Linear and nonlinear optical properties of Langmuir–Blodgett ...

Ž . 

Materials Science and Engineering C 8–9 1999 527–537 

www.elsevier.comrlocatermsec 

Linear and nonlinear optical properties of Langmuir–Blodgett 

multilayers from chromophore-containing maleic acid anhydride 

polymers 

Sigurd Schrader a,) , Vismants Zauls a , Birgit Dietzel b , Costel Flueraru c , Dietrich Prescher b , 

Jurgen Reiche a , Hubert Motschmann c , Ludwig Brehmer a 

¨ 

a 

UniÕersitat ¨ Potsdam, Institut fur ¨ Physik, Lehrstuhl Physik kondensierter Materie, Am Neuen Palais 10, Potsdam D-14469, Germany 

b 

Institut fur ¨ Dunnschichttechnologie ¨ 

und Mikrosensorik Teltow, Forschungsgruppe Fluorpolymere, Rudower Chaussee 5, Berlin-Adlershof D-12484, 

Germany 

c 

Max-Planck Institut fur ¨ Kolloid- und Grenzflachenforschung, ¨ 

Rudower Chaussee 5, Berlin-Adlershof D-12484, Germany 

Abstract 

Organized polymer films prepared from amphiphilic side-chain polymers were investigated with respect to their application in passive 

or active photonic devices. The polymers were synthesized by means of polymer analogous reaction of azo chromophores with a maleic 

acid anhydride polymer which bears saturated hydrophobic side chains. The resulting polymer contains, besides hydrophilic groups and 

hydrophobic side chains, azo chromophores with high second-order hyperpolarizability which are attached to the main chain via a spacer. 

In order to increase the hydrophobic character of the chromophore, a fluorinated moiety was chosen as acceptor of the chromophore. 

Langmuir–Blodgett Ž LB. technique was applied for preparation of multilayer structures with distinct supramolecular architecture from 

these polymers. They are characterized by high anisotropy and a special combination of linear and nonlinear optical Ž NLO. properties. 

The polymers were deposited both in Y-type and Z-type LB layers. The Y-type layers of the azopolymer show high birefringence and 

dichroism but small nonlinear response which originates from the first deposited layer only. Multilayers of the azopolymer deposited in 

Y-type configuration are characterized by high internal order which can be concluded from a pronounced Bragg-peak and Kiessig-fringe 

patterns as obtained from gracing incidence X-ray scattering. Experiments to deposit the active polymer directly in Z-type configuration 

by passing a blank water compartment before layer deposition in a double compartment LB-trough led to the result that well-ordered, 

anisotropic films are obtained only if the number of deposited monolayers is limited to a value of about 30. The layers deposited in Z-type 

manner show low order. This can be concluded from the fact that only a weak Bragg-peak is observed in the X-ray scattering curves, and 

Kiessig-fringes are very weak or missing. The same conclusion can be drawn from ellipsometric and second-harmonic generation Ž SHG. 

experiments. A second kind of Z-type multilayers was prepared by alternate deposition of the active azopolymer and of another maleic 

acid anhydride polymer without chromophores. They show similar linear optical properties like Y-type layers but due to the 

noncentrosymmetric internal structure, a NLO second-order response grows proportional to the square of the sample thickness. Again, 

thick layers of the azopolymer prepared by alternate deposition LB-technique were prepared, which can be used as active waveguide for 

second-order NLO application. q 1999 Elsevier Science S.A. All rights reserved. 

Keywords: Nonlinear optics; Second-order effects; Fluorine-containing chromophore; Azobenzene; Side-chain polymers 

1. Introduction 

In order to satisfy the growing demand for a high 

bandwidth for optical data transfer and data processing 

systems, a variety of passive and active components like 

polarization-retaining waveguides wx 1 or fast-operating de- 

) Corresponding author. Tel.: q49-3319771503; fax: q49- 

3319771457; E-mail: sschrader@rz.uni-potsdam.de 

0928-4931r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. 

Ž . 

PII: S0928-4931 99 00088-0 

vices like electro-optic amplitude modulators, phase modu- 

lators or directional couplers w2–4x is required. Organic 

materials are due to their high optical nonlinearities and 

their low dielectric constant promising candidates to real- 

ize fast optical devices w2–6 x. 

These devices operate on the 

basis of second-order nonlinear optical Ž NLO. effects. 

Prerequisites for the appearance of a significant macroscopic 

second-order NLO susceptibility x 2 in the active 

material are a sufficient molecular second-order hyperpolarizability 

and the presence of a noncentrosymmetric in-

528 

ternal structure both on the molecular and the supramolec- 

ular level w2–7 x. 

Chromophores with high second-order 

hyperpolarizability b are usually linear conjugated chromophores 

which bear a strong donor and a strong acceptor 

in p–p X -position, and which have a long, undistorted pelectron 

system. Azobenzene chromophores are known to 

represent a good compromise between high nonlinear response, 

and other technologically important properties like 

sufficient solubility in common solvents and good photo- 

chemical stability w8–18 x. 

In order to obtain material properties 

required for waveguide preparation, the chromophores 

can be chemically bound to polymeric chains. In 

principle, a huge variety of materials can be synthesized in 

this way which involves main- or side-chain polymers 

suitable for spin-coating, amphiphilic polymers for Langmuir–Blodgett 

Ž LB. deposition or polyionenes which can 

be used for deposition by self-assembly from polyelec- 

trolyte solutions w2–5,19–25x w26,27 x. 

In the present study, 

amphiphilic side-chain polymers which bear new fluorinecontaining 

chromophores were synthesized and used for 

preparation of anisotropic multilyer structures by LB deposition. 

