- Email: [email protected]
Quadratically enhanced second harmonic generation in polymer-dye Langmuir-Blodgett films: A new bilayer architecture
Synthetic Metals. 28 (1989) D683 D688
D683
QUADRATICALLY ENHANCED SECOND HARMONIC GENERATION IN POLYMER-DYE LANGMU1R-BLODGETTFILMS: A NEW BILAYERARCHITECTURE
B.L. Anderson*, R.C. Hall***, B.G. Higgins*, G. Lindsay***, P. Stroeve*, and S.T. Kowel**. *Department of Chemical Engineering, **Department of Electrical Engineering and Computer Science, University of California, Davis, CA 95616, U.S.A. ***Polymer Science Branch, Chemistry Division, Research Department, N a v a l Weapons Center, China Lake, CA 93555.
ABSTRACT A new double bilayer ABCC dipping sequence for the Langmuir-Blodgett deposition process has resulted in a one hundred-fold increase in second harmonic signal generation for ten ABCC sequences compared to one ABCC sequence. The AB bilayers are comprised of hemicyanine side chains on a polyether backbone. Two different polymer-dye compositions are used in each bilayer in order to have dipoles pointing in the same direction upon "Y" type deposition. Hydrophilic-hydrophilic interactions between polymer-dye bilayers are prevented by interleaving with a CC behenic acid bilayer, which also provides interlayer local field insulation.
INTRODUCTION To achieve large second-order nonlinear optical responses in organic and polymeric thin film systems, the active dye chromophore must possess a large m o l e c u l a r hyperpolarizability and be organized in a film architecture that i s noncentrosymmetric [1]. The Langmuir-Blodgett (L-B) technique provides a method for fabricating thin film systems with this required asymmetry [2]. In addition, the technique allows one to tailor the film architecture so that any cancelling of local field effects from interactions between chromophores in the same layer or adjacent layers is minimized [3]. In this way the overall second-order bulk susceptibility of the film can be optimized. Many L-B dipping protocols have been proposed to realize the theoretically predicted quadratic enhancement in second harmonic generation (SHG) intensity with the number of layers. For a film consisting of a single material, there are two dipping protocols that result in noncentrosymmetric films, X-type (deposition on the 0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
D684
downstroke only) and Z-type (deposition on the upstroke only). Our group [4-5] and others [6] have shown that Z-type multilayers often do not give the expected quadratic increase in the SHG intensity with the number of layers deposited. Quadratic enhancement of SHG intensity in X-type multilayers of up to five layers has been reported, however [7-8], and more recently, Popovitz-Biro et al. [9] describe a class of amphiphiles that form Z-type multilayers that show quadratic enhancement. Noncentrosymmetric Y-type films have been made by interleaving an optically active layer of dye with an inert layer of fatty acid. Girling et al. [10-11] found that when a cyanine dye was interleaved with c0-tricosenoic acid, the enhancement in SHG intensity for 3 dye layers was slightly less than quadratic. Our group has shown that a hemicyanine dye interleaved with behenic acid gave quadratic enhancement for up to 6 dye layers [12], and a hemicyanine dye-substituted polyether interleaved with behenic acid gave quadratic enhancement for 3 polymer layers [13]. To optimize the density of active layers in interleaved structures, some groups have replaced the inert fatty acid layer with a second optically active layer, resulting in a Ytype film. The film is made noncentrosymmetric by biasing the two chromophores. Neal et al. [14] showed that when a hemicyanine dye is interleaved with a long chain nitrostilbene, the enhancement of SHG intensity was greater than quadratic for a film of 3 hemicyanine and 2 nitrostilbene layers. Tredgold et al. [15] showed enhancement of SHG intensity for up to 70 bilayers of a merocyanine dye interleaved with an optically active p r e f o r m e d polymer. However, this SHG enhancement was approximately linear. The lack of quadratic enhancement was attributed to a space charge effect that results from ionization of chromophores. In this paper we report on the deposition of noncentrosymmetric Y-type films formed by interleaving two preformed polymers. Each polymer consists of a hemicyanine chromophore substituted onto a polyether backbone. Two film architectures are studied, interleaved bilayers and non-interleaved bilayers. Enhancement of SHG intensity with the number of bilayers for the two architectures is investigated. In addition, the interaction between chromophores in the same bilayer is examined by studying SHG in single bilayer noncentrosymmetric assemblies. CHEMICAL STRUCTUREAND COMPRESSIONISOTHERMS The chemical structures of the hemicyanine dye-substituted polymers are shown in Fig. 1. The synthesis of these materials is based on the chemical modification of poly (epichlorohydrin) (PECH). Details of the synthesis are given elsewhere [13, 16]. Polymer A has 43 repeat units per molecule with a chromophore attached to 47% of the units. The electron-donating group of the chromophore is adjacent to the polymer
D685
backbone. Polymer B has 11 repeat units per molecule with a chromophore attached to 33% of the units. Note that in polymer B the electron-accepting group is adjacent to the polymer backbone. The hemicyanine chromophores absorb visible light with an absorption peak at 390 nm and an absorption edge near 500 nm (10-4 M in chloroform). The polymers are deep red-colored glasses stable to about 230°C in nitrogen. No crystalline or liquid crystalline transitions were detected with differential scanning calorimetry.
