5.1 Introduction

Silica-based glass is one of the important optoelectronic and photonic materials. It exhibits extremely low optical loss over broad transmission band from the ultraviolet to the near infrared; it has high optical damage threshold; and it is compatible with the current fiber technology. So silica-based glass has very important applications as optical waveguides and other optoelectronic devices. ^[1] - ^[6]Generally speaking, there should be no second-order optical nonlinearity in glass because of its inversion symmetry. This has usually brought the glass material only to the passive usages like fibers in photonics, while the second-order optical nonlinearity is the property that absolutely required for active applications in photonics.

In 1986, however, Osterberg and Margulis discovered an efficient second harmonic generation (SHG) in silica-based optical fiber by irradiation with a laser light. Its second harmonic conversion efficiency was as high as 5%. ^[7] Later, in 1991, Myers et al. found that a permanent and large second-order optical nonlinearity could be generated in silica glass by thermal poling method. ^[1] A second-order nonlinear susceptibility ? ⁽²⁾ as large as 1 pm/V was induced in bulk silica glass. The large second-order optical nonlinearity induced in fibers and glasses is of great interest since it opens new frontiers in optical material research. Since then, a large number of investigati/pons have been done to search for novel glass materials and optimum poling conditions for the highest nonlinearity, and to make practical applications feasible. ^[8]

The second-order optical nonlinearity in...