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Studies on the second harmonic generation of bilayer assembly structures
Thin Solid Films, 179 (1989)381-385
381
STUDIES ON THE SECOND HARMONIC GENERATION OF BILAYER ASSEMBLY STRUCTURES FENGGANGTAO AND LINXIAOXU Department of Chemistry, Fudan University, Shanghai (China)
GANG CHEN, XINLIANGYANG~GONGMINGWANG, JIABIAOZHENG, ZHIMINGZHANG AND WENCHENGWANG Department of Physics, Fudan University, Shanghai (China}
(ReceivedApril 25. 1989;acceptedMay25, 1989) Bilayer assembled Langmuir-Blodgett (LB) films were prepared with 11methacrylaminoundecanonic acid and stearic acid as the layer materials and the magnitudes and signs of the effective non-linear optical coefficients were studied by the optical second harmonic generation technique. It was found that the polarizabilities of these two compounds have opposite signs and their second-order nonlinear coefficients were superposed in the bilayer assembled LB films. This simple model offers us a new way to construct LB films with high non-linear polarizabilities by alternating the assembled multilayers. Furthermore, the polymerization of the LB film of 11-methacrylaminoundecanonicacid under irradiation with UV light was also investigated by the attenuated total reflection method.
1. INTRODUCTION In recent years, Langmuir-Blodgett (LB) films have been investigated intensively because of their potential applications 1. In addition, because more and more organic compounds have been found to have excellent non-linear optical properties 2, research on the non-linear optical properties of LB films has received increasing attention because they offer a way to prepare non-linear optical film devices. It is well known that the molecular arrangement in the LB films exhibits a definite symmetry so that non-linear polarization in the vertical direction of the layer plane can be produced in this direction under an external field and the corresponding second-order non-linear polarization should be orientational. However, in most LB films with multilayers, the arrangement of the molecules in the adjacent layers is usually Y type, i.e. arranged with hydrophilic end to hydrophilic end and hydrophobic end to hydrophobic end. Therefore, their overall second-order non-linear susceptibilities ~({2} should be equal to zero because of the opposite orientations of the non-linear polarizations in the adjacent layers. This means that LB films with high second-order non-linear coefficients are almost impossible to be obtained by multideposition of the same layer material. However, the alternating LB film assembly technique using two or more different layer materials with opposite 0040-6090/89/$3.50
© ElsevierSequoia/Printedin The Netherlands
382
E. T A O et al.
signs of the polarizability seems to be a feasible method for overcoming this difficulty. In this work, two simple layer materials, 1l-methacrylaminoundecanonic acid (molecule A) and stearic acid (molecule B), were used to prepare the bilayer assembly structure. The magnitudes and signs of the effective non-linear optical coefficients of these two layer materials and their two different assembly patterns were studied by the optical second harmonic generation (SHG) technique. 2. EXPERIMENTAL DETAILS
2.1. The preparation of layer materials 11-methacrylaminoundecanonic acid was prepared by Bats and Koldehoff's method 3. After purification by column chromatography, 1 l-methacrylaminoundecanonic acid with over 99~ purity was obtained by high performance liquid chromatography. The melting point was 66-68 °C. Values of 6 (ppm) were 1.25 (m, 16H, 8 × CH2), 2.00 (bs, 3H, CH3), 2.35 (t, 2H, CH2CO), 3.31 (m, 2H, CH2N ), 5.275.60 (m, 2H, CH2=). These values were the same as those reported in the literature 3. Stearic acid was purified by column chromatography.
2.2. Optical measurements The experimental arrangement for SHG measurement is shown in Fig. 1. A mode-locked Nd:YAG laser giving 30 ps pulses of 1064 nm wavelength at 10 Hz was used. The intensity and polarization of the laser beam were adjusted by a pair of Glam prisms. A p-polarized laser beam with an output energy of 0.1 mJ per pulse and a 1 mm beam diameter was incident onto the sample, which was placed on a rotating stage. The angle of incidence 0 could be changed by rotating the stage. The filter was used to ensure that only second harmonic radiation (or the fundamental frequency) was detected. The SHG (or attenuated total reflection (ATR)) signals of the sample were detected by a photomultiplier tube (PMT) and were fed into the boxcar averager. Meanwhile, another laser beam as a reference signal was also fed into the boxcar after passing through a quartz crystal to reduce the influence caused by fluctuation of the laser intensity on the experimental results.
Mode-locked Nd:YAG laser , spliter t _.
~
prism
filter
h~. . . . ])Mr
si., I prism
~
. . . . fi~er
~
s
I z ~ r a L--w --a
F--]
sample p l a t e X-Yrecorder Fig. 1. The experimental arrangement.
