562 Poling of Glasses and Optical Fibers in thermally poled waveguides