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Third-Order Nonlinear Optical Susceptibility Measurement in 1-Methyl-1'Octadecyl-2,2'-Cyanine Perchlorate Langmuir-Blodgett Films by Means ofElectroabsorption Spectroscopy
Hidetoshi Murakamia; Ryuji Moritaab; Takashi Watanabec; Keisuke Asaibc; Itaru Honmabd; HaoshenZhoud; Mikio Yamashitaab; Kenkichi Ishigurec; Hidemi ShigekawabeaDepartment of Applied Physics, Hokkaido University, Sapporo, JAPAN b CREST, Japan Science andTechnology Corporation (JST), c Department of Quantum Engineering and Systems Science, The
University of Tokyo, Tokyo, JAPAN d Energy division, Electrotechnical Laboratory, AIST, Tsukuba,Ibaraki, JAPAN e Institute of Materials Science, Center for TARA, University of Tsukuba, Tsukuba,
JAPANFirst published on: 01 February 1999
To cite this Article Murakami, Hidetoshi , Morita, Ryuji , Watanabe, Takashi , Asai, Keisuke , Honma, Itaru , Zhou,Haoshen , Yamashita, Mikio , Ishigure, Kenkichi and Shigekawa, Hidemi(1999) 'Third-Order Nonlinear Optical
Susceptibility Measurement in 1-Methyl-1'-Octadecyl-2,2'-Cyanine Perchlorate Langmuir-Blodgett Films by Means ofElectroabsorption Spectroscopy', Molecular Crystals and Liquid Crystals, 327: 1, 31 — 35
To link to this Article: DOI: 10.1080/10587259908026774
URL: http://dx.doi.org/10.1080/10587259908026774

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Mol. Cvsr. 4 Crysr.. 1999. Vol. 327. pp. 31-35

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Third-Order Nonlinear Optical Susceptibility
Measurement in l-Methyl-1’Octadecyl-2,2’-CyaninePerchlorate
Langmuir-Blodgett Films by Means ofElectroabsorption Spectroscopy
HIDETOSHI MURAKAMIa, RYUJI MORITAaVe,
TAKASHI WATANABE~,KEISUKEA S A I ~ , ~ITARU,
HONMA~.~,HAOSHEN ZHOU~,MIKIO YAMASHITA~.~,KENKICHI ISHIGURE~and HIDEMI SHIGEKAWAd7e
aDepartment of Applied Physics, Hokkaido University, Kita-13, Nishi-8, Kita-ku,
Sapporo 060-8628, JAPAN, bDepartment of Quantum Engineering and SystemsScience, The University of Tokyo, 2-11-16, Yayoi, Bunkyo-ku, Tokyo 113-0032,JAPAN, ‘Energy division, Electrotechnical Laboratory, AISI: 1 - 1 4 Umezono,Tsukuba, Ibaraki, 305-8568, JAPAN, %stitute of Materials Science, Centerf o r TARA, University of Tsukuba 1-1-1, Tennodai, Tsukuba 305-8573, JAPANand ‘CREST Japan Science and Technology Corporation (JST)(Received June 30, 1998; In final form July 15, 1998)

By an electroabsorption spectroscopy technique, we determined the third-order nonlinear
optical susceptibilities dispersion in I-methyl-I ’-octadecyL2,2’-cyanine perchlorate Langmuir-Blodgett (J-aggregate) films in the range of 1.8 to 2.7 eV. In addition, from the values ofthe polarizability change in the monomer and aggregate films, the number of molecules in anaggregate was evaluated to be 3-4.
Keywords: J-aggregate; Langmuir-Blodgett film; cyanine dye; electroabsorption spectroscopy; nonlinear optical susceptibility

INTRODUCTION
l-niethyl-l’-octadecyl-2,2’-cyanine perchlorate (S120) molecules inLangmuir-Blodgett (LB) films form J-aggregates. In each molecule, in31HIDETOSHI MURAKAMI et af.

32

termoleculu dipole interaction couples electronic excitation and forms an
exciton state delocalized over an aggregate. The excitonic transition exhibits a very sharp absorption spectrum (J-band) red-shifted from themonomer absorption. Because of thread-like form of J-aggregates, linear
and nonlinear properties of quasi-one-dimensional excitons can be investigated in S120 LB films. In the present study, we measured the third-order( O,O,w) of J-aggregates in ,5120
nonlinear optical susceptibilities x ( ~ )-w;
LB films by use of an electroabsorption (EA) spectroscopy technique.

