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Optics Communications 273 (2007) 435–440

Two-photon induced anisotropy in PMMA film dopedwith Disperse Red 13

C.R. Mendonca *, U.M. Neves, L. De Boni, A.A. Andrade, D.S. dos Santos Jr.,F.J. Pavinatto, S.C. Zilio, L. Misoguti, O.N. Oliveira Jr.

Instituto de Fısica de Sao Carlos, Universidade de Sao Paulo, Caixa Postal 369, 13560-970 Sao Carlos, SP, Brazil

Received 3 July 2006; received in revised form 28 December 2006; accepted 17 January 2007

Abstract

In this paper we present optical anisotropy induced by 150-fs laser pulses operating at 775 nm in poly(methyl methacrylate) (PMMA)films doped with Disperse Red 13 (DR13). Two-photon absorption of DR13 induces reversible birefringence in the sample, a featureassociated with the molecular orientation caused by isomerization of the azochromophores. No irreversible anisotropy, associated withphotobleaching of the chromophore was observed. The slope of 2 obtained for the induced birefringence as a function of the excitationirradiance corroborates the two-photon nature of the process.� 2007 Elsevier B.V. All rights reserved.

Keywords: Birefringence; Azopolymers; Two-photon absorption

1. Introduction

Optical properties of azochromophores, particularly inpolymeric structures, have been widely investigated in thelast few years owing to their potential applications basedon properties such as photo-induced birefringence, surfacerelief gratings and nonlinear optics [1]. Motivated by thespatial localization of the excitation, multi-photon absorp-tion processes, such as two-photon absorption (2PA),started to be studied as an option to induce optical proper-ties in azoaromatic compounds [2–6].

The interest in two-photon absorption processes hasincreased enormously in the last few years, leading to asearch for new organic materials with high 2PA cross-sec-tions [7–9]. This process involves the simultaneous absorp-tion of two photons, at wavelengths far from the moleculelinear absorption region, whenever the material has anelectronic excited level at twice the frequency of the inputbeam. In 2PA the rate of energy absorbed is proportional

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doi:10.1016/j.optcom.2007.01.035

* Corresponding author. Tel.: +55 16 2712012; fax: +55 16 2739511.E-mail address: crmendon@if.sc.usp.br (C.R. Mendonca).

to the square of the excitation intensity, which allows forspatial resolution in applications such as microfabrication[10,11], two-photon fluorescence imaging [12,13], two-pho-ton photodynamic cancer therapy [14,15] and three-dimen-sional (3D) optical storage [14,16–18].

Recently, 2PA induced molecular orientation of photoi-somers was demonstrated and theoretically studied [2,3].Orientation hole burning of chromophores by two-photonabsorption was reported for Disperse Red 1 in poly(methylmethacrylate) films [4,5], a irreversible process related tothe photoselective bleaching of DR1 by multi-photonabsorption anisotropic absorption.

Motivated by these results and the recent demonstrationof two-photon induced photoisomerization in azodyes[6,19], we investigate here the two-photon induced anisot-ropy in poly(methyl methacrylate) films doped with Dis-perse Red 13 an azodye whose molecular structure isdisplayed in the inset of Fig. 1. We observed a reversibleoptical birefringence induced by 150-fs laser pulses operat-ing at 775 nm, a feature associated with the molecular orien-tation caused photoisomerization of the azochromophores[1,20–22]. In the present case, the process was induced by

300 400 500 600 700 800 9000.0

0.4

0.8

1.2

N

N

O2N

N

CH3

OH

Cl

abso

rban

ce

wavelength (nm)

Fig. 1. UV–Vis spectra of DR13/PMMA film. The inset shows the DR13molecular structure.

detector

analizer

sample polarizer mirror

polarizer

mirror

mirror

mirrorwriting laser

reading laser

Fig. 2. Optical storage experimental set-up. The reading beam is a He–Nelaser at 632.8 nm. The writing beam is a low-power cw laser at 532 nm orthe pulsed Ti:sapphire laser at 775 nm, for resonant and nonresonantrecording case, respectively. Two crossed polarizers are used to read thebirefringence induced by writing beam. The polarization of recordingbeam is set 45� off to the reading beam.