383
SHG OF BILAYER ASSEMBLY STRUCTURES
2.3. The preparation of Langmuir-Blodgen films The silver film was evaporated onto a substrate of dense flint glass. The thickness of the silver film was about 50 nm. The dense flint substrate was placed in optical contact with the prism with an adequate matching oil. The LB films were deposited on the surface of the silver film by the usual LB technique. 3.
RESULTS AND DISCUSSION
Molecule A was prepared as follows: CH 3
CH 3
I
t
HEC=C__COOH
soo2
, H2C = C - - - C O C I CH3
I NH2(CH2)toCOOH , H 2 C = C _ _ C O . ~ N H ( C H 2 ) l o C O O H
The methacrylic acid was converted into the corresponding methacryl chloride by using thionyl chloride, which was then reacted with ~-aminoundecanonic acid to provide A. The surface-enhanced optical SHG of LB films was measured by taking advantage of the excitation of surface plasmon waves (SPWs) with the Kretschmann configuration 4,s as shown in Fig. 2. When the bare silver film was measured, the enhanced signal of the reflected SHG originated either from the second-order susceptibility Z(2) at the surface of the silver film or from the contributions of electric quadrupoles and magnetic dipoles in the bulk silver. When the LB film was deposited on the silver film, the second-order susceptibility of the sample should be the superposition of those from both the bare silver and the LB film. The silver film in this case played two roles: (1) the enhanced SHG produced by the excitation of SPWs at the surface of the silver film could be measured more easily; (2) the SHG signal of the bare silver could be used as a standard. The magnitudes and signs of the second-order susceptibilities ;((2)of the LB films deposited on the silver film could be obtained by comparing the signal of the silver film with that of the LB film. It was found that the relative SHG intensity lsn G obtained from the monolayer of molecule A deposited on the silver film was less than that of the bare silver film, 2co
.
.
.
.
.
F t 1.
lllllll L.-B F'i.lm Fig. 2. The Kretschmannconfigurationfor SPW excitation.
384
F. TAO et al.
but that for the monolayer of molecule B was greater than that of the bare silver film. This means that the sign of the effective second-order non-linear coefficient of molecule A was opposite to that of molecule B. As expected, Isn~ of the bilayer systems prepared with either molecule A or molecule B was close to that of the bare silver film because of self-cancellation of the contributions from each layer (Fig. 3). However, when the bilayer was assembled as Ag + A + B, Isno became even less than that from a monolayer ofAg + A whereas, when the bilayer was assembled as Ag + B + A, its IsriG value was much greater than that from the monolayer ofAg + B (Fig. 4). It was shown in both cases that the second-order non-linear coefficients of the two different layer materials in the bilayer assembly structures were superposed on each other.
0
Ag Ag i A
~
Ag.t-B+A~
+B Ag+R I +B AR+A+ O X
37"
36.5 °
37"
0 0 Fig. 3. Comparisons oflsa ~ values of LB films consisting of a single molecular species with that of a bare silver film. Fig. 4. Comparisons of Is. ~ values ot assembled LB films with that of a bare silver film.
Using the value of the second-order non-linear polarizability of stearic acid, i.e. ~(2) ¢;~¢B) = 0.07 × 10- 3o e.s.u, which was measured on a water surface by Rasing et a]. 6, we could evaluate ~ for molecule A: /SHG OC IpNLI 2
=
pNL_t_ ~oNL ~Ag ~ ~LB
2
pr~L oc ~¢2):EE = N~(3~:EE where Pis the non-linear polarization intensity of the silver film or the LB film, N is the molecular surface density of the LB film, Eis the electric field intensity, ~(2) is the second-order susceptibility tensor of the LB film and a (2) is the second-order molecular polarizability tensor. To take account of the decay of the electric field, the electric field intensity in the centre of the molecule was taken as the average intensity. Also only the ot~ component was considered within a~t2). Then A/'N(2) ~' 2 e x p ( _ d c / 6 2 ) PLNL OC,, ~¢~¢~0
where Eo is the electric field intensity on the surface of the silver film, N is the
385
SHG OF BILAYER ASSEMBLY STRUCTURES
molecular surface density, ~2is the decay length and dc is the thickness of the LB film. Here the correction for local fields is not considered. In our experiments, the molecular surface densities N of molecules A and B were 4 x 1014 cm-2. Therefore the non-linear polarizability of molecule A could be obtained: (2)
{IsH°(Ag+A)}l/2--{lsH°(Ag)}l/2exp
~ ( A ) = (isrm(Ag + B)} 1/2 _ (isrm(Ag)} 1/2
de(B)- d~(A)~ (2) -~2 j ~¢~(B)
