Ž .Thin Solid Films 323 1998 227–234

Properties of polymeric Langmuir–Blodgett films containingsulphonyl-substituted azobenzene moieties for second harmonic

generation

David West a,), Damien Dunne b, Philip Hodge b, Neil B. McKeown b, Ziad Ali-Adib b

a Department of Physics and Astronomy, Schuster Laboratory, UniÕersity of Manchester, Manchester M13 9PL, UKb Department of Chemistry, Brunswick Street, UniÕersity of Manchester, Manchester M13 9PL, UK

Received 5 August 1997; accepted 6 November 1997

Abstract

Polymeric Langmuir–Blodgett films incorporating a sulphonyl-substituted azobenzene chromophore have been studied by secondharmonic generation. These films are more stable to light and more transparent than alternatives based on hemicyanine and other azo dyesthat have been used previously. The initial 100 nm of polymeric LB film, nearest the substrate, exhibits a nonlinear susceptibility ofx Ž2.s6.3 pm Vy1 and a chromophore nonlinear vector susceptibility of magnitude bs46=10y50 C 2 my3 Vy2. A loss ofpp

performance is observed for molecular layers a distance greater than 100 nm from the substrate, which is reflected in a reduced butuniform nonlinearity in the layers deposited later in the sample fabrication process. q 1998 Elsevier Science S.A. All rights reserved.

Keywords: Polymeric Langmuir–Blodgett films; Second harmonic generation; Hemicyanine; Azo dyes

1. Introduction

The control of lightwave signals guided within thinw xtransparent films is desired in many applications 1 . Of

these, second harmonic generation and electro-opticswitching of guided lightwave signals require closely re-lated materials, both types being dependent on quadraticoptical nonlinearity that is usually achieved through non-

w xcentrosymmetric film structures 2–5 . Polymers are attrac-tive materials for use in the manufacture of guided-wave

w xnonlinear optical devices 6–8 : in many cases, the poly-mers are poled electrically whilst held at a temperatureabove the glass transition T to create a non-centrosym-g

metric structure. The poled films resulting from this proce-dure can have limited lifetimes due to structural relaxation,especially in the elevated temperatures that are possiblewith optical components with non-zero absorption, whenunder high irradiances.

Ž .Langmuir–Blodgett LB films are deposited onemolecular layer at a time and each layer is orientedpowerfully by hydrophilic and hydrophobic interactions at

) Corresponding author.

w xroom temperature 9,10 . An alternating layer structure isused to achieve non-centrosymmetry. If the material de-posited is a polymer then it may resist subsequent molecu-

w xlar reorganisation 11 . Macroscopic LB film structure iswell defined in planes, one molecular repeat unit thick.The amorphous structure of the individual planes in thepolymeric films can reduce the guided-wave propagationlosses due to a reduction in domain formation and conse-quent Rayleigh scatter.

A nonlinear optical chromophore must be incorporatedinto the polymer for active devices to be made possible;we have used previously the hemicyanine and Disperse

w xRed chromophores 12–14 . Normally chromophores mustbe well elevated in orientation angle away from the filmplane for optimum device performance, as in most casesapplications require a macroscopic nonlinear response tofields applied perpendicular to the film plane. This is thedirection of the optic axis of an ideal uniaxial LB film, andthe possibility exists of using the birefringence for guided

w xwave phase matching schemes 15 .In earlier work we have deposited 600 layer alternating

LB films incorporating a hemicyanine chromophore withuniformly highly-oriented molecular layers throughout the

0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved.Ž .PII S0040-6090 97 00903-6

( )D. West et al.rThin Solid Films 323 1998 227–234228

w xthickness of the film 16 . Such films have birefringence ofaround 3%, typical of organic LB film structures, and arethick enough for multimode waveguide transmission. Theuniform cross-sectional structure reduces mode-to-modecoupling and simplifies the modal properties for phase

w xmatching or quasi-phase-matching device designs 17 .However the hemicyanine and Disperse Red chro-

mophores were found to interact with light. The azo dyesexhibited photoisomerism leading to reorientation in po-larised light, and the hemicyanine chromophore was liableto photodimerisation and loss of nonlinear optical responsew x12–14 . Hemicyanine is partially resonant and absorbs thegreen light from frequency doubling any of the nearinfrared lasers such as the Nd:YAG laser. A higher opticaltransparency and a greater long-term stability are required,which will be maintained in the working environment oflikely devices. Chromophores based on sulphonyl-sub-stituted azobenzene chromophores have been shown topossess nonlinear optical response comparable to hemicya-nines and azo dyes, with increased transparency over mostof the visible region of the spectrum and enhanced photo-

w xchemical stability 18 . In this paper we report the prepara-tion and study of polymeric LB films containing suchgroups.