Polymer A c12H25
Polymer B c,.u~7
~(~.~Br-
O i CH_CI CH. d(--CH-CH~ O ~" " C H - C H1~O -- - 4 t
O
CH^CI ~ C' H ~: - CH20~
CICH^ , ,¢C H 2 0 ~ l -x C[-1-
Fig. 1. Structure of hemicyanine dye-substituted polymers. The polymers were spread from a chloroform solution containing approximately 1.5 mgm1-1 of polymer onto a water subphase (pH=5.7+0.3, T=23.5°C). A single compartment commercial trough (Joyce-Loebl, model 4) housed in a class 100 laminar flow modular clean room was used in all the experiments. The compression isotherms for the two polymers showed that at a surface pressure of 35 mNm q the surface area per chromophore unit is 27 ~2 for polymer A and 40 ~2 for polymer B. The area per molecule for the hemicyanine chromophore is approximately 30 ,~2 at 35 m N m q [12]. The results suggest that the hydrophilic polyether repeat units that do not have a chromophore side chain may be looping down into the subphase and not occupying any space at the air-water interface, similar as found for other polymers with spacer groups [17]. LANGMUIR-BLODGETTBILAYERS Two bilayer architectures were deposited, interleaved and non-interleaved. For the non-interleaved bilayers, polymer B was deposited on the upstroke at a surface pressure of 30 mNm -1, and polymer A on the downstroke at 35 mNm -1. Up to eight bilayers were deposited in this manner. The average deposition ratio for polymer B was 0.98, and for polymer A it was 0.94. To prevent peeling when the substrate was removed from the subphase, a fatty acid monolayer (arachidic or behenic) was deposited on the last upstroke. All multilayer films were deposited on a hydrophilic glass substrate prepared by placing two clean microscope slides back-to-back. In this way it was possible to prepare two multilayer films under identical dipping conditions.
D686
The dipping speed was 0.63 cm min -1. The stroke length for the first bilayer was twice as long as that for subsequent bilayers. This procedure ensured that one portion of the substrate was always coated with a single bilayer. For the interleaved bilayers a similar dipping procedure was followed except that two layers of behenic acid were deposited between each bilayer as a Y-type film. The average deposition ratio for behenic acid was 0.99. The architectures for the films are shown schematically in Fig. 2.
behenic or arachidic acid POLVMEFi A POLYMER B
Fig. 2. Bilayer architectures: (a) interleaved, (b) non-interleaved. SECOND HARMONIC GENERATION A Nd:YAG Q-switched laser with a pulse width of 10ns and a repetition rate of 10 Hz was used for the optical experiments. The fundamental beam (1064 nm) was polarized in the plane of incidence, and the angle of incidence the beam made with the substrate was 60 °. The second harmonic radiation (532 nm) was detected in transmission. Results are reported for SHG radiation polarized in the plane of incidence. Details of the experimental set-up are given elsewhere [18]. All measurements were made within 30 minutes of completing film deposition. The enhancement of SHG intensity with the number of polymer bilayers is given in Fig. 3 for the interleaved and non-interleaved architectures. The SHG intensity data have been n o r m a l i z e d with the average SHG intensity d e t e r m i n e d from SHG measurements on single bilayers. The non-interleaved bilayer films exhibit quadratic enhancement of SHG intensity up to 4 bilayers, after which the enhancement is essentially linear. On the other hand, the interleaved bilayer films show quadratic enhancement for up to 10 bilayers. To date this is the largest number of bilayers we have attempted to deposit with a single compartment trough. The difference in SHG enhancement for the two film architectures is quite similar to our previous results published for interleaved and non-interleaved layers of a hemicyanine dye [4, 5, 12, 13]. The polymers used in this study exhibited no deposition problems for the thickest films attempted. Thus the reduction in the SHG enhancement observed for non-interleaved bilayers cannot be attributed to poor deposition. It is possible that the non-interleaved bilayers u n d e r g o successive disordering because of imperfect registry between bilayers. Another possibility is nonconstructive interactions between adjacent chromophores clue to local field effects
D687
~
120.
