permanent polarization gratings in elastomer azo-dye systems: comparison of layered and mixed...

Ciuchi et al. Vol. 19, No. 11 /November 2002 /J. Opt. Soc. Am. B 2531

Permanent polarization gratings in elastomerazo-dye systems:

comparison of layered and mixed samples

Federica Ciuchi, Alfredo Mazzulla, and Gabriella Cipparrone

Istituto Nazionale di Fisica della Materia, Dipartimento di Fisica, Universita della Calabria, I-87036,Rende (CS), Italy

Received February 6, 2002; revised manuscript received May 17, 2002

An investigation was carried out of permanent polarization gratings recorded in films of polymer (elastomer)and azo dye written by two orthogonally circularly polarized Ar-ion laser beams. The films prepared were oftwo kinds, namely, a mixture of dye and polymer and a layered structure, i.e., a thin polymer film covered withdye. In both samples gratings with long-time stability were observed. The layered sample had higher dif-fraction efficiency than the mixed sample for low writing intensities when no reliefs were present and purepolarization holograms were recorded. For higher recording intensities, reliefs were observed in both cases:The mixed sample’s efficiency was much higher than that of the layered sample, suggesting that layered sys-tems inhibit formation of reliefs. The stability of the recorded structures can be attributed to molecular in-teractions between polymer and dye. © 2002 Optical Society of America

OCIS codes: 160.5470, 050.0050, 090.2900, 230.5440.

1. INTRODUCTIONThe prospect of recording holographic gratings in systemsthat contain azobenzene chromophores has been proposedin a number of papers.1–7 Special attention has been de-voted to polarization holography; it follows from theoreti-cal considerations1–3 that pure polarization holographicgratings can be used for numerous applications,1,2 for ex-ample, in polarimetry.1,2,8

Side-chain azobenzene polymers and azo-dye guest–host polymer systems are widely investigated because ofthe relatively large value of their photoinducedbirefringence,1–7,9 Dn ' 0.1, which could provide highsensitivity for grating recording. The basic mechanismthat is responsible for the storage effect in these materi-als is the reorientation of azobenzene chromophores thatis induced by light.10,11 In such systems, anisotropicgratings that are due to photoinduced linear and circularbirefringence are recorded simultaneously with topo-graphical relief gratings; the latter dramatically affect thepolarization properties of the former.1–7 Several mecha-nisms have been proposed12–14 to explain the formation ofsurface-relief gratings (SRGs). In particular, a mecha-nism is proposed to explain the SRGs written by two or-thogonally polarized beams that uses a model based on aphotoinduced ordering of the dye molecules: the mol-ecules are subjected to anisotropic intermolecular interac-tions, leading to mass transport, even when the intensityis spatially uniform.15

For many materials1 the lifetime of the photoinducedeffects is too short to produce a stable memory effect. Inother systems long-time memory has been observed; how-ever, the memory is strongly influenced by temperature orby material parameters such as the polymer glass-transition point. In such cases SRG generallyappears.4,16,17

0740-3224/2002/112531-07$15.00 ©

Recently a study was made of thin permanent phasegratings recorded by use of a polarization light patternobtained by superposition of two opposite circularpolarizations18 in Langmuir–Blodgett (LB) films com-posed of amphiphilic azo-dye molecules. Polarizationanalysis of the diffracted beams has shown that thesestructures are pure phase gratings without topographicalreliefs, a conclusion that is supported also by investiga-tion with an atomic-force microscope (AFM). This factsuggests to us that layered systems may inhibit the for-mation of reliefs. Molecular motion is allowed inside thesame layer but is inhibited among layers. Furthermore,in LB films no reliefs appear, even at high intensity; per-haps the reason is the highly ordered one-dimensionalcrystallographic structure of the films. This idea is alsosupported by observations reported in Ref. 19, which de-scribed the inhibition of surface-relief formation inazobenzene functionalized polymers by the superpositionof a layer of polyelectrolytes. Although these results pro-vide a substantially different explanation for the reliefs’inhibition in layered systems, they are consistent withour idea.

In this paper we present the results of measurementsof gratings recorded by a polarization holographic tech-nique in samples made from elastomer and azo dyes. Ex-periments were performed on thin elastomer films cov-ered by azo dye and on homogeneous mixtures of thesame materials.

In both cases, above a writing-intensity threshold, grat-ings that have remained highly stable for at least oneyear have been recorded. Owing to the experimental ge-ometry, i.e., uniform illumination and polarization pat-terns, the optical modulation obtained can be attributedto molecular reorientation of chromophores.