Fluorine-containing chromophores exhibit lower refractive 

index than fluorine-free chromophores but show, 

in case of medium or strong donor and acceptor sub- 

stituents, high b-values w28,29 x. 

Therefore, this material 

class is of special interest for electro-optic and other 

photonic applications where the figure-of-merit is proportional 

to the square of the second-order nonlinear suscepti- 

bility and to the inverse cube of the refractive index w2–4 x. 

Many photonic devices are based on the principle of 

optical waveguiding w1–4,30–33 x. 

A typical waveguide 

thickness reaches values around one or a few micrometers. 

Preparation of waveguides can easily be done by spin-coating 

of soluble polymers. This technique does not allow for 

a fine-tuning of internal architecture of the waveguide. The 

structure of waveguides prepared by means of LB technique, 

however, can be controlled on a molecular scale. 

For instance, a gradient of refractive index can easily be 

introduced through layer-by-layer deposition of suitable 

amphiphilic polymers. Therefore, the LB deposition technique 

is the method of choice for fabrication of waveguides 

having molecularly controlled architecture. This 

method involves the compression of a monolayer of organic 

molecules spread on the surface of a liquid subphase, 

e.g., on water w34 x. 

After reaching a sufficient surface 

pressure, the monolayer attains a molecular density which 

( ) 

S. Schrader et al.rMaterials Science and Engineering C 8–9 1999 527–537 

resembles the density of a two-dimensional solid. By 

dipping a pre-treated substrate into the subphase covered 

with the compressed monolayer, a material transfer takes 

place. During the transfer process, the surface pressure is 

kept constant by moving the trough barriers. The transferred 

layers have about the thickness of the compressed 

monomolecular layer on the subphase. Monolayers can be 

deposited onto the substrate both on immersion and withdrawal, 

resulting in a centrosymmetric arrangement of both 

layers Ž Y-type deposition. or just on immersion ŽX-type 

deposition. or on withdrawal Ž Z-type deposition. which 

provides noncentrosymmetric structures. The control of 

molecular orientation and of supramolecular arrangement 

is crucial for two aspects — the kind of molecular packing 

perpendicular to the layer plane is important to obtain 

noncentrosymmetric or centrosymmetric structures, while 

the molecular orientation with respect to the substrate 

plane is important to obtain an anisotropic refractive index. 

If the amphiphilic material deposited by LB-technique 

is a polymer, then two basic approaches are possible w34 x: 

– the polymer is synthesized prior to its deposition and 

then transferred in its compressed state onto the substrate; 

or 

– the monomeric film is transferred first to the substrate 

and polymerization takes place after deposition, e.g., by 

photochemical polymerization. 

For the present study, the first approach was used, i.e., 

the azochromophore-containing polymers were synthesized 

and characterized before their deposition. 

The aim of the present study is the evaluation of these 

polymers with respect to their potential for fabrication of 

passive and active waveguide structures by means of LB 

technique. 

2. Materials 

2.1. Chromophore synthesis 

The functionalized chromophores used for polymer syn- 

thesis were 3-w4-Ž 4-trifluoromethyl-phenyl-azo. phenoxyx- propan-1-ol Ž TPPP. and 3-w4-Ž4-trifluoromethyl-phenylazo. phenoxyx-undecan-1-ol Ž TPPU . . The chromophore, 

TPPP, was synthesized according to the scheme given in 

Fig. 1, and synthesis of TPPU was carried out via an 

w Ž . x Ž . 

Fig. 1. Synthesis of the functionalized chromophore, 3- 4- 4-trifluoromethyl-phenylazo -phenoxy -propan-1-ol TPPP .

equivalent route. At first, 4-trifluoromethyl-4 X -hydroxyazobenzene 

Ž. 1 was prepared by diazotation of trifluoromethyl-aniline 

Ž Aldrich. and subsequent coupling with 

phenol analogues according to a method reported previ- 

ously w35 x. Then, equimolar amounts of compound Ž. 1 and 

of 3-bromopropan-1-ol were heated in dried acetone up to 

608C under vigorous stirring for 35 h together with the 

fivefold amount of dried K 2CO3 and a small amount of 

KI. 

The inorganic salts were filtered off and the volume of 

the filtrate reduced by evaporation in vacuum. By adding 

heptane to the organic phase, compound Ž. 2 was precipitated. 

The precipitate was filtered off and purified by 

recrystallization from heptane. An amount of 4 mmol 

Ž 78% yield. yellow-colored crystals of compound Ž 2. were 

obtained Ž mp)2408C, decomposed . . 

2.2. Polymer synthesis 

Polymer synthesis was realized via a polymer analogous 

Ž 

reaction. One millimole of poly maleic anhydrid-co-alt-1- 

( ) 

S. Schrader et al.rMaterials Science and Engineering C 8–9 1999 527–537 529 

octadecene.Ž. I having a weight average of molar mass of 

Mw s 40 000 was converted together with 1 mmol 

azochromophore Ž II. Ž3-w4-Ž 4-trifluoromethyl-phenylazo. - 

phenoxyx -propan-1-ol. and 210 ml Ž three drops. of dimethylaminopyridine 

as catalyser into a chromophore-containing 

side-chain polymer Ž III. Ž cf. Fig. 2 . . The reaction was 

carried out in dry acetone under stirring for 30 h at 808C. 

In order to precipitate the resulting polymer, the reaction 

mixture was subsequently poured onto ice. 

The precipitated product was washed several times with 

water and dried at 508C in vacuum for 4 h. The soluble 

by-products were extracted from the polymer with acetone 

for 10 h. From the elemental analysis, a fluorine weight 

content of 3.75% was found for the polymer, which was 

insoluble in acetone. 