436 C.R. Mendonca et al. / Optics Communications 273 (2007) 435–440

two-photon absorption excitation [6,19]. Such birefringenceexhibited a quadratic dependence on the excitation irradi-ance, confirming its two-photon nature. This process allowsthree-dimensional optical storage in bulk materials, as dem-onstrated through a volumetric optical storage experiment.

2. Experimental

Amorphous poly(methyl methacrylate) films containingDR13 (4 0-[ethylamino]-2-chloro-4-nitroazobenzene) [23]were prepared by casting the polymer-dye chloroform solu-tion on glass slides. The films were first dried at room tem-perature for 6 h and then heated at 100 �C for 12 h. Thethickness of the film used in our optical storage experi-ments was 150 lm.

[4 0[[2-(Meth-acryloyloxy) ethyl] ethylamino]-2-chloro-4-nitroazobenzene methacrylate] DR13Ma (Aldrich) wasused as obtained. Methyl methacrylate (MMA)(Aldrich)was distilled before use. The bulk polymerization was car-ried out in a glass ampoule in which 18.72 g (0.1872 mol)of MMA, 0.021 g (5.037 · 10�6 mol) of DR13Ma and0.0032 g of 2,2-azobisisobutyronitrile (AIBN) were added.The ampoule was cooled in liquid nitrogen and placed ina vacuum before sealing. The ampoule was allowed toattain room temperature and then heated to 60 �C in anoven for 2 h. The ampoule was broken and the solidmaterial was cut in cubes of 1 · 2 · 0.5 cm3 and polishedusing a wax base for polishing. The absorption spectrain the UV–Vis region were obtained with a Cary 17spectrophotometer.

The optical anisotropy was induced in the guest–hostfilm using the linearly polarized 150 fs pulses at 775 nm(writing beam), delivered by a Ti:sapphire chirped pulseamplified system operating at a 1 kHz repetition rate.The polarization angle of the writing beam was set at 45�with respect to the linear polarization orientation of theprobe beam (reading beam). A low-power He–Ne laserlight at 632.8 nm passing through crossed polarizers wasused as the reading beam to measure the induced birefrin-

gence in the sample. Fig. 2 presents a diagram of this exper-imental set-up. The optically induced birefringence can bedetermined by measuring the probe beam transmission

(T = I/I0) and using Dn ¼ kpd sin�1

ffiffiffiII0

q, where k is the wave-

length of the incident radiation, d is the film thickness, I0 isthe incident beam intensity and I is the intensity after thesecond polarizer (analyzer). When resonant excitation isemployed, the same experimental set-up is used. However,in this case a cw 532 nm Nd:YAG laser is used as the writ-ing beam.

The volumetric optical storage experiment was per-formed in the bulk PMMA/DR13 sample using the femto-second laser as the writing beam (irradiance of 25 GW/cm2). In this experiment the anisotropy was induced inthe focal plane by focusing, with a 12 cm cylindrical lens,the writing beam that passes through a 1 mm iris. With thisapproach, the focalization spot has about 1 mm of widthand 1.5 mm of depth, which helps the pattern visualization.The pattern was draw by scanning the sample using atranslation stage at a constant speed of 1.7 mm/min.

3. Results

Fig. 1 shows the UV–Vis spectrum of the DR13/PMMAguest–host film. The absorption band observed at ca.500 nm is due to the p! p* electronic transition responsi-ble for the photoisomerization process. We also observedthat the film is completely transparent in the near infraredregion.

