Langmuir-Blodgett films and non-linear optics

Langmuir-Blodgett films and non-linear optics

Thin Solid Films, 152 (1987) 265 274 265 L A N G M U I R - B L O D G E T T FILMS A N D N O N - L I N E A R OPTICS* GARO KHANARIAN Hoechst-Celanese ...

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Thin Solid Films, 152 (1987) 265 274

265

L A N G M U I R - B L O D G E T T FILMS A N D N O N - L I N E A R OPTICS* GARO KHANARIAN

Hoechst-Celanese Corporation, 86 Morris Avenue, Summit, NJ07901 (U.S.A.)

Recent work on the non-linear optical and electro-optical properties of Langmuir-Blodgett monolayers and multilayers is reviewed. The fundamental theory of non-linear optical susceptibilities and interactions is discussed, as well as their relationship to molecular hyperpolarizabilities. Among the non-linear optical experiments discussed are second and third harmonic generation, d.c. electric field induced second harmonic generation and the Pockels linear electro-optic effect. It is shown that surface second harmonic generation is a powerful tool for studying the orientation of molecules and their hyperpolarizabilities. Second harmonic generation from multilayers is also useful in understanding their structure and interactions. The interactions between monolayers and metallic surfaces are also studied by surface-plasmon-enhanced second harmonic generation. Finally, third harmonic generation is used to study the electronic spectra and resonances (twoand three-photon absorption) on very thin multilayers of polydiacetylenes. It is concluded that non-linear optics is a powerful and useful tool for the study of Langmuir-Blodgett films, and that there is a good understanding of fundamental theoretical and experimental problems. However, the details of the interpretation of non-linear optical experiments are not fully understood. Consistent results are not always obtained from the same sample. The internal optical electric field needs to be understood more fully. Electro-optic studies are still in their infancy, and they would complement the non-linear optical studies.

l. INTRODUCTION Langmuir-Blodgett (LB) films have received great attention in recent years 1-3 as a means of fabricating controlled and highly oriented superlattice structures. The ability to add one monolayer at a time means that supermolecular structures on a nanometer scale can be built up in a controlled fashion. These films have been studied by many techniques, including non-linear optics and electro-optics. Apart * Paper presented at a Workshop on the Molecular Engineeringof Ultrathin PolymericFilms, Davis, CA, U.S.A., February 18-20, 1987. 0040-6090/87/$3.50

© ElsevierSequoia/Printedin The Netherlands

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from measuring the non-linear susceptibilities, these techniques are particularly useful because they are sensitive to the symmetry of molecular packing and the orientation to the surface. In this paper, we review some fundamental aspects of non-linear optics as it pertains to LB films and discuss some recent non-linear optical experiments. We assess the present status of this rapidly growing area of research, highlight the areas that need more attention and recommend future directions. 2. NON-LINEAR POLARIZATION AND WAVES4

When a non-linear optical medium is subject to an electric field, it is polarized. The polarization is expanded in powers of the electric field, thus P = Z~).E+z~):EE+V.(2~):EE)+Z~)iEEE

(1)

Z~d) in eqn. (1) denotes the linear susceptibility of the medium, whereas the remaining terms denote the non-linear polarization. The term that is quadratic in the electric field has two contributions arising from dipolar Zip2) and quadrupolar Z~ ) susceptibilities 5. For a clean interface with no LB film on it, second-order interactions (for example, second harmonic generation) arise solely from the quadrupolar term whose physical origin lies in the dielectric constant and material discontinuity at the interface, the rapid variation in the electric field across the interface and the non-local magnetic dipole contribution from the bulk of the material 6'7. When there is an asymmetric LB film on a substrate, then the dipolar dominates the quadrupolar susceptibility, and second harmonic generation can be used to study the LB film itself. The third-order susceptibility can also be used to study thin films although, in general, it is less sensitive to symmetry and order than the second-order tensor Z~ ). The second- and third-order susceptibility tensors give rise to a number of nonlinear optical and electro-optical effects, which are summarized in Table I. The Pockels effect is the change in the optical dielectric constant in an applied d.c. electric field; second harmonic generation and third harmonic generation are the frequency doubling and tripling of light respectively, and electric-field-induced second harmonic generation will be described in Section 7. It is emphasized that both the Pockels effect and second harmonic generation occur in non-centrosymmetric media, and this has important implications for the fabrication of and interpretation of results for LB films. Having described the nature of the non-linear polarization of thin films, we now describe the propagation of electromagnetic waves. For a monochromatic wave TABLE I RELATION OF N O N - L I N E A R SUSCEPTIBILITY TENSORS TO N O N - L I N E A R O P T I C A L EFFECTS