2. Material synthesis

General experimental details have been described previ-w x 1ously 19 . All products gave satisfactory H NMR and

FT-IR spectra.

( )2.1. Synthesis of polymer 1

The reactions used are outlined in Scheme 1a.

( ) ( )2.1.1. 4- 6-Hydroxyhexylsulphonyl acetanilide 4Ž .To a mixture of sodium sulphite 27.2 g, 0.21 mol and

Ž .water 300 ml was added 4-acetamidobenzenesulphonylŽ .chloride 43.9 g, 0.19 mol over a period of 25 min. The

temperature was maintained at 45–508C and the pH atŽ .8.5–9.0 by the addition of NaOH 30% wrv as necessary.

The solution was then stirred at 508C for 2 h. 6-Hydroxy-Ž .hexyl bromide 34.6 g, 0.19 mol was added and the pH

kept at 6.5–7.5 by the addition of sodium hydrogen car-Ž .bonate ;15 g . The mixture was stirred overnight. The

precipitate was collected and recrystallised from 95%Ž . Ž .ethanol to give 4- 6-hydroxyhexylsulphonyl acetanilide 4 ,

yield 52%, m.p. 102–1038C. Found: C 55.9%, H 7.1%, N4.4% and S 10.3%. Calculated for C H NO S: C 56.2%,14 21 4

H 7.0%, N 4.7% and S 10.7%.

( ) ( )2.1.2. 4- 6-Hydroxyhexylsulphonyl aniline 5Ž . Ž .The above anilide 4 15.8 g, 0.053 mol was treated

Ž . Žwith a mixture of concentrated HCl 75 ml and water 75.ml at 1008C for 4 h. The mixture was cooled to 808C and

Žthe pH brought to 5.5–6.0 by the addition of NaOH 30%.wrv . The mixture was left to cool overnight. The precipi-

tate was collected and recrystallised from ethanolrwaterŽ . Ž . Ž .1:1 . This gave 4- 6-hydroxylsulphonyl aniline 5 , yield80%, as white crystals, m.p. 798C. Found: C 55.5%, H7.3%, N 5.4% and S 12.1%. Calculated for C H NO S:12 19 3

C 56.0%, H 7.4%, N 5.4% and S 12.4%.

X ( )2.1.3. 4-Hydroxy-4 - 6-hydroxyhexylsulphonyl azobenzene( )6

Ž .The above aniline 5 was diazotized and the diazoniumsalt coupled with phenol using the general procedures

w x X Žgiven in Vogel 20 . 4-Hydroxy-4 - 6-hydroxyhexyl-. Ž . Ž .sulphonyl azobenzene 6 62% yield was obtained as a

light yellow powder with m.p. 193–1958C. Found: C59.1%, H 5.9%, N 7.4% and S 8.6%. Calculated forC H N O : C 59.7%, H 6.1%, N 7.7% and S 8.8%.18 22 2 4

X ( )2.1.4. 4-Octyloxy-4 - 6-hydroxyhexylsulphonyl azobenzene( )7

Ž . Ž .The above phenol 6 5.82 g, 16.1 mmol , potassiumŽ . Žcarbonate 2.25 g, 16.1 mmol , potassium iodide 0.16 g,. Ž .1.0 mmol and 1-bromooctane 3.32 g, 17.1 mmol in

Ž .acetone 40 ml was heated at reflux, with vigorous stirringand under nitrogen, for 72 h. The mixture was allowed tocool and then filtered. The precipitate was collected andtwice recrystallised from methanol to give 4-octyloxy-4X-Ž . Ž .6-hydroxyhexylsulphonyl azobenzene 7 as a light yel-low powder, m.p. 127–1298C, yield 88%. Found: C 65.3%,H 7.6%, N 5.7% and S 6.4%. Calculated for C H N O S:26 38 2 4

C 65.8%, H 8.0%, N 5.9% and S 6.7%.

