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Probing of spatial orientational correlations between chromophores in polymer films by femtosecond hyper-Rayleigh scatterring
26 April 1996
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 253 (1996) 135-140
Probing of spatial orientational correlations between chromophores in polymer films by femtosecond hyper-Rayleigh scattering Geert Olbrechts a, Erik J.H. Put a, Koen Clays a, Andre Persoons l,a, Naoki Matsuda b a Laboratory of Chemical and Biological Dynamics, Center for Research in Molecular Electronics and Photonics, Department of Chemistry, Unioersity of Leuoen, Celestijnenlaan 200D, B-3001 Leuoen, Belgium b Department of Analytical Chemistry, National Institute of Materials and Chemical Research, 1-1, Higashi, Tsukuba, lbaraki 305, Japan
Received 15 February 1996
Abstract
Femtosecond hyper-Rayleigh scattering has been used to probe the spatial orientational fluctuations between nonlinear optical chromophores as dopants in spincoated polymer films. The fluctuation in the second-order incoherently scattered light intensity upon micro-translating the solid sample is indicative of the degree of spatial correlation between the individual chromophores. The decay of the autocorrelation function of this fluctuating signal is characterized by a spatial correlation length. Electric-field poling of dipolar chromophores is shown to increase this correlation length. The longer correlation length after poling results from a higher degree of spatial orientational correlation between the individual chromophores.
I. Introduction
Since the introduction of hyper-Rayleigh scattering (HRS) as a fast and reliable measurement technique for the first hyperpolarizability (second-order polarizability) of molecules in solution [1,2], the importance of fluctuations in the orientational distribution was realized [3]. Translational fluctuations are required in linear light scattering. These density fluctuations are the basis of the experimental determination of the hydrodynamic volume of molecules in dynamic light scattering. Translational fluctuations do not change the isotropic, on average centrosymmetric, orientational distribution of molecules. Only
l Also at Optical Sciences Center, University of Arizona, Tucson, AZ 85721, USA
rotational fluctuations can cause deviations from the average centrosymmetry. For a measurement in solution, a combination of spatial and temporal fluctuations causes the instantaneous and local deviation from centrosymmetry. The intensity of the hyperRayleigh scattered light can then be related to a value for the first hyperpolarizability of the molecules in solution. The specific advantages of HRS over the electric-field induced second-harmonic generation (EFISHG) method have been discussed and HRS has become the method of choice for the experimental determination of fl [4-1 1]. After the measurement of the molecular hyperpolarizability, the individual molecules must be assembled in a macroscopic non-centrosymmetric structure. Only a few molecules with the potential for nonlinear optical (NLO) applications arrange in a
0009-2614/96/$12.00 © 1996 Elsevier Science B.V. All rights reserved PH S0009-261 4(96)002 19-9
136
G. Olbrechts et a l . / Chemical Physics Letters 253 (1996) 135-140
crystal with the right symmetry. For other molecules, special schemes must be devised to (partially) orient them over certain macroscopic distances. Electricfield poling of dipolar chromophores in a polymer matrix is a useful approach. The chromophores are dispersed in the matrix, or covalently linked to the polymer, either as pending side-groups or incorporated in the main chain. The dipoles of the molecules interact with an applied electric field. Upon heating the polymer above the glass transition temperature, the chromophores gain mobility and orient in the field. Lowering the temperature with the orienting field on stabilizes the non-centrosymmetric orientation of the chromophores. The partial order is subject to relaxation, and different schemes have been worked out to increase the order and retard this relaxation. Current fields of research include liquid crystalline polymers and locking the chromophores by cross-linking. Another possibility is to incorporate the NLO chromophore in an amphiphilic molecule. Langmuir-Blodgett (LB) deposition of these types of molecules results in macroscopic structures with large order parameters. Domain formation in the micrometer size range is, however, typical in LB films and limits applications in waveguide nonlinear optics. Different schemes to orient dipolar molecules in non-centrosymmetric structures have been reviewed [ 12]. The efficiency of a phase-matched second-order