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EXPERIMENTAL
Samples for electric-field modulation measurement were prepared as follows. While a monomer film was spin-coated on the conductive I T 0 (Indium Tin Oxide)-deposited glass, a 26-layer LB film of S120 (Z-type) wasdeposited onto a ITO-deposited glass which was coated with a single layerof arachidic acid prior to the S120 deposition, Moreover, semi-transparent
aluminum was deposited on the film surface as a counter electrode. Inthe EA measurement, an ac electric field on the order of lo5 V/cm wasapplied between the I T 0 glass and the electrode.
The third-order nonlinear optical susceptibility x ( ~ --w;) ( 0, 0,w) can
be calculated from the absorption (A') spectrum normalized by the photon energy and its Kerr-effect EA ( A A ' ) spectrum through the KramersKronig relation. AA' is generally represented by a linear combination of
the absorption spectrum itself and its first- and second-derivatives as[']A A ' ( E , F )=l%l21dA'1#A'A'+ j A d 2 - t - ( A p F ) dE 2
dE2 '(1)where E, F, An and A p are the photon energy, the applied electric field,
linear polarizability change and permanent dipole nioment change, respectively. M and A M F are the matrix element of transition dipole moment
between the excited mid ground states and its first-order change with theapplied electric field, respectively.

~'~0,'0, o)(MEASUREMENT4;
IN S120 LB FILMS

33

RESULTS AND DISCUSSION

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Absorption and EA spectra (normalized by the photon energy) of S120 LB
filnis are shown in Figure 1 (a) and (b), respectively. It is found that theEA spectrum has a similar profile to the second-derivative of the absorption spectrum, that is, the third term of Equation (1). Thus we conclude
that the spectral change in the presence of the electric field is mainly due
to the permanent dipole change. Decomposing the EA spectrum into thelinear combination expressed as Equation (l),we evaluated the Aa andAp values to be -1.5 x
cm3 and 1.6 D, respectively. Absorption
and EA spectra of S120 monomer films are shown in Figure 1 (c) and(d), respectively. In the similar manner, but in less S/N ratio experiniental condition, Aa and Ap values for the monomer were evaluated to be-(4.0-5.2)x lowz3cm3 and 0.70-0.83D, respectively. From the ratio between Aor values for the monomer and aggregate, the number of molecules
in an aggregate was evaluated to be 3-4.
Besides, the dispersion of real and imaginary parts of x ( ~ -w;) ( 0, 0,w)
in the range of 1.8 to 2.7 eV were also determined, as shown in Figure 2.
S120 LB films have a large I x ( ~ ) ( -w; O,O,w)l value of 2.8 x lo-'' esu at
2.10 eV corresponding to the J-band.

CONCLUSION
We determined the third-order nonlinear optical susceptibilities dispersionof in l-1nethyl-l'-octadecyl-2,2'-cyanineperchlorate Langmuir-Blodgett(J-aggregate) flms in the range of 1.8 to 2.7 eV, by use of an electroabsorption spectroscopy technique. &om the values of the polarizability change
in the monomer and aggregate, the number of molecules in an aggregate
was evaluated to be 3-4.

HIDETOSHI MURAKAMI er al.

34

6x 1 0-24 -p:

200.00
2,

111,

111

2.4

2.6

2.80

Downloaded At: 01:44 13 January 20112 -2
1.8 2.0

E

2.2

2.4

2.6

2.0

lev1

2.2

3.0

E [eVIFIGURE 1 (a) Absorption spectra of the S120 LB film (solid line)
and S120/acetone solution (broken line), and its (b) electroabsorption
spectrum (solid line: experimental result, broken line: theoreticalfit). (c) Absorption spectrum of the S120 monomer film, and its (d)electroabsorption spectrum (solid line: experimental result, broken
line: theoretical fit ).

w
I-2*-30,1.8

2.0

2.2

2.4

2.6

E [eVIFIGURE 2 Dispersion of , ~ ( ~ ) ( - wO,O,w);
for S120 LB fihns.

~ ' ~0,'(0,4w) MEASUREMENT;IN S120 LB FILMS

35References

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[ I ] W. Liptay, in Excited States, edited by E. C. Lim (Academic Press, New York, 1974). p.
129.