This is equivalent to a conversion rate of 30% of the 

anhydride moieties along the polymer chain. Other analytical 

data obtained from infrared Ž IR. investigations support 

this result. Therefore, the statistical co-polymer Ž III. 

Žazochromophore-containing maleic acid side-chain polymer 

— AMS-1. can be described by an average sum 

formula as given in Fig. 2 with ms0.3 and ns0.7, 

Fig. 2. Synthesis of the azochromophore-containing side-chain polymer Ž III. Ž AMS-1. from polymer Ž I. and chromophore Ž II. Ž3-w4-Ž4-trifluoromethyl- phenylazo. -phenoxyx -propan-1-ol. by polymer analogous reaction.

530 

respectively. In the same way, another polymer was synthesized 

which differs only in the length of the spacer 

between chromophore and main chain Ž AMS-2 . . This 

means that the chromophore, TPPP, is replaced by the 

chromophore, TPPU, and the polymer, AMS-2, has, consequently, 

a spacer of 11 CH 2-units 

in the side chain. 

2.3. Film preparation by LB technique 

The LB film deposition was performed on a NIMA 622 

alternating trough Ž NIMA Technology Coventry, UK. 

equipped with a Wilhelmy plate surface pressure sensor. 

The spreading solutions were obtained by dissolving 0.5 

mg of the polymer in 1.0 ml of a N, N-dimethylacetamide 

Ž DMAc. rbenzene Ž 1:1. mixture ŽMerck, 

high-grade purity 

. . The monolayer was formed by spreading 100–200 

ml of the solution on the water subphase provided by a 

Milli-Q system Ž Millipore . . The subphase temperature was 

held constant to 218C. The films were compressed with a 

constant barrier speed of 10 cm2rmin. The surface pressure 

— area isotherm of polymer Ž III . — is plotted in Fig. 

3, where a statistically averaged repeat unit with ms0.3 

and ns0.7 according to Fig. 2 was assumed for calculation 

of the area per repeat unit. The monolayer shows a 

high collapse pressure of about 50 mNrm for both kinds 

of polymers. They form stable monolayers on water. The 

isotherms are reproducible. 

For further investigations, LB multilayers have been 

prepared on different substrate materials. Multilayer films 

of the polymers were obtained by depositing the LB 

monolayers at a target pressure of 25 mNrm and a dipper 

speed of 5 and 10 mmrmin. A transfer ratio between 0.8 

and 1.0 was observed. 

Silicon wafers having a native oxide layer of 3 nm 

thickness, silicon wafers coated with gold by vacuum 

evaporation, and quartz glass plates used as substrate 

Fig. 3. Surface pressure — area isotherm of azochromophore-containing 

side-chain polymer AMS-1 for the statistically averaged repeat unit of the 

polymer. 

( ) 


material, were hydrophobized by treatment with hexamethyldisilazane 

Ž HMDS. prior to deposition. Multilayers of 5, 

10, 20, 30, 40, and 60 monolayers were prepared on these 

substrates. These layers were of Y-type or Z-type, depen- 

dent on the selected way of layer deposition w34 x. 

In 

addition, Z-type layers have been prepared by alternating 

deposition of two different materials. In that case, alternating 

monolayers of AMS-1 and of a chromophore-free 

maleic acid anhydride polymer have been deposited in 

order to form noncentrosymmetric multilayers. 

3. Experimental 

The LB multilayers were characterized by X-ray scattering 

and various linear and NLO techniques in order to 

obtain information about sample thickness, chromophore 

orientation, degree of order and about their optical constants. 

3.1. X-ray characterization 

X-ray specular reflectiÕity ( XSR) measurements carried 

out under gracing incidence provide information about 

sample thickness and molecular periodicity inside the LB 

multilayers. For these measurements, the radiation of a 

fine focus copper tube with nickel filter Žwavelength 

ls 

0.154 nm. is used. The radiation is collimated by slit 

systems both in the incident and the reflected beam. A 

goniometer HZG-4 Ž Seifert-FPM, Freiberg, Germany. with 

proportional counter was operated in uy2u mode with 

u-steps of 0.018. 

In Fig. 4, the X-ray reflectivity of an 18 layer Y-type 

LB-film of AMS-2 is plotted. A pronounced first-order 

001 peak, as well as Kiessig-fringes in the low-angle 

region, are visible. The bilayer thickness derived from the 

position of the 001 peak using the Bragg equation is 3.75 

nm. The observation of Kiessig-fringes indicates a low 

value of roughness both of the substrate-film and the 

film–air interface. The overall thickness of the film derived 

from the Kiessig-fringes is in agreement with a 

perfect transfer of nine bilayers. Y-type layers of AMS-1 

are characterized by very similar XSR curves. The bilayer 

thickness derived from the position of the Bragg-peak is 

somewhat higher and has a value of 3.9 nm. This indicates 

a slightly lower density of the material which can be the 

result of a less perfect packing of the LB layers. This is 

indeed reasonable, since the short-chain spacer of AMS-1 

causes a stronger coupling to the main chain in comparison 

to the polymer AMS-2 where the longer spacer allows a 

decoupling between polymer main chain and chromophore 

packing. 

Investigation of multilayers of AMS-1 deposited in 

Z-type configuration shows a different behaviour. The

( ) 


Fig. 4. X-ray reflectivity curve of an 18 layer LB-film of the azochromophore-containing polymer, AMS-2, deposited in Y-type configuration on silicon. 

X-ray reflectivity curve is characterized by a weak Braggpeak 

and almost missing Kiessig-fringes. This indicates a 

much higher roughness and low order in these films. The 

reason for this behaviour could be a structural transformation 

which occurs during the deposition process. 