Z(2) ( - 2co, co, o~) Z(3) ( - 2o), ~ , ~ , 0 ) Z(3) ( - 36'2,o9, oh co)

Pockels linear electro-optic effect Second harmonic generation D.c. electric-field-induced second harmonic generation (DCSHG) Third harmonic generation

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incident on a thin optically non-linear film bounded by linear media (say air or glass), the time-independent non-linear Maxwell equation is (o 2 {(V × V × )-- ~ - ~ } E ( k , (o) = 47~(o2, NL ~T-

(2)

In eqn. (2), pNL is the non-linear part of the polarization of eqn. (1), k is the propagation vector, (o is the angular frequency, c the speed of light and e is the dielectric constant. Equation (2) is solved subject to the boundary conditions at the interfaces, for the electric field E and magnetic field B at an interface. In general, there are three electric fields present in the non-linear medium; a transmitted wave/~ and reflected wave E r , and a bound wave F~ which arises from the non-linear polarization 8'9. Thus F_.NL = /~--~--Erq-F_.b

(3)

4np NL ~((o)- e(2(o) It should be noted that the dielectric constant of the non-linear medium enters directly in the problem of evaluating the electric fields. It also enters in the final expressions when the boundary conditions are taken into account. Thus there is a need to know the dielectric constants of an LB film accurately to evaluate its nonlinear optical properties. The final expression for the output intensity is given by

I = c8n e l F~LI2

(4)

It should be noted that, from eqns. (1), (3) and (4), we deduce that second harmonic generation scales as the square of the input intensity, while for third harmonic generation it scales as the cube of the input intensity. 3. RELATION OF THE SUSCEPTIBILITY TO MOLECULAR HYPERPOLARIZABILITY Molecules with large hyperpolarizabilities have a large charge transfer on optical excitation, causing a large difference between the ground and excited state dipole moments. These excitations involve ~ to n* transitions in one predominant direction, causing one term fl,zz to dominate over all the other terms 1°. This is usually along the dipole moment direction for molecules that have cylindrical symmetry (for example, merocyanine dye). In an LB film, such a molecule will be tilted at an angle 0 relative to the interface normal. Thus, the relation between the molecular hyperpolarizability and the susceptibility tensor components becomes 11-1s Zt2) = m f 3 f l ( c o s 3 0 )

(5) Zt2L = N f 3 fl(sin20 cos 0) 2 In eqn. (5), N is the surface number density of molecules, fl is the dominant

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contribution to the hyperpolarizability and ( ) denotes averages of orientation functions. If one can assume that the distribution of orientations is very narrow (say, about 0o), then o~e can ignore averaging ( ) and so, from a measurement Of Z~2)~and Z~2) .... one can deduce the two unknowns, fl and the molecular tilt angle 0 o. f denotes the internal field factor, which is usually approximated by the Lorentz formula f -

nZ+2 3

(6)

In the above equation, n is the refractive index. This formula was derived for a molecule in a spherical environment. In fact, molecules in LB films find themselves in highly ordered states causing a change in the local optical electric field not accounted for by eqn. (6). Thus it has been shown 6'17 that the effective ~(2) of a highly ordered arrangement of molecules changes considerably with the linear susceptibility Ztl). This is because of induced dipole-induced dipole interactions between molecules. Local field effects need to be studied more extensively, theoretically and experimentally, to enable meaningful molecular parameters such as hyperpolarizabilities and tilt angles to be extracted. 4.

