(a*x*).

Fig. 19. The diffraction efficiency of parallel linearly polarization holograph recording at different ORR: (a) kinetic curves (*without auxiliary light*); (b) maximum values compare (*without auxiliary light*); (c) kinetic curves(*with auxiliary light*); (b) maximum values and stable values compare (*with auxiliary light*)

Holographic Image Storage with a 3-Indoly-Benzylfulgimide/PMMA Film 163

 Experimental result Fitting curve

**-40 -30 -20 -10 0 10 20 30 40**

θ / degree

From the Fig.22, it can be seen that the angular selectivity ΔΘmeasured=38°, if it is taken as the angle between the first minimum diffractions at both sides of the fitting curve's peak. But Side lobes do not exist in the experimental curve, so the angular selectivity can be taken as

2 2 2 cos

650nm the refractive index difference between E-form and C-form is Δ*n*≈1.84×10-2, so it can

It can be seen from the experimental value of angular selectivity is greater than the calculated value. One of the reasons is that, for this calculation the incident beam is considered as an infinite plane wave, however, the spot diameter is small in actual storage experiment. According to diffraction theory, the limited size of the beam would inevitably

In the reflection type holographic image storage (where the *I*c is used as reference beam, shown in section 4.1), the angles between the two recording waves is 173°, so it is proved

From the diffraction dynamic curves, the optimum exposure can be obtained. From Fig.15 it can be seen that he optimum exposure of the Fulgide is 590mJ/cm2, the similar data also can be obtained from the changing of the diffracted spots' pattern, details please see in Reference [9].

that this sample can store the gratings with spatial frequency of 6300lines/mm.

Fig. 22. The curves showing the DE dependence on the reading beam incidence angle

the angular width at the 1/10 of the maximum diffraction efficiency, ΔΘ10dB=30°.

π νλ

*nd r s* . In this experiment, the

lead to an angle broadening, so the measured curves are broadened.

**3.5 The spatial resolution measurement of fulgide film** 

**3.6 Measurement of the optimum exposure of the sample** 

π

In theory[10], the angular selectivity of the gratings ΔΘcalculated is:

ΔΘmeasured

ΔΘ10dB

**15**


m, the refractive index of the sample is about *n*≈1.5, and for this sample at

 θθ

θ

<sup>−</sup> ΔΘ = − (1)

λ

=650nm, the thickness of the

*nd r s*

*y*=*y*<sup>0</sup> ¡Ásinc2 (a*x*)

fitting with

**19**

**0**

**1**

η max / 10

**2**

diffracted intensity / a. u.

Where /( cos cos )

be calculated that ΔΘcalculated≈27.3°.

μ

 λ  θθ

ν = Δπ

sample is *d*=10

**3**

**4**

Fig. 20. The diffraction efficiency of orthogonal circularly polarization holograph recording at different ORR: (a) Kinetic curves(*without auxiliary light*); (b) maximum values and stable values compare(*without auxiliary light*); (c) Kinetic curves (*with auxiliary light*); (b) stable values compare (*maximum value basically same with the stable values, with auxiliary light*)

Fig. 21. The measurement set-up of angular selectivity of the sample

**0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14**

> **0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045**

η **/ %**

η **/ %**

(a) (b)

(c) (d) Fig. 20. The diffraction efficiency of orthogonal circularly polarization holograph recording at different ORR: (a) Kinetic curves(*without auxiliary light*); (b) maximum values and stable values compare(*without auxiliary light*); (c) Kinetic curves (*with auxiliary light*); (b) stable values compare (*maximum value basically same with the stable values, with auxiliary light*)

1 1:1 2 1:0.9 3 1:0.3 4 1:0.05 **0123**

*I* ref / *I*obj

**01234**

*I***ref /** *I***obj**

**0.16** Orthogonal circularly recording

 DE max DE fix

**0 20 40 60 80**

**0 20 40 60 80**

Fig. 21. The measurement set-up of angular selectivity of the sample

3 2

**t / s**

4

1 1:1 2 1:0.9 3 1:0.3 4 1:0.05

**t / s**

**0.07** Orthogonal circularly + UV recording

**0.15** Orthogonal circularly recording

**0.00**

**-0.01 0.00 0.01 0.02 0.03 0.04 0.05 0.06**

η **/ %**

**0.05**

4

1

3

2 1

**0.10**

η **/ %**

Fig. 22. The curves showing the DE dependence on the reading beam incidence angle

