Holographic grating recording in azobenzene polymer films

Optical Materials 27 (2004) 527–532 www.elsevier.com/locate/optmat

Holographic grating recording in azobenzene polymer films Jun Yang

b

a,*

, Jiangying Zhang a, Jian Liu b, Pei Wang a, Hui Ma a, Hai Ming a, Zengchang Li b, Qijin Zhang b

a Department of Physics, University of Science and Technology of China, Hefei 230026, PR China Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China

Received 5 January 2004; accepted 15 April 2004 Available online 21 October 2004

Abstract The photo-induced birefringence grating and the surface grating in azobenzene polymer films were studied. The photoinduced birefringence (about 0.02 at 633 nm) was investigated at various intensities of laser (532 nm) beam. The effect of laser-induced heating has been introduced to the processes of photo-induced reorientation in azo polymer. The holographic grating period and the peak-to-peak value of the sinusoidal surface profile are 0.8 mm and 40 nm, respectively. The maximum diffractive efficiency of the grating is 2%. The mass diffusion responsible for the formation of the surface grating due to the temperature rise is analyzed. Ó 2004 Elsevier B.V. All rights reserved. PACS: 42.40.Eq; 42.70.Jk; 42.70.Ln; 78.20.Fm Keywords: Azo polymers; Photo-induced birefringence; Holographic grating

1. Introduction Azobenzene polymers have received increasing attention due to their interesting properties and possible application in optical storage [1], optical communication [2,3], nonlinear optics [4,5], and diffractive optical elements [6]. The azo group in azobenzene polymer is one kind of photoactive group, which can change from trans to cis and from cis to trans, under the action of linear light and heat. With this character, we can illuminate azobenzene polymers by a given wavelength of linear light to make them carry out photo-alignment by trans–cis–trans isomerization cycles. Azobenzene falls perpendicularly to the direction of the electric field vector of the linear polarized light [4,7]. This reorientation induces anisotropy and the difference of refractive index *

Corresponding author: Tel.: +86 551 3603504; fax: +86 551 3603504. E-mail address: [email protected] (J. Yang). 0925-3467/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.04.013

parallel and perpendicular to the irradiating laser polarization direction. In this paper we discuss the basic photo-induced physical properties of azo films, and the effects of laser-induced heating have been introduced to the buildup of photo-induced birefringence in azobenzene-side-chain copolymer. The birefringence grating and surface grating in azo films are studied by using near-field scanning optical microscope (NSOM). The near-field transmittance image and the sinusoidal surface profile of the grating can both be obtained by NSOM at the meantime. It is shown that stable grating can be written not only by the optically induced birefringence but also by optically altering the surface profile of the films. The grating formation mechanism is analyzed by investigation of the diffraction efficiency change with irradiation time. The mass diffusion responsible for the formation of the surface grating resulting from the temperature rise at irradiated area is analyzed.

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J. Yang et al. / Optical Materials 27 (2004) 527–532 CH3

70

( C

60

C )n O

C O

Absortance

50

( CH2 ) 2

40 30

P-CN

N

20

N

10 0

CN

200

250

300

350

400

450

500

550

600

650

Wavelength (nm) Fig. 1. The UV–VIS absorption spectrum of azobenzene polymer film before irradiation and chemical structure.

2. Materials The polymers poly [2-(4-(4-cyanophenyl) diazenyl phenyloxy) ethoxyl methacrylate], which contains a common azobenzene moiety in the side chain, were used and obtained through chemical synthesis as described in Refs [16]. Its glass transition temperature (Tg) is 120 °C, n
1.68 (at 632.8 nm). The thin films are prepared by spinning dilute solutions of the azo-polymer in tetrahydrofuran onto freshly cleaned glass substrates and then remove the solvent. The thickness of the films is about 1lm. The UV–VIS absorption spectrum (the absorption of the substrate is deducted) of the film before irradiation and chemical structure are shown in Fig. 1.

