Vol.1, No.2,136-141 (2009) Natural Science
http://dx.doi.org/10.4236/ns.2009.12017
Investigation on Third-Order Optical Nonlinearities of
Two Organometallic Dmit2- Complexes Using Z-Scan
Technique
He-Liang Fan1, Quan Ren2,*, Xin-Qiang Wang1, Ting-Bin Li3, Jing Sun1, Guang-Hui Zhang1,
Dong Xu1, *, Gang Yu1, Zhi-Hua Sun1
1State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, Jinan 250100, China;
[e-mail], [e-mail]
2Department of Optics, Shandong University, Jinan 250100, China
3Department of Materials and Chemical Engineering, Taishan University, Taian 271021, China
Received 9 August 2009; revised 27 August 2009; accepted 29 August 2009.
ABSTRACT
The third-order nonlinear optical properties of
two dmit organometallic complexes, [(CH3)4N]
[Au(C3S5)2] (MeAu) and [(CH3)4N][Ni(C3S5)2] (Me
Ni) in acetone solutions, were characterized us-
ing a short pulse Z-scan technique at 1064 nm
wavelength. Self-defocusing effects were found
in both samples and stronger saturable absorp-
tion was observed in MeNi solution comparing
with that of MeAu. The origins were analyzed for
the differences between the results. Two figures
of merit W and T were also calculated to evalu-
ate the suitability of two materials for all-optical
integrated devices. The results of W=22.84 and
T≈0 of MeAu make it an excellent candidate for
the all-optical applications.
Keywords: Z-scan Technique; Third-order
Nonlinearity; Metal-dmit Complexes;
Figure of Merit.
1. INTRODUCTION
As the development of optical communication networks
progress, the demand for ultrafast optical switching with
femtosecond or picosecond response time operation is
rising. materials with large third-order nonlinear optical
(NLO) properties and ultrafast response time have arose
great interest for its widespread applications in optical
switching, signal processing, ultrafast optical communi-
cations and optical limiting [1-4]. In recent years,
π-conjugated organometallic complexes have emerged as
a promising class of third-order nonlinear optical (NLO)
materials because of their architectural flexibility with a
variety of combinations of central metals and ligands as
well as the charge-transfer nature of the metal-ligand
bonds, which can further enhance the nonlinearity [5-6].
Special π-conjugated electron systems, like
4,5–dithiolato-1,3-dithiole-2-thione (dmit) complexes,
have been used as building blocks for organic, or-
ganometallic and coordination-complex electrical con-
ductors and superconductors [7-10]. Currently, more
attention has been paid to the third-order NLO proper-
ties of these materials [11-13]. These structures that
contain transition metal ions may exhibit new properties
due to the richness of various excited states present in
these systems in addition to the tailorability of
metal-organic ligand interactions. Also the π-electron
delocalization and the transfer of electron densities be-
tween metal atom and the ligands make this kind of
compounds exhibit a large molecular hyperpolarizabil-
ity which can contribute to ultrafast optical response
capability and larger third NLO effects. Usually, as-
sessing the suitability of a material for all-optical
switching devices is evaluated through two figures of
merit: 20
WnI



and T2 n

 (2 n is the
nonlinear refractive index,
I
is incident light intensity,
0

is the linear absorption coefficient,

is wavelength
and

is the nonlinear absorption coefficient) [2,14]. In
order to satisfy the requirement, it is necessary to
achieve W>>1 and T<<1. In this paper, the third-order
optical nonlinearity of two dmit organometallic com-
plexes, MeAu and MeNi, were reported using a Z-scan
technique at 1064 nm with 20 ps pulse duration and 10
Hz repetition rate. Additionally, W and T of these sam-
ples were obtained which were used to evaluate their
feasibility of to be applied in all-optical device field.
The Z-scan technique which was firstly reported by M.
Sheik-Bahae et al. [15], is a simple and sensitive single
beam method for measurement of third-order nonlinear
optical coefficients. It is based on the self-focusing or

