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in which the CT transition from Br 4p-valence band to the Ni-3d upper Hubbard
band corresponds to the optical gap.
In the following subsections, we review spectroscopic studies on
in Ni-X
(3) is significantly enhanced due to the specific
feature of photoexcited states in Ni-X chains originating from the strong electron
correlation [ 3 , 10 , 11 ]. We also review the demonstration of all-optical switching
using thin film samples of a Ni-Br chain compound, in which the ultrafast control
of the transmittance of the film with a 1 THz repetition was achieved [ 5 ].
chain compounds and show that
7.2 Evaluation of Third-Order Optical Nonlinearity
Third-order nonlinear optical phenomenon is a result of the third-order nonlinear
polarization P ð 3 Þ , which is governed by the third-order nonlinear susceptibility
w ð 3 Þ .
w ð 3 Þ is defined as a function of three incident electric fields, Eðo 1 Þ
, Eðo 2 Þ
, and Eðo 3 Þ
with the frequencies,
o 1 ,
o 2 , and
o 3 , as follows:
P ð 3 Þ ðo p Þ¼Ke 0 w ð 3 Þ ðo p ; o 1 ; o 2 ; o 3 ÞEðo 1 ÞEðo 2 ÞEðo 3 Þ
o p ¼ o 1 þ o 2 þ o 3 is the frequency of the nonlinear polarization, and K
is the constant depending on the measurement configuration [ 12 ]. By changing the
frequencies of the incident electric fields, o 1 , o 2 , and o 3 , we can observe various
kinds of third-order nonlinear optical phenomena. In this chapter, four kinds of the
nonlinear optical measurement methods are presented. Each technique is briefly
introduced below.
7.2.1 Electroreflectance Method
In the electroreflectance (ER) method, we measure the electric-field induced change
of the reflectivity as shown in Fig. 7.2a . On the surface of the crystal, two electrodes
are made with carbon paste or silver paste and with the gap of a few hundred
micrometers. Using these two electrodes, the low frequency-alternating electric
field with a typical frequency of 1 kHz is applied. The amplitude of the electric field
is typically a few tens of kV/cm. The induced change
DR of the reflectivity R is
measured using the lock-in detection method. The obtained quantity is
DR=R and
its typical magnitude is 10 6 -10 3 . By applying Kramers-Kronig (KK) transfor-
mation to the
DR=R spectrum, the change of the imaginary part of dielectric
De 2 ) is obtained.
De 2 is directly connected to
w ð 3 Þ ðo;
; oÞ
constants (
by the
following relation:
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