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Fig. 7.3 Detailed
experimental configurations
of Z-scan method. Open
(a) and closed (b) aperture
experiments. Adapted
from [ 5 ]
a
Open Aperture
Z=0
Z
b
Closed Aperture
Z < 0
Z
7.2.3 Pump-Probe and Z-Scan Methods
In the pump-probe method (Fig. 7.2c ), picosecond or femtosecond laser pulses
are used, and absorption changes for the probe light induced by the pump
light are detected. This type of nonlinear optical processes is determined by Im
w ð 3 Þ ðo 1 ; o 2 ; o 2 ; o 1 Þ
o 2 are the frequencies of the probe and
pump lights, respectively. By changing the delay time of the probe pulse relative to
the pump pulse, the time characteristic of the nonlinear responses can also be
measured. In this method, however, it is difficult to evaluate precise values of Im
w ð 3 Þ ðo 1 ; o 2 ; o 2 ; o 1 Þ
, in which
o 1 and
. It is because we can hardly use a reference sample, in
contrast to the THG method.
In order to evaluate precise values of this type of
(3) , the Z-scan method is
generally used. In this method, both real and imaginary parts of
w
w ð 3 Þ ðo; o; o; oÞ
can be quantitatively measured from the efficiencies of the optical Kerr effect
(nonlinear change of the refractive index n ) and of the nonlinear absorption,
respectively [ 13 ]. The experimental setups of the measurements are shown in
Fig. 7.3 .
In the Z-scan measurements, a sample is moved along the optical path (the z
axis). In this procedure, the electric field strength on the sample is changed
depending on the sample position z . Thus, one can evaluate the optical nonlinearity
by detecting the transmitted light as a function of z . In the open aperture condition
shown in Fig. 7.3a , nonlinear absorption changes can be measured from the
transmittance changes of the sample, while in the partially closed aperture condi-
tion shown in Fig. 7.3b , the optical Kerr effect can be measured from the intensity
changes of the light passing through the aperture, which are induced by the changes
in the direction of the transmitted lights. By comparing those two kinds of trans-
mittance changes of the sample as a function of z to those of a reference, we can
obtain both the real and imaginary parts of
w ð 3 Þ ðo; o; o; oÞ
of the sample. The
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