,;."
100 a~
80 E~
6O
=..=.
40
.t~"
20 0 0
1 2 3 4 5 6 7 8 9 1 0 1 Number of Bilayers
Fig. 3. Enhancement of SHG power for interleaved dye bilayers (O) and noninterleaved bilayers (5); quadratic enhancement (....). [3] or ionization of chromophores [15]. If present these effects would be minimized in an interleaved bilayer architecture, and our results seem consistent with this view. Experiments were also undertaken to determine if there was any measurable interaction between adjacent polymer dye layers. SHG measurements were made on the two bilayer assemblies shown in Fig. 4. In bilayer I the alkyl chains are nearest neighbors, while in bilayer II the polymer backbones are nearest neighbors. Bilayer I thus provides the largest possible separation for the chromophores. Although the signal-to-noise ratio was low, our preliminary results show that bilayer I has a SHG intensity about 12% larger than bilayer II. m
m Polymer
= ~ ~ ~ • .......
I
''==
0. . . . , oke \
A
Polymer
B
behenic
acid
II
\0o. d o2 °k°
upstroke
Fig. 4. Noncentrosyrnmetric bilayer assemblies. CONCLUDING REMARKS This study has shown that it is possible to make Langmuir-Blodgett polymeric films that have nonlinear optical properties, and through control of the film architecture it is possible to obtain quadratic enhancement of SHG intensity when the optically active bilayers are interleaved with an optically inert material. These results are encouraging
D688
since future
device application
a r e l i k e l y to r e q u i r e
materials
with
mechanical
p r o p e r t i e s s u p e r i o r to t h o s e of o r g a n i c s [19].
ACKNOWLEDGEMENTS F u n d i n g for this r e s e a r c h w a s p r o v i d e d in p a r t b y t h e L a w r e n c e L i v e r m o r e N a t i o n a l L a b o r a t o r y a n d the N a t i o n a l Science F o u n d a t i o n (CBT Division).
REFERENCES 1.
J. Zyss, ]. Mol. Electron., 1~ 25 (1985).
2.
G.L. Gaines, Jr., "Insoluble Monolayers at Liquid Gas Interfaces," Interscience, New York, 1966.
3.
I.R. Girling, N.A. Cade, P.V. Kolinsky, R.J. Jones, I.R. Peterson, M.M. Abroad, D.B. Neal, M.C. Petty, G.G. Roberts, W. J. Feast, J. Opt. Soc. Am. B, 4, 950 (1987).
4.
L.M. Hayden, S.T. Kowel, M.P. Srinivasan, Optics Comm., 6_!,15 (1987).
5.
P. Stroeve, M.P. Srinivasan, B.G. Higgins, and S.T. Kowel, Thin Solid Films, ~
6.
I. Ledoux, D. Josse, P. Vidakovic, J. Zyss, R.A. Hann, P.F. Gordon, B.D. Bothwell, S.K. Gupta, S.
209 (1987).
Allen, P. Robin, E. Chastaing, and J.C. Dubois, Europhys. Lett., 3_, 803 (1987). 7.
O.A. Aktisipetrov, N.N. Akhmediev, I.M. Baranova, E.D. Mishina, and V.R. Novak, Sov. Tech.
Phys. Lett., 11(5), 249 (1985). 8.
O.A. Aktsipetrov, N.N. Akhmediev, I.M. Baranova, E.D. Mishina, and V.R. Novak, Sov. Phys
JETP, 62(3), 524 (1985). 9.