The gratings grow in few minutes at intensity valuesabove threshold. For layered systems, pure polarization

2002 Optical Society of America

2532 J. Opt. Soc. Am. B/Vol. 19, No. 11 /November 2002 Ciuchi et al.

gratings, with a diffraction efficiency of 0.4%, are re-corded at low intensities. For the mixed systems, purepolarization gratings are observed in the same intensityrange but the diffraction efficiency is small, 0.2%. Athigh intensity the formation of reliefs changes thegrating-polarization properties. Relief formation is eas-ily tested by polarization analysis of the diffracted beams:In the layered samples the efficiency is less than in themixed samples and does not increase with intensity. Inmixed samples, regular and quite deep relief gratings areobserved; their depth is controlled by the recording inten-sity and the exposure time.

The results obtained from this study, as we report be-low, lead to some interesting conclusions: (a) It is pos-sible that permanent gratings may be recorded by use ofelastomers; (b) the recorded grating is a pure polarizationhologram or a SRG, depending on the writing intensity;and (c) whereas, in stratified system, better diffraction ef-ficiency is obtained for pure polarization gratings (low-intensity recording), for the mixed system the higher effi-ciency obtained at high intensity indicates that deeperSRGs are recorded, whose depth can be controlled by thewriting intensity and the exposure time.

These observations support our initial idea that strati-fied systems inhibit the formation of SRGs.

2. EXPERIMENTThe materials that we used are an azo dye (Methyl Red,Aldrich) and a thermoplastic elastomer, i.e., styrene–butadiene copolymer (glass-transition temperature Tg ,,220 °C, measured by differential scanning calorimetry).We prepared the layered structures that were depositedupon glass substrates, by dissolving the dye in chloro-form, depositing it onto the polymer layer, and letting itevaporate under vacuum. The dye layer had a thicknessof ;0.8 mm as measured with an AFM. The mixedsamples were prepared from a solution of polymer andMethyl Red, which was then evaporated in vacuum. Forthe two samples the dye concentrations were chosen suchthat their optical densities at l 5 514.5 nm, the Ar-ionlaser wavelength, were the same.

The apparatus for recording and analyzing the polar-ization gratings is shown schematically in Fig. 1: Twocircularly polarized (right and left) Ar-ion laser beamswith equal intensities crossing at u 5 4° angle impingeupon a film placed in the superposition region. If Ecl isthe field of the left-hand circularly polarized wave and Ecris the field of the right-hand circularly polarized wave, theresultant field, E 5 Ecl 1 Ecr , is linearly polarized andcan be written as

E 52

A2Ucos~d/2!

sin~d/2!U. (1)

In this configuration the intensity distribution wasnearly uniform and the light was linearly polarized withuniform rotation of the polarization direction along thegrating wave vector (Fig. 1, inset).

The corresponding spatial periodicity of the gratingwas 8 mm. We used a He–Ne laser beam (l 5 632.8nm) linearly polarized along the grating wave vector as aprobe to investigate the diffracted light fields. The probebeam did not influence the recorded gratings because itwas not absorbed by the azo dye. The polarization stateof the diffracted beams was verified with four-detectorpolarimeter,20 as shown in Fig. 1. Permanent polariza-tion holographic gratings were recorded for light powerdensities ranging from 1 to 10 W/cm2 for exposure timesof a couple of minutes.

Diffraction efficiency h, defined as the intensity ratioof the first diffracted beam to the incident beam,h61 5 I61 /I0 , was measured. SRGs and thicknesses ofthe films were also probed with the AFM.

In these materials, photoinduced linear birefringenceoccurs as a result of the orientation of the dye chro-mophore, as we checked by erasing (by homogenous illu-mination) and rewriting the grating many times. Thepure phase gratings produced by use of the configurationdescribed above can be recognized from their polarizationproperties1–3 as well as from the formation of relief grat-ings.

To characterize the gratings recorded by the polariza-tion holographic technique we measured diffraction effi-ciencies for various values of writing intensity. More-over, to verify whether they were pure phase polarizationgratings we made a polarization state analysis of the dif-fracted beams.

3. RESULTS AND DISCUSSIONFor gratings recorded with the experimental geometry ofFig. 1, the complete transmission matrix enclosing bothphotoinduced birefringence (TP) and relief contributions(TR) has the following form:

T 5 TPTR . (2)

Here

Fig. 1. Experimental setup: WPs, quarter-wave plates; S,sample; Phs, photodiodes; FDP, four-detector polarimeter. In-set, the polarization pattern.


