Optics and Photonics Journal, 2011, 1, 81-84
doi:10.4236/opj.2011.12013 Published Online June 2011 (http://www.SciRP.org/journal/opj/)
Determination of the Triplet State Lifetime of C60 / Toluene
Solution and C60 Thin Films by Pump-Probe Method
Cheng Bao Yao1,2, Elisee Kponou1, Yun Dong Zhang1*, Jin Fang Wang1, Ping Yuan1
1National Key La boratory of Tunable Laser Te ch nology, Institute of Optoelectronics, Harbin Institute of Techn ology,
Harbin, China
2Heilongjiang Key Laboratory for Advanced Functional Materials and Excited State Processes, School o f P hys i c s an d
Electronic Engineering, Harbin Normal University, Harbin, China
Received April 6, 2011; revised May 4, 2011; accepted May 12, 2011
Abstract
Excited state lifetimes of C60/toluene solution and C60 films macromolecular were measured by pump-probe
method. Relation between optical switching effect of material and pulse width of pumping field is briefly
described. It is found that the faster switching speed of light is, the triplet state lifetime is shorter. A He-Ne
laser, as a probe, passed through the sample in the pump-probe experiment. All-optical switching effect was
realized. Changing the optical power of the pumping field, switching response of the sample and modulation
depth were investigated. In certain experimental conditions, relation between transmission through the sam-
ple and response were measured by an oscilloscope. Triple state lifetime of the molecule is speculated. The
result showed that C60/toluene solution and C60 film have a fast response time. They would be utilized in
some applications, such as optical switches, photonic devices.
Keywords: Excited State Lifetimes, All-Optical Switching, Fullerenes
1. Introduction
The nonlinearities of C60 have been paid considerable at-
tention for their promising use in the applications of
photonic devices due to its unique opto-electronic proper-
ties, its reverse saturable absorption phenomenon was
found by Guiliano and Hess [1]. In recent years, the poten-
tial applications of RSA materials in optical limiting and
switching have attracted more interests because of the
growing needs for laser protection [2]. RSA behaviours
have been successively investigated in numerous materials.
The C60 shows a strong and broad spectrum absorption
induced in the visible domain. As an view of application,
this makes C60 an interesting candidate for optical power
limiting purposes [3-7]. On the other hand there exist a
multitude of studies concerning the relaxation dynamics
following a photo excitation of C60 in liquid solutions
and in thin films. In this paper, we compared with func-
tionalized fullerene solutions, scattering contributions to
the observed limiting performance, and possible limiting
in the near infrared region, we got a clear picture of op-
tical switching properties of liquid C60/toluene solution
and solid C60 thin films.
2. Experiment and Theory
The mechanism of optical switch of excited state absorp-
tion is that the molecules at excited states pumped by
laser strongly absorbs photons at some wavelength,
while the ground state absorption is very weak. So when
the pump field stimulated the medium, the probe light is
strongly absorbed, the output is in low state (in the
closed state); on the contrary, when no pump exists,
molecules in the ground state, prob e light is not absorb ed
(or little absorbed), output in highly excited state (the
switch turned on).
The experimental arrangement is shown in Figure 1.
A doubled-frequency Nd : YAG laser with repetition rate
of 10 Hz, at 532 nm, is employed to provide exciting
pulse, and its pulse width is 10 ns. The laser beam first
though the beam splitters 1 (BS1), part of the beam
splitted was detected by detector D1, which is used as the
trigger pulse for the oscilloscopes. The other part of the
laser, which is used as the pump beam, passed through
the beam splitters 2 (BS2), was focused on the sample by
a lens (f = 300 mm). A He-Ne laser at 632.8 nm is used
as the probe light. The probe beam first through the sam-

C. B. YAO ET AL.
82
ple, then focu sed by the lens, splitted by BS2. And at last
the probe beam is detected by D2, then is recorded on a
digital oscilloscope.
Figure 2 is the energy level diagram of C60 molecules
in the laser irradiation, the ground state molecu le absorbs
photon and transits to the first singlet excited state vibra-
tional level S1, then rapidly relaxes through the inter-
system crossing transition to the first excited triplet state
T1, molecules of the excited state T1 absorbs photon can
transition to a higher excited level Tn, then return to the
energy levels of T1. Molecules of the level T1 may have a
longer lifetime, an d can be back to the gr ound state in the
form of non-radiative transition.
Considering the role of the pump light, the rate equa-
tions take th e following:
00 0
() 11
LST
lSOTO
dNI t
N
NN
dt h


