Modification of the third-order nonlinearity in poled silica fibers

Silica glass can be poled either thermally or with UV exposure during application of a strong electric field. Such treatment allows electret formation. So normally isotropic glass can become anisotropic via formation of a frozen-in field. This produces non-zero second-order nonlinearity in glass. After such poling treatment a change in the third- order nonlinearity has been observed. In this paper we examine if modification of the third-order nonlinearity is real or some artifact. To do this the DC third-order nonlinearity was measured before poling, after poling and then after erasure of the second-order nonlinearity. It was found that modification of the third-order nonlinearity remains after erasure of the frozen-in field. The reason for modification of the third-order nonlinearity is still not understood. It may be due to some kind of structural modification of the glass. It is known that impurity ionic species are moved through the glass during poling. This movement of ions under the high field may be sufficient to modify the glass structure. From our results, it is clear that the second-order nonlinearity is predominantly caused by formation of a frozen-in field. The increase of the third-order nonlinearity is independent of existence of a frozen-in field after poling.