Third-Order Optical Nonlinearity of Halogen-Bridged Nickel(III) Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 90

Chapter 7
Third-Order Optical Nonlinearity
of Halogen-Bridged Nickel(III) Compounds

Hideo Kishida and Hiroshi Okamoto

7.1 Introduction

In this chapter, we focus on the third-order optical nonlinearity of one-dimensional
(1D) halogen (X)-bridged transition metal (M)-chain compounds (MX chain
compounds). Third-order optical nonlinearity is an important physical property to
achieve all-optical switching devices. The material having large third-order optical
nonlinearity can show the large change of the optical constants by irradiation of
light. Using such behaviors, we can control the intensity or traveling direction of a
light beam in an ultrafast time scale. The enhancement of the performance of all-
optical ultrafast switching is desired because they can be a key technology of the
future ultrafast all-optical communications.
In order for the large optical nonlinearity to be obtained, extensive studies have
been devoted to the development of third-order nonlinear optical materials. The
simplest but sound way is the usage of low-dimensional electronic systems. Espe-
cially, in a 1D system, a clear van Hove singularity and/or a large excitonic effect is
expected to enhance the oscillator strengths of the absorption near the band-edge or
the absorption due to the lowest exciton transition. This necessarily enhances the
third-order optical nonlinearity not only at around the optical gap energy but also
the transparent region within the gap.
In 1990s, Pt-X chain compounds were studied from the viewpoints of nonlinear
optical materials. Wada et al. clarified nature of electronic excited states of Pt-X chain
compounds near the optical gap energies using the electroreflectance method [ 1 ].