SECOND HARMONIC GENERATION FROM MONOLAYERS

Shen and coworkers have pioneered the study of monolayers and interfaces by second harmonic generation 4'6'7'11':5'18 21. Heinz e t al. 15 studied resonanceenhanced second harmonic generation from rhodamine dye adsorbed on fused silica. By changing the polarization of the incident radiation and studying the output polarization, they measured Z=~ t2) a n d X~xx. t2) Th erefore, from eqn. (5), they deduced that 0o = 34 ° and fl = 30 x 10 -3o esu. These studies were extended by Heinz e t al. 11 to p-nitrobenzoic acid adsorbed on silica with either air or ethanol at the interface. They deduced that the tilt angle 0o was equal to 38 ° and 70 ° for the ethanol-silica and air-silica interfaces respectively. The hyperpolarizability fl was 0.7 × 10-3o esu at 1.06 ~tm incident radiation, in fair agreement with calculations :°. Rasing e t al.X 8 used similar techniques to study the orientation of a monolayer of sodium dodecylnaphthalene sulphonate in a Langmuir trough as a function of surface pressure. The reflected second harmonic generation signal was measured from the m o n o l a y e r - a i r interface as the film was compressed. They concluded that the average polar tilt angle decreased from 35 ° to 30 ° going from the gas phase to the compressed state of the monolayer. Rasing e t al. 2° and Berkovic e t al. 2 ~ extended the study of compressed monolayers on water to cyanobiphenyis and fatty acids with different alkane chain lengths. Using a frequency-doubled YAG laser beam (0.53 ~tm), they deduced the hyperpolarizabilities and average tilt angles. For the cyanobiphenyls, they found fl = 25 x 10 -3o esu and 00 = 71 °, while for the fatty acids fl = 0.1 x 10 3o esu and 0o = 50 °. They found no dependence of these two parameters on the length of the alkane chain. Berkovic e t a l ) 9 showed the power and versatility of surface second harmonic generation in studying chemical problems by following the kinetics of polymerization of a monolayer of vinyl stearate and octadecyl methacrylate on a water surface. The initial monomer gave a larger second harmonic generation signal than the polymer and so there was a

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decrease in second harmonic generation with time after polymerization was initiated. Girling e t al. 22 studied the deposition of a monolayer and multilayers of a merocyanine dye by second harmonic generation. These dyes protonate when exposed to the atmosphere and lose their non-linear optical activity. They regain their activity in an ammonia atmosphere. From polarization studies, they deduced that the molecules were at an angle of 0o = 10° to the glass surface normal and fl = 500 x 10- 30 esu. The incident radiation was 0.53 ~tm and so there was resonant enhancement of ft. A bilayer gave no second harmonic generation, indicating a centrosymmetric film, which is consistent with the Y-type deposition characteristics of this molecule. 5. SECOND H A R M O N I C G E N E R A T I O N FROM MULTILAYERS

Second harmonic generation is seen from multilayers if the film is noncentrosymmetric. This can be obtained in LB films with Z-type deposition which gives inherently polar films but which occurs infrequently in nature because molecular dipoles prefer antiparallel alignment. The other approach is Y-type deposition where the non-linear optical molecule is deposited alternately with an inert molecule, or with a molecule whose hyperpolarizability is of the opposite sign. In the latter case, the dipoles are antiparallel, which is energetically preferred, but the hyperpolarizabilities are additive. Girling e t al. 23 reported second harmonic generation from multilayers of merocyanine dyes alternating with ~o-tricosenoic acid. As in the earlier experiment 22, they performed their experiment in an ammonia atmosphere. They observed a superlinear dependence of second harmonic generation intensity with the number of layers, but it was not quadratic as expected from theory. In a more detailed study, they chose the more stable hemicyanine dye 24 as the active layer, alternating with ~o-tricosenoic acid. They performed second harmonic generation experiments in transmission and reflection mode with all possible combinations of input and output polarizations. They deduced fl = 170 × 10-30 esu with an average tilt angle of 23 ° relative to the normal. As in the earlier study 23, they did not see a quadratic dependence of second harmonic generation signal with the number of layers, and this was interpreted to mean that the first layer was more highly ordered than the second, third etc. layers. They also discussed the fact that they did not obtain self-consistent results for fl and 0o from all their polarization experiments. The main reasons are the uncertainty of the optical dielectric constants of the LB films and the internal field. The alternation with a fatty acid which is not optically active is not very efficient for second harmonic generation. A more efficient method is described by Neal e t a/. 25'26, who alternated the hemicyanine dye with an amino-nitrostilbene dye. The latter molecule has the property that its hyperpolarizability is of the opposite sign to the hemicyanine dye molecule. Thus, even though the deposition is Y type and the film is non-polar, the net hyperpolarizabilities are additive. Experimentally, they showed that there was an enhancement in the second harmonic generation signal from the LB film. They deduced that the hyperpolarizabilities fl for hemicyanine and