In Fig. 3 (solid circles) it is depicted a typical result forthe time evolution of the reading beam normalized trans-mission through the analyzer placed after the poly(methylmethacrylate)/DR13 (PMMA/DR13) film. The writingbeam irradiance used in this experiment was 25 GW/cm2

(Paverage (cm2) = 1.9 W/cm2). Before the writing beam isswitched-on no probe beam transmission is observed, indi-cating random orientation of the chromophores. However,when the writing beam is switched-on at time A, transmis-sion increases as a consequence of the induced anisotropy.When the writing beam is switched off at time B, transmis-sion decreases sharply to a nearly constant value, approxi-mately 20% of the maximum value (time C), remaining

0 200 400 600 8000.0

0.4

0.8

1.2

B

C

A

tran

smitt

ed s

igna

l (ar

b. u

nits

)

time (s)

Fig. 3. Writing sequence obtained in PMMA/ DR13 guest–host film viaone-photon (open circle) and two-photon (solid circle) absorption process.

C.R. Mendonca et al. / Optics Communications 273 (2007) 435–440 437

stable for a long period (� several weeks). Fig. 3 (open cir-cles) also shows an optical storage measurement obtainedunder resonant excitation (0.1 W/cm2) using a cw Nd:YAGlaser operating at 532 nm as the writing beam.

Fig. 4 shows the quadratic dependence (slope � 2)observed for the two-photon induced birefringence as afunction of the writing beam irradiance.

The volumetric pattern (acronym of the University ofSao Paulo – USP) recorded via two-photon absorption inthe PMMA/DR13 thick sample is displayed in Fig. 5.

3 4 5 6 7 8 90.1

1

Δn x

104

Irradiance (GW/cm2)

Fig. 4. Quadratic dependence between the induced birefringence and thewriting beam irradiance.

Fig. 5. Volumetric birefringence pattern recorded via two-photon absorp-tion. When the sample orientation is settled at 45� with respect to thepolarizer axis the pattern can be visualized (i). When the sample is placedparallel (ii) to the polarizer axis the picture can not be seen.

The pattern can only be observed if the sample is placedbetween two crossed polarizers at correct orientation. Ifpolarizers were not used, no pattern could be seen. Fig. 5shows a top view of the sample settled at 45� (i) and parallel(ii) to the polarizer axis. The pattern is located at 3 mmfrom the sample surface, which can be seen in a side view,demonstrating that the optically induced anisotropy is con-fined in the bulk of the sample. This pattern could be com-pletely erased by overwriting the test spot with circularlypolarized light, using the same irradiance employed forthe writing process. Erasing was also achieved by heatingthe sample to a temperature close to the polymer glass-transition temperature for approximately 20 min.

4. Discussion

One interesting property of azoaromatic compounds isthe reversible cis–trans photoisomerization of the azogroup, that can be used to induce molecular orientation[1,24,25]. The photoisomerization, and subsequent molecu-lar orientation, can be induced in polymers containingDR13 by excitation at the p! p* transition, locatedaround 500 nm as showed in Fig. 1. Upon shining linearlypolarized light of appropriate wavelength on an azopoly-mer film, the chromophore absorbs light and undergoes atrans–cis isomerization, unless it is oriented perpendicularto the polarization direction. The cis form thermally relaxesto the more stable trans form, a process that is accompa-nied by a change in the chromophore orientation. Thenet result is an excess of molecules oriented perpendicularlyto the laser polarization direction, thus inducing a dichro-ism or birefringence. In general, birefringence can be readwith a probe beam in the transparent region [26–29], whiledichroism is measured with resonant light [5]. The inducedanisotropy can be completely erased by overwriting the testspot with circularly polarized light or by heating the sampleclose to the glass-transition temperature of the polymer.

Usually, this anisotropy is achieved with a writing beamwavelength at the absorption region of the azochromo-phore [23,26–28,21], around 500 nm for the compoundstudied here (Fig. 1). In this case, photoisomerization andmolecular orientation are one-photon induced processes.Because of the large linear absorption of the azo-group,in this regime anisotropy is attained even at low laser inten-sities. On the other hand, resonant excitation does notallow data localization in a small volume within the sam-ple. In this context, two-photon induced anisotropy isattractive for recording data in an erasable medium, sinceit allows information to be written not on the material sur-face, but within the volume [16,30], without erasing oroverwriting the data already stored on neighboring posi-tions. With this approach, the data density achieved byoptical memory devices would be extremely increased.