NLO effect is highly dependent on the length over which the nonlinear interaction can be maintained. In single crystals, this length is the dimension of the crystal. In artificially ordered systems, such as the poled polymer and LB films, this length is much more determined by the length over which the molecular order is constant, than by the physical dimension of the film. Although refractive index measurements can indicate that the structure is phase-matched over the total length of the film, the efficiency is largely determined by the constancy of the order in the structure. A measurement scheme to determine the degree of spatial correlation between the chromophores would, therefore, be instrumental in the development of reliable and reproducible devices. To be able to study this degree of orientational correlation without any limitations, a technique that does not rely on a minimal degree of order to obtain
the signal must be used. For this reason coherent second-harmonic generation can not be used, since for a system with limited orientation, virtually no coherent signal will be generated. Incoherent scattering is always present, even for a macroscopically, on average, centrosymmetric structure. The applicability of incoherent femtosecond HRS for the study of the spatial correlation between chromophores in solid samples has been demonstrated in a preliminary study. The short pulse duration ensures high peak power to observe nonlinear optical effects from low total energy to avoid optical damage to the solid samples. Three typical ensembles of chromophores were studied, each with a different degree of spatial orientational correlation: a single crystal, an isotropic dispersion of a chromophore in a polymer matrix, and chromophores deposited in an LB film are characterized by a long, a very short and an intermediate correlation length, respectively [13,14]. We have now extended the principle and demonstrated that the technique can be used to study the technologically much more important (poled) polymeric thin films. Electric-field poling of dipolar chromophores in a thin polymer film is shown to increase the correlation length substantially.
2. Experimental For the reasons outlined above, femtosecond HRS must be used for this study. The experimental apparatus is a slight modification of the one that has been used for the determination of the first hyperpolarizability of molecules in solution [6]. The essential change is the possibility to micro-translate the condensing system and the detector perpendicular to the beam (see Fig. 1) with a resolution of 0.13 /.tm. The condensing system, consisting of a concave reflector, an aspheric lens, a low-pass filter and a planoconvex lens, is used to maximize the amount of incoherent scattered light detected by the photomultiplier tube (PMT). The overall spatial resolution of the measurement is determined by the resolution of micro-translation (3 /zm) and by the specifications in the optics used (focal distance of the lens, divergence and beam diameter). For a lens with 20 mm focal distance and a beam diameter of 4 mm, this results in a beam
137
G. Olbrechts et al. / Chemical Physics Letters 253 (1996) 135-140 4000
CRC--~---q//
3500 "
,~
a
3000 250O
'FI~ I ~ L*',
b
"~" 2000 --
1500 IOO0
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5O0
I phase-sen
0
[ detector
Fig. 1. Schematic of the essential change in the experimental apparatus to the femtosecondhyper-Rayleigh scattering apparatus. ASL, aspheric lens; INT, interference filter; LPF, low-pass filter; PCL, planoconvex lens; PMT, photomultiplier tube; REF, reference frequency; CR, concave reflector; 2 to, beam waist diameter; f, focal length.
400
600
x(p.m)
800
tO00
Fig. 2. HRS intensities as a function of position for spincoated films after corona poling: (a) 9 kV (high field strength); (b) 8 kV (intermediate field strength); (c) 0 kV (zero field strength).
sample and detector is moved perpendicular to the beam. To study the effect of poling on the domain sizes estimated by a correlation length deduced from the measured fluctuations of the second-order scattered light as a function of position, a comparison was made between spincoated polymer films (4-1 /~m thickness) corona poled at different poling voltages to induce different effective field strengths: 0 kV (zero field strength), 8 kV (intermediate field strength), 9 kV (high field strength). These corona voltages were chosen based on the voltage for the onset of corona discharge (7 kV) and the lower limit
waist diameter of 5 /zm and a Rayleigh length of 50 /xm. The spot size then becomes 20 /zm 2. Angular deviations of the sample relative to the beam make further increase of the spatial resolution useless.