= -0.16 x 10 -3° e.s.u. Similarly, the second-order non-linear polarizabilities of the bilayer assembly ~2~ B + A) structures were calculated to be ct~(A + B) = -0.25 x 10-ao e.s.u, and ere;c( = 0.23 x 10- 30 e.s.u. When the LB film of molecule A was prepared, its polymerization under UV light irradiation (253.7 nm, 30 W) was also investigated by use of the ATR method. It was found that the ATR spectrum of this LB film changed substantially during the polymerization process. According to a method reported in the literature 7, the thickness of the LB film could be determined from its ATR spectra. The thickness was found to be 1.25 nm for each monolayer of the unpolymerized molecule A and 2.14 nm for that of the polymerized molecule A according to the corresponding ATR spectra, of which the latter was identical to the theoretical thickness (about 2.1 nm by calculation). This indicated that the monomeric molecules were arranged in the layers in such a way that they were not perpendicular to the plane of the substrate but rather that they maintained a certain gradient (about 36 °) to it. Although the second-order non-linear polarizabilities of the assembled LB films in the present work are still not high enough, the experimental evidence has been provided that LB films with high non-linear polarizabilities can be constructed by the assembly technique. Our further research into the construction of assembled LB films using the layer materials with higher values of at~ is now under way. REFERENCES 1 M. Sugi, J. Mol. Electron., 13-17 (1985) 3. 2 D.S. Chemla and J. Zyss, Nonlinear Optical Properties of Organic Molecules and Crystals, 2 vols., Academic Press, Orlando, FL, 1987, and references cited therein. 3 H. Bats and J. Koldehoff, Makromol. Chem., 177 (1976) 683. 4 H.J. Simon, D. E. Mitchell and J. G. Watson, Phys. Rev. Left., 33 (1974) 1531. 5 Z. Chen, W. Chen, J. Zheng, W. Wang and Z. Zhang, Opt. Commun., 54 (1985) 305, 6 T. Rasing, G. Berkovic, Y. R. Shen, S. G. Grubb and M. W. Kim, Chem. Phys. Lett., 130 (1986) 1. 7 X. Yang, D. Li Chen and W. Wang, Acta Opt. Sin., 5 (1985) 557.
381
STUDIES ON THE SECOND HARMONIC GENERATION OF BILAYER ASSEMBLY STRUCTURES FENGGANGTAO AND LINXIAOXU Department of Chemistry, Fudan University, Shanghai (China)
GANG CHEN, XINLIANGYANG~GONGMINGWANG, JIABIAOZHENG, ZHIMINGZHANG AND WENCHENGWANG Department of Physics, Fudan University, Shanghai (China}
(ReceivedApril 25. 1989;acceptedMay25, 1989) Bilayer assembled Langmuir-Blodgett (LB) films were prepared with 11methacrylaminoundecanonic acid and stearic acid as the layer materials and the magnitudes and signs of the effective non-linear optical coefficients were studied by the optical second harmonic generation technique. It was found that the polarizabilities of these two compounds have opposite signs and their second-order nonlinear coefficients were superposed in the bilayer assembled LB films. This simple model offers us a new way to construct LB films with high non-linear polarizabilities by alternating the assembled multilayers. Furthermore, the polymerization of the LB film of 11-methacrylaminoundecanonicacid under irradiation with UV light was also investigated by the attenuated total reflection method.
1. INTRODUCTION In recent years, Langmuir-Blodgett (LB) films have been investigated intensively because of their potential applications 1. In addition, because more and more organic compounds have been found to have excellent non-linear optical properties 2, research on the non-linear optical properties of LB films has received increasing attention because they offer a way to prepare non-linear optical film devices. It is well known that the molecular arrangement in the LB films exhibits a definite symmetry so that non-linear polarization in the vertical direction of the layer plane can be produced in this direction under an external field and the corresponding second-order non-linear polarization should be orientational. However, in most LB films with multilayers, the arrangement of the molecules in the adjacent layers is usually Y type, i.e. arranged with hydrophilic end to hydrophilic end and hydrophobic end to hydrophobic end. Therefore, their overall second-order non-linear susceptibilities ~({2} should be equal to zero because of the opposite orientations of the non-linear polarizations in the adjacent layers. This means that LB films with high second-order non-linear coefficients are almost impossible to be obtained by multideposition of the same layer material. However, the alternating LB film assembly technique using two or more different layer materials with opposite 0040-6090/89/$3.50