3.2. Optical characterization 

Optical characterization of the polymeric LB multilayers 

involves both linear optical methods like ultraviolet– 

visible spectra Ž UV–Vis. absorption spectroscopy and ellipsometry, 

and second-order NLO measurements. 

3.3. Linear optical measurements 

Linear optical measurements on LB samples provide 

not only the optical material constants but also information 

Fig. 5. Ellipsometric angles delta Ž D. and psi Ž C . in dependence on the 

angle of incidence for five monolayers of AMS-1 on hydrophobized 

silicon Ž squares — experimental values; lines — theoretical curves . . 

about the internal structure of the material, e.g., the average 

thickness of a LB monolayer. The two applied techniques 

were ellipsometry and absorption spectroscopy. 

Ellipsometry is a method which is mainly applied for 

determination of refractive index and thickness of thin 

transparent layers. In this paper, we used a standard ellipsometer 

AFE-400 Ž ZWG, Berlin, Germany. with a rotating 

analyser and retarder which operates at the wavelength of 

the He–Ne Laser at 632.8 nm. Both azimuth angles and 

four fixed angles of incidence between 458 and 82.58 are 

known with a precision of 0.018. Furthermore, free adjustable 

angles of incidence are known with an accuracy of 

0.058. Two examples of spectra of multiple angle ellipsometry, 

as measured for AMS-1 on different substrates, are 

presented in Figs. 5 and 6. 

Ellipsometric information about refractive index, absorption 

and thickness of the layers under investigation is 

Fig. 6. Ellipsometric angles delta Ž D. and psi Ž C . in dependence on the 

angle of incidence for 30 monolayers of AMS-1 on gold substrate 

Ž squares — experimental values; lines — theoretical curves . .

532 

Fig. 7. UV–Vis absorption spectra of AMS-1 Z-type multilayers on 

quartz glass substrate measured under perpendicular incidence of light: 5, 

10, 20, 30, 40, 60 monolayers. 

condensed into the two ellipsometric angles delta Ž D. and 

psi Ž C . . They express the complex ratio of reflectivities of 

S- and P-polarized light, i.e., polarized perpendicular and 

parallel to the plane of incidence, respectively. Ellipsometric 

data were analysed by the use of an own simulation and 

data processing program. This program calculates C and 

D values for an assumed input optical model of the 

sample, and then optimizes the difference between measured 

and calculated C and D values by varying model 

parameters, like layer thicknesses, refractive indices of the 

different layers or number of layers. 

The quality of fit is judged by the mean square difference 

Ž MSD . : 

1 

MSDs ½ 2 Nypy1 

Ž . 

= 

N 

1r2 

2 2 

Ý Ž c m. i Ž c m. 

i 5 

i 

( ) 


D yD q C yC , 

where D , C and D , C are the calculated and the 

c c m m 

measured values of the ellipsometric angles, respectively. 

N represents the number of experimental points and p is 

the number of unknown model parameters. Different points 

were measured in order to check the uniformity of the 

film. In all cases, the fit is accepted if MSD is below 10 y2 . 

For measurements carried out at one angle of incidence, 

the question about the existence and the uniqueness of the 

solution is very important. From the physical point of 

view, a solution obviously exists. The nonuniqueness of 

the solution results from the investigated physical mechanisms 

that are potential sources for multiple stationary 

solutions. Using different incident angles, it is possible to 

find simultaneously the minimum error between simulated 

and experimental spectra for all angles of incidence. Measurements 

with different incident angles do not produce 

the effect of enhancing information content. However, the 

multiple-angle measurements will help choose the true 

solution. 

Absorption spectra of LB multilayers on quartz glass 

substrates in the UV–Vis range were recorded with a 

Perkin-Elmer Lambda 18 spectrophotometer at perpendicular 

incidence of light. In Fig. 7, the UV–Vis spectra of 

samples having different numbers of monolayers are presented. 

Three main absorption peaks can be seen as expected 

for the azochromophore. The weak absorption band around 

450 nm can be assigned to the p–p U transition while the 

two strong absorption bands centered at 350 and 248 nm 

are connected with p–p U transitions of the chromophore. 

The absorption spectra show an increase of absorbance 

with the number of deposited monolayers. 

3.4. NLO measurements 

Second-harmonic generation Ž SHG. measurements have 

been carried out in order to determine second-order NLO 

susceptibility of the LB multilayers. Samples were prepared 

on quartz glass substrates for transmission measurements 

or on silicon or gold substrates for measurements in 

reflection geometry. These NLO measurements do not 

only provide x 2 values but also information about chromophore 

orientation and about the internal structure of LB 

multilayers. The light source for SHG measurements has 

been an actively–passively mode-locked Nd:YAG laser 

Ž BM Industries, France. which emits laser pulses of 30 ps 

duration at a wavelength of 1064 nm, of an energy of 7 mJ 

per pulse, and with a repetition rate of 10 Hz. For SHG 

measurements in transmission, the sample was rotated on a 

computer-controlled rotation stage with the rotation axis 

perpendicular to the laser beam. 

The detected second-harmonic Ž SH. intensity shows the 

typical dependence on the angle of incidence ŽMaker 

fringes w2–4 x. , as can be seen in Fig. 8 where the SHG 

Fig. 8. The SH intensity of an LB-film containing alternate monolayers of 

AMS-1 Ž 10 monolayers. and octadecyl archidate Ž nine monolayers . . For 

comparison, the rescaled signal of a quartz reference is also plotted.