X ( )2.1.5. 4-Octyloxy-4 - 6-bromohexylsulphonyl azobenzene( )8

Ž . Ž .Compound 7 0.61 g, 1.29 mmol was dissolved inŽ .dry toluene 10 ml . This solution was stirred and ice-

Ž .cooled. Thionyl bromide 0.80 g, 3.86 mmol was thenslowly added. The mixture was refluxed for 30 min. and

Ž .pyridine 0.10 g, 1.29 mmol was added. Heating underreflux was continued for another 2.5 h. The cold mixture

Ž .was extracted with diethyl ether ;100 ml . The solventwas evaporated to give the crude product. Recrystallisation

X Žfrom methanol gave 4-octyloxy-4 - 6-bromohexyl-. Ž .sulphonyl azobenzene 8 as a yellow powder m.p. 109–

1108C, yield 55%. Found: C 57.6%, H 6.6%, N 5.0%, S5.4% and Br 14.7%. Calculated for C H BrN O S: C26 37 2 3

58.1%, H 6.9%, N 5.2%, S 6.0% and Br 14.9%.

[ ]2.1.6. Zwitterionic polymerisation 21 using compound( )8 and 4-Õinylpyridine

Ž . Ž .Compound 8 3.0 g, 0.56 mmol was placed in aŽ .sealed tube with freshly distilled 4-vinylpyridine 15 ml

Ž .and hydroquinone monomethyl ether 10 mg, 0.08 mmol .The mixture was degassed by repeated freeze–thaw cyclesŽ .5 or 6 . The tube was then sealed under vacuum andheated at 508C for 1 h, then left at room temperature for 5h. Vinylpyridine was distilled off. The crude product was

( )D. West et al.rThin Solid Films 323 1998 227–234 229

Ž . Ž . Ž . Ž .Scheme 1. a Outline of the synthesis of polymer 1 : b synthesis of polymer 2 .

dissolved in chloroform and precipitated into diethyl etherŽ .400 ml . The polymer was collected and dried under

Ž .vacuum for several days. Polymer 1 was obtained as anorange powder, yield 90%. Found: C 57.8%, H 7.1%, N6.3%, Br 11.2% and S 4.9%. Calculated forŽ .C H BrN O S : C 61.7%, H 6.8%, N 6.5%, Br 12.5%33 44 3 3 n

and S 5.0%. Such polyelectrolytes are known to retainwater.

( )2.2. Synthesis of polymer 2

The reactions used are as outlined in Scheme 1b.

( )D. West et al.rThin Solid Films 323 1998 227–234230

( )2.2.1. N-Methyl-N-octylaniline 9ŽA mixture of freshly distilled N-methylaniline 19.6 g,

. Ž .0.18 mmol , 1-bromooctane 35.9 g, 0.186 mol , potassiumŽ .iodide 0.76 g, 5.0 mmol and n-butanol were heated at

reflux temperature under nitrogen with vigorous mechani-cal stirring for 4 days. The cold mixture was filtered.Solvent was removed on the rotary evaporator. The crudeyellow oil was distilled under reduced pressure to yield

Ž .N-methyl-N-octylaniline 9 , yield 54%, as a light yellowŽ .oil, b.p. 110–1208C -0.1 mm Hg .

( ) X (2.2.2. 4- N-Methyl-N-octylamino -4 - 6-hydroxyhexyl-) ( )sulphonyl azobenzene 10Ž .Aniline 5 was diazotised and the diazonium salt cou-

pled with N-methyl-N-octylaniline using the general pro-w x Ž .cedures given by Vogel 20 . 4- N-methyl-N-octylamino -

X Ž . Ž .4 - 6-hydroxyhexylsulphonyl azobenzene 10 was ob-tained as a red powder, m.p. 80–828C, yield 80%. Found:C 59.9%, H 8.2%, N 8.1% and S 6.3%. Calculated forC H N O S: C 66.5%, H 8.4%, N 8.6% and S 6.6%.27 41 3 3

