Optics and Photonics Journal, 2013, 3, 25-28
doi:10.4236/opj.2013.32B006 Published Online June 2013 (http://www.scirp.org/journal/opj)
Study on the Third-order Nonlinear Optical Properties of
bis(tetraethylammonium)
bis(1,3-dithiole-2-thione-4,5-dithiolato)cadium
Hongliang Yang1, Fujun Zhang1, Xinqiang Wang2, Guanghui Zhang2
1Department of Optics, Shandong University, Jinan,250100,China
2State Key Laboratory of Crystal, Shandong University, Jinan,250100,China
Received 2013
ABSTRACT
A dmit2- salt: bis(tetraethylammoniu m)bis(1,3 -dith io le-2-thion e-4,5 -dith io lato)cadiu m (CADMIT) was synthesized. The
Optical Kerr Effect (OKE) signal of its acetonitrile solution was measured by femtosecond optical Kerr gate technique.
Using CS2 OKE signal as reference signal measured under identical conditions, the third-order optical nonlinear suscep-
tibility, (3), of the sample solution was obtained to be about 2.98  10-14 esu at the concentration of 1.57  10-3 M. The
second-order hyperpolarizability of its molecular was estimated to be as large as 1.23  10-32 esu. Its response time was
about 195 fs, which is believed to be the contribution from the delocalized electrons.
Keywords: Optical Nonlinearity; Kerr Gate Technology; Response Time
1. Introduction
Recently, nonlinear optical materials have attracted much
interest because of their potential in fabricating ultrafast
optical switching and processing devices [1-3]. For this
purpose, many materials, including semiconductors, poly-
mers, nanomaterials and inorganic materials, have been
studied. Among them, -conjugated organic material has
been paid great attention for its high nonlinear optical
(NLO) properties and ultrafast response [4]. Especially,
the organometallic and coordination material has been
attracting great concern, because it has the advantages of
architectural flexibility, ease of fabrication and tailoring,
and high NLO properties of organics. Simultaneously, it
possesses good transmissivity, temporal and thermal sta-
bility of inorganic matter. Such material also has strongly
enhanced second-order hyperpolarizability by introduc-
ing a metal atom.
Currently, the synthesis and characterization of 1,3-
dithiole-2-thione-4,5-dithiolate (dmit) complexes and
related selenlum- and oxygen-substituted isologs have
been studied. As a special -electron delocalization con-
jugated system, dmit and related ligand complexes have
been used in the assembly of highly electrically conduct-
ing radical anion salts and charge-transfer complexes.
They are generally used as important building blocks for
organic, organometallic and coordination complex elec-
trical conductors and superconductors. The -electron
delocalization in conjugated systems can also contribute
to the ultrafast response capability and large third-order
nonlinearity. Recently, some of these complexes pos-
sessing good second-order[5,6] and third-order[7-11]
NLO properties have been reported. In this paper, a
dmit2- salt: bis(tetraethylammonium)bis(1,3-dithiole-2-
thione-4,5-dithiolato)cadium was synthesized. The Opti-
cal Kerr Effect (OKE) signal of its acetonitrile solution
was measured by femtosecond optical Kerr gate tech-
nique.
2. Material Preparation and Experiment
Method
The preparation method of CADMIT crystal was a modi-
fication according to literature method [12]. The sample
solution was prepared using acetonitrile as the solvent
whose concentration is 1.57  10-3 M.
The femtosecond optical Kerr gate technique was ap-
plied in this experiment whose measurement setup is
illustrated in Figure 1. The light source, centered at 800
nm, is a Ti:sapphire femtosecond laser system (Mira
900F, Coherent, USA), which is pumped by a multiline
Ar+ laser system (Innova 400, Coherent, USA) at 11 W.
The repetition rate is 76 MHz and its pulse width is 120
fs. The femtosecond laser beam whose average power
was about 0.64 W is split into a probe beam and a pump
beam by a beam splitter. The intensity ratio for the probe
beam and the pump beam is 1:10. The polarization of the
probe beam is carefully adjusted at 45o to the linear polar-