The interest in third-order nonlinear optical propertiesin azoaromatic compounds is a recent subject [30,31].Recently we have reported a study on the 2PA process inseveral azoaromatic compounds [31,32]. We observed that

438 C.R. Mendonca et al. / Optics Communications 273 (2007) 435–440

the 2PA spectrum of DR13 presents a peak at twice thewavelength of the one-photon absorption band [31,32]. Itwas also observed an increase of the 2PA cross-sectionbelow 800 nm due to the resonance enhancement of thenonlinearity. Considering these features of the 2PA absorp-tion spectrum, we choose 775 nm as the excitation wave-length to be employed in the two-photon opticallyinduced anisotropy experiments. Moreover, as observedin the film absorption spectrum (Fig. 1), two-photons of775 nm will excite the p! p* band responsible for thephotoisomerization [6,19].

The 2PA cross-section for DR13 at 775 nm is �300 GM,the same order of magnitude reported for organic com-pounds with moderate two-photon absorption cross-sec-tions [33]. This cross-section is also consistent with thosequoted in the literature for azobenzene and azopolymericsolutions [34]. Although the 2PA cross-section for DR13is only moderate, it is sufficient to excite the azochromo-phore p! p* transition and induce the isomerization. Withthe accumulative nature of the molecular reorientationprocess, in several cycles of trans–cis–trans isomerizationan effective anisotropy may be induced via 2PA [35,36].

In order to test this possibility, we used fs-laser pulses asa writing beam and a cw He–Ne laser as a reading beam.The power of the reading beam was reduced to a minimum(�10 lW) to avoid any effect this beam could cause. Asshowed in Fig. 3 (solid circles), an optically induced anisot-ropy was indeed observed. This anisotropy may be attrib-uted to an optically induced birefringence, once the linearabsorption at the probe beam wavelength (632.8 nm) isvery small, as can be seem in the absorption spectrum(Fig. 1). By switching off the fs-laser, time B in Fig. 3,the transmission decreased to a value of �20% of its max-imum, indicating a randomization of some of the chro-mophores. This low residual storage efficiency is typicalfor optically induced birefringence in guest–host films,where the azochromophores are not covalently attachedto the polymeric chain and, therefore, there is a large freevolume for the rotational diffusion mechanism take place[1]. Is worthy mentioning that the optically induced anisot-ropy observed in Fig. 3 has a well-determined orientation,with one refraction axis always perpendicular to the writinglaser polarization, as expected for a birefringence process.For comparison, in Fig. 3 (open circles) we presented a typ-ical optically induced birefringence curve obtained with cwresonant excitation (532 nm), where the same general fea-tures observed for the results obtained under nonresonantfs-laser excitation (closed circles) can be seen. The samefinal birefringence level is expected to be achieved forone- and two-photon induced processes, although with dif-ferent dynamics. Such distinct temporal evolution canclearly be seen in Fig. 3. In our experiments, the one-pho-ton process takes about six minutes to reach a birefringenceof 10�4, while for the two-photon process it takes approx-imately 1 hour to achieve a birefringence of 10�5. The two-photon excited optical storage is slower than that using res-onant excitation due to two main factors: (i) the smaller

isomerization rate at the nonresonant 2PA and (ii) thedecreased molecular mobility as less heating is generatedin the 2PA. Point (ii) is also the reason why the residual sig-nal is higher and the relaxation slower for the two-photonprocess.

The two-photon induced anisotropy exhibited in Fig. 3could be completely erased by overwriting the test spotwith circularly polarized light, using the same irradianceemployed for the writing process. This reversible natureof the observed effect rules out any processes related tothe chromophore photobleaching which are permanent.Erasing could also be achieved if the sample was heatedto a temperature close to the polymers glass-transitiontemperature.

To verify that the process observed in Fig. 3 wasobtained by means of two-photon excitation, we analyzedthe dependence of the induced birefringence as a functionof the writing beam intensity, which was varied from 1 to25 GW/cm2. The quadratic dependence observed inFig. 4 clearly indicates the two-photon nature of theinduced birefringence.