laser (diode-pumped, frequency-doubled, Nd:YVO4 laser) beam at 532 nm, which polarized at 45° in relation to the polarization direction of the reading beam, and at room temperature (20 °C). The reading beam was a low power semiconductor laser at 650 nm, which passed through a pair of crossed polarizers with the sample between them. Before irradiation, the polymer film was not transparent and showed no angular-dependent transmittance. After irradiation with 532 nm linear polarized light, a clearly reorientation was obtained. The change in transmission of the reading light was measured with Newport Dual-channel Meter (Model-2832c), and data was collected with PC. Fig. 3 shows the buildup of transmission (the polymer birefringence) at various laser intensities at room temperature (20 °C). All transmission curves were normalized by maximum value. The curves for the buildup of birefringence have much difference between at high light intensities and at lower light intensities. We can find that when laser energy density is increased above a threshold, the curves for the buildup of birefringence appear a peak, and then decreased. Further more, lower light intensity can induce higher saturated birefringence. Under pulsed irradiation, photo-induced birefringence appears the same rule [18,19]. We observed the buildup and decay of the polymer birefringence in detail at various illuminating time at power of density 30 mw/cm2 (Fig. 4). The writing laser was switched off at point A. As shows in Fig. 4(d), the curve for the buildup of birefringence has a maximum value, and then decreased with the laser illumination. The different shape of decay curves between C–A and A–D shows the two processes following distinct principles. We have observed the same rule in azobenzene doped polymethyl methacrylate (azo-PMMA).

3. The photo-induced birefringence

P3

normalized transmission

The experimental setup is shown in Fig. 2. The photo-induced isomerization was induced by a cw P2

P1

polarizer (p1,p 2,p 3)

45 P2

Sample

P1 Probe Light

Power meter Beam expander P3 Computer

Pum pi

ng L i

ght

Fig. 2. The experimental setup for measuring buildup the photoinduced birefringence.

Fig. 3. Buildup the photo-induced birefringence at various laser intensity for P–CN films. All curves were normalized by their own maximum value.

J. Yang et al. / Optical Materials 27 (2004) 527–532 A

1.0

A 1.0

0.194 0.8

normalized transmission

529

0.056

0.8

0.6

0.6

0.4

0.4

0.2

0.2

a

0.0

b

0.0

0

5

10

15 T(minute)

1.0

20

25

30

0

10 15 T(minute)

20

25

30

1.0

0.074

0.8

5

0.9

0.314 A

C

0.8

A

0.7

0.6

D

0.6 0.5

0.4

0.4 0.3

0.2

c

d

0.2 0.1

0.0

0.0

0

5

10

15

20

25

30

0

5

10

15

20

25

30

35

T(minute)

T(minute)

Fig. 4. The curve for the buildup of birefringence. The writing laser was switched off at point A. (Power: 28 mw/cm2; T: observation time).

Many researches focus on the photo-alignment and theories of azobenzene side-chain polymers [4,6,7]. With continuous-wave irradiation to the samples, the curves for the buildup of birefringence were fitted with biexponential functions of the form [4]. y ¼ Að1  et=sf Þ þ Bð1  et=ss Þ;

ð1Þ

where A and B are the pre-exponential factors, t the time and ss and sf the time constants for writing of the slow and the fast processes, respectively. Biexponential functions are normally employed for representing the photoisomerization buildup process. They are related to two processes of orientation. The first is much fast and is attributed to the local movement of the azobenzene groups in the trans–cis–trans photoisomerization. Such a fast process depends on the size of the azobenzene group, on the local free volume and on the polymer chain character. The slow process is associated with the movement on the azobenzene groups linked to the polymeric chain. However, the curves for the buildup of birefringence (shown in Figs. 3, 4) could not be fitted with biexponential functions with high energy and cw irradiation to the sample for long time. The reason is that heating effects were neglected in biexponential functions. Azobenzene polymer liquid crystals can be aligned by light under the glass transition temperature (Tg) and an important factor is thought to be the heat produced by the illumination of the laser [8]. The heat causes the temperature within the polymer to rise high enough to reach Tg. Furthermore, the azobenzene groups can reorient and be perpendicular to the electric field vector of laser. It can be assumed that the photo-alignment is such a dynamic process: when the azobenzene polymer is illuminated by

