Jpn. J. Appl. Phys. Vol. 39 (2000) pp. 5838–5841
Part 1, No. 10, October 2000
c
°2000
The Japan Society of Applied Physics

Determination of Third-Order Optical Nonlinearity Dispersion of 1-Methyl-10 -Octadecyl-2,20 -Cyanine
Perchlorate Langmuir-Blodgett Films Using Electroabsorption Spectroscopy
Hidetoshi M URAKAMI1 , Ryuji M ORITA1,5, ∗ , Takashi WATANABE2 , Keisuke A SAI2,5 , Itaru H ONMA3,5 , Haoshen Z HOU3 ,Mikio YAMASHITA1,5 , Kenkichi I SHIGURE2 and Hidemi S HIGEKAWA4,5
1 Department of Applied Physics, Hokkaido University, Kita-13, Nishi-8, Kita-ku, Sapporo 060-8628, Japan
2 Department of Quantum Engineering and Systems Science, The University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
3 Energy Division, Electrotechnical Laboratory, AIST, 1-1-4 Umezono, Tsukuba, Ibaraki, 305-8568, Japan
4 Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8573, Japan
5 CREST, Japan Science and Technology Corporation (JST)(Received March 7, 2000; accepted for publication July 18, 2000)

The absorption and electroabsorption (EA) spectra of 1-methyl-10 -octadecyl-2,20 -cyanine perchlorate (S120) LangmuirBlodgett films were measured.
From these results, dispersion of the third-order nonlinear optical susceptibility
χ (3) (−ω; 0, 0, ω) in the range of 1.8 to 2.7 eV was determined. In addition, the linear-polarizability difference 1α and thepermanent dipole-moment difference 1µ between excited and ground states were evaluated. Upon comparison with the resultsof measurement of the S120 monomer film, it was found that the third-order nonlinear optical susceptibility of the S120 LB
film is strongly enhanced by a factor of 1000. It was concluded that this strong enhancement is attributed to the fact that S120
molecules with a short methyne chain that tightly binds two quinoline rings form a rigid J-aggregate with a one-dimensional
structure that hardly twists, so as to yield a sharp absorption band with a large transition moment. Finally, we evaluated theaverage aggregation number of the S120 LB film to be 3–4.
KEYWORDS: J-aggregate, Langmuir-Blodgett film, cyanine dye, electroabsorption spectroscopy, third-order optical nonlinearity

1. Introduction
J-aggregates of dye molecules1, 2) have attracted much attention because these systems are expected to exhibit mesoscopic properties which are a crossover between microscopic(i.e., single molecules) and macroscopic (i.e., bulk materials)
properties. Moreover, they are also attractive because of theirlarge optical nonlinearities. In each molecule, the intermolecular dipole interaction couples with electronic excitation toform an exciton state delocalized over an aggregate. As a result, the absorption spectrum of J-aggregates shows a new,very sharp and strong peak, the so-called J-band, shifted to
the lower energy side with respect to a monomer absorption
band. Because of the thread-like structure of the J-aggregate,significant insight into the linear and nonlinear optical properties of quasi-one-dimensional excitons is anticipated to begained.
1-Methyl-10 -octadecyl-2,20 -cyanine perchlorate (S120), as
shown in Fig. 1, is one of the materials that form J-aggregatesand has the following characteristics. First, because of its
short methyne chain, few stereoisomers exist. Second, themolecule is almost planar because the two quinoline rings arecoupled strongly by the short methyne chain and thus shows
one-dimensional (1D) confined electronic properties. Third,
dye molecules have substituents and counter ions (ClO−
4)
which assist in the alignment of the molecules. Hence, S120
molecules form J-aggregates in which molecules bind more