From the Fig.22, it can be seen that the angular selectivity ΔΘmeasured=38°, if it is taken as the angle between the first minimum diffractions at both sides of the fitting curve's peak. But Side lobes do not exist in the experimental curve, so the angular selectivity can be taken as the angular width at the 1/10 of the maximum diffraction efficiency, ΔΘ10dB=30°. In theory[10], the angular selectivity of the gratings ΔΘcalculated is:

$$
\Delta\Theta = \frac{2\sqrt{\pi^2 - \nu^2}\lambda}{\pi nd} \frac{\cos\theta\_s}{|\sin(\theta\_r - \theta\_s)|}\tag{1}
$$

Where /( cos cos ) ν = Δπ λ θθ *nd r s* . In this experiment, the λ=650nm, the thickness of the sample is *d*=10μm, the refractive index of the sample is about *n*≈1.5, and for this sample at 650nm the refractive index difference between E-form and C-form is Δ*n*≈1.84×10-2, so it can be calculated that ΔΘcalculated≈27.3°.

It can be seen from the experimental value of angular selectivity is greater than the calculated value. One of the reasons is that, for this calculation the incident beam is considered as an infinite plane wave, however, the spot diameter is small in actual storage experiment. According to diffraction theory, the limited size of the beam would inevitably lead to an angle broadening, so the measured curves are broadened.

#### **3.5 The spatial resolution measurement of fulgide film**

In the reflection type holographic image storage (where the *I*c is used as reference beam, shown in section 4.1), the angles between the two recording waves is 173°, so it is proved that this sample can store the gratings with spatial frequency of 6300lines/mm.

#### **3.6 Measurement of the optimum exposure of the sample**

From the diffraction dynamic curves, the optimum exposure can be obtained. From Fig.15 it can be seen that he optimum exposure of the Fulgide is 590mJ/cm2, the similar data also can be obtained from the changing of the diffracted spots' pattern, details please see in Reference [9].

Holographic Image Storage with a 3-Indoly-Benzylfulgimide/PMMA Film 165

continuously adjustable attenuators A1, A2 and A3 are used to adjust the intensities of *I*O, *I*<sup>R</sup> and *I*C. Polarizer P1, P2 and P3 are used to adjust the polarization states of *I*O, *I*R and *I*C. Polarizer P4 in front of CCD is used to filter scattered light. For the ordinary holographic storage the Polarizers are not used. The intensities of object wave and reference wave are both 6mW/cm2 in experiment. The angles between the two recording waves are 7° and 173°. The intensity of reconstruction beam is 60μW/cm2. And the diameter of hologram is about

In Fig.24, the phase conjugated beam is used as reconstruction beam. *I*R and *I*C are conjugate with each other. For transmission type holographic recording, reference and reconstruction beams are *I*R and *I*C respectively, which are exchanged with each other, for reflection type holographic recording experiment. Diffracted beam *I*D is conjugated with object beam *I*O. After being reflected by BS3, the diffracted image can be detected by CCD real-timely. Other

In Fig.25, the reconstructed images of parallel linearly polarized transmission type hologram

(a) (b) Fig. 25. The reconstructed images of parallel linear polarized transmission hologram and reflection hologram: (a) reconstructed image of transmission recording hologram; (b)

It can be seen that: compared with transmission-type hologram, reflection-type hologram has higher SNR. This is because that the noise in the reconstructed image of transmissiontype hologram is come from the forward scattering, but that of reflection-type holographic recording hologram is come from the backward scattering. Usually the forward scattering is always larger than backward scattering, so the reflection-type hologram has smaller noises.

In Fig.26, the reconstructed images of reference beam reconstruction hologram and conjugated beam reconstruction hologram are shown. It can be seen that: compared with reference beam reconstruction, the phase conjugated beam reconstruction can effectively correct the phase aberration caused by the mis-adjustment of optical setup and the real-time

conditions are same with that of set up shown in Fig.23.

reconstructed image of reflection recording hologram

But reflection-type hologram has lower diffraction efficiency.

**4.1.2.2 Reference beam reconstruction hologram and phase conjugated beam** 

detection of the changing progress of the diffraction image can be realized.

**4.1.2.1 Transmission type hologram and reflection type hologram** 

and reflection type hologram (recorded in setup shown in Fig.23) are shown.

**4.1.2 Results and discussions** 

**reconstruction hologram** 

2mm.