3. M e a s u r e m e n t o f d o m a i n sizes
The primary experiment is the measurement of the incoherently second-order scattered light as a function of position (see Fig. 2). During a measurement the translation stage with condensing system, 2000
200
3500t
R=0 900
f- 1 575
2725~
¢-
1175
oO
y
~
.
...%
,.'" ~......~ ...
• ..
.
~o) 1 1 5 0 oo
"0 m~
,oo
725
30~0'0
,;o
,;oo
x(~m) r
L
7215
;,oo
.. -. ".~..-'~::
.;-:
•
•
.
::"
.-
....
_.
"
.
.
.
"
,
I 1 150
12~ (x) of first
,
I 1 575
2000
run
Fig. 3. Reproducibility test and correlation between two consecutive measurements. The inset shows the consecutive primary data of the fluctuating intensities as a function of position for a spincoated film after poling.
138
G. Olbrechts et aL / Chemical Physics Letters 253 (1996) 135-140
for damage to the poled film (slightly higher than 9 kV). The observed fluctuations in intensity as a function of position are caused by changing interactions between the polarized beam and the molecules in the focus. By micro-translating the sample rather than varying the fundamental intensity, measuring hyperpolarizability values becomes impossible, however, by doing so, access to correlation measurements is obtained. To prove that the fluctuations in intensity are related only to the fluctuations in orientation of the molecules, reproducibility tests as a function of position were performed. Fig. 3 shows reproducible runs (inset) and the degree of correlation between two consecutive measurements on an LB film of 2docosylamino-5-nitropyridine (DCANP) [15]. Lower correlation between runs on polymer films is believed to be due to local heating and relaxation. This is a topic for further investigation. The application of femtosecond HRS for the study of polymer films allows a study of the increase in correlation length as a function of degree of poling. Before poling, no correlation exists and the measurement results in a random fluctuating signal corresponding to the random oriented molecules in the polymer film. Virtually no coherent signal would be generated for such an unpoled film.
'°l\,
'
4. C a l c u l a t i o n o f correlation length
From the primary data (HRS intensity as a function of position), the normalized autocorrelation (NAC) function of the fluctuation of the variable is calculated. To deduce the NAC function of the fluctuations one starts by subtracting from each value of i n t e n s i t y 12o, the average value ( 1 2 ~ ) . This way we get a correlation function with a starting amplitude 2 2 . . . . of (I2~o) -- (I2~o) which decays to 0. By dlvldmg each point by the starting amplitude (I22o,) - ( 12,o)2, the function becomes normalized and decays from 1 to 0. For the measurement of a randomly fluctuating variable with a spatial increment A x (3.25 /.tm) small enough with respect to the correlation length ~:cor over a sufficient total distance L, the autocorrelation function is single exponentially decaying and the NAC function is completely determined by this correlation length. Fig. 4 shows the NAC functions for the 4methoxy-4'-nitrostilbene (MONS) molecule in a poly(methylmethacrylate) matrix at a loading of 4.3 % by weight, unpoled, after intermediate poling with 8 kV and after full poling at 9 kV. From these figures it is clear how the correlation length increases by poling. To deduce correlation lengths for the different curves on the graph, a double exponential decay
,
0.8
--~" 0,6
~
-
\
.--
~: o.+ Z
.
b
O.2
0.0
i
0
50
i
100
150
200
250
300
~(.um) Fig. 4. Normalized autocorrelation functions of the fluctuating intensities for spincoated films at the indicated corona voltages: (a) 9 kV (high orienting field strength); (b) 8 kV (intermediate orienting field strength); (c) 0 kV (zero orienting field strength).