N+

CH

N

CH3

ClO4-

C18H37

Fig. 1. Molecular structure of S120.
∗ E-mail: morita@eng.hokudai.ac.jp

tightly to one another and become one of the typical 1Dconfined Frenkel exciton systems. In contrast to this, in thecase where the methyne chain is long, J-aggregates show twodimensional electronic properties and are much more complicated to investigate. Concerning the J-aggregates of cyanine
dyes, although only the origin of nonlinearity in spin-coatedfilms3) and nonlinearity in a disperse system4–6) was studied,nonlinearity in LB films or dispersion of nonlinearity has notbeen investigated so far.
Modulation absorption spectroscopy using an external
electric field is a powerful technique for the investigation of
the subtle difference in electronic structure between aggregates and monomers. The permanent dipole moment and thelinear polarizability are key parameters that govern nonlinear
optical properties as well as linear optical properties. Fromthe electroabsorption (EA) measurement, we can evaluate thechange in these parameters for the electronic transition froma ground state to an excited state.7) Moreover, we can determine the dispersion of the third-order nonlinear optical susceptibility χ (3) (−ω; 0, 0, ω) through the Kramers-Kronig relation.8, 9) Thus, we can clarify the origin of nonlinearities ofmesoscopic materials.
The purpose of the present work is to clarify the mechanismof the third-order optical nonlinearity in the S120 multilayer
film fabricated by the Langmuir-Blodgett (LB) method andto evaluate the coherence size or the aggregation number of
S120 molecules in the LB film from a comparison of experimental results between S120 J-aggregated and monomer samples. Finally, we determine the dispersion of the third-ordernonlinearity for S120 LB film which is an ideal 1D exciton
system.
2. Experiment and Analysis
The cyanine dye S120 used in this work was purchasedfrom Hayashibara Biochemical Laboratories Inc. (NK-3025).
Conductive indium tin oxide (ITO) coated quartz glass was
employed as the substrate. Cyanine dye S120 J-aggregatedsamples for measurement were prepared by the Langmuir-

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Jpn. J. Appl. Phys. Vol. 39 (2000) Pt. 1, No. 10

H. M URAKAMI et al.

Blodgett method; S120 monolayers were transferred onto thesubstrate after coating with cadmium arachidate to render thesubstrate surface hydrophobic. The fabricated sample has 26
layers and forms a Z-type LB film. For the S120 monomer
sample, S120-doped tetraethyl orthosilicate thin film was fabricated by the spin-coating method. Semi-transparent aluminum was deposited over the film surface as a counter electrode. The thickness of the J-aggregated sample was about
0.07 µm, which was measured using a multiple-beam interferometer (Mizojiri BM-4).
EA spectroscopy was carried out at room temperature using
the previously employed system10) with the introduction ofa vacuum chamber in order to avoid sample degradation in
air. The spectral range for EA measurement was from 1.80 to
2.70 eV for the J-aggregated sample and from 2.00 to 3.00 eV
for the monomer sample.
When an electric field is applied to the sample, the change
in the absorption spectrum is described by a linear combination of the absorption spectrum and its first- and secondderivatives as follows:7, 11, 12)
µ ¶
· µ ¶
A
A
d
+ a1
1A(E, F)/E = a0
E
dE E
¸
2 µ ¶
A
d
+ · · · R(θ )[L(0)F]2 ,+a2
d E2 E(1)
where E is the photon energy h̄ω. a0 , a1 and a2 are, respectively, given by
X2
2Y
,
(2)
+
2
µ
µ
1
2X
1µ + 1α,
(3)
a1 =
µ
2
1
(4)
a2 = (1µ)2 .
2Here, µ, X, Y , 1α and 1µ are the transition-dipole moment,
the transition-dipole-moment polarizability, its hyperpolarizability, the linear-polarizability difference and the permanent
dipole-moment difference between excited and ground states,
respectively. They generally have a relationship expressed in
the dyadic notation:a0 =µ(F int ) = µ + X · F int + F int · Y · F int + · · · ,(5)where F int is the internal electric field,
F int = L(0)F,(6)

and where L(Ä) is the Lorentz local-field factor given by²(Ä) + 2,(7)3
where ²(Ä) is a dielectric constant at an angular frequencyof the applied field. For J-aggregates, molecules are aligned
parallel to each other; thus, vectors µ and 1µ, and tensors
X, Y and 1α are reduced to scalars µ, 1µ, X, Y and 1α,
respectively. R(θ ) is a general sensitivity function given by1[5 + (3 cos2 ξ − 1)(3 cos2 θ − 1)],(8)
R(θ ) =
15
where ξ is the angle between the directions of 1µ and µ, andθ is the angle between the directions of the applied electricfield and the polarization vector of the incident light.
L(Ä) =

