Study on the Third-order Nonlinear Optical Properties of bis(tetraethylammonium) bis(1,3-dithiole-2-thione-4,5-dithiolato)cadium

doi:10.4236/opj.2013.32B006

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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

Email: [email protected], [email protected]

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.

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.

SCd

N)2

((H5

C2)4

Figure 2. Molecular structure of CADMIT.

Figure 3. Absorption spectrum of acetonitrile solution of

CADMIT.

(3)(3) 1/22

()()



 (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



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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