TP 5 F cos~Df ! 1 i sin~Df !cos d i sin~Df !sin d

i sin~Df !sin d cos~Df ! 2 i sin~Df !cos dG
is the phase matrix,1 Df 5 pDnd/l, Dn is the photoin-duced birefringence, d is the film thickness, and l is thewavelength of the writing waves.
TR 5 Fexp@~iDc!cos~d 1 d1!# 0

0 exp@~iDc!cos~d 1 d1!#G

is the relief matrix,21 Dc 5 (2p/l)d1(np 2 na)/2 is thephase shift that is due to the surface relief, d1 is the sur-face modulation depth, np is the polymer’s refractive in-dex, na is the air’s refractive index, and d1 is a phase con-stant that accounts for the spatial phase shift betweenthe two gratings described by the above matrices.

In the low-intensity regime, only TP is present; whenthe incident beam is left (right) circularly polarized,1/A2(2i

1 )@(1/A2)( i1)#, only the 21st (11st) diffracted order

appears; in both instances the polarization is circular:right for 21st (left for 11st) order. For the left-handbeam, the diffracted fields are

E0 1 E11 1 E21 5 TPEi

5 F cos~Df ! 1 i sin~Df !cos d i sin~Df !sin d

i sin~Df !sin d cos~Df ! 2 i sin~Df !cos dG 1

A2S 1

2i D5 cos~Df !

1

A2S 1

2i D 1 isin~Df !

2F exp~id! 1 exp~2id! i@exp~2id! 2 exp~id!#

i@ exp~2id! 2 exp~id!# 2@exp~id! 1 exp~2id!#G 1

A2S 1

2i D ,

E0 5 cos~Df !1

A2S 1

2i D , E11 5 0, E21 5 i sin~Df !S 1i D . (3)

The same procedure applies to the right circularly polar-ized beam.

The two writing beams are orthogonally circularly po-larized; thus, when only a pure polarization grating isgrowing, the pump diffracted beams exactly overlie thetransmitted beams. The latter hide the former. The for-mation of a relief grating during the writing process caneasily be detected by the simultaneous appearance of11st and 21st diffracted orders of the pump beams thatare not overlaid upon the transmitted beams.

The curves in Fig. 2(a) represent the dynamics of the1st-order diffraction efficiency for the layered sample atfive intensity values from 2.2 to 8.8 W/cm2. For all therecording intensities the gratings were pure polarizationgratings, as verified by the absence of one of the first or-der from the writing beams and also by polarimetric mea-surements of the diffracted probe beams: The gratingswere found to be opposite circularly polarized, as ex-pected. The writing beams were removed when the effi-ciency reached its maximum value; further presence of awriting beams would induce oscillations (much more vis-ible in curves 1 and 5) with a subsequent reduction in ef-ficiency. This behavior has been confirmed by repetitionof the measurements several times. After the writingbeams are removed, the efficiencies stabilize to a value a

little lower than the maximum, and no further reductionis observed. The diffracted beam grows faster, increasingthe intensity, because the rise time diminishes from 250to 50 s. The final value of the efficiencies grows to a writ-ing intensity of 5 W/cm2 (curves 1–3) and decrease forhigher values (curves 4 and 5).

In Fig. 2(b) are shown the same kinds of measurementas for Fig. 2(a) but carried out with films made from adye–elastomer mixture. Now the diffraction efficienciesat small intensities (curves 1–3) are lower than the corre-sponding efficiency of the layered sample. The dynamicsshow a shorter rise time. The result for writing intensi-ties above 5 W/cm2 (curves 4 and 5) almost reproduces theresult obtained for a stratified sample, a similar rise time,and a final efficiency value. In both kinds of sample theintensity threshold for recording the gratings is ;1W/cm2.

For recording intensity above 10 W/cm2, both diffrac-

tion orders appear on the transmitted Ar-ion laser beamsafter less than 1-min exposure time, indicating formationof a SRG. In Fig. 3 we show the time evolution of theprobe beam’s 1st-order diffraction efficiency for bothsamples, layered (curve 1) and mixed (curve 2), at 15.5W/cm2 writing intensity. In both samples, after few sec-onds, pure polarization gratings are formed; they reachdiffraction efficiency values similar to those shown incurves 4 and 5 of Fig. 2. Afterward the efficiency isslowly reduced; during this time diffracted beams fromthe pump laser are observed, indicating the start of SRGformation. Later on the curves begin to rise. As Fig. 3shows, the growth of the mixed sample is more evident:Four orders of diffraction are clearly observed. The 1st-order efficiency is 2.2%. The steady diffraction efficiencyvalue is ;10 times that achieved by the layered sample.Bleaching of the irradiated regions is observed at thisstage for both samples.