a polarized laser, the probability of absorbing a photon by an azobenzene group is related to cos2, where is the angle between the dipole of the azobenzene group and the electric vector of illuminating light. The azobenzene group that has absorbed the photon changes from the trans state to the cis state and then back from the cis state to the trans state. The absorbed energy of light turns into heat energy, which makes the temperature of the film rise. Once the temperature reaches Tg, the polymerÕs chain-segment can move freely. The azobenzene groups begin to align by the effect of light. The probability of absorbing a photon decreases and the thermal energy, which is transformed from the energy of light, also gets smaller [9]. When the azo groups are orthogonal to the electric vector of illuminating light, the azo groups no longer absorb photons. Photoisomerization cycles results in a net dichroism and birefringence induced in the material due to the absorbance and refractive index difference in the parallel and perpendicular direction (to polarization of light). Many experiments have showed the obvious heating effects [17,19], and it is found that the heat produced in the irradiated region of sample can lead to a temperature rise as high as over 40 °C by academic simulation [17,20]. The results that we have got can be explained by local thermal heating of the polymer caused by laser. With the increase of temperature, the mobility of the mesogens for angular motion as well as cis-trans isomerization rate of azobenzene groups increases. Which results in faster reorientation with increased values of the order parameter. At the same time, however, with the increase in temperature, the thermal movement can take place more easily. These contradictory factors may be approximately described as the Gauss-shaped profiles between

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J. Yang et al. / Optical Materials 27 (2004) 527–532

order parameter and temperature [7]. When achieving thermal balance, the birefringence of polymer films reaches stabilization. The analysis given above can better explain the change in the transmission of the reading light. We consider there are three processes of orientation. The first process is attributed to the local movement of the azobenzene groups in the trans-cis-trans photoisomerization. The second process is associated with the movement of the azobenzene groups linked to the polymeric chain [6,7]. We introduce the third process that is thermal movement caused by heating effects. The curves for the buildup of birefringence can be approximately fitted with biexponential curves and Gaussian curves, y ¼ Að1  et=sf Þ þ Bð1  et=ss Þ þ Cðeðtt0 Þ

2

=s2t

 1Þ

ð2Þ

where the sum of A, B and C are proportional to attribution to photo-induced isomerization in the three processes, respectively. sf, ss and st are the time constants for writing. The experimental curves and the fitted curves are shown in Fig. 5, which shows B–G (biexponential and Gaussian) function is good fitting for the curves for the buildup of birefringence. The fitting parameters are list in Table 1. With increasing laser intensity, sf and ss decreased, and namely, the mobility of the molecules in-

creased and the process quickened. The result was consistent with the known facts. Negative B means the slow process brings polymer to isotropy at high temperature t0 represents the speed of heating and st represents the magnitude of the heat effect. These parameters relate to laser intensity and the thermal characters of the material. Only when the large laser-induced temperature distribution in azobenzene was substantially established, would the effects of heat be obvious and be clearly observed.

4. Laser-induced grating Measurements of diffraction efficiency of transmission holograms are carried out using the system described in Fig. 6. The gratings were recorded by twobeam interference with polarization. A 532 nm Nd:YVO4 double frequency laser is chosen as the writing light source. The laser beam is expanded and split into two beams. The two beams interfere in the recording plane where the film is placed. The angular separation of the two writing beams is about 40°. A 650 nm diode laser is used as probe light, before which a polarizer is placed to adjust its polarization direction. The power of the probe light is 2 mw. Behind the film, a filter is arranged to block the recording light and only let the probe light pass through. The first-order diffraction intensity is collected by a detector, which is connected to a computer by A/D converter. To study the formation mechanism of the grating, we measured the first-order diffraction efficiencies during irradiation at writing power density 36.4 mW/cm2, 65.0 mW/cm2 and 124.8 mW/cm2. The three curves have similar profile (Fig. 7). Parts of the recorded grating are investigated under near-field scanning optical microscope.

532nm:YVO4 Double

Mirror

Frequency laser Polarizer Fig. 5. The comparison between the experimental curves (dot) and the fitting curves (solid).

Computer

Table 1 Parameters obtained by fitting the birefringence data Pump power (mW/cm2)

A

40.2 30.0 25.2 20.0

1.304 1.046 0.77 0.654

B

C

sf

ss

650nm Diode Laser

Polarizer st

t0

Filter detector

0.431 0.306 0.315 0.214

0.146 0.122 0.070 0.058

40.33 52.60 69.54 70.38

251.26 352.12 348.60 320.00

35.24 53.14 54.00 57.89

73.94 96.99 114.06 117.35

Fig. 6. The experimental setup for creating holographic grating and measuring diffraction efficiency of grating changed with time.