© ElsevierSequoia/Printedin The Netherlands
382
E. T A O et al.
signs of the polarizability seems to be a feasible method for overcoming this difficulty. In this work, two simple layer materials, 1l-methacrylaminoundecanonic acid (molecule A) and stearic acid (molecule B), were used to prepare the bilayer assembly structure. The magnitudes and signs of the effective non-linear optical coefficients of these two layer materials and their two different assembly patterns were studied by the optical second harmonic generation (SHG) technique. 2. EXPERIMENTAL DETAILS
2.1. The preparation of layer materials 11-methacrylaminoundecanonic acid was prepared by Bats and Koldehoff's method 3. After purification by column chromatography, 1 l-methacrylaminoundecanonic acid with over 99~ purity was obtained by high performance liquid chromatography. The melting point was 66-68 °C. Values of 6 (ppm) were 1.25 (m, 16H, 8 × CH2), 2.00 (bs, 3H, CH3), 2.35 (t, 2H, CH2CO), 3.31 (m, 2H, CH2N ), 5.275.60 (m, 2H, CH2=). These values were the same as those reported in the literature 3. Stearic acid was purified by column chromatography.
2.2. Optical measurements The experimental arrangement for SHG measurement is shown in Fig. 1. A mode-locked Nd:YAG laser giving 30 ps pulses of 1064 nm wavelength at 10 Hz was used. The intensity and polarization of the laser beam were adjusted by a pair of Glam prisms. A p-polarized laser beam with an output energy of 0.1 mJ per pulse and a 1 mm beam diameter was incident onto the sample, which was placed on a rotating stage. The angle of incidence 0 could be changed by rotating the stage. The filter was used to ensure that only second harmonic radiation (or the fundamental frequency) was detected. The SHG (or attenuated total reflection (ATR)) signals of the sample were detected by a photomultiplier tube (PMT) and were fed into the boxcar averager. Meanwhile, another laser beam as a reference signal was also fed into the boxcar after passing through a quartz crystal to reduce the influence caused by fluctuation of the laser intensity on the experimental results.
Mode-locked Nd:YAG laser , spliter t _.
~
prism
filter
h~. . . . ])Mr
si., I prism
~
. . . . fi~er
~
s
I z ~ r a L--w --a
F--]
sample p l a t e X-Yrecorder Fig. 1. The experimental arrangement.
383
SHG OF BILAYER ASSEMBLY STRUCTURES
2.3. The preparation of Langmuir-Blodgen films The silver film was evaporated onto a substrate of dense flint glass. The thickness of the silver film was about 50 nm. The dense flint substrate was placed in optical contact with the prism with an adequate matching oil. The LB films were deposited on the surface of the silver film by the usual LB technique. 3.
RESULTS AND DISCUSSION
Molecule A was prepared as follows: CH 3
CH 3
I
t
HEC=C__COOH
soo2
, H2C = C - - - C O C I CH3
I NH2(CH2)toCOOH , H 2 C = C _ _ C O . ~ N H ( C H 2 ) l o C O O H
The methacrylic acid was converted into the corresponding methacryl chloride by using thionyl chloride, which was then reacted with ~-aminoundecanonic acid to provide A. The surface-enhanced optical SHG of LB films was measured by taking advantage of the excitation of surface plasmon waves (SPWs) with the Kretschmann configuration 4,s as shown in Fig. 2. When the bare silver film was measured, the enhanced signal of the reflected SHG originated either from the second-order susceptibility Z(2) at the surface of the silver film or from the contributions of electric quadrupoles and magnetic dipoles in the bulk silver. When the LB film was deposited on the silver film, the second-order susceptibility of the sample should be the superposition of those from both the bare silver and the LB film. The silver film in this case played two roles: (1) the enhanced SHG produced by the excitation of SPWs at the surface of the silver film could be measured more easily; (2) the SHG signal of the bare silver could be used as a standard. The magnitudes and signs of the second-order susceptibilities ;((2)of the LB films deposited on the silver film could be obtained by comparing the signal of the silver film with that of the LB film. It was found that the relative SHG intensity lsn G obtained from the monolayer of molecule A deposited on the silver film was less than that of the bare silver film, 2co
.
.
.
.
.
F t 1.
lllllll L.-B F'i.lm Fig. 2. The Kretschmannconfigurationfor SPW excitation.
