Third-Order Optical Nonlinearity of Halogen-Bridged Nickel(III) Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 105

pulse applied 1 ps after the first pulse produces the same changes of the transmit-
tance, DT / T , as the first pulse. This result indicates that an optical modulation of

more than 1 THz repetition is possible. Such high repeatability of optical switching
by the TPA process as well as the large

(3) , demonstrates that the [Ni( L ) 2 Br]Br 2
thin film has a really high potential as an optical switching medium.

w

7.4 Summary

The third-order optical nonlinearity in one-dimensional Mott-Hubbard insulators
of Ni-X chain compounds is reviewed from the experimental viewpoint. The large
w ð 3 Þ values in Ni-X chain compounds were revealed by the electroreflectance
method. The spectral analyses based on the discrete-level models clarified that
the strong electron correlation results in the degeneracy of odd-parity and even-
parity CT excited states. Such degeneracy of excited states leads to the large
transition dipole moment between those excited states and is the origin for the
large third-order nonlinear susceptibilities w ð 3 Þ in Ni-X chain compounds. In other

one-dimensional semiconductors belonging to Peierls or band insulators, on the
other hand, the lowest two excitons are not degenerated and, therefore,

w ð 3 Þ is
relatively small. In Ni-X chain compounds, detailed ER and THG measurements
revealed the presence of another essential state with odd-parity, which corresponds
to the edge of the continuum state. Such information also made clear the excitonic
effects in Ni-X chain compounds. Z-scan experiments on the thin film samples
evidenced that Ni-Br chain compounds show large optical nonlinearity at optical
communication wavelength. Moreover, the pump-probe experiments demonstrated
that all-optical switching operation is possible at 1 THz repetition rate. Thus, Ni-X
chain compounds show hopeful ultrafast third-order optical nonlinearity based
upon the novel mechanism resulting from strong electron correlation, which is
different from that in other one-dimensional semiconductors.

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