Chemistry Reference
In-Depth Information
a
b
c
Electroreflectance
(ER)
Third Harmonic Generation
(THG)
Pump-Probe
(
)
3
(
3
)
(
3
)
χ
(
−
ω
;
ω
)
χ
(
−
3
ω
;
ω
,
ω
,
ω
)
χ
(
−
ω
;
ω
,
−
ω
,
ω
)
1
2
2
1
Electric
field
(
ω
≈
0
)
ω
ω
−
ω
2
2
ω
(
3
)
P
(
ω
)
(
3
)
ω
NL
(
3
)
P
(
ω
)
P
(
1
ω
)
ω
NL
NL
ω
ω
ω
2
ω
ω
(
ω
≈
0
)
ω
ω
3
(3)
Fig. 7.2 Experimental methods and optical configurations for the evaluations of
w
w
ð
3
Þ
ðo;
2
De
2
¼
3Im
0
;
0
; oÞfEð
0
Þg
(7.2)
The experiments to measure the electric field-induced change of the transmit-
tance are also used, especially in thin film samples. That is called the electroab-
sorption (EA) method. Both ER and EA give the same nonlinear susceptibility,
w
ð
3
Þ
ðo;
0
;
0
; oÞ
. The most important advantage of those techniques is that wide
w
ð
3
Þ
spectra are easily obtained, as compared to other nonlinear
spectroscopies. Therefore, they are frequently used for the evaluations of spectral
shapes of
ranges of
w
ð
3
Þ
in nonlinear optical materials.
7.2.2 Third-Harmonic Generation Method
Third-harmonic generation (THG) method (Fig.
7.2b
) is also a general method to
evaluate
w
ð
3
Þ
. In this method, a nonlinear optical material is irradiated by a laser
pulse with the frequency of
o
, and the light with triplicate frequency 3
o
is emitted
to forward and backward directions, which is detected in the transmission and
reflection configuration, respectively. In most of THG experiments, the TH light
in the forward direction is measured. However, in the case that the sample is bulky
crystal and not transparent in the frequency region of THG, the TH light in the
backward direction is useful to determine
w
ð
3
Þ
. The THG experiments presented in
this chapter were all performed in the backward configuration as shown in Fig.
7.2b
.
For the evaluation of
w
ð
3
Þ
in the THG method, THG intensities in the reference
w
ð
3
Þ
of which is known, should be measured in the same experimental
condition as the sample. As the reference sample in the following studies, SrTiO
3
substrates were used.
sample,
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