384
F. TAO et al.
but that for the monolayer of molecule B was greater than that of the bare silver film. This means that the sign of the effective second-order non-linear coefficient of molecule A was opposite to that of molecule B. As expected, Isn~ of the bilayer systems prepared with either molecule A or molecule B was close to that of the bare silver film because of self-cancellation of the contributions from each layer (Fig. 3). However, when the bilayer was assembled as Ag + A + B, Isno became even less than that from a monolayer ofAg + A whereas, when the bilayer was assembled as Ag + B + A, its IsriG value was much greater than that from the monolayer ofAg + B (Fig. 4). It was shown in both cases that the second-order non-linear coefficients of the two different layer materials in the bilayer assembly structures were superposed on each other.
0
Ag Ag i A
~
Ag.t-B+A~
+B Ag+R I +B AR+A+ O X
37"
36.5 °
37"
0 0 Fig. 3. Comparisons oflsa ~ values of LB films consisting of a single molecular species with that of a bare silver film. Fig. 4. Comparisons of Is. ~ values ot assembled LB films with that of a bare silver film.
Using the value of the second-order non-linear polarizability of stearic acid, i.e. ~(2) ¢;~¢B) = 0.07 × 10- 3o e.s.u, which was measured on a water surface by Rasing et a]. 6, we could evaluate ~ for molecule A: /SHG OC IpNLI 2
=
pNL_t_ ~oNL ~Ag ~ ~LB
2
pr~L oc ~¢2):EE = N~(3~:EE where Pis the non-linear polarization intensity of the silver film or the LB film, N is the molecular surface density of the LB film, Eis the electric field intensity, ~(2) is the second-order susceptibility tensor of the LB film and a (2) is the second-order molecular polarizability tensor. To take account of the decay of the electric field, the electric field intensity in the centre of the molecule was taken as the average intensity. Also only the ot~ component was considered within a~t2). Then A/'N(2) ~' 2 e x p ( _ d c / 6 2 ) PLNL OC,, ~¢~¢~0
where Eo is the electric field intensity on the surface of the silver film, N is the
385
SHG OF BILAYER ASSEMBLY STRUCTURES
molecular surface density, ~2is the decay length and dc is the thickness of the LB film. Here the correction for local fields is not considered. In our experiments, the molecular surface densities N of molecules A and B were 4 x 1014 cm-2. Therefore the non-linear polarizability of molecule A could be obtained: (2)
{IsH°(Ag+A)}l/2--{lsH°(Ag)}l/2exp
~ ( A ) = (isrm(Ag + B)} 1/2 _ (isrm(Ag)} 1/2
de(B)- d~(A)~ (2) -~2 j ~¢~(B)
= -0.16 x 10 -3° e.s.u. Similarly, the second-order non-linear polarizabilities of the bilayer assembly ~2~ B + A) structures were calculated to be ct~(A + B) = -0.25 x 10-ao e.s.u, and ere;c( = 0.23 x 10- 30 e.s.u. When the LB film of molecule A was prepared, its polymerization under UV light irradiation (253.7 nm, 30 W) was also investigated by use of the ATR method. It was found that the ATR spectrum of this LB film changed substantially during the polymerization process. According to a method reported in the literature 7, the thickness of the LB film could be determined from its ATR spectra. The thickness was found to be 1.25 nm for each monolayer of the unpolymerized molecule A and 2.14 nm for that of the polymerized molecule A according to the corresponding ATR spectra, of which the latter was identical to the theoretical thickness (about 2.1 nm by calculation). This indicated that the monomeric molecules were arranged in the layers in such a way that they were not perpendicular to the plane of the substrate but rather that they maintained a certain gradient (about 36 °) to it. Although the second-order non-linear polarizabilities of the assembled LB films in the present work are still not high enough, the experimental evidence has been provided that LB films with high non-linear polarizabilities can be constructed by the assembly technique. Our further research into the construction of assembled LB films using the layer materials with higher values of at~ is now under way. REFERENCES 1 M. Sugi, J. Mol. Electron., 13-17 (1985) 3. 2 D.S. Chemla and J. Zyss, Nonlinear Optical Properties of Organic Molecules and Crystals, 2 vols., Academic Press, Orlando, FL, 1987, and references cited therein. 3 H. Bats and J. Koldehoff, Makromol. Chem., 177 (1976) 683. 4 H.J. Simon, D. E. Mitchell and J. G. Watson, Phys. Rev. Left., 33 (1974) 1531. 5 Z. Chen, W. Chen, J. Zheng, W. Wang and Z. Zhang, Opt. Commun., 54 (1985) 305, 6 T. Rasing, G. Berkovic, Y. R. Shen, S. G. Grubb and M. W. Kim, Chem. Phys. Lett., 130 (1986) 1. 7 X. Yang, D. Li Chen and W. Wang, Acta Opt. Sin., 5 (1985) 557.