P. Popovitz-Biro, K. Hill, E.M. Landau, M. Lahav, L. Leiserowitz, J. Sagiv, H. Hsiung, G.R. Meredith and H. Vanderzeele, J. Am. Chem. Soc., ~
2673 (1988).
10.
I.R. Girling, P.V. Kolinsky, N.A. Cade, J.D. Earls, I.R. Peterson, Optics Comm., ~
11.
I.R. Girling, P.V. Kolinsky, N.A. Cade, J.D. Earls, I.R. Peterson, and G.H. Cross, Thin Solid Films,
4 (1985).
101 (1985). 12.
L.M. Hayden, B.L. Anderson, J.Y.S. Lain, B.G. Higgins, P. Stroeve, and S.T. Kowel, Thin Solid
Films, 160. xxx (1988). 13.
R.C. Hall, G.A. Lindsay, S.T. Kowel, L.M. Hayden, B.L. Anderson, B.G. Higgins, P. Stroeve, and M.P. Srinivasan, Proc. Photo-Opt. Instrum. Engrs. (SPIE), ~
14.
121 (1987).
D.B. Neal, M.C. Petty, G.G. Roberts, M.M. Ahmad, W.J. Feast, I.R. Girling, N.A. Cade, P.V. Kolinsky, and I.R. Peterson, Electron. Lett., 2_2,2460 (1986).
15.
R.H. Tredgold, M.C.J. Young, R. Jones, P. Hodge, P. Kolinsky, and R.J. Jones, Electron. Lett., ~ 308 (1988).
16.
R.C. Hall, G.A. Lindsay, B.L. Anderson, S.T. Kowel, B.G° Higgins, and P. Stroeve, Proc. Mat. Res.
17.
H. Ringsdorf, G. Schmidt, and J. Schneider, Thin Solid Films, 152, 207 (1987).
Soc., ~
351 (1988).
18.
L.M. Hayden, Ph.D. Thesis, University of California, Davis, 1987.
19.
S.T. Kowel, R. Selfridge, C. Eldering, N. Matloff, P. Stroeve, B.G. Higgins, M.P. Srinivasan, L.B. Coleman, Thin Solid Films, 151,377 (1987).
D683
QUADRATICALLY ENHANCED SECOND HARMONIC GENERATION IN POLYMER-DYE LANGMU1R-BLODGETTFILMS: A NEW BILAYERARCHITECTURE
B.L. Anderson*, R.C. Hall***, B.G. Higgins*, G. Lindsay***, P. Stroeve*, and S.T. Kowel**. *Department of Chemical Engineering, **Department of Electrical Engineering and Computer Science, University of California, Davis, CA 95616, U.S.A. ***Polymer Science Branch, Chemistry Division, Research Department, N a v a l Weapons Center, China Lake, CA 93555.
ABSTRACT A new double bilayer ABCC dipping sequence for the Langmuir-Blodgett deposition process has resulted in a one hundred-fold increase in second harmonic signal generation for ten ABCC sequences compared to one ABCC sequence. The AB bilayers are comprised of hemicyanine side chains on a polyether backbone. Two different polymer-dye compositions are used in each bilayer in order to have dipoles pointing in the same direction upon "Y" type deposition. Hydrophilic-hydrophilic interactions between polymer-dye bilayers are prevented by interleaving with a CC behenic acid bilayer, which also provides interlayer local field insulation.