G. Olbrechts et al./ Chemical Physics Letters 253 (1996) 135-140
instead of a single exponential, as predicted from theory, is required due to additional persistent correlations. The length of the first decaying component is then the correlation length. This yields a correlation function decaying to zero within the spatial resolution of the experiment (5 /zm) for the unpoled polymer film and a length of 20 and 121 /.tm for the 8 and 9 kV poled polymer films, respectively. The value for the correlation length of the fully poled polymer film (121 /zm) corresponds well with values for the interaction length, deduced from experimental phase-matching curves (230 /xm) [16]. Although the poled polymeric waveguides are (quasi-)phase-matched over their complete physical length, efficient interaction occurs only over a fraction of this length. The same phenomenon is observed in LB films (120 and 160 /xm interaction length) [17]. In a preliminary study, we have also investigated the correlation length for DCANP LB films [13,14]. These LB films, however, always exhibit correlation through the LB deposition process itself. Their domain sizes can, hence, be studied by coherent SHG also. Good agreement between the values for the domain size obtained by SHG [l 8] and the correlation length, obtained by femtosecond HRS [14] was found.
5. Conclusions and perspectives We have shown that femtosecond hyper-Rayleigh scattering can be used to study the correlation between dipolar nonlinear optical chromophores dispersed in polymer films. The normalized autocorrelation functions are characterized by a correlation length. Corona poling at increasing voltages is shown to induce an increase in this correlation length. The longer correlation length is indicative of a higher degree of spatial orientational correlation between the chromophores. Extension of this study to side-chain polymers with the chromophores pending from the polymer main-chain will reveal the influence of the covalent binding to the backbone on the degree of orientational correlation. This study will complement the hyperpolarizability study by HRS on this type of polymer [19].
139
The cause of the lower reproducibility of the results on spincoated polymer films, when compared to those on LB films of DCANP, assumed to be local heating followed by relaxation, can be deduced by dispersion studies. Measurements in and out of the electronic absorption band should reveal a different degree of reproducibility. Lowering the fundamental power could also be envisaged, but, for this secondorder nonlinear optical measurement, this would lower the signal-to-noise ratio to a significant extent. We have started a comparative study of the relaxation of the order in poled films by the technique demonstrated in this Letter and by coherent secondharmonic generation.
Acknowledgements KC is a Senior Research Associate of the Belgian National Fund for Scientific Research. EJHP acknowledges financial support from the Flemish Institute for the Advancement of the Scientific and Technological Research in Industry. This research was supported by research grants from the Belgian National Fund for Scientific Research (G.2103.93 and 9.0011.92), from the Belgian government (IUAP-16) and from the University of Leuven ( G O A / I / 9 5 ) .
References: [1] K. Clays and A. Persoons, Phys. Rev. Letters 66 (1991) 2980. [2] K. Clays and A. Persoons, Rev. Sci. Instrum. 63 (1992) 3285. [3] K. Clays, A. Persoons and L. De Maeyer, in: Modern nonlinear optics, Part 3, 85, eds. M. Evans and S. Kielich (Wiley, New York, 1994) p. 455. [4] K. Clays, E. Hendrickx, M. Triest, T. Verbiest, A. Persoons, C. Dehu and J.-L. Br6das, Science 262 (1993) 1419. [5] J. Zyss, C. Dhenant, T. Chauvan and I. Ledoux, Chem. Phys. Letters 206 (1993) 409. [6] K. Clays and A. Persoons, Rev. Sci. lnstrum. 65 (1994) 2190. [7] T. Verbiest, K. Clays, C. Samyn, J. Wolff, D. Reinhoudt and A. Persoons, J. Am. Chem. Soc. 116 (1994) 9320. [8] J. Zyss and I. Ledoux, Chem. Rev. 94 (1994) 77. [9] C. Dhenaut, 1. Ledoux, I.D.W. Samuel, J. Zyss, M. Bourgault and H.L. Bozec, Nature 374 (1995) 339. [10] M.A. Pauley, C.H. Wang and A.K.-Y. Jen, J. Chem. Phys. 102 (1995) 6400.