( ) X (2.2.3. 4- N-Methyl-N-octylamino- -4 - 6-bromohexyl-) ( )sulphonyl azobenzene 11

Ž . Ž .Compound 10 0.30 g, 0.62 mmol and carbon tetra-Ž .bromide 0.26 g, 0.80 mmol were dissolved in dry

Ž . Ždichloromethane 10 ml . Triphenylphosphine 0.21 g, 0.8. Ž .mmol in dry dichloromethane 10 ml was added carefully

and the mixture was stirred at room temperature overnight.The solvent was evaporated off and the product purified bychromatography using silica and a chloroformrethyl ac-etate mixture as effluent. Suitable fractions were collected

Žand recrystallised from methanol. This gave 4- N-methyl-. X Ž .N-octylamino- -4 - 6-bromohexylsulphonyl azobenzene

Ž .11 as red crystals, m.p. 87-888C, yield 68%. Found: C58.4%, H 7.2%, N 7.4%, S 54% and Br 13.9%. Calculatedfor C H BrN O S: C 58.9%, H 7.3%, N 7.6%, S 5.8%27 40 3 2

and Br 14.5%.

[ ] ( )2.2.4. Zwitterionic polymerisation 21 of compound 11and 4-Õinylpyridine

The procedure described above for the synthesis ofŽ . Ž .polymer 1 was followed but using bromide 11 in place

Ž . Ž .of bromide 8 . Polymer 2 was obtained as a dark orangepowder, yield 84%. Found: C 60.8%, H 7.6%, N 8.2%, Br

Ž .11.5% and S 4.2%. Calculated for C H BrN O S : C34 47 4 2 n

62.3%, H 7.2% N 8.5%, Br 12.2% and S 4.9%. Suchw xpolyelectrolytes are known to retain water 20 .

( )2.3. Synthesis of polymer 3

ŽThis polymer was synthesised from poly 4-vinylpyri-. Ž .dine Polysciences, Mn 50,000 and docosyl bromide

w x 1using the procedure described before 19 . By H NMRŽ .spectroscopy and elemental analysis 14.8% Br it was

70% quaternised.

3. Characterisation techniques

X-ray reflectivity measurements were performed as de-w xscribed previously 22 . The refractive index, absorption

coefficient and film thickness were modelled from ellipso-metric measurements in a reflection configuration usingmultiple angles of incidence, at a wavelength of 633 nm.The second harmonic generation from a Q-switchedNd:YAG laser polarised in the plane of incidence andreflection at incident wavelength 1064 nm was resolved asa function of angle of incidence onto the sample as de-scribed previously to determine the internal structure of the

w xfilms 23 . Results were calibrated against the signal fromquartz. Techniques involving transmission of light throughthe film at varying angles of incidence are useful for thestudy of materials with potential guided-wave applications,because they enable us to determine the effective nonlinearresponse x Ž2. of the film in the experimental geometry,pp

the mean tilt c of the chromophores from the perpendicu-lar, the extent of departure from ideal uniaxial symmetry,the chromophore nonlinear response b and the uniformityof the structure for films of different thickness. All of theseparameters are significant in guided-wave applications.

4. Properties of monolayers of polymers

Monolayers were prepared at the air–water interface ona Langmuir–Blodgett trough using chloroform as thespreading solvent. The surface pressure—area isothermsfor each were measured and are shown in Fig. 1. Allpolymers were stable as monolayers at the surface pressure

y1 Ž . Ž .30 mN m . Polymers 1 and 3 can be seen to compresssteadily with increasing surface pressure, whilst polymerŽ .2 shows a steep isotherm and a rapid increase of surface

Ž . Ž . Ž .Fig. 1. Surface pressure—area isotherms for polymers 1 , 2 and 3 asmonolayers spread at the water surface of a Langmuir–Blodgett deposi-tion trough.