H. L. Fan et al. / Natural Science 1 (2009) 136-141
137
defocusing of a distorted beam of known spatial struc-
ture induced by moving a nonlinear sample along the
light-propagation direction (Z-axis). Using this method,
the magnitude and sign of both the real (nonlinear re-
fraction, NLR) and imaginary (nonlinear absorption,
NLA) parts of the nonlinearity of transparent mediums
can be immediately obtained based on the relationship of
the variation of transmittance in the far field and the
sample position. The Z-scan technique can be signed
two types: closed-aperture Z-scan and open-aperture
Z-scan. For closed-aperture Z-scan both NLR and NLA
can be measured simultaneously while for another, the
NLA can be independently measured more accurately.
To date the Z-scan technique is becoming an increas-
ingly popular approach on measurement of nonlinear
optical responses for its convenient operation, higher
sensitivity and simple apparatus comparing with other
method such as degenerated four-wave mixing, optical
Kerr gate, nonlinear interference and so on.
2. EXPERIMENTAL
The molecular structures of MeAu and MeNi were illus-
trated in Figure 1. The synthesized procedures were
respectively referred by the literatures [14,16]. The lin-
ear UV-Vis-NIR absorption spectra of 1×10-4 mol/L so-
lution of two materials in acetone were recorded using a
scanning spectrophotometer (Hitachi U-4100, Japan).
Figure 2 shows the results with the wavelength region
330-1500 nm at room temperature. Both MeAu and
MeNi represent several absorptive peaks in the UV-Vis
region which can be regarded as attributing to the n-π
transition and the d-p interaction [16,17]. In another
words, for MeAu, there was so wider a transparent win-
dow with no absorption in the wavelength region longer
than 500 nm. While for MeNi, it also exhibits a strong
absorption band with the peak at about 1137 nm in the
NIR region (800-1500 nm) which may be assigned to the
low-energy π-π* transition [18].
CH3
CH3
H3C
H3C
S
S
S
S
S
Au
S
S
S
S
S
CH3
CH3
H3C
H3C
S
S
S
S
S
Ni
S
S
S
S
S
N
-
+
N
-
+
Figure 1. Molecular structures of MeAu and MeNi.
4006008001000 1200 1400
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Absorbance
Wavelength (nm)
MeAu
MeNi
Figure 2. UV-Vis-NIR absorption spectra of acetone solutions of MeAu (solid
line) and MeNi (dot line) with a concentration of 1×10-4 mol/L at room tem-
perature.

H. L. Fan et al. / Natural Science 1 (2009) 136-141
138
The Z-scan technique was used to characterize the
third-order nonlinear optical response of the acetone
solutions of two materials. In our measurement, a
mode-locked Nd:YAG laser (Leopard-10, Continuum)
was employed as the Gaussian light source with a repeti-
tion rate of 10 Hz, pulse width of 20 ps and wavelength
of 1064 nm. The sample is moved along the optic axis
(the Z-direction) through the focus of the lens, which has
a focal length of 150 mm, while the energy transmitted
through an aperture in the far-field is recorded as a func-
tion of the sample position. The radius of the beam waist
(w0) was determined to be 39 μm. Accordingly, the
Rayleigh length (z0) was calculated to be 4.5 mm, much
larger than either the thickness of a quartz cell with the
optical length of 1 mm. The incident beam intensity (I)
performed on the samples was set to be 5.3(±0.3)
GW/cm2. Before measuring the samples, the system was
calibrated using a standard CS2 solution in quartz cell as
reference. Measurements on the pure solvent (acetone)
in the cell were also performed under the same measur-
ing condition to verify that the peak-valley configuration
in the Z-scan curves all originated from the material, but
not from the solvent or the quartz cell [19].
3. RESULTS AND DISCUSSION
The concentration of samples used in Z-scan measure-
ments is 1×10-4 mol/L. Figure 3 exhibits the
closed-aperture (CA) and open-aperture (OA) data for
MeAu in acetone solution. The symmetrical val-
ley-to-peak configuration of the CA curve and the hori-
zontal straight line of the OA curve reveal that the sam-
ple shows obvious self-defocusing effect and tiny NLA,
which is considered a potential feature for all-optical
switching. Then the Z-scan curves for MeNi are shown
in Figure 4 (a) and (b). The configurations of CA and
OA curves in Figure 4 (a) both demonstrate single peak,
-40 -30 -20 -10010203040
0.90
0.95
1.00
1.05
1.10
CA Z-scan curve
OA Z-scan curve
Normalized Transmittance
Z (mm)
Figure 3. Normalized Z-scan transmittance curves of
MeAu in acetone solution with concentration of 1×10-4
mol/L. The solid line is the theoretical fitting curve. Z is
the sample position away from the focus.
-40 -30 -20 -100102030
0.85
0.90
0.95
1.00
1.05
1.10
1.15
1.20
1.25
1.30
1
.
35
CA Z-scan curve
OA Z-scan curve
Normalized Transmittance
Z(mm)
(a)