5839

The first, second and third terms in eq. (1) correspond to thechange in oscillator strength, the spectral shift and the spectral
broadening induced by the applied electric field, respectively.
When the change in linear polarizability is significant, the first
derivative of the absorption spectrum is dominant in the EA
spectrum. In contrast, when the permanent dipole-moment
difference is significant, the second derivative is dominant.
This formula is valid for an isolated absorption band due to asingle electronic transition.
The third-order nonlinear optical susceptibility χ (3) (−ω;0, 0, ω) is obtained from the absorption spectrum A(ω), theKerr-effect EA spectrum 1A(ω), the refractive index n(ω)
and the refractive index change 1n(ω).8, 9) The refractive
index n(ω) is related to the absorption coefficient throughthe Kramers-Kronig (K-K) relation, and the refractive index
change 1n(ω) due to electromodulation is calculated using
the following derivative form of the K-K relation:
Z ∞1A(ω0 )cdω0 ,(9)1n(ω) = P
0 2 − ω2πω0where P denotes the Cauchy principal value of the integral
and c is the velocity of light. This relation is valid for the casewhere the frequency Ä of the applied electric field is sufficiently low compared with the optical frequency ω, and thesystem is regarded to be in the steady state. The third-ordernonlinear optical susceptibility χ (3) (−ω; 0, 0, ω) is expressedas1[(n(ω)1n(ω) − k(ω)1k(ω))χ (3) (−ω; 0, 0, ω) =6π F 2+i(n(ω)1k(ω) + k(ω)1n(ω))] ,(10)
where F is the amplitude of the applied electric field, and k
and 1k are the extinction coefficient and its change due toelectromodulation, respectively, represented by
λ
A(ω),
(11)
k(ω) =4π
λ
1A(ω),(12)
1k(ω) =
4π
where λ is the wavelength.
3. Results and Discussion
Figure 2(a) shows the absorption spectrum of the S120 LB
film together with that of the S120/acetone solution. The
S120 LB film has a new absorption band in the low-energy
side of the lowest absorption band of the S120 solution. The
fact that the new absorption band is extremely strong andsharp indicates that S120 molecules form stable J-aggregates,
and that the S120 molecule has a short methyne chain. Figures 2(b) and 2(c) show the first and second derivatives of theabsorption spectrum, respectively. The EA spectrum of theS120 LB film and its fit using eq. (1) are shown in Fig. 2(d).
It is found that the EA spectrum has a simple profile dominated by the second-derivative of the absorption spectrum,that is, the third term of eq. (1). Thus, we conclude that the
spectral change in the presence of the electric field is mainly
due to the permanent dipole-moment difference 1µ. Here,
we assume that the component proportional to the absorption
spectrum in the measured EA spectrum is negligible, that is,a0 = 0 in eq. (1). Decomposing the EA spectrum into the5840

Jpn. J. Appl. Phys. Vol. 39 (2000) Pt. 1, No. 10

1.0

(a)

H. M URAKAMI et al.

0.15

0.5

0.10

0.4

0.4

0.05

A

0.6

A

A

0.8

(a)

0.3

0.2
0.0

dA/dE

20

(b)

0.1
0.0
0.5

10

(b)

0
0.0

0.5

∆A

-10
1.0

d2A/dE2

0.2

0.00

(c)

-0.5

0.0

-1.0x10-1.0-1.5x10

-4

3
-13

(d)

∆A

0
-2

Experimental data
Theoretical fit

-4

-4x10

2x10

-10

Reχ(3)Imχ(3)|χ(3) |(e)χ(3) (−ω,0,0,ω) [esu]

2x10

2

χ(3) (−ω,0,0,ω) [esu]Experimental data
Theoretical fit

-0.5

(c)

1
0
Reχ(3)Imχ(3)|χ (3)|

-1
-2
2.0

2.2

2.4

2.6

2.8

3.0

E [eV]

0
-2
1.8

2.0

2.2

E

2.4

2.6

Fig. 3. (a) Absorption spectrum of the S120 monomer film, and its (b) EA
spectra [solid line: experimental result, broken line: fit by eq. (1)]. (c)Dispersion of χ (3) (−ω; 0, 0, ω) for the S120 monomer film (solid line:
real part of χ (3) , broken line: imaginary part of χ (3) , dotted line: |χ (3) |).

[eV]

Fig. 2. (a) Absorption spectrum of the S120 LB film (J-aggregated sample,
solid line) together with that of the S120/acetone solution (monomer sample, broken line), and its (b) first and (c) second derivatives. (d) EA spectraof the S120 LB film [solid line: experimental result, broken line: fit by eq.
(1)]. (e) Dispersion of χ (3) (−ω; 0, 0, ω) for the S120 LB film (solid line:
real part of χ (3) , broken line: imaginary part of χ (3) , dotted line: |χ (3) |).

linear combination expressed as eq. (1), we evaluate the 1α
and 1µ values to be −1.5 × 10−22 cm3 and 1.6 D, respectively. Fig. 2(e) shows the dispersion of the real and imaginary parts of χ (3) (−ω; 0, 0, ω) in the range of 1.80 to 2.70 eV
obtained from the absorption and EA spectra using eq. (10).
The χ (3) value also changes markedly around the J-band. The
maximum value of |χ (3) (−ω; 0, 0, ω)| of the S120 LB film is
2.8 × 10−10 esu at 2.10 eV where the J-band is.
The absorption and EA spectra of the S120 monomer film
are shown in Figs. 3(a) and 3(b), respectively. We evaluate the 1α and 1µ values for the S120 monomer film to be−(4.0–5.2) × 10−23 cm3 and 0.70–0.83 D in a manner similar
to the S120 LB film, respectively, with a lower signal-to-noise
ratio. Figure 3(c) shows the dispersion of the real and imaginary parts of χ (3) (−ω; 0, 0, ω) in the range of 2.00 to 3.00 eV.
The maximum value of |χ (3) (−ω; 0, 0, ω)| is 2.7 × 10−13 esu
at 2.34 eV.