( )D. West et al.rThin Solid Films 323 1998 227–234 231

Table 1Ž . Ž . Ž .Summary of the properties of monolayers and of LB films deposited from floating monolayers of polymers 1 , 2 and 3

Film Surface Surface Thickness Volume Nonlinear Molecularamaterial pressure area per of associated susceptibility hyperpola-

b Ž2. cŽmN repeat bilayer with each x risabilityppy1 y1 y50˚. Ž . Ž . Žm unit at A chromo-phore pm V b =10

3 2 y3 y2˚Ž . .stated A C m Vpressure

2˚Ž .A

Ž .1 30 35 50.6 1770Ž .1 22 40 52.9 2120Ž .2 30 46.7 43.8 2050Ž .3 30 32.5 45.0 yŽ . Ž .3 downstroke; 1 on upstroke 30 49.2 1720 0.65 4

dŽ . Ž .3 downstroke; 2 on upstroke 30 46.0 2150 6.3 46eŽ . Ž .3 downstroke; 2 on upstroke 30 46.0 2150 1.75 13

Ž . Ž .3 downstroke; 2 on upstroke 15 48.5 2980 3.4 32

a Film deposition ratio 1.00"0.05 in all cases.b From X-ray diffractions from samples of 100 bilayers, confirmed by ellipsometry and waveguide mode studies.c Based on a single vector approximation to the tensor molecular nonlinear response, neglecting local field effects in film.d First 20 bilayers only: this film exhibits positive uniaxial birefringence with ordinary index 1.57, extraordinary index 1.61.eFor the layers deposited after first 20 bilayers: there is departure from uniaxial symmetry and a reduction in nonlinearity.

pressure as the monolayer is compressed. Some propertiesof the monolayers are included in Table 1.

5. LB multilayer deposition of individual polymers

Each polymer was deposited as a Y-type LB film inŽ .which all layers contain the same material. Polymer 1

was deposited from the trough at surface pressure 30 mNy1 ˚m to give multilayers with a bilayer spacing of 50.6 A

measured by X-ray reflectivity, with one diffraction peakobserved. At a reduced surface pressure of 22 mN my1

multilayers were deposited in which the bilayer spacing˚was found from one X-ray peak to be 52.9 A. As surface

pressure at deposition is reduced the volume associatedwith each repeat unit in the film increases. The reducedsurface pressure corresponds to a reduced degree of orderin the monolayer. A summary of the properties of the filmsis given in Table 1.

Ž . y1Polymer 2 was deposited at 30 mN m surface˚pressure to give multilayers with bilayer spacing 43.8 A

calculated from a single X-ray peak. The layer spacing isŽ .smaller than found for polymer 1 , but a larger area per

repeat unit is observed at the deposition surface pressureŽ .leading to a 10% lower density in polymer 2 . A lower

density implies that the structure of Y-type films of poly-Ž .mer 1 are more ordered than corresponding films of

Ž .polymer 2 , in which there is more free volume, see Fig.2.

Ž .Polymers of the type of polymer 3 have been de-w xscribed before 19 : in the present study three peaks are

observed in the X-ray reflectivity from Y-type films ofŽ .polymer 3 , indicating that a high degree of layer order is

present in these polymer films perpendicular to the film

˚plane. The bilayer spacing observed is 45 A. This materialis not strongly nonlinear in optical response, and actsessentially as a passive spacer material capable of separat-ing nonlinear layers in an alternating layer structure.

6. LB deposition of alternating layer films

Deposition of alternating multilayer LB films of poly-Ž . Ž . y1mers 1 and 3 at surface pressure 30 mN m led to

˚films of bilayer spacing 49.2 A which exhibit a singleŽ .X-ray peak. Alternating multilayer films of polymers 2

Ž .and 3 deposited at the same surface pressure have a˚bilayer spacing of 46 A measured from a single X-ray

Ž .reflectivity peak. When polymer 2 is deposited at areduced surface pressure of 15 mN my1 the volumeoccupied by each repeat unit in the alternating films of

Ž . Ž .polymers 2 and 3 increases, as expected for a film withreduced order. Relevant data is summarised in Table 1.The small increase that is observed in the thickness of thefilms deposited at reduced pressures is perhaps surprising,but this result from X-ray diffraction studies has beenconfirmed independently by ellipsometry. One possibleexplanation is that at the lower surface pressure disorder inthe side chains may allow the polymer backbone to buckleaway from the water surface on the deposition trough, socontributing to the layer thickness of the deposited film. Atthe higher surface pressure of deposition the close packingof the side chains may require the backbone to stay at thewater surface.