 (1)
00
() 11
SL ST
lSOST
dNI t
N
NN
dt h



(2)
11
TST
ST TO
dN NN
dt

 (3)
Boundary conditions:
0(,)
N
tzN
(,)(.)0
ST
Ntz Ntz
0
(, )()
L
ItzIft (4)
where N0, NS and NT represent the number densities of
states S0, S1 and T1, respectively. N is the total number
density of the molecules, 
L is the pump frequency, f(t) =
exp[-c(t/t)] is described time function of pump pulse
shape（c = 2）. Through the medium of the probe light can
be expressed as:
Figure 1. The experimental setup.
Figure 2. The five-energy-level diagram showing optical
excitation.
( )()()
pTTp
dI pN tIt
dz
  (5)
Boundary conditions: (, 0)
p
po
I
tz I 
where 
T (p) is the absorption cross sections for the probe
light of T1 state . Ipo is the optical power density of incident
to the sample s urface for t he detection.
The change of population along with time in the first
triplet state are described by solving Equations (1) - (5). We
obtain simulated transmission as shown in Figure 3.
3. Results and Discussion
The numerical result for the case of solutions is in good
agreement with published resul ts summarized i n Table 1.
The data for i of the C60/toluene is taken from Ref.
[8-10], S1 and T1 is the S1 and T1 state absorption cross
sections. Th e higher ex cited states Sn and Tn are assumed to
have a very short lifetime [11-13] compared to our pulse
duration, such that they can always be considered to be
empty. Electrons relax back to the S1 state almost immedi-
ately and the first excited triplet state T1 is populated via
intersystem crossing with a time constant 
ST. The lifetime
of the T1 state To of the investigated compounds have been
reported previously and ranged between100 ns and 300 s.
In the pumping processing, the pumping field excites
molecules at ground state to the excited states. The absorption
of excited state leads to the decrease of probe field energy.
When molecule returns to ground state from excited states.
The absorption at excited states dispears. The probing power
resumes the initial valu e. The time correspondin g this process
is considered as the lifetime of the excited state, which is equal
to the closing time. W e implemented similar measuremen t on
C60 toluene solutions in thin quartz cells. The result of the
pump–probe experiment on C60/toluene solu tions is shown in
Figure 3(a). It is shown that the triplet state lifetime in our
samples is 0.28 s. The parameters of interest are summarized
in Table 1. The experimental results ag ree well with theoreti-
cal analyses. The output of the digital oscilloscope in our
pump–probe e xperime nt for a C 60 thin film is shown in Figure
3(b). The results show that the triplet state lifetime in the sam-
ples is 0.98 s. The numerical value of lifetime is between the
two values of 100ns or 300 s [14]. One difference in our
experiment al conditions is t he fact that we used film as toluene
solvent, which leading to a longer lifetime of these states.
Table 1. The input data for simulations.
aranetersP Datas

S0 (cm2) 1.2410–18 cm2

S1(cm2) 6.8110–18 cm2

To(cm2) 4.010–18 cm2

T(cm2) 2.4810–17 cm2
I0(W/cm2) 1.34105 W/cm2
c(mol/L) 4.010–4 mol/L
l(mm) 1 mm

C. B. YAO ET AL.
83
(a)
(b)
Figure 3. Transmission changes during a pump—probe ex-
periment for a C60 thin film.
Under identical conditions, increasing the concentra-
tion and the th ickness o f the sa mple, the switch ing ti me is
not changed and the contrast ratio is increased. These are
essential characteristics of an RSA process involving a
long-lived excited state. Our experimental method is well
adapted to the determination of these important parame-
ters by a purely opti cal method.
4. Conclusions
We studied the triplet state lifetime of C60/toluene solu-
tion and C60 thin films. From the experimental results we
conclude following conclusion: The lifetime of the ex-
cited state is about 0.28 s and 0.98 s of C60/toluene
solution and C60 thin films, respectively. The theoretical
fits are in good agreement with experimental results. This
is of advantage for RSA materials used as optical limiting.
The absorption cross section of the excited state is larger
than that of the ground state at both 532 nm and 632.8 nm.
Furthermore, we showed with pump-probe experiments,
that the T1 state of C60 decays exponentially with a time
constant which depends on the vibrational excitation en-
ergy of the molecule. This also allowed us to find the
unambiguous assignment of the number of absorbed
photons associated with a specific lifetime.
5. Acknowledgements
The research is supported by the National Natural Science
Foundation of China under Grant Nos. 61078006 and
60878006, the National High Technology Research and
Development Program (“863”Program) of China under
Grant No. 200 7AA 12Z112.
The research is supported by the National You th Natu-
ral Science Foundation of China under Grant Nos.
51002041.
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