Polarizing microscope observations of layered andmixed samples (between crossed polarizers) are shown inFigs. 4 and 5, respectively. Figures 4(a) and 5(a) showthe films before illumination, Figs. 4(b) and 5(b) showthem after low-intensity exposure (polarization gratingrecording), and Figs. 4(c) and 5(c) show them after high-intensity exposure (relief grating recording).


Figure 4(a) shows that the dye layer deposited on thepolymer film is organized in well-defined domains of di-mensions of hundreds of micrometers, yielding a typicaltile structure that is somewhat less evident in the mixedsample [Fig. 5(a)]. After irradiation at low intensity,when polarization holograms are recorded the tile struc-ture is still evident and the observed grating lies over it[Fig. 4(b)]. After irradiation at higher intensity [Fig.4(c)] the polymer film is modified; in fact the domainstructure disappears and a dim grating remains. How-ever, for the mixed sample the grating observed in Fig.5(c) is much better defined.

The last observation regarding the layered sample sug-

Fig. 2. Dynamics of efficiency for (a) the layered and (b) themixed samples at several writing intensities. Curves 1, 2, 3, 4,and 5: 2.2, 3.4, 4.4, 5.6, and 8.8 W/cm2, respectively.

Fig. 3. Dynamics of efficiency for (1) layered and (2) mixedsamples at 15.5-W/cm2 writing intensity.

gest that for high-intensity exposure the effects of tem-perature should be considered; i.e., the structural varia-tion of the dye layer can be attributed to mixing of the twocomponents close to the interface.

The grating surface reliefs were also investigated byAFM: No SRGs were observed for low-intensity record-ing, regular and rather deep (;140-nm) structures wereobserved in mixed samples, and irregular surface defor-mations of ;40-nm depth were found in layered samples.

The most important results obtained for polarizationholographic recording in elastomer azo-dye systems canbe summarized as follows: (1) it is possible to record re-programmable permanent polarization gratings, (2) thereis a threshold that is generally not found in similar azo-dye and polymer systems, and (3) SRG formation in thelayered structure is inhibited.

The recorded structures are stable for long times. Thischaracteristic is independent of the type of sample prepa-ration but should depend on the kind of interaction be-tween polymer and dye. Results obtained for samples,both mixed and layered, prepared with another polymer

Fig. 4. Optical polarization microscope images (1003) of thelayered sample: (a) before irradiation, (b) during low-intensityrecording, and (c) during high-intensity recording.


[poly(methyl methacrylate)] and the same dye show purepolarization gratings similar to those observed at low in-tensity in the elastomer but that have a completely differ-ent dynamics. The grating grows quickly, in ,1 s, indi-cating interaction between the poly(methyl methacrylate)and the dye that is weaker than the interaction betweenthe elastomer and the dye. Furthermore, the lifetimes ofthe photoinduced structures are rather short: The dif-fracted beams vanish after few seconds, probably relatedto cis–trans relaxation. Finally, using samples preparedwith the elastomer and a different azo dye (Disperse Red),we observed no polarization or relief gratings.

Another important observation is the presence of theintensity threshold for the recording process: Below 1W/cm2 no gratings are produced, even for long-time expo-sure (tens of minutes). This result strengthens the ideathat the interaction between polymer and azo-dye mol-ecules is significant for optical storage.

A result that gives rise to numerous questions is thatSRG formation is more evident for mixed samples and isalmost entirely inhibited in layered samples. The origin

Fig. 5. Optical polarization microscope images (1003) of themixed sample: (a) before irradiation, (b) during low-intensityrecording, and (c) during high-intensity recording.

of SRG, although it has been widely studied,12,15,19 is notyet well understood, particularly in our experimental ge-ometry for which the recording intensity is spatially uni-form.

Let us assume that the formation of a polarization grat-ing (low intensities) is due mainly to the photoinducedorientation of dye chromophores. In the layered samplemost of the dye molecules are in the upper layer. Be-cause these molecules interact only at the interface, theycan easily be reoriented, and a greater efficiency can bereached whose its value depends on the dye layer’s thick-ness. In the mixed sample, however, the fewer dye mol-ecules near the surface should be bond less than those inthe bulk; then, at low intensity, only the less-bonded mol-ecules can be involved in the process, thus providinglower efficiency.