Ellipsometry and waveguide coupling measurementshave been used to determine the thickness and refractive

Ž . Ž .index of alternating layer films of polymers 2 and 3deposited at 30 mN my1, indicating the refractive indices

( )D. West et al.rThin Solid Films 323 1998 227–234232

Ž . Ž . Ž .Fig. 2. Structures of polymers 1 , 2 and 3 .

to be n s1.57 and n s1.61 for perpendicular polari-ord ext˚sations, and confirming a thickness per bilayer of 46.4 A.

7. Nonlinear optical properties of the alternating films

Two sets of alternating layer LB film samples rangingfrom 20 to 100 bilayers in thickness were prepared at

y1 Ž . Ž .surface pressure 30 mN m from polymers 1 and 3 ,Ž . Ž .and polymers 2 and 3 , respectively. Consistent and

uniform deposition of multilayers of the latter films wasconfirmed by a check on the optical density at the absorp-tion maximum at 442 nm wavelength, which can be seenin Fig. 3 to be linear with the number of bilayers de-posited.

Ideally the intensity of second harmonic generation

Fig. 3. Magnitude of the UV absorbance peak at 442 nm wavelength,expressed as a function of the number of bilayers in a sample film

Ž . Ž .consisting of alternating layers of polymers 2 and 3 .

should depend on the square of the number of bilayers inthe sample, for samples less than a micron in thickness.The square root of the maximum second harmonic signalfrom each sample is plotted in Fig. 4 as a function of thenumber of bilayers in the samples. Individual results areconsidered accurate to "5%, and the additional scatter ofthe results indicates the extent to which different samplesare reproducible in structure. The straight lines suggest thatthere is a uniform and consistent structure in cross-sectionthrough the films, but the failure of the lines to pass closeto zero prompted further studies which are described be-low.

Second harmonic generation results are usually anal-ysed using a simplified vector model of the tensor nonlin-

Fig. 4. The square root of the peak intensity of second harmonicgenerated from the sample films, expressed as a function of the numberof bilayers in sample films consisting of alternating layers as shown.

( )D. West et al.rThin Solid Films 323 1998 227–234 233

ear hyperpolarisability of the chromophores within thefilms. The films are assumed to be far from resonance andeach chromophore is tilted a characteristic angle from theperpendicular, this characteristic angle being well-definedin the film. A good fit to the incidence angle dependencyof the second harmonic signal is obtained using this model.

Ž .The structure of alternating films of polymers 1 andŽ .3 is not totally consistent for all samples, and we observea variation of tilt angle between samples in the range 458

to 548. This leads to some fluctuation in the nonlinearresponse between samples. From the rate of change of

Ž . Ž .intensity of second harmonic from polymers 1 and 3 asextra layers are added to the structure, we calculate a bulknonlinear susceptibility in the experimental geometry ofx Ž2.s0.65 pm Vy1. In the approximation that the localpp

field correction within the films is negligible, this corre-sponds to a vector chromophore nonlinear susceptibility ofmagnitude bs3.6=10y50 C 2 my3 Vy2 .

Ž .The series of alternating layer films of polymers 2 andŽ .3 show a rate of increase of second harmonic signal withincrease in film thickness which corresponds to a bulknonlinear susceptibility for these films of x Ž2.s1.75 pmpp

Vy1. This corresponds in this case to a vector chro-mophore nonlinear susceptibility of magnitude bs13=

10y50 C 2 my3 Vy2 . However, as can be seen in Fig. 4,the variation in second harmonic with thickness of film for

Ž . Ž .alternating films of polymers 2 and 3 does not passclose to the origin as should be expected for a film of zerothickness deposited on a glass substrate. This behaviourhas been investigated further with a thinner series of

Ž . Ž .samples of polymers 2 and 3 from a single bilayer to 20bilayers in thickness, with results shown in Fig. 5. It isseen that the first 20 bilayers deposit with a better nonlin-ear susceptibility, observed as an increase in the gradientof the signal variation with film thickness. The first 20

Fig. 5. Comparison of the square root of peak intensity of secondŽ .harmonic from thin -100 nm thick and thicker LB film samples

Ž . Ž .consisting of alternating layers of polymers 2 and 3 .