Fig. 9. P- and S-polarized reflected SHG intensity as function of polarization 

azimuth of fundamental beam. Angle of incidence is 458 and zero 

polarization azimuth corresponds to the S-polarized wave. 

intensity of a Z-type multilayer of AMS-1 deposited in 

alternation with a chromophore-free maleic acid anhydride 

polymer is plotted vs. the angle of incidence. The envelope 

of this curve contains information about NLO susceptibility 

x 2 while the oscillation results from interference of 

harmonic light from layers on both sides of the substrate. 

The nonlinear fit to the envelopes provides the main 

tensorial components of x 2 of the LB multilayer. Details 

of data analysis are given elsewhere w25 x. 

The SHG measurements have been also carried out in 

reflection using samples on silicon or gold substrate. For 

this kind of measurements, the angle of incidence of the 

laser beam was kept constant at 458. The sample could be 

rotated with respect to the laser beam, and the polarisation 

of incident light could be varied with respect to the plane 

of incidence Ž variation of polarization azimuth. as well. 

The polarization state of the incident fundamental beam 

was rotated by a Glan polarizer and half-wave plate combination. 

Reflected SH intensity for P- and S-orientations 

behind an analyzer has been recorded by a photomultiplier 

after spectral selection by an appropriate set of filters and a 

monochromator. A digital storage scope was used for 

signal acquisition and averaging. Data readout, rotation of 

the sample platform, half-wave plate and analyzer have 

been performed by a measurement control computer. 

A typical result of such an experiment is plotted in Fig. 

9 where the SH intensity for a sample with five monolayers 

of AMS-1 on silicon is shown. 

The SHG experiments in reflection provide additional 

information about the internal structure, especially about 

chromophore orientation inside the LB layers. From the 

growth of SH intensity with the number of layers, information 

about the type and quality of chromophore ordering 

can be extracted. 

A second kind of SHG measurements in reflection 

keeps the polarization state of the exciting laser beam 

constant but analyzes the generated harmonic light by the 

use of an rotating analyzer. This kind of experiment pro- 

( ) 


vides the same information as the experiment described 

before, but it is not reported here. 

4. Results and discussion 

The absorbance of the azopolymer multilayers in the 

UV–Vis spectral range is plotted in Fig. 7. It depends on 

the number of monolayers deposited on the substrate in 

Z-type. The deposition of LB films to solid quartz glass 

substrates was carried out with an average transfer ratio of 

0.9 to 1.0. For a transfer ratio of 1.0, one would expect an 

almost linear increase of absorbance with number of layers. 

This behaviour is, in fact, observed as can be seen in 

Fig. 10 where the integrated absorbance of the two strong 

absorption bands at 350 and 248 nm are plotted vs. the 

number of monolayers. 

Since the integrated spectra were not corrected for 

reflection losses, the linear plots do not cross the y-axis at 

zero but at a value which includes substrate absorption and 

reflection at the different interfaces of the sample. 

A somewhat different result is obtained from ellipsometric 

measurements. Here, an almost linear increase of 

the calculated sample thickness with the number of monolayers 

is only observed for the first monolayers. Fig. 11 

represents the ellipsometrically determined thickness in 

dependence on the number of AMS-1 monolayers on 

silicon substrate. As can be seen, it is not possible to 

calculate the thickness of monolayers with high accuracy 

because the slope of this dependence is not constant and it 

does not cross the origin of the graph, too. The same 

behaviour is observed for the samples on quartz glass ŽFig. 

12. and on gold substrates Ž Fig. 13 . . For the latter samples, 

the calculated thickness per monolayer amounts to 1.6 nm 

if the range between 5 and 10 monolayers is considered. 

For samples on silicon Ž Fig. 11. and quartz glass substrates 

Ž Fig. 12 . , the calculated thicknesses per monolayer 

are 1.7 and 1.3 nm, respectively. 

Fig. 10. Integrated absorbance of the two strong absorption bands of 

AMS-1 at 350 and 248 nm in dependence on the number of monolayers.

534 

Fig. 11. Thickness of AMS-1 multilayers on silicon in dependence on the 

number of monolayers determined by ellipsometry at a wavelength of 

632.8 nm. 

The value ns1.557 of the refractive index was found 

to be the same for these layers, independent of the kind of 

substrate. 

For samples with more than 20 monolayers, the thickness 

per monolayer, as calculated above, cannot be used 

since no reasonable solution can be found for thicker 

samples on the base of these parameters. As a consequence, 

a new ellipsometric model with two virtual layers 

has been used. The first virtual layer is built up of 10 

monolayers of polymer AMS-1, and the next layer covers 

the remaining number of monolayers deposited on the 

substrate. This procedure was applied to the various samples 

on different substrates. It was found that the thickness 

per monolayer for the upper virtual layer is about three 

times smaller than that for the lower virtual layer. Our 

evaluation for samples on three different substrates led to 

the conclusion that, the thickness per monolayer decreases 

with increasing number of monolayers. If the number of 

monolayers exceeds 40, the film starts to collapse — 

Fig. 12. Thickness of AMS-1 multilayers on quartz glass in dependence 

on the number of monolayers. 