INTRODUCTION To achieve large second-order nonlinear optical responses in organic and polymeric thin film systems, the active dye chromophore must possess a large m o l e c u l a r hyperpolarizability and be organized in a film architecture that i s noncentrosymmetric [1]. The Langmuir-Blodgett (L-B) technique provides a method for fabricating thin film systems with this required asymmetry [2]. In addition, the technique allows one to tailor the film architecture so that any cancelling of local field effects from interactions between chromophores in the same layer or adjacent layers is minimized [3]. In this way the overall second-order bulk susceptibility of the film can be optimized. Many L-B dipping protocols have been proposed to realize the theoretically predicted quadratic enhancement in second harmonic generation (SHG) intensity with the number of layers. For a film consisting of a single material, there are two dipping protocols that result in noncentrosymmetric films, X-type (deposition on the 0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
D684
downstroke only) and Z-type (deposition on the upstroke only). Our group [4-5] and others [6] have shown that Z-type multilayers often do not give the expected quadratic increase in the SHG intensity with the number of layers deposited. Quadratic enhancement of SHG intensity in X-type multilayers of up to five layers has been reported, however [7-8], and more recently, Popovitz-Biro et al. [9] describe a class of amphiphiles that form Z-type multilayers that show quadratic enhancement. Noncentrosymmetric Y-type films have been made by interleaving an optically active layer of dye with an inert layer of fatty acid. Girling et al. [10-11] found that when a cyanine dye was interleaved with c0-tricosenoic acid, the enhancement in SHG intensity for 3 dye layers was slightly less than quadratic. Our group has shown that a hemicyanine dye interleaved with behenic acid gave quadratic enhancement for up to 6 dye layers [12], and a hemicyanine dye-substituted polyether interleaved with behenic acid gave quadratic enhancement for 3 polymer layers [13]. To optimize the density of active layers in interleaved structures, some groups have replaced the inert fatty acid layer with a second optically active layer, resulting in a Ytype film. The film is made noncentrosymmetric by biasing the two chromophores. Neal et al. [14] showed that when a hemicyanine dye is interleaved with a long chain nitrostilbene, the enhancement of SHG intensity was greater than quadratic for a film of 3 hemicyanine and 2 nitrostilbene layers. Tredgold et al. [15] showed enhancement of SHG intensity for up to 70 bilayers of a merocyanine dye interleaved with an optically active p r e f o r m e d polymer. However, this SHG enhancement was approximately linear. The lack of quadratic enhancement was attributed to a space charge effect that results from ionization of chromophores. In this paper we report on the deposition of noncentrosymmetric Y-type films formed by interleaving two preformed polymers. Each polymer consists of a hemicyanine chromophore substituted onto a polyether backbone. Two film architectures are studied, interleaved bilayers and non-interleaved bilayers. Enhancement of SHG intensity with the number of bilayers for the two architectures is investigated. In addition, the interaction between chromophores in the same bilayer is examined by studying SHG in single bilayer noncentrosymmetric assemblies. CHEMICAL STRUCTUREAND COMPRESSIONISOTHERMS The chemical structures of the hemicyanine dye-substituted polymers are shown in Fig. 1. The synthesis of these materials is based on the chemical modification of poly (epichlorohydrin) (PECH). Details of the synthesis are given elsewhere [13, 16]. Polymer A has 43 repeat units per molecule with a chromophore attached to 47% of the units. The electron-donating group of the chromophore is adjacent to the polymer
D685
backbone. Polymer B has 11 repeat units per molecule with a chromophore attached to 33% of the units. Note that in polymer B the electron-accepting group is adjacent to the polymer backbone. The hemicyanine chromophores absorb visible light with an absorption peak at 390 nm and an absorption edge near 500 nm (10-4 M in chloroform). The polymers are deep red-colored glasses stable to about 230°C in nitrogen. No crystalline or liquid crystalline transitions were detected with differential scanning calorimetry.
Polymer A c12H25
Polymer B c,.u~7
~(~.~Br-
O i CH_CI CH. d(--CH-CH~ O ~" " C H - C H1~O -- - 4 t
O
CH^CI ~ C' H ~: - CH20~
CICH^ , ,¢C H 2 0 ~ l -x C[-1-
Fig. 1. Structure of hemicyanine dye-substituted polymers. The polymers were spread from a chloroform solution containing approximately 1.5 mgm1-1 of polymer onto a water subphase (pH=5.7+0.3, T=23.5°C). A single compartment commercial trough (Joyce-Loebl, model 4) housed in a class 100 laminar flow modular clean room was used in all the experiments. The compression isotherms for the two polymers showed that at a surface pressure of 35 mNm q the surface area per chromophore unit is 27 ~2 for polymer A and 40 ~2 for polymer B. The area per molecule for the hemicyanine chromophore is approximately 30 ,~2 at 35 m N m q [12]. The results suggest that the hydrophilic polyether repeat units that do not have a chromophore side chain may be looping down into the subphase and not occupying any space at the air-water interface, similar as found for other polymers with spacer groups [17]. LANGMUIR-BLODGETTBILAYERS Two bilayer architectures were deposited, interleaved and non-interleaved. For the non-interleaved bilayers, polymer B was deposited on the upstroke at a surface pressure of 30 mNm -1, and polymer A on the downstroke at 35 mNm -1. Up to eight bilayers were deposited in this manner. The average deposition ratio for polymer B was 0.98, and for polymer A it was 0.94. To prevent peeling when the substrate was removed from the subphase, a fatty acid monolayer (arachidic or behenic) was deposited on the last upstroke. All multilayer films were deposited on a hydrophilic glass substrate prepared by placing two clean microscope slides back-to-back. In this way it was possible to prepare two multilayer films under identical dipping conditions.