Fig. 6. Extent of departure from uniaxial symmetry in the film structure,expressed as a function of the number of layers in a sample, for samples

Ž . Ž .consisting of alternating layers of polymers 2 and 3 .

bilayers, corresponding to the first 100 nm of film de-posited next to the substrate, exhibit a nonlinear perfor-mance equivalent to a bulk susceptibility x Ž2.s6.3 pmpp

Vy1 and a chromophore nonlinear vector susceptibility ofmagnitude bs46=10y50 C 2 my3 Vy2 . The gradient ofthis section of the line is seen to pass close to the origin asis expected.

Ž .The thicker multilayers of alternation of polymers 2Ž .and 3 become increasingly biaxial in symmetry, as a

macroscopic component of the quadratic nonlinear re-sponse in the plane of the film begins to build up as thenumber of bilayers is increased. Normally LB films aremodelled as uniaxially symmetric structures in which theazimuthal orientations w of the nonlinear chromophoresare random. One way of introducing an azimuthal asym-metry into the model of the structure is to postulate anon-uniform azimuthal orientation probability distribution

P w dws 1qA cos w dwr2p 1Ž . Ž . Ž .leading to a correction to the form of the projection factor

w xfor the chromophore orientations within the film 24 . Thisapproach may be valid for small values of the asymmetryparameter A<1. Incorporating this modified azimuthaldistribution into a model for the variation of second har-monic signal with incidence angle, we can find the asym-metry parameter A shown in Fig. 6 as a function of thenumber of bilayers in the sample. There is a gradualincrease in the departure from a pure uniaxial structure asthe additional bilayers of reduced nonlinear susceptibilityare deposited. The first 20 bilayers are observed to beuniaxial in symmetry. The different nonlinear responsesare not caused by different angles of chromophore tilt inthe films, as for all samples of alternating layers of poly-

Ž . Ž .mers 2 and 3 the tilt angle from the perpendicular waswell defined as 53"28. The tilt variation corresponds to a

( )D. West et al.rThin Solid Films 323 1998 227–234234

variation in bulk nonlinear susceptibility of "3% only. Itis probable that other factors such as the degree of order orthe local field enhancement in the films are responsible forthe change of response.

ŽInterestingly, films from the same materials polymersŽ . Ž .. Ž2 and 3 deposited at half the surface pressure 15 mN

y1 .m do not show such a change in structure in layersdeposited later in the deposition process, but produce filmsof reduced nonlinearity x Ž2.s3.4 pm Vy1 as might bepp

expected from the lower degree of order and the reduceddensity of nonlinear chromophores in this case. Takinginto account the reduced chromophore density we calculatea hyperpolarisability bs32=10y50 C 2 my3 Vy2 for the

Ž . Ž .alternating films of polymers 2 and 3 , deposited at thelower pressure.

8. Thermal and photo-stability

UV–visible absorption spectra indicate an absorptionpeak in these materials at 442 nm wavelength, away fromthe harmonic wavelength used of 532 nm where the mate-rials are almost transparent. Sample films consisting of

Ž . Ž .alternating layers of polymers 2 and 3 have been storedunder mercury fluorescent lighting at room temperature for

Ž .over 36 months without significant )5% loss of nonlin-ear performance.

9. Conclusions

The azobenzene chromophores containing the sulphonylgroup are more transparent at visible wavelengths thanalternatives such as the hemicyanine and azo dyes thathave been used in studies previously of LB films bysecond harmonic generation. As expected the chromophoreincorporating the more strongly electron-donating amine

Ž .substituent as in polymer 2 gave a stronger nonlinearity.The initial 100 nm of the alternating LB films prepared

Ž . Ž .from polymers 2 and 3 , nearest the substrate, haveexhibited a bulk nonlinear susceptibility of x Ž2.s6.3 pmpp

Vy1 and a chromophore nonlinear vector susceptibility ofmagnitude bs46=10y50 C 2 my3 Vy2 that is compara-ble to results for the more resonant hemicyanine-based LBpolymer films. The films may support several opticalwaveguide modes. The reduced absorption may extend theuseful lifetime of devices through a lower operating tem-perature and improve performance stability. A curious lossof performance is observed for molecular layers a distancegreater than 100 nm from the substrate, which is reflected

in a reduced but uniform nonlinearity in the layers de-posited later in sample fabrication.

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