( ) 


Fig. 13. Thickness of AMS-1 multilayers on gold in dependence on the 

number of monolayers. 

which means that the next monolayers have a thickness 

below 0.2 nm in terms of the applied model which includes 

a third virtual layer covering all monolayers above 

40. In Table 1, the results are summarised. 

In most cases, the LB deposition is sensitive to the 

temperature dependence of surface pressure of the floating 

monolayer, to the composition of the subphase, and to the 

surface properties of the substrate. In order to reduce the 

number of unknown parameters, the LB deposition was 

always carried out under the same conditions. Therefore, 

the change in the deposition behaviour with increasing 

number of monolayers should have its origin in material- 

inherent properties. In fact, Robinson et al. w36x observed a 

similar behaviour for 22-tricoseneic acid and arachidic 

acid. Using contact angle measurements, he confirmed the 

change in surface free energy relative to the substrate for 

successive layers. The surface free energy for 22-tricoseneic 

acid and arachidic acid decreases with increasing 

deposition pressure until triple-line pinning occurs. Then 

an increase of surface energy with number of monolayers 

has been measured. Hence, with increasing number of 

monolayers, the influence of substrate decreases, which 

leads to a loss of molecular order especially in the upper 

layers. It is, therefore, not possible to deposit oriented 

multilayers from this kind of organic molecule alone, 

because the deposition is, in this case, sensitive to the 

surface energy which depends on thickness. 

Table 1 

Thickness per monolayer for Z-type multilayers of AMS-1 dependent on 

the number of monolayers deposited on different kinds of substrates 

determined by multiple angle ellipsometry 

Substrate Sample with different numbers of monolayers Ž nm. 

5 10 20 30 40 60 

Silicon 1.64"0.12 0.55"0.04 about 0.1 

Gold 1.70"0.25 0.55"0.13 about 0.1 

Quartz 1.29"0.24 0.52"0.01 about 0.1

Fig. 14. Reflected SHG of Z-type AMS-1 multilayers on silicon in 

dependence on the number of monolayers. The expected N 2 -dependence 

is shown by the dashed line. 

In case of AMS-1 Z-type multilayers, the chromophores 

of the first 10 monolayers are oriented almost perpendicular 

to the substrate plane, but they become more and more 

tilted as the number of monolayers increases. Obviously, a 

similar situation, as described by Robinson, is valid also 

for the present case. The reason could be that the polar 

groups of the polymer main chain cannot compensate the 

large dipole moments of the chromophores when the interaction 

with the substrate loses its influence. As a result, 

disorder in the upper layers increases in comparison to the 

lower layers. 

The investigation of molecular order by SHG led to the 

same result. The reflected SHG intensity of an adsorbate– 

surface system can schematically be given as a sum of 

contributions from substrate surface nonlinear susceptibil- 

( ) 


ity x 2 s and from the susceptibility of deposited multilayers 

x 2 m: 

< 2 2 2 < 2 

I2 v A xs qxm I v . 

For perfectly aligned LB multilayer, x 2 m is proportional 

to the number of deposited layers N and as a result, a 

dependence according to I AN 2 2 v should be observable 

( 2 v 

as linear increase in a I plot vs. N. SHG reflection 

measurement were carried out by the use of a circularly 

polarized fundamental light beam and without an analyzer 

in the detector path in order to average contributions of all 

x 2 tensor components. The increase of the SHG signal 

m 

follows the expected N 2-dependence only for the first 30 

monolayers, as shown by the ( I2 v vs. N plot in Fig. 14. 

Comparing this dependence with the results of ellipsometric 

measurements, it is clear that orientation and ordering 

of chromophores are increasingly reduced for Z-type 

multilayers above a thickness of 30 monolayers. 

This is consistent with results of SHG experiments 

dependent on the polarization of the incident laser beam. 

For a sample of five monolayers Ž Z-type, AMS-1 . , the 

detected SH signal in S- and P-polarisation in dependence 

on the polarization angle of incident fundamental beam is 

plotted as polar diagram in Fig. 15. The observed intensity 

is described by the following expression w37 x: 

ž / 

2p 2 sec 2 2 v 

u 

2 l´ 0 c 

< ˆ2 

v Ž . 

2 

s ˆv Ž . ˆv 

Ž 

2 < 2 . v 

I s e L 2v 

Px :e L v e L v I , 

where l is the wavelength, c is the speed of light, ´ 

stands for vacuum permittivity, the term, e ˆV , is a unit 

polarization vector and LŽ V . are appropriate linear local 

field correction factors at frequencies Vsv and Vs2v, 

respectively. 

Ž . 

Fig. 15. Reflected SH intensity of a Z-type AMS-1 multilayer sample five monolayers in dependence on the polarizer azimuth of the incident 

fundamental beam. Both S- and P-polarized SH intensities are plotted.

536 

( ) 


Fig. 16. Reflected SH intensity of a Z-type AMS-1 multilayer covering 60 monolayers. 

Model calculations show that this intensity pattern resembles 

a situation where the chromophore is tilted towards 

the normal of the substrate plane by an angle of 

about 308. The intensity distribution for a sample with 60 

monolayers is shown in Fig. 16. 

The features, as visible in Fig. 15, are still present as 

fingerprint of the lower layers but this signal is superimposed 

by a second signal of layers having a clearly different 

average tilt angle, which deviates considerably from 

the normal direction to the substrate plane, indicating a 

more disordered state. A detailed analysis will be pub- 

lished elsewhere w38 x. 

Since multilayers of AMS-1 prepared as Z-type layers 

collapse if a large number of layers is deposited, the 

second approach is used where alternating layers of the 

azopolymer and of the chromophore-free maleic acid anhydride 

polymer are prepared. Effective NLO coefficients 

Ž 2 d s0.5x . of these multilayer films have been deter- 

33 33 

mined by means of Maker-fringe technique. A typical 

pattern, as received from SHG measurements, is shown in 

Fig. 9. Films were deposited on quartz glass substrates and 

SHG measurements were made in transmission for P–P 

and S–P polarization geometry. The signal from 1-mm 

thick quartz reference plate was used for calibration. We 

found d values of about 10 pmrV for our samples 

33 

having 30 or less monolayers. Investigations on thicker 

multilayer samples are still in progress. 