Considering that the linear polarizability is proportional tothe occupied volume of a molecule,13) from the ratio of thelinear-polarizability difference between the S120 monomer
film and the LB film, we evaluate the average aggregation
number to be three to four molecules.
We would like to emphasize that the maximum value of|χ (3) (−ω; 0, 0, ω)| of the S120 LB film is about 1000 times
larger than that of the S120 monomer film. It is clear that thisextremely large χ (3) enhancement is caused by the formationof a J-aggregate by S120 molecules in the LB films. Thisenhancement can be explained as follows.
For the two-level model, the third-order nonlinear optical susceptibility χ (3) (−ω; 0, 0, ω) under the resonant condition is represented by the following equation based on thequantum-mechanical perturbation theory:14)
χ (3) (−ω; 0, 0, ω)reso ∝1|µ|2 (1µ)2,h̄ 3 (ωeg − ω − i0eg )3(13)

where 0eg is the relaxation rate between the excited state andthe ground state. h̄ωeg is the energy difference between the
excited state and the ground state. From eq. (13), it is foundthat there are three possibilities for the χ (3) enhancement of
the J-aggregate sample. They are: 1) an increase in 1µ, 2)
an increase in µ and 3) a decrease in 0eg . The experimentJpn. J. Appl. Phys. Vol. 39 (2000) Pt. 1, No. 10

showed that the 1µ value of the S120 LB film slightly increases compared with that of the S120 monomer film. In
contrast, the µ value is large (corresponding to the strong absorption intensity and hence the large absorption cross section) and the 0eg value is small (corresponding to the very
sharp absorption spectrum). This suggests that the significant χ (3) -enhancement of the S120 LB film is due to the latter points, 2) and 3). This finding is based on the fact that theformed aggregate has a hardly twisting and rigid 1D structure
because S120 molecules with the short methyne chain bindtightly with one another unlike the other aggregates. Thatis, this aggregate yields the well-behaved 1D Frenkel exciton that delocalizes over the N -independent molecular sitesof one J-aggregate due to the intermolecular dipole-dipole
interaction during optical excitation. As a result, the typical 1D exciton produces a strong absorption band on thelower energy side and its coherent delocalization causes thedecrease in 0eg . In spite of the rigid 1D structure, the χ (3)value of S120 LB film is smaller than those of previously reported pseudoisocyanines or cyanine J-aggregates.4–6) This is
ascribed to the following two reasons. One is that the aggregation number is limited by the long alkyl chain whichis introduced to form the LB film configuration. The longer
the alkyl chain, the lower the aggregation number. The other(3)component was measured using
reason is that while the χiiii
4, 5)
the four-wave mixing or the Z-scan6) technique in previous
works, a comparatively small χi(3)j j i (i 6 = j) component wasmeasured for S120 LB films since the direction of the applied
electric field to S120 molecules in LB film was perpendicular
to the transition dipole moment of S120 molecules in our configuration. Therefore, the absolute value in our measurement
cannot be directly compared with those in previous measurements. However, the comparison of the relative value, that is,the comparison of the magnitude of the χ (3) enhancement between our result and the previous one is reasonable even if themeasurements were carried out in different ways. In fact, ourresult that χ (3) is enhanced by a factor of 1000 is comparable
to the previous result4) of the H-aggregate which has the same
origin of χ (3) enhancement as the J-aggregate, which is dueto the near-resonant effect and the delocalized exciton in an

H. M URAKAMI et al.

5841

aggregate unit.
4. Conclusion
We measured the absorption and EA spectra for 1-methyl10 -octadecyl-2,20 -cyanine perchlorate Langumuir-Blodgett
films and monomer films. From these results, we evaluated the linear-polarizability difference 1α and the permanent dipole-moment difference 1µ. In addition, we determined the dispersion of the third-order nonlinear susceptibility χ (3) (−ω; 0, 0, ω) through the Kramers-Kronig relation.
Consequently, it was found that the χ (3) value is extremely
enhanced, i.e., the maximum value of |χ (3) (−ω; 0, 0, ω)| of
the S120 LB film is about 1000 times larger than that of themonomer film. This result is ascribed to the fact that S120
molecules, possessing the short methyne chain, bind tightly toform a rigid J-aggregate. Finally, from the ratio of the valuesof the linear-polarizability difference between the monomer
and the J-aggregate, we evaluated the average aggregation
number of the S120 molecules to be 3–4.

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