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amido-nitrostilbene dyes w e r e 300×10 -30 esu and 55 × 10 -30 esu respectively. Interestingly, the effective fl for the bilayer (850 x 10- 30 esu) was considerably more than the sum of.the two component molecules, indicating a possible interaction between the two layers. This study was extended by Neal e t al. 43 to study up to four alternating multilayers, and they confirmed that there was an interaction between the hemicyanine and amino-nitrostilbene dye molecules. They showed that the second harmonic generation intensity dependence on the number of layers was more than quadratic. In the same paper, they also studied a monolayer of a mixture of hemicyanine dye with a fatty acid. They measured second harmonic generation intensity as a function of mole fraction of the hemicyanine dye. They found the remarkable result 44 that the maximum second harmonic generation signal occurred for a film containing equimolar quantities of the optically active dye and the fatty acid and not for the 100~o hemicyanine dye film. They observed four times the intensity for the equimolar film than for the 100~ hemicyanine dye film. The most likely explanation for this result is the change in the local optical electric field as a function of the concentration of the optically active species. Preliminary studies of second harmonic generation from multilayers have recently been published by Allen e t al. 27 and Kowel et al. 2s In the first study, they deposited an azo dye molecule and observed both Y-and Z-type deposition. In both instances they observed second harmonic generation, indicating that the first layer was responsible for second harmonic generation. This was corroborated by the fact that they did not see the expected increase in second harmonic generation with the number of layers. In the second study 2s, a mixture of a hemicyanine dye with polymethylmethacrylate was deposited and Z-type deposition obtained. Again, as in other studies, they failed to see a quadratic dependence of the second harmonic generation signal on the number of layers, indicating a disordering of the multilayer. 6.

SURFACE-ENHANCED SECOND HARMONIC GENERATION FROM PLASMONS

From earlier work on surface-enhanced Raman scattering 29, it was shown that there was an enhancement of the local electric field near a metallic particle. This occurred near the plasmon resonance frequency of a metal particle. It was demonstrated by Chen and coworkers 3°'31 that the same mechanism also leads to enhanced surface second harmonic generation from metal particles and surfaces. This has been used to study an adsorbed submonolayer of pyridine molecules on silver surfaces by Heskett e t al. 32 It can also be shown that electromagnetic waves can propagate along a metal surface 4. Surface polaritons have two properties, namely that the electric vector is normal to the surface and also that it is confined to a thin waveguide leading to an enhancement of the electric field intensity. For an LB monolayer deposited on a thin silver film, these two properties lead to efficient coupling of the electric field with the largest component ;tt:2~of the tensor ;(2~ and also to an increase in second harmonic generation since it depends on the electric field intensity. Experimentally, a 500/~ silver film is deposited on one face of a glass prism and the LB monolayer is deposited on the metal film. The Kretschmann prism coupling method is used 4 to couple radiation into and out from the prism. Initially, a linear

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optical experiment is carried out to determine accurately the dielectric constants of the metal layer and then that of the LB film, since the coupling angle is a very sensitive function of the dielectric constants and the thickness of the films. These parameters are important for the interpretation of the non-linear second harmonic generation experiment. Then the second harmonic generation intensity is measured as a function of the input coupling angle. By fitting the data to theoretical expressions, it is possible to deduce ZTM and, hence, the hyperpolarizability. Chen et al. 33 used the above technique to study arachidic acid on silver and obtained fl = 3 × 10 -3° esu. Girling et al. 34 and Cross et al. 35 have studied the hemicyanine dye monolayer on silver by this technique and obtained a fl value which was somewhat lower than the value that they had obtained from an earlier study, using second harmonic generation in the transmission mode. They say that the final result is very sensitive to the values used for the dielectric constants and the thicknesses of the films. Thus far, we have reviewed second harmonic generation studies to measure Zt2~ using the surface plasmon technique. Far from resonance, Zt2~ can also be measured by the Pockels linear electro-optic effect. Experimentally, the Kretschmann prism configuration can be easily modified to measure the Pockels effect. The silver layer described above acts as one of the electrodes. The LB monolayer is deposited on the silver. Then a thin spacer is inserted to separate it from an electrode-coated glass plate, which acts as the other electrode. Thus the applied d.c. electric field is normal to the silver surface. The dielectric constant of the LB film changes linearly with the applied voltage, and this causes a change in the input coupling angle into the prism. Cross et al.36 have reported the Pockel's effect from a monolayer of hemicyanine dye molecule, and their Zt2~ value is in reasonable agreement with second harmonic generation studies on the same film. 7. THIRD-ORDER EFFECTS IN LANGMUIR--BLODGETT FILMS Third-order non-linear optical effects in LB films arising from their third-order s u s c e p t i b i l i t y ~(3) have been studied less than second-order effects. In general, there