5. Conclusions 

The LB multilayers, prepared from side-chain polymers 

of maleic acid anhydride which carry fluorine-containing 

NLO azochromophores in the side chain, show high second-order 

NLO susceptibility if they are deposited as 

Z-type layers. These layers can be prepared from the 

azopolymer alone or by alternating deposition with other 

materials, e.g., with a chromophore-free maleic acid anhydride 

polymer. Multilayers prepared from the azopolymer 

alone tend to collapse if the number of layers exceeds a 

value of 30, which is confirmed both by linear and by 

NLO measurements. The ellipsometrically determined 

thickness per monolayer reduces gradually for increasing 

number of deposited layers, and the second-order NLO 

response grows less rapid with sample thickness than 

expected. Both results reveal that the ‘‘Z-type’’ layers of 

the pure azopolymer are characterized by high disorder 

which can be due to structural rearrangement during the 

process of layer deposition. This behaviour can be explained 

by insufficient screening of the strong chromophore 

dipoles, and related to that by strong changes of 

surface energy with increasing thickness. Alternating LB 

layers prepared from the azopolymers and other amphiphiles 

are promising candidates for fabrication of thicker 

LB layers with high anisotropy and high NLO susceptibility. 

Investigations on these materials are still in progress. 

Typical values of nonlinear bulk optical coefficients d 33 

are in the range of 10 pmrV for LB multilayers prepared 

from azochromophore-containing maleic acid anhydride 

polymer and a respective chromophore-free polymer by 

alternating Z-type deposition. 

Acknowledgements 

Financial support of the Deutsche Forschungsgemeinschaft 

under project number Schr 462r3-1, andr3-2, and 

project Kn 439r2-1 and of Ministry of Education, Culture 

and Science of the Country of Brandenburg is gratefully 

acknowledged.

References 

wx 1 G.P. Agrawal, Nonlinear Fiber Optics, 2nd edn., Academic Press, 

San Diego, 1995, 592 pp. 

wx 2 P.N. Prasad, D.J. Williams, Introduction to Nonlinear Optical Effects 

in Molecules and Polymers, Wiley, New York, 1991. 

wx 3 C.-C. Tang, High-speed electro-optic modulators from nonlinear 

optical polymers, in: H.S. Nalwa, S. Miyata Ž Eds. . , Nonlinear Optics 

of Organic Molecules and Polymers, CRC Press, Boca Raton, FL, 

1997, pp. 441–464. 

wx 4 M.G. Kuzyk, C.W. Dirk Ž Eds. . , Characterization Techniques and 

Tabulations for Organic Nonlinear Optical Materials, Marcel Dekker, 

New York, 1998. 

wx 5 D.J. Williams, Organic polymeric and nonpolymeric materials with 

large optical nonlinearities, Angew. Chem., Int. Ed. Engl. 23 Ž 1984. 

690. 

wx 6 D.S. Chemla, J. Zyss Ž Eds. . , Nonlinear Optical Properties of Organic 

Molecules and Crystals, Vols. 1 and 2, Academic Press, New York, 

1987. 

wx 7 R. Boyd, Nonlinear Optics, Academic Press, San Diego, 1992. 

wx 8 M. Barzoukas, A. Fort, G. Klein, C. Serbutoviez, L. Oswald, J.F. 

Nico, Conformational dependence of quadratic hyperpolarizabilities 

of a series of push–pull stilbenes — characterization and investigation 

of empirical correlations, Chem. Phys. 64 Ž.Ž 3 1992. 395–406. 

wx 9 L.-T. Cheng, W. Tam, S.H. Stevenson, G.R. Meredith, G. Rikken, 

S.R. Marder, Experimental investigations of organic molecular nonlinear 

optical polarizabilities: 1. Methods and results on benzene and 

stilbene derivatives, J. Phys. Chem. 95 Ž 26. Ž 1991. 10631–10643. 

w10x L.-T. Cheng, W. Tam, S.R. Marder, A.E. Stiegman, G. Rikken, 

C.W. Spangler, Experimental investigation of organic molecular 

nonlinear optical polarizabilities: 2. A study of conjugation dependence, 

J. Phys. Chem. 95 Ž 26. Ž 1991. 10643–10652. 

w11x K. Clays, A. Persoons, Hyper-Rayleigh scattering in solution, Phys. 

Rev. Lett. 66 Ž 23. Ž 1991. 2980. 

w12x K. Clays, A. Persoons, Hyper-Rayleigh scattering in solution, Rev. 

Sci. Instrum. 63 Ž.Ž 6 1992. 3285–3289. 

w13x C.R. Moylan, S.A. Swanson, C.A. Walsh, J.I. Thackava, J.R. Twieg, 

R.D. Miller, V.Y. Lee, From EFISH to electro-optic measurements 

of nonlinear chromophores, Proc. SPIE 2025 Ž 1993. 192–201. 

w14x C.R. Moylan, R.J. Twieg, V.Y. Lee, S.A. Swanson, K.M. Betterton, 

R.D. Miller, Nonlinear optical chromophores with large hyperpolarizabilities 

and enhanced thermal stabilities, J. Am. Chem. Soc. 115 

Ž 26. Ž 1993. 12599–12600. 

w15x J.L. Oudar, Optical nonlinearities of conjugated molecules: stilbene 

derivatives and highly polar aromatic compounds, J. Chem. Phys. 67 

Ž.Ž 2 1997. 446–457. 

w16x J.L. Oudar, H. LePerson, Second-order polarizabilities of some 

aromatic molecules, Opt. Commun. 15 Ž 29. Ž 1975. 258–262. 

w17x M.A. Pauley, H.W. Guan, C.H. Wang, A.K.Y. Jen, Determination of 

first hyperpolarizability of nonlinear optical chromophores, J. Chem. 