H. L. Fan et al. / Natural Science 1 (2009) 136-141
139
-40-30-20-100 102030
0.85
0.90
0.95
1.00
1.05
1.10
1.15
1.20
1.25
1.30
1.35
CA/OA Z-scan curve
Normalized Transmittance
Z(mm)
(b)
Figure 4. Normalized Z-scan transmittance curves of MeNi in
acetone solution with concentration of 1×10-4 mol/L. The solid
line is the theoretical fitting curve. Z is the sample position away
from the focus.
Table 1. Nonlinear optical parameters of MeAu and MeNi at 1064 nm.
Sample α0 (mm-1) n2 (10-19 m2/W) β (10-12 m/W) χ(3)(10-13 esu) γ (10-30 esu) W T
MeAu 0.0021 -9.50 -9.17×10-3 4.58 2.65 22.84 ≈0
MeNi 4.60 -5.68 -10.34 5.03 2.91 0.013 1.94
suggestion that the NLA which is regarded as the satur-
able absorption, covers the contribution of the NLR and
plays a dominant role in the third-order nonlinear proc-
ess of the sample, which is a very attractive feature for
laser mode-locking, laser Q-switching and optical bista-
bility applications [20-22]. Figure 4 (b) shows the divi-
sion curve of the CA by OA data of MeNi, which also
reveals a self-defocusing effect for the sample.
In general, to distinguish the NLR and NLA, the nor-
malized transmittance dependence can be presented as
follows [23-24]:
 

2
22 22
23
4
191 91
x
x
Txx xx

  
  (1)
with
20 eff
kn IL  (2)
02 eff
IL

  (3)
where T is the normalized transmittance of the sample,
 and  is the nonlinear phase shifts due to the
NLR and NLA, respectively. Here x=z/z0, indicates the
dimensionless relative position from the waist, k is wave
vector,

00
1exp
eff
LL

  

is the effective
thickness (L denotes its real thickness).
The real and imaginary parts of the third-order
nonlinear susceptibility χ(3) are related to the NLR and
NLA coefficients by [15, 25]:




33 3
i
R
I

  (4)

2
32
0
2
2
() (mW)
120
R
cn
esu n
  (5)

22
30
2
() (mW)
240
I
cn
esu


 (6)
where ω is the angular frequency of the light field and c
is the velocity of the light in vacuum.
Forwards, the second-order hyperpolarizability γ of
the sample molecule can be estimated through the equa-
tion [26]

3
cc
NL

  (7)
where c N is the number density of molecules and c L
is the local field correction factor which equals

4
2
023n




.
According to above-mentioned procedure, the nonlin-
ear parameters of the two sample MeAu and MeNi n2, β,
χ(3), and γ can be obtained in succession. Additionally,
the results of two figures of merit W and T were also
calculated basing on the NLO parameters. All the pa-
rameters were listed in Table 1. We can see that both

H. L. Fan et al. / Natural Science 1 (2009) 136-141
140
MeAu and MeNi show larger third-order nonlinear opti-
cal properties because of the delocalized electronic states
formed by the overlapping between p-p and d orbits [10].
But the nonlinear absorption coefficient of MeNi is rap-
idly larger than that of MeAu. The resonant wavelength
of 1064 nm of MeNi gives to the stronger saturable ab-
sorption comparing with the weaker nonlinear absorp-
tion of MeAu at 1064 nm which locates on the
off-resonant field of linear absorption [27]. The figures
of merit of MeAu were calculated to be W=22.84 and
T≈0, which finely satisfy the requirement of suitability
for all-optical switching devices W>>1 and T<<1. So the
material can be considered to be an excellent candidate
to be applied in integrated optics field as all-optical
switching devices. While for MeNi, W=0.013 and
T=1.94, the values of two figures of merit don’t satisfy
the requirement of all-optical devices but may be applied
in laser mode-locking, laser Q-switching and optical
bistability fields because of its saturable absorption
properties.
4. CONCLUSIONS
The third-order nonlinear properties of two metal-dmit
complexes MeAu and MeNi were investigated using a
Z-scan technique at 1064 nm with 20 ps pulse width and
10 Hz repetition rate. Z-scan curves indicated that both
MeAu and MeNi show negative nonlinear refraction
which are regarded as self-defocusing effects. Mean-
while, tiny nonlinear absorption and stronger saturable
absorption was found in MeAu and MeNi, respectively.
The figures of merit W and T of two materials were cal-
culated to judge the suitability as all-optical switching
devices. The values of MeAu W=22.84 and T≈0 were
considered to be appropriate for applications in
all-optical integrated field. While for MeNi, the stronger
saturable absorption comparing with nonlinear refraction
makes it a fine material to be applied in laser
mode-locking, laser Q-switching, optical bistability field
and so on.
5. ACKNOWLEDGEMENTS
This research work is supported by the grants (Nos.
50772059, 60778037, 60608010 and 50872067) of the
National Natural Science Foundation of China (NNSFC)
and the Foundation for the Author of National Excellent
Doctoral Dissertation of P. R. China (No. 200539).
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