140
G. Olbrechts et a l . / Chemical Physics Letters 253 (1996) 135-140
[11] O.K. Song, C.H. Wang, B.R. Cho and J.T. Je, J. Phys. Chem. 99 (1995) 6808. [12] D.J. Williams, Introduction to nonlinear optical effects in molecules and polymers, (Wiley, New York, 1991). [13] K. Clays, M. Wu and A. Persoons, Proc. Soc. Photo-Optical Instrum. Eng. 2527 (1995) 209. [14] K. Clays, M. Wu and A. Persoons, J. Nonlinear Optical Phys. Mater. 5 (1996). [15] M. Fli~rsheimer, M. Kiipfer, C. Bosshard, H. Looser and P. Giinter, Adv. Mater. 4 (1992) 795.
[16] G. Khanarian, R.A. Norwood, D. Haas, B. Feuer and D. Karim, Appl. Phys. Letters 57 (1990) 977. [17] T.L. Penner, H.R. Motschmann, N.J. Armstrong, M.C. Ezenyilimba and DJ. Williams, Nature 367 (1994) 49. [18] C.A. Bosshard, Ph.D. Thesis (Swiss Federal Institute of Technology, 1991). [19] K. Clays, E. Hendrickx, S. Houbrechts, M. Triest, T. Verbiest and A. Persoons, Proc. Soc. Photo-Optical lnstrum. Eng. 2025 (1993) 182.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 253 (1996) 135-140
Probing of spatial orientational correlations between chromophores in polymer films by femtosecond hyper-Rayleigh scattering Geert Olbrechts a, Erik J.H. Put a, Koen Clays a, Andre Persoons l,a, Naoki Matsuda b a Laboratory of Chemical and Biological Dynamics, Center for Research in Molecular Electronics and Photonics, Department of Chemistry, Unioersity of Leuoen, Celestijnenlaan 200D, B-3001 Leuoen, Belgium b Department of Analytical Chemistry, National Institute of Materials and Chemical Research, 1-1, Higashi, Tsukuba, lbaraki 305, Japan
Received 15 February 1996
Abstract
Femtosecond hyper-Rayleigh scattering has been used to probe the spatial orientational fluctuations between nonlinear optical chromophores as dopants in spincoated polymer films. The fluctuation in the second-order incoherently scattered light intensity upon micro-translating the solid sample is indicative of the degree of spatial correlation between the individual chromophores. The decay of the autocorrelation function of this fluctuating signal is characterized by a spatial correlation length. Electric-field poling of dipolar chromophores is shown to increase this correlation length. The longer correlation length after poling results from a higher degree of spatial orientational correlation between the individual chromophores.
I. Introduction
Since the introduction of hyper-Rayleigh scattering (HRS) as a fast and reliable measurement technique for the first hyperpolarizability (second-order polarizability) of molecules in solution [1,2], the importance of fluctuations in the orientational distribution was realized [3]. Translational fluctuations are required in linear light scattering. These density fluctuations are the basis of the experimental determination of the hydrodynamic volume of molecules in dynamic light scattering. Translational fluctuations do not change the isotropic, on average centrosymmetric, orientational distribution of molecules. Only
l Also at Optical Sciences Center, University of Arizona, Tucson, AZ 85721, USA
rotational fluctuations can cause deviations from the average centrosymmetry. For a measurement in solution, a combination of spatial and temporal fluctuations causes the instantaneous and local deviation from centrosymmetry. The intensity of the hyperRayleigh scattered light can then be related to a value for the first hyperpolarizability of the molecules in solution. The specific advantages of HRS over the electric-field induced second-harmonic generation (EFISHG) method have been discussed and HRS has become the method of choice for the experimental determination of fl [4-1 1]. After the measurement of the molecular hyperpolarizability, the individual molecules must be assembled in a macroscopic non-centrosymmetric structure. Only a few molecules with the potential for nonlinear optical (NLO) applications arrange in a
0009-2614/96/$12.00 © 1996 Elsevier Science B.V. All rights reserved PH S0009-261 4(96)002 19-9
136
G. Olbrechts et a l . / Chemical Physics Letters 253 (1996) 135-140