are many mechanisms that contribute to third-order effects, including molecular orientation, vibrational and electronic effects. The large non-resonant electronic contribution t o ~(3) has attracted great attention in cases of conjugated polymers such as polydiacetylenes and polyacetylenes 37'38. An experimental technique for studying the electronic contribution to Zt3) is third harmonic generation. Third harmonic generation measurements On polydiacetylene LB films have been reported by Kajzar and coworkers 39-41. Experimentally, multilayers of cadmium salts of diacetylene m o n o m e r are deposited onto glass substrates and then polymerized by UV radiation. Because the multilayers are very thin and absorption is low, it is possible to measure third harmonic generation when either the fundamental or the harmonics correspond to a one-, two- or three-photon absorption. They show that there is a two-photon resonance at a wavelength of 1.35 ~tm and Z~3~ = 1.5 × 10- zo esu, and a three-photon resonance at a wavelength of 1.9 ~tm with g t3~ = 2.2 x 10- lo esu. Linear conjugated polymers are also known to be photoconductive. To study

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this effect experimentally, electrodes are placed across a polymer and the conductivity is measured as a function of laser light intensity. The generation of photoconductors in the polymer can lead to interesting polarization effects inside the polymer. These effects were studied by Chollet e t al. 4z by the DCSHG technique. The polymer was the same polydiacetylene used in the earlier third harmonic generation studies 39 41. An LB film was deposited on a glass plate that had two strips of electrodes so that a voltage was applied across the LB film. The d.c. electric field breaks the symmetry of the otherwise isotropic material and second harmonic generation is observed. Since no molecular motion is present, this technique also probes the electronic component ofz ~3).The value they obtained by DCSHG agreed closely with that obtained by third harmonic generation. However, the intense laser beam used for the second harmonic generation experiment also acts to generate photoconductors in the material. They found the interesting result that, when the d.c. electric field is switched off, the second harmonic generation signal did not go to zero immediately, but persisted for several minutes. This was interpreted by a model where the photoconductors are separated by the d.c. electric field, and so they set up their own polarization field inside the polymer. When the d.c. electric field is switched off, there is a remnant electric field until the photoconductors recombine. 8. CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE W O R K

Non-linear optical and electro-optical techniques are powerful methods for the characterization of LB monolayers and multilayers. Second harmonic generation studies from monolayers have provided a wealth of information about adsorption of molecules on surfaces, their orientation relative to the surface and their molecular hyperpolarizabilites. In the case of asymmetric multilayers, second harmonic generation has provided structural information about the deposition and multilayering process. Second harmonic generation has also provided insight into the interaction between metal surfaces and monolayers. Third-order processes (third harmonic generation and DCSHG) have confirmed the electronic origin of the large non-linear susceptibility of conjugated polymers and provided insight into their photoconductivity. At this time, the basic theoretical and experimental approaches to the study of the non-linear optical properties of LB films is well understood. However, the specific details of interpretation are not well understood. For example, LB films are modelled as dielectric slabs which ignore interactions between layers. However, recent results 43 show that this model is too simplistic. Another area is the internal optical electric field in multilayers. Some work on monolayers 17 indicates that induced dipole-induced dipole interactions can modify the effective hyperpolarizability. This will also influence the effective polar tilt angles that we deduce from second harmonic generation studies. Experimentally, electro-optic effects (Pockels and Kerr) in LB films have not been studied extensively. These studies will complement the non-linear optical second and third harmonic generation work. Second harmonic generation has not yet been applied extensively to the study of monolayers on water surfaces. It is envisioned that it can be used for continuous monitoring of LB films and deposition processes.

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ACKNOWLEDGMENTS

I wish to thank Dr. Gunilla Gillberg and Dr. James B. Stamatoff for their comments on this paper. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12

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