Optical anisotropy induced by multi-photon absorptionin PMMA films doped with the azochromophore DR1 wasalso observed by Kawata and coworkers [4,5]. The dichro-ism they observed was attributed to a photoselectivebleaching of DR1 by multi-photon anisotropic absorption.The photobleaching involves a permanent degradation ofthe chromophores by the writing beam. Such irreversibleprocess was indeed observed in Kawatas’s papers, wheneven after turning off the writing beam, the absorption ofthe sample does not recovers the initial state. This featureis different form the one observed here for PPMA/DR13films, where, as can be seen in Fig. 3, when the laser beamis switched off the induced anisotropy relaxes toward theisotropic initial state. Besides, as mentioned previouslythe induced anisotropy can be completely erased, whichalso shows the reversible characteristic of the effectobserved here. Furthermore, it is interesting to point outthat after erasure, or even after point C, the anisotropycan be induced again without any appreciable amplitudechange.

Photodegradation of azochromophores is a complexsubject, and has been shown to depend on the host poly-mer, irradiation wavelength, temperature, atmosphericenvironment and chromophore structure [37–39]. System-atic studies demonstrated that photo-induced oxidation isa dominant mechanism for photodegradation of azobenz-enes [37–39]. It has been demonstrated a strong correlationbetween the oxidation potential and the azodye degrada-tion, with those molecules that have larger oxidationpotentials being the most stable ones [37]. This might bean explanation for the different results obtained here whencompared to Kawata’s [4,5], once DR13 presents a higheroxidation potential than DR1. The Cl group in DR13 isan electron-withdrawing substituent, and its presence raisesthe oxidation potential, explaining why photobleaching, anirreversible process, was not observed in our work for the

C.R. Mendonca et al. / Optics Communications 273 (2007) 435–440 439

range of irradiances employed. On the other hand, DR1that presents a smaller oxidation potential will also exhibitsa smaller photobleaching threshold.

Another remarkable distinction between our results andthe one reported by Kawata [5] is the slope of the processwith the excitation intensity. As displayed in Fig. 4, a slopeof �2 (two-photons) was obtained for the birefringence asa function of the excitation irradiance. In Ref. [5], however,it was obtained a slope of 3.8 for the absorption anisotropywith the intensity. This result point out that higher orderprocesses are taking place in the effect reported by Kawata,which could also explain the distinct nature of the phenom-ena observed.

The 2PA process was then used to produce a localizedbirefringence pattern in the volume of an azopolymericsample, as illustrated in Fig. 4. Since the 2PA probabilityis proportional to the squared intensity of the incidentlight, photoisomerization – and consequently birefringence– is induced only in the focal volume, where the intensity issufficiently high. Care must be taken to avoid white-lightcontinuum generation [40] during the pattern recording,as it could lead to resonant molecular orientation. Thistwo-photon induced pattern could be completely erasedby circularly polarized light or heating, as well as theanisotropy induced in the films (Fig. 3). Using the sameapproach, a 3D pattern could be fabricated employing az-axis stage.

5. Conclusion

We have obtained anisotropy in DR13 dispersed inpoly(methyl methacrylate) films via nonresonant two-pho-ton absorption. This effect leaded to a reversible opticallyinduced birefringence, which was attributed to the molecu-lar reorientation induced by the photoisomerization of theazochromophores, once no irreversible processes thatcould indicate chromophore photobleaching wereobserved.

The quadratic dependence of the induced birefringenceon the excitation irradiance confirms the two-photon nat-ure of the process. This confines the excitation to the focalvolume of the writing beam, which allows for two-photoninduced three-dimensional birefringence. The capabilityof the process in the sample used here is limited by thesmall birefringence conserved after relaxation, owing tothe large freedom for angular diffusion typical of guestmoieties. This limitation can in principle be overcome ifthe azochromophore is attached as a side chain in a poly-mer backbone [23,27]. Additionally, the efficiency of thetwo-photon induced birefringence process can be tunedby choosing distinct azochromophores or excitationwavelength.

Acknowledgements

This work was supported by the FAPESP, CNPq andCapes from Brazil.

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