crystal with the right symmetry. For other molecules, special schemes must be devised to (partially) orient them over certain macroscopic distances. Electricfield poling of dipolar chromophores in a polymer matrix is a useful approach. The chromophores are dispersed in the matrix, or covalently linked to the polymer, either as pending side-groups or incorporated in the main chain. The dipoles of the molecules interact with an applied electric field. Upon heating the polymer above the glass transition temperature, the chromophores gain mobility and orient in the field. Lowering the temperature with the orienting field on stabilizes the non-centrosymmetric orientation of the chromophores. The partial order is subject to relaxation, and different schemes have been worked out to increase the order and retard this relaxation. Current fields of research include liquid crystalline polymers and locking the chromophores by cross-linking. Another possibility is to incorporate the NLO chromophore in an amphiphilic molecule. Langmuir-Blodgett (LB) deposition of these types of molecules results in macroscopic structures with large order parameters. Domain formation in the micrometer size range is, however, typical in LB films and limits applications in waveguide nonlinear optics. Different schemes to orient dipolar molecules in non-centrosymmetric structures have been reviewed [ 12]. The efficiency of a phase-matched second-order NLO effect is highly dependent on the length over which the nonlinear interaction can be maintained. In single crystals, this length is the dimension of the crystal. In artificially ordered systems, such as the poled polymer and LB films, this length is much more determined by the length over which the molecular order is constant, than by the physical dimension of the film. Although refractive index measurements can indicate that the structure is phase-matched over the total length of the film, the efficiency is largely determined by the constancy of the order in the structure. A measurement scheme to determine the degree of spatial correlation between the chromophores would, therefore, be instrumental in the development of reliable and reproducible devices. To be able to study this degree of orientational correlation without any limitations, a technique that does not rely on a minimal degree of order to obtain
the signal must be used. For this reason coherent second-harmonic generation can not be used, since for a system with limited orientation, virtually no coherent signal will be generated. Incoherent scattering is always present, even for a macroscopically, on average, centrosymmetric structure. The applicability of incoherent femtosecond HRS for the study of the spatial correlation between chromophores in solid samples has been demonstrated in a preliminary study. The short pulse duration ensures high peak power to observe nonlinear optical effects from low total energy to avoid optical damage to the solid samples. Three typical ensembles of chromophores were studied, each with a different degree of spatial orientational correlation: a single crystal, an isotropic dispersion of a chromophore in a polymer matrix, and chromophores deposited in an LB film are characterized by a long, a very short and an intermediate correlation length, respectively [13,14]. We have now extended the principle and demonstrated that the technique can be used to study the technologically much more important (poled) polymeric thin films. Electric-field poling of dipolar chromophores in a thin polymer film is shown to increase the correlation length substantially.
2. Experimental For the reasons outlined above, femtosecond HRS must be used for this study. The experimental apparatus is a slight modification of the one that has been used for the determination of the first hyperpolarizability of molecules in solution [6]. The essential change is the possibility to micro-translate the condensing system and the detector perpendicular to the beam (see Fig. 1) with a resolution of 0.13 /.tm. The condensing system, consisting of a concave reflector, an aspheric lens, a low-pass filter and a planoconvex lens, is used to maximize the amount of incoherent scattered light detected by the photomultiplier tube (PMT). The overall spatial resolution of the measurement is determined by the resolution of micro-translation (3 /zm) and by the specifications in the optics used (focal distance of the lens, divergence and beam diameter). For a lens with 20 mm focal distance and a beam diameter of 4 mm, this results in a beam
137
G. Olbrechts et al. / Chemical Physics Letters 253 (1996) 135-140 4000
CRC--~---q//
3500 "
,~
a
3000 250O
'FI~ I ~ L*',
b
"~" 2000 --
1500 IOO0
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5O0
I phase-sen
0
[ detector
Fig. 1. Schematic of the essential change in the experimental apparatus to the femtosecondhyper-Rayleigh scattering apparatus. ASL, aspheric lens; INT, interference filter; LPF, low-pass filter; PCL, planoconvex lens; PMT, photomultiplier tube; REF, reference frequency; CR, concave reflector; 2 to, beam waist diameter; f, focal length.