D686
The dipping speed was 0.63 cm min -1. The stroke length for the first bilayer was twice as long as that for subsequent bilayers. This procedure ensured that one portion of the substrate was always coated with a single bilayer. For the interleaved bilayers a similar dipping procedure was followed except that two layers of behenic acid were deposited between each bilayer as a Y-type film. The average deposition ratio for behenic acid was 0.99. The architectures for the films are shown schematically in Fig. 2.
behenic or arachidic acid POLVMEFi A POLYMER B
Fig. 2. Bilayer architectures: (a) interleaved, (b) non-interleaved. SECOND HARMONIC GENERATION A Nd:YAG Q-switched laser with a pulse width of 10ns and a repetition rate of 10 Hz was used for the optical experiments. The fundamental beam (1064 nm) was polarized in the plane of incidence, and the angle of incidence the beam made with the substrate was 60 °. The second harmonic radiation (532 nm) was detected in transmission. Results are reported for SHG radiation polarized in the plane of incidence. Details of the experimental set-up are given elsewhere [18]. All measurements were made within 30 minutes of completing film deposition. The enhancement of SHG intensity with the number of polymer bilayers is given in Fig. 3 for the interleaved and non-interleaved architectures. The SHG intensity data have been n o r m a l i z e d with the average SHG intensity d e t e r m i n e d from SHG measurements on single bilayers. The non-interleaved bilayer films exhibit quadratic enhancement of SHG intensity up to 4 bilayers, after which the enhancement is essentially linear. On the other hand, the interleaved bilayer films show quadratic enhancement for up to 10 bilayers. To date this is the largest number of bilayers we have attempted to deposit with a single compartment trough. The difference in SHG enhancement for the two film architectures is quite similar to our previous results published for interleaved and non-interleaved layers of a hemicyanine dye [4, 5, 12, 13]. The polymers used in this study exhibited no deposition problems for the thickest films attempted. Thus the reduction in the SHG enhancement observed for non-interleaved bilayers cannot be attributed to poor deposition. It is possible that the non-interleaved bilayers u n d e r g o successive disordering because of imperfect registry between bilayers. Another possibility is nonconstructive interactions between adjacent chromophores clue to local field effects
D687
~
120.
,;."
100 a~
80 E~
6O
=..=.
40
.t~"
20 0 0
1 2 3 4 5 6 7 8 9 1 0 1 Number of Bilayers
Fig. 3. Enhancement of SHG power for interleaved dye bilayers (O) and noninterleaved bilayers (5); quadratic enhancement (....). [3] or ionization of chromophores [15]. If present these effects would be minimized in an interleaved bilayer architecture, and our results seem consistent with this view. Experiments were also undertaken to determine if there was any measurable interaction between adjacent polymer dye layers. SHG measurements were made on the two bilayer assemblies shown in Fig. 4. In bilayer I the alkyl chains are nearest neighbors, while in bilayer II the polymer backbones are nearest neighbors. Bilayer I thus provides the largest possible separation for the chromophores. Although the signal-to-noise ratio was low, our preliminary results show that bilayer I has a SHG intensity about 12% larger than bilayer II. m
m Polymer
= ~ ~ ~ • .......
I
''==
0. . . . , oke \
A
Polymer
B
behenic
acid
II
\0o. d o2 °k°
upstroke
Fig. 4. Noncentrosyrnmetric bilayer assemblies. CONCLUDING REMARKS This study has shown that it is possible to make Langmuir-Blodgett polymeric films that have nonlinear optical properties, and through control of the film architecture it is possible to obtain quadratic enhancement of SHG intensity when the optically active bilayers are interleaved with an optically inert material. These results are encouraging
D688
since future
device application
a r e l i k e l y to r e q u i r e
materials
with
mechanical
p r o p e r t i e s s u p e r i o r to t h o s e of o r g a n i c s [19].
