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Fig. 7.12 Demonstration of
the ultrafast optical
switching. (a) The absorption
spectrum and energies of
pump (
gray
) and probe
(
black
) pulses. (b) The
transmittance modulation by
the two pump pulses with an
interval of 1 ps. From [
5
]
w
ð
3
Þ
ðo; o; o; oÞ
w
ð
3
Þ
ðo; o; o; oÞ
lines. From these profiles, Re
and Im
at
10
9
esu, respectively,
both of which are also very large. Taking the discussions in the previous
subsections into account, the two-photon state (1.6 eV) corresponds to the even-
parity states within the continuum. The large TPA associated with the even-parity
states could be also explained by the fact that the degeneracy of the odd- and even-
parity states enhances the dipole moment
10
9
and 0.56
1.55
m
m were evaluated to be
1.5
<
1
jxj
2
>
between them.
7.3.4 Ultrafast Optical Switching [
5
]
High repeatability of the switching operation is the most important condition for
all-optical switching devices. To investigate the performance of Ni-X chain
compounds, the repeatability of the TPA phenomenon in the [Ni(
L
)
2
Br]Br
2
film
was investigated by double pump pulses. Energies of the pump and probe pulses
were set at 0.56 and 0.93 eV, respectively, both of which are located in the
transparent region as shown in Fig.
7.12a
. Figure
7.12b
shows the time characteris-
tic of the transmittance change
DT
/
T
. The first pulse gives rise to an instantaneous
decrease in the transmittance, which is followed by its ultrafast recovery. This
indicates that the decay time of the photoexcited states is much shorter than the time
resolution of the measurement system, which is about 180 fs. The second pump
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