400
600
x(p.m)
800
tO00
Fig. 2. HRS intensities as a function of position for spincoated films after corona poling: (a) 9 kV (high field strength); (b) 8 kV (intermediate field strength); (c) 0 kV (zero field strength).
sample and detector is moved perpendicular to the beam. To study the effect of poling on the domain sizes estimated by a correlation length deduced from the measured fluctuations of the second-order scattered light as a function of position, a comparison was made between spincoated polymer films (4-1 /~m thickness) corona poled at different poling voltages to induce different effective field strengths: 0 kV (zero field strength), 8 kV (intermediate field strength), 9 kV (high field strength). These corona voltages were chosen based on the voltage for the onset of corona discharge (7 kV) and the lower limit
waist diameter of 5 /zm and a Rayleigh length of 50 /xm. The spot size then becomes 20 /zm 2. Angular deviations of the sample relative to the beam make further increase of the spatial resolution useless.
3. M e a s u r e m e n t o f d o m a i n sizes
The primary experiment is the measurement of the incoherently second-order scattered light as a function of position (see Fig. 2). During a measurement the translation stage with condensing system, 2000
200
3500t
R=0 900
f- 1 575
2725~
¢-
1175
oO
y
~
.
...%
,.'" ~......~ ...
• ..
.
~o) 1 1 5 0 oo
"0 m~
,oo
725
30~0'0
,;o
,;oo
x(~m) r
L
7215
;,oo
.. -. ".~..-'~::
.;-:
•
•
.
::"
.-
....
_.
"
.
.
.
"
,
I 1 150
12~ (x) of first
,
I 1 575
2000
run
Fig. 3. Reproducibility test and correlation between two consecutive measurements. The inset shows the consecutive primary data of the fluctuating intensities as a function of position for a spincoated film after poling.
138
G. Olbrechts et aL / Chemical Physics Letters 253 (1996) 135-140
for damage to the poled film (slightly higher than 9 kV). The observed fluctuations in intensity as a function of position are caused by changing interactions between the polarized beam and the molecules in the focus. By micro-translating the sample rather than varying the fundamental intensity, measuring hyperpolarizability values becomes impossible, however, by doing so, access to correlation measurements is obtained. To prove that the fluctuations in intensity are related only to the fluctuations in orientation of the molecules, reproducibility tests as a function of position were performed. Fig. 3 shows reproducible runs (inset) and the degree of correlation between two consecutive measurements on an LB film of 2docosylamino-5-nitropyridine (DCANP) [15]. Lower correlation between runs on polymer films is believed to be due to local heating and relaxation. This is a topic for further investigation. The application of femtosecond HRS for the study of polymer films allows a study of the increase in correlation length as a function of degree of poling. Before poling, no correlation exists and the measurement results in a random fluctuating signal corresponding to the random oriented molecules in the polymer film. Virtually no coherent signal would be generated for such an unpoled film.