ACKNOWLEDGEMENTS F u n d i n g for this r e s e a r c h w a s p r o v i d e d in p a r t b y t h e L a w r e n c e L i v e r m o r e N a t i o n a l L a b o r a t o r y a n d the N a t i o n a l Science F o u n d a t i o n (CBT Division).
REFERENCES 1.
J. Zyss, ]. Mol. Electron., 1~ 25 (1985).
2.
G.L. Gaines, Jr., "Insoluble Monolayers at Liquid Gas Interfaces," Interscience, New York, 1966.
3.
I.R. Girling, N.A. Cade, P.V. Kolinsky, R.J. Jones, I.R. Peterson, M.M. Abroad, D.B. Neal, M.C. Petty, G.G. Roberts, W. J. Feast, J. Opt. Soc. Am. B, 4, 950 (1987).
4.
L.M. Hayden, S.T. Kowel, M.P. Srinivasan, Optics Comm., 6_!,15 (1987).
5.
P. Stroeve, M.P. Srinivasan, B.G. Higgins, and S.T. Kowel, Thin Solid Films, ~
6.
I. Ledoux, D. Josse, P. Vidakovic, J. Zyss, R.A. Hann, P.F. Gordon, B.D. Bothwell, S.K. Gupta, S.
209 (1987).
Allen, P. Robin, E. Chastaing, and J.C. Dubois, Europhys. Lett., 3_, 803 (1987). 7.
O.A. Aktisipetrov, N.N. Akhmediev, I.M. Baranova, E.D. Mishina, and V.R. Novak, Sov. Tech.
Phys. Lett., 11(5), 249 (1985). 8.
O.A. Aktsipetrov, N.N. Akhmediev, I.M. Baranova, E.D. Mishina, and V.R. Novak, Sov. Phys
JETP, 62(3), 524 (1985). 9.
P. Popovitz-Biro, K. Hill, E.M. Landau, M. Lahav, L. Leiserowitz, J. Sagiv, H. Hsiung, G.R. Meredith and H. Vanderzeele, J. Am. Chem. Soc., ~
2673 (1988).
10.
I.R. Girling, P.V. Kolinsky, N.A. Cade, J.D. Earls, I.R. Peterson, Optics Comm., ~
11.
I.R. Girling, P.V. Kolinsky, N.A. Cade, J.D. Earls, I.R. Peterson, and G.H. Cross, Thin Solid Films,
4 (1985).
101 (1985). 12.
L.M. Hayden, B.L. Anderson, J.Y.S. Lain, B.G. Higgins, P. Stroeve, and S.T. Kowel, Thin Solid
Films, 160. xxx (1988). 13.
R.C. Hall, G.A. Lindsay, S.T. Kowel, L.M. Hayden, B.L. Anderson, B.G. Higgins, P. Stroeve, and M.P. Srinivasan, Proc. Photo-Opt. Instrum. Engrs. (SPIE), ~
14.
121 (1987).
D.B. Neal, M.C. Petty, G.G. Roberts, M.M. Ahmad, W.J. Feast, I.R. Girling, N.A. Cade, P.V. Kolinsky, and I.R. Peterson, Electron. Lett., 2_2,2460 (1986).
15.
R.H. Tredgold, M.C.J. Young, R. Jones, P. Hodge, P. Kolinsky, and R.J. Jones, Electron. Lett., ~ 308 (1988).
16.
R.C. Hall, G.A. Lindsay, B.L. Anderson, S.T. Kowel, B.G° Higgins, and P. Stroeve, Proc. Mat. Res.
17.
H. Ringsdorf, G. Schmidt, and J. Schneider, Thin Solid Films, 152, 207 (1987).
Soc., ~
351 (1988).
18.
L.M. Hayden, Ph.D. Thesis, University of California, Davis, 1987.
19.
S.T. Kowel, R. Selfridge, C. Eldering, N. Matloff, P. Stroeve, B.G. Higgins, M.P. Srinivasan, L.B. Coleman, Thin Solid Films, 151,377 (1987).