G. Ma et al. / Solid State Communications 118 (2001) 633±638
637
response for a MgPc ®lm. The bleaching recovery processes
consist of three components, an ultrafast process with time
constant less than 200 fs, a slow process of 6 ps time scale
and a very slow process with recovery time about hundreds
of ps to even ns scale.
800 nm of the CuPc ®lms, a reverse saturable absorption
(RSA) with ultrafast response was observed. If the material
has a larger cross section of excited state (sex) than that of
ground state (sg), and the incident light is suf®ciently
intense, there will be a signi®cant population in the excited
state, resulting in RSA. It was reported that the sex and sg of
the CuPc monomolecule were about 5.2 £ 10218 and
2.0 £ 10218 cm2 at 590 nm, respectively [25]. Although
both the sex and sg of CuPc depends on the wavelength
as observed from the photoinduced absorption spectrum,
the low absorbance at 800 nm would result in RSA process
as well. The ultrafast decay of the photo-induced absorption
in the CuPc ®lm is much faster than that of bleaching
recovery in MgPc ®lm. As the decay of the photo-induced
absorption should also strongly relate to the relaxation of the
singlet excited state, it means that the relaxation process in
the CuPc ®lm should be much faster than that in the MgPc
®lm. In ®lm of CuPc, due to electron coupling between the
un®lled d orbit of central Cu ion and PC22 ligand, the
ground state of CuPc is singdoublet state (2G), the ®rst
excited state also become singdoublet (2Q), and the triplet
state is split into tripdoublet (2T) and tripquartet (4T) which
The transient bleaching of the MgPc ®lm could come
from the saturable absorption (SA) or the stimulated
emission (SE). Under photoexcitation, the singlet excited
state would be populated, and the SA will take place if the
absorption cross section of the excited state is smaller than
that of ground state. As we did not observe detectable photo-
luminescence (PL) emission from the MgPc ®lms, SE
process could be excluded. The ground state bleaching
should arise from the SA effect.
In general, the recovery of ground state bleaching
involves several decay mechanisms: (a) exciton±phonon
coupling (internal conversion) [20]; (b) exciton±exciton
annihilation [15,21]; (c) ¯uorescence[22]; (d) inter-system
crossing (ISC) transfer from singlet state to triplet state
[10,15] and (e) nonradiative recovery through triplet
states[8,10];
The ultrafast component may be assigned to the internal
conversion due to a strong interaction between exciton and
phonon [9,20]. The exciton±exciton annihilation can also
dominate the ultrafast process under strong excitation
density [15,21]. But in our experiment, the excitation
density is about 0.1 GW/cm2, and the absorbance of MgPc
at 800 nm is very small. In this case, we consider that the
contribution to the ultrafast component is not a bimolecular
process but the electron±phonon interaction. Yuan et al.
found that for an HDVPC LB ®lm, the ultrafast component
of OKE signal increased dramatically with the temperature
of the sample, and it was also reported that for polydiacetyl-
ene, phonon emission process could be as fast as 50±180 fs.
The phonon emission is actually a process in which energy
redistributes between the phonons and excitons, and so it
can be considered as a kind of exciton±phonon coupling.
The recovery process of photo-induced bleaching also
re¯ects the relaxation of the population in the singlet excited
state S1, other relaxation processes should include the
process of S1 ! T1(triplet state) by ISC, S1 ! S0 via
radiative transition or the nonradiative transition. As
mentioned earlier, there is nearly no PL emission observed
from the MgPc ®lm, the radiative process is really ineffec-
tive in the MgPc ®lm. Some papers pointed out that ISC
process in MPc materials could be ranged in ps domain [22],
even in fs scale [23], which would dominate the lifetime of
the singlet excited state. Usually, the ISC rate in a solid
material is larger than that in a solution. A very effective
crossing transfer rate (F , 1) and long lifetime of triplet
state (,ns) were reported in various Pc ®lms [15,20,22,24].
Therefore, the slow component with recovery time about
6 ps can be assigned to the decay of singlet excited state
via ISC process, and the very slow component is due to the
nonradiative relaxation from triplet state to the ground state.
Fig. 5b shows the temporal transmittance change at
2
2
states lie between G and Q [8,15]. As a result, the inter-
2
system crossing process from 2S ! T/4T becomes very
fast, which results in very short lifetime of the singlet
excited state, so the ultrafast relaxation process could result
from fast ISC process in the CuPc ®lm.
4. Conclusions
In summary, we have investigated experimentally the
NLO response of the CuPc and MgPc ®lms by femtosecond
OKE and pump-probe technique. These results indicate that
the central metal atom affects greatly the magnitude of the
nonlinear optical susceptibility of MPc ®lms. In CuPc with
planar structure, the un®lled d valance orbit probably couple
with p-electrons of the Pc rings and enlarge the conjugation
system, as a result, the CuPc ®lms show large third-order
nonlinear optical property, while the enhanced third-order
optical nonlinearity of the MgPc ®lm arises mainly from its
pyramidal structure. From the pump-probe experiment, the
RSA and SA process was found in the CuPc and MgPc ®lm,
respectively. The ultrafast OKE response and decay of
photo-induced RSA of the CuPc ®lm is assigned to fast
intersystem crossing from singlet to triplet state. For
MgPc ®lm, long decay of both OKE signal and photo-
induced bleaching would arise from the ISC process from
the singlet state to triplet state and relaxation process from
the triplet state to ground state.
Acknowledgements
The authors, from Fudan University, are grateful for the