'°l\,
'
4. C a l c u l a t i o n o f correlation length
From the primary data (HRS intensity as a function of position), the normalized autocorrelation (NAC) function of the fluctuation of the variable is calculated. To deduce the NAC function of the fluctuations one starts by subtracting from each value of i n t e n s i t y 12o, the average value ( 1 2 ~ ) . This way we get a correlation function with a starting amplitude 2 2 . . . . of (I2~o) -- (I2~o) which decays to 0. By dlvldmg each point by the starting amplitude (I22o,) - ( 12,o)2, the function becomes normalized and decays from 1 to 0. For the measurement of a randomly fluctuating variable with a spatial increment A x (3.25 /.tm) small enough with respect to the correlation length ~:cor over a sufficient total distance L, the autocorrelation function is single exponentially decaying and the NAC function is completely determined by this correlation length. Fig. 4 shows the NAC functions for the 4methoxy-4'-nitrostilbene (MONS) molecule in a poly(methylmethacrylate) matrix at a loading of 4.3 % by weight, unpoled, after intermediate poling with 8 kV and after full poling at 9 kV. From these figures it is clear how the correlation length increases by poling. To deduce correlation lengths for the different curves on the graph, a double exponential decay
,
0.8
--~" 0,6
~
-
\
.--
~: o.+ Z
.
b
O.2
0.0
i
0
50
i
100
150
200
250
300
~(.um) Fig. 4. Normalized autocorrelation functions of the fluctuating intensities for spincoated films at the indicated corona voltages: (a) 9 kV (high orienting field strength); (b) 8 kV (intermediate orienting field strength); (c) 0 kV (zero orienting field strength).
G. Olbrechts et al./ Chemical Physics Letters 253 (1996) 135-140
instead of a single exponential, as predicted from theory, is required due to additional persistent correlations. The length of the first decaying component is then the correlation length. This yields a correlation function decaying to zero within the spatial resolution of the experiment (5 /zm) for the unpoled polymer film and a length of 20 and 121 /.tm for the 8 and 9 kV poled polymer films, respectively. The value for the correlation length of the fully poled polymer film (121 /zm) corresponds well with values for the interaction length, deduced from experimental phase-matching curves (230 /xm) [16]. Although the poled polymeric waveguides are (quasi-)phase-matched over their complete physical length, efficient interaction occurs only over a fraction of this length. The same phenomenon is observed in LB films (120 and 160 /xm interaction length) [17]. In a preliminary study, we have also investigated the correlation length for DCANP LB films [13,14]. These LB films, however, always exhibit correlation through the LB deposition process itself. Their domain sizes can, hence, be studied by coherent SHG also. Good agreement between the values for the domain size obtained by SHG [l 8] and the correlation length, obtained by femtosecond HRS [14] was found.
5. Conclusions and perspectives We have shown that femtosecond hyper-Rayleigh scattering can be used to study the correlation between dipolar nonlinear optical chromophores dispersed in polymer films. The normalized autocorrelation functions are characterized by a correlation length. Corona poling at increasing voltages is shown to induce an increase in this correlation length. The longer correlation length is indicative of a higher degree of spatial orientational correlation between the chromophores. Extension of this study to side-chain polymers with the chromophores pending from the polymer main-chain will reveal the influence of the covalent binding to the backbone on the degree of orientational correlation. This study will complement the hyperpolarizability study by HRS on this type of polymer [19].
139
The cause of the lower reproducibility of the results on spincoated polymer films, when compared to those on LB films of DCANP, assumed to be local heating followed by relaxation, can be deduced by dispersion studies. Measurements in and out of the electronic absorption band should reveal a different degree of reproducibility. Lowering the fundamental power could also be envisaged, but, for this secondorder nonlinear optical measurement, this would lower the signal-to-noise ratio to a significant extent. We have started a comparative study of the relaxation of the order in poled films by the technique demonstrated in this Letter and by coherent secondharmonic generation.
Acknowledgements KC is a Senior Research Associate of the Belgian National Fund for Scientific Research. EJHP acknowledges financial support from the Flemish Institute for the Advancement of the Scientific and Technological Research in Industry. This research was supported by research grants from the Belgian National Fund for Scientific Research (G.2103.93 and 9.0011.92), from the Belgian government (IUAP-16) and from the University of Leuven ( G O A / I / 9 5 ) .
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