H. L. YANG ET AL.
26
ized pump beam. The pump beam passes through a delay
line driven by a step motor. Then it is carefully reflected
parallel to the probe beam at an adjacent configuration.
The two beams are focused by a convex lens. At the fo-
cus of the lens, the two beams overlap each other. This
focus would be enclosed within the sample cell of 1 mm
in thickness during the measurement. After transmitting
through the sample cell, the pump beam is blocked, while
the probe one passes an analyzer, of which the transmis-
sion axis is strictly perpendicular to that of the probe
beam. Finally, the OKE signal was recorded as a function
of pump-probe delay by an amplified photodiode and
recorded by a digital lock-in amplifier (SR830, Stanford,
USA) referenced to the chopper frequency. Both delay
stage and lock-in amplifier were controlled by a personal
computer.
3. Results and Discussions
The molecular structure of CADMIT is illustrated in
Figure 2 and the absorption spectrum of its acetonitrile
solution is in Figure 3. Its absorption coefficient at 800
nm is very small and suggest that 800 nm is far from its
resonant band. And the absorption around 400 nm is also
not srong which indicates its two-photon absorption at
800 nm has small effect on our measurement result. In
the OKE experiment, CS2 was used as reference. Since
the sample is prepared as a solution, the measured third-
order NLO susceptibility, (3), was the combination of
the response from both the sample solute and solvent.
Therefore in the experiment, we measured the OKE sig-
nals of CS2, the solvent, and sample solution, consecu-
tively. With the solvent signal was removed from the
sample solution signal, we obtained the pure signal of
sample. Then we used the following equ ation to calculate
its third-order NLO susceptibility (3) [13-15].
Figure 1. Experimental Schematic drawing for femtosecond
Optical Kerr Effect.
S
S
S
S
SCd
S
S
S
S
S
N)2
((H5
C2)4
Figure 2. Molecular structure of CADMIT.
Figure 3. Absorption spectrum of acetonitrile solution of
CADMIT.
(3)(3) 1/22
()()
SS
SR
RR
I
n
I
n
  (1)
The subscripts S and R represent the sample of CAD-
MIT and reference sample CS2. I indicates the intensity
of OKE and n is the refractive index. The concentration
of the sample is very low in the solvent, so we use the
refractive index of acetonitrile as the index of solution,
which is 1.34. The n for CS2 is 1.62. The third order
nonlinear susceptibility of CS2 is estimated to be 1 
10-13 esu in femtosecond time scale [16]. Using the equa-
tion, we can get the third-order NLO suscep tibility (3) of
sample directly by measuring the OKE signal intensity of
both sample and reference under identical conditions.
The OKE signal of the sample solution was measured
and illustrated in Figure 4, in which the small contribu-
tion from the solvent was subtracted. Using Eq. (1) and
the measured signals of CS2 and sample solution, the
third-order susceptibility (3) of sample solution was ob-
tained to be 2.98  10-14 esu at the concentration of 1.57
 10-3 M. The second-order hyperpolarizability, , of the
sample molecule may be estimated through the equation
[17,18]
(3) /( )NL
  (2)
where N is the concentration of the solution and L is the
local field correction factor which is defined as

4
22/3n




 (n is the refractive index of solution). By Eq.
(2), we may calculate  of CADMIT as 1.23  10-32 esu
using the measured (3) = 2.98  10-14 esu.
From Figure 4, we obtained the response time of
bis(tetraethylammonium)bis(1,3-dithiole-2-thione-4,5-dit
hiolato)cadium to be about 195 fs, which is commonly
accepted to be the contribution from the transient motion
of the -conjugated-electron distribution. Normally, the
time domain response of the sample should contain sev-
eral processes of different time scales. Using the femto-
second OKE measurement system, some subpicosecond

H. L. YANG ET AL. 27
Figure 4. OKE signals of CADMIT.
processes can be distinguished. The third-order NLO
response time induced by delocalized electrons is be-
lieved to be 10-14 - 10-15 s.While the response time in-
duced by molecular reorientation is 10-11 - 10-12 s and that
induced by density change is 10-8 - 10-9 s.
4. Conclusions
CADMIT was synthesized by making some modifica-
tions to the literature method. The absorption sp ectrum of
its acetonitrile solution was measured, which shows that
the salt has good transmittance from 600 nm to infrared
band. Its third-order optical nonlinearity (3) was studied
by a transient OKE measurement and was about 2.98 
10-14 esu at the concentration of 1.57  10-3 M. Its third-
order optical nonlinearity response time was obtained
and be about 195 fs, which is commonly accepted to be
the contribution from the transient motion of the
-conjugated-electron distribution. Its second-order hy-
perpolarizability, as large as 1.23  10-32 esu, was esti-
mated. These features indicate CADMIT is a potential
photonics material in future.
5. Acknowledgements
The authors acknowledge the financial support of the
National Natural Science Foundation (Grant No. 1100
4123).
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