Assuming this hypothesis, let us compare the photoin-duced birefringence modulation that we have evaluatedfrom the diffraction efficiency in the layered sample andthe photoinduced birefringence values measured for oursample and generally reported in the literature for thiskind of azo dye. The diffraction efficiency of polarizationholograms for a linearly polarized probe beam is18

h61 5 ~sin Df !2/2. (4)

Using this expression, we found a value of Dn ; 23 1022. We obtained the same value by measuring theprobe’s transmittance through crossed polarizers by irra-diating the sample with linearly polarized Ar-ion laserlight at 45°. The measured birefringence was consistentwith the usual azo-dye values reported in theliterature.22–24 This test supports the assumption thatthe polarization grating is given by the reorientation ofthe dye chromophores.

The idea proposed above could also explain the behav-ior of curves 4 and 5 of Fig. 2(a). In the layered structurethe reduction in efficiency relative to the recorded inten-sity can be attributed to a rise in temperature that firstinhibits the orientation of the chromophores, randomizingtheir direction, and then (above 10 W/cm2) produces amixture of dye and polymer just near the interface, asshown by the optical microscope observations. Thereforethe formation of a topographical relief grating can be seenas the effect produced in a mixed region that has differentthicknesses (thin for layered samples and thick for mixedsamples). Subsequently, in the general case of mixedsamples the chromophores embedded in the matrix wouldbe the principal causes of SRG formation. The increasein temperature in the thermoplastic elastomer bulk mayallow a higher possibility for the chromophores to rotate.The consequence is a morphological deformation of thepolymer, which causes a topographical relief as a result ofinteractions of the chromophores.

This hypothesis agrees with the results obtained withLB films in which the structure of the material remainsstratified and no SRGs are produced and also with the re-sults presented in Ref. 19 in which the photoinduced for-mation of SRG is supported by a surface-initiated mecha-nism.

Our results suggest that SRG could originate in thebulk and that a layered structure will intrinsically pre-


vent morphological deformation. All the results pre-sented above accounted can be illustrated in the followingway:

At low intensity, when polarization holograms are re-corded in both kinds of sample, only the dye molecules inthe outer layer are involved, no bleaching is observed, andthe effect can be attributed to the orientation of the chro-mophores. At higher writing intensities, nonnegligiblethermal effects can possibly lead to polymer softening anddeformation of the film, producing the reliefs.

Films illuminated by two orthogonally circularly polar-ized beams, as in our experimental geometry, are irradi-ated with a spatially constant intensity with only a varia-tion of the total polarization state. A mean field model15

that was proposed to explain the presence of photoin-duced surface reliefs demonstrates that the photoinducedordering of dye molecules that have been subjected to an-isotropic intermolecular interactions results in masstransport even when the intensity of light is spatially uni-form. In the presence of a force gradient along the direc-tion of the grating wave vector, a deformation of the poly-mer can be stored when the writing beams are removed.In fact, mass transport may take place during the poly-mer’s softening. Formation of a relief seems to be a bulkprocess that induces surface deformation. In fact, in thelayered sample the volume of the mixed layer is smallcompared with the total volume: Shallow and irregularsurface deformations are produced. In the mixedsample, in which the whole bulk is involved, deeper andregular SRGs are observed.

4. CONCLUSIONSWe have carried out investigations of polarization grat-ings recorded in azo-dye elastomer films of two kinds, lay-ered and mixed. Interesting results were obtained: po-larization gratings were recorded with long timestabilities at lower recording intensities in both samples;at higher intensities, SRGs were produced. The strati-fied system seems to inhibit the formation of topographi-cal relief gratings, indicating that SRG formation is abulk process. These results are also supported by ourprevious observations with LB films and suggest that, fora stratified structure, the mass transport that generallyoccurs in azo-dye polymer systems is prevented. The dif-fraction efficiency obtained was low but was consistentwith the linearly photoinduced birefringence of the mate-rials used.

The long-time stability of the recorded structures, notobserved in other kinds of doped polymer, suggests thatthe choice of the elastomer is particularly significant forthe process described and needs further investigation.

ACKNOWLEDGMENTSThe authors thank N. Scaramuzza for supplying the ma-terials, G. Carbone for making the AFM measurements,and D. Pucci for differential scanning calorimetry mea-surements.

G. Cipparrone’s e-mail address is [email protected].

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permanent polarization gratings in elastomer azo-dye systems: comparison of layered and mixed...

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