J. Yang et al. / Optical Materials 27 (2004) 527–532

Fig. 7. The measured first order diffraction efficiency of the grating changed with the irradiation time.

The near-field transmittance image, as shown in Fig. 8(a), indicates the photo-induced phase distribution of the photo-induced grating. The thickness change is far more less than the total thickness of the film. The distribution of the refractive index is due to the photo-induced birefringence. Before irradiation, the index is uniform at the whole area of the film. With the generation of birefringence due to alignment of the azobenzene moieties, an ellipsoid of refractive index is formed. So, with the polarization of the probe light, the first order diffraction efficiency is shown in Fig. 9. Here the angle a is defined as the angle between the polarization directions of the recording and the probe light. The diffractive efficiency can reach maximum value 0.8%. This measurement was taken when grating was recorded at power density 124.8 mW/cm2 for only 10 s, no surface

531

Fig. 9. The first-order diffraction efficiency of the grating changed with the polarization direction of the probe light.

profile formed yet. Here the slight index change caused by the density of the molecule, is neglectable. The sinusoidal surface profile of the grating (124.8 mW/cm2 for 2000 s) is shown in Fig. 8(b). This means a surface grating was formed [10–15]. The grating period and the peak-to-peak value of the sinusoidal profile are about 0.8 lm and 40 nm, respectively. The surface profile is related to the light intensity in a very complex way. The intensity distribution results in a temperature distribution which is as high as 40 °C [17,20]. The mass of the polymer at high intensity area expands, diffuses and deposits above the low intensity area due to the increase of internal stress. For a sinusoidal varying intensity, this would result in a sinusoidal height profile with the intensity distribution. The notch forms at the high intensity area and heave occurs at low intensity

Fig. 8. (a) The transmittance image of the grating scanned by NSOM, the bright area indicates high transmittance. (b) The sinusoidal surface profile of the grating by NSOM, the notch forms at the high intensity area and heave at low intensity area.

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J. Yang et al. / Optical Materials 27 (2004) 527–532

area, which corresponds with the dark and bright lines in Fig. 8 (b), respectively. The above analysis is in coincidence with the diffractive efficiency of the grating measured in Fig. 7. The first order diffractive efficiency increasing with irradiation time at the beginning is due to the birefringence grating caused by photo-induced alignment of azobenzene moieties, which is polarization sensitive. After the first peak, the first order diffractive efficiency decreases because the surface profile formed. The surface grating is 180° outof-phase with the birefringence grating due to the temperature distribution at the film. A few minutes later, with the amplitude of the surface profile grows, it increases again and then reaches to the stable values. The temperature at the sample gets balanced with the environment around. The transition from the birefringence grating to the surface grating is the reason for the decrease in diffraction efficiency after first peak in Fig. 7. The noise in Fig. 7 comes from the slight intensity undulation of the irradiation light, azoÕs heat motion and the thickness difference of the film. The stable values of diffractive efficiency grow with the writing power density. At 36.4 mW/cm2 and 65.0 mW/cm2, the total effect of the refractive grating and surface grating is lower than that caused by only the birefringence grating at the first peak. With the writing intensity increases to 124.8 mW/cm2, the temperature rise, the effect of surface grating becomes larger than that of the refractive grating. After the writing light cut off, the diffraction efficiencies of the gratings decrease a little and then keep stable values, which vary with the writing energy density, as indicated by the arrows in Fig. 7. The little decline results from the rapid decrease of the temperature at the moment when irradiation is cut off. The maximum diffractive efficiency in our experiment is 2%.

5. Summary The polymer poly [2-(4-(4-cyanophenyl) diazenyl phenyloxy) ethoxyl methacrylate], has been investigated, and shown a reversible trans-cis-trans isomerization in-

duced by linear visible light. We introduce the effects of laser-induced heating in the processes of reorientation and present three processes in photo-induced birefringence. Biexponential function has been modified. A hologram grating was recorded on azobenzene polymer films using two linearly polarized laser beam of various intensities. Birefringence grating and the surface grating have been studied.

Acknowledgments This subject was supported by National Science Foundation of China, No. 90201013, No. 90201016, Hi-Tech Research and Development Program of China(863), No. 2002AA313030, Natural Science Foundation of Anhui Province No. 03042402.

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