Phys. 104 Ž 20. Ž 1996. 7821. 

w18x P.C. Ray, P.K. Das, Dissociation constants of weak organic acids in 

protic solvents obtained from the first hyperpolarizabilities in solution, 

J. Phys. Chem. 99 Ž 51. Ž 1995. 17891. 

w19x M. Amano, T. Kaino, Second-order nonlinearity of novel diazo-dye 

attached polymer, J. Appl. Phys. 68 Ž 12. Ž 1990. 6024. 

w20x L.R. Dalton, A.W. Harper, R. Ghosn, W.H. Steier, M. Ziavi, H. 

Fellerman, Y. Shi, R.V. Mustacich, A.K.Y. Jen, K.J. Shen, Synthesis 

and processing of improved organic second-order nonlinear 

( ) 


optical materials for applications in photonics, Chem. Matter 7 

Ž 1995. 1060–1081. 

w21x A.K.Y. Jen, V.P. Rao, K.J. Drost, Y.M. Cai, R.M. Mininni, J.T. 

Kenney, E.S. Binkley, L.R. Dalton, S.R. Marder, Progress on heteroaromatic 

chromophores in high temperature polymers for 

electro-optic applications, Proc. SPIE 2143 Ž 1994. 30–40. 

w22x C.R. Moylan, R.J. Twieg, V.Y. Lee, R.D. Miller, W. Volksen, J.L. 

Thachava, C.A. Walsh, Synthesis and characterization of thermally 

robust electro-optic polymers, Proc. SPIE 2285 Ž 1994. 17–30. 

w23x K.D. Singer, H.E. Katz, J.E. Sohn, C.W. Dirk, L.A. King, H.M. 

Gordon, Second-order nonlinear optical properties of donor- and 

acceptor-substituted aromatic compounds, J. Opt. Soc. Am. B 6 

Ž 1989. 1339. 

w24x S. Schrader, K. Pfeiffer, F. Reuther, H.-J. Lorkowski, Stable polymer 

chromophore systems for nonlinear optics, in: E. Silinsh, A. 

Medvids, A. Lusis, A. Ozols Ž Eds. . , Proc. SPIE — Optical Organic 

and Semiconducting Inorganic Materials, Vol. 2968, 1997, pp. 61– 

66. 

w25x S. Schrader, D. Prescher, V. Zauls, New chromophores and polymers 

for second-order nonlinear optics, in: M. Eich Ž Ed. . , Second- 

Order Organic Nonlinear Optics, Proc. SPIE, Vol. 3474, 1998, pp. 

160–171. 

w26x M. Florsheimer, ¨ M. Kupfer, ¨ C. Bosshard, H. Looser, P. Gunter, ¨ 

Phase-matched optical second-harmonic generation in Langmuir– 

Blodgett film waveguides by mode conversion, Adv. Mater. 4 Ž 12. 

Ž 1992. 795–798. 

w27x G. Decher, F. Klinkhammer, Makromol. Chem. Macromol. Symp. 

46 Ž 1991. 19. 

w28x S. Schrader, D. Prescher, Molecular substitution and nonlinear optical 

properties of chromophores, in: E. Silinsh, A. Medvids, A. Lusis, 

A. Ozols Ž Eds. . , Proc. SPIE — Optical Organic and Semiconductor 

Inorganic Materials, Vol. 2968, 1997, pp. 44–49. 

w29x S. Schrader, D. Prescher, K. Lukaszuk, R. Wortmann, A.H. Otto, 

Solvatochromy and electro-optical study of new fluorine-containing 

chromophores, in: M. Eich Ž Ed. . , Second-Order Organic Nonlinear 

Optics, Proc. SPIE, Vol. 3474, 1998, pp. 14–22. 

w30x C.N. Ironside, J.S. Aitchison, J.M. Arnold, An all-optical switch 

employing the cascaded second-order nonlinear effect, IEEE J. 

Quantum Electron. 29 Ž 10. Ž 1993. 2650–2654. 

w31x Y. Baek, R. Schiek, G.I. Stegeman, G. Krijnen, I. Baumann, W. 

Sohler, All-optical integrated Mach–Zehnder switching due to cascaded 

nonlinearities, Appl. Phys. Lett. 68 Ž 15. Ž 1996. 2055–2057. 

w32x R. Schiek, Y. Baek, G.I. Stegeman, Second-harmonic generation and 

cascaded nonlinearity in titanium-undiffused lithium niobate channel 

waveguides, J. Opt. Soc. Am. B 15 Ž.Ž 8 1998. 2255–2268. 

w33x L. Palchetti, S. Sottini, D. Grando, E. Giogetti, R. Ricceri, G. 

Gabriellli, Electro-optic modulation of laser diode by mode interference 

in a multilayer waveguide including a 2-docosylamino-5nitropyridine 

Langmuir–Blodgett film, Appl. Phys. Lett. 72 Ž. 8 

Ž 1998. 873. 

w34x R.H. Tredgold, Order in Thin Organic Films, Cambridge Univ. 

Press, Cambridge, 1994. 

w35x D. Prescher, T. Thiele, R. Ruhmann, G. Schulz, J. Fluorine Chem. 

74 Ž 1995. 185. 

w36x I. Robinson, J.-R. Sambles, N.A. Cade, Thin Solid Films 178 Ž 1989. 

125. 

w37x Y.R. Shen, The Principles of Non-linear Optics, Wiley, New York, 

1992. 

w38x V. Zauls, S. Schrader, C. Flueraru, H. Motschmann, in preparation.

Linear and nonlinear optical properties of Langmuir–Blodgett ...

Create successful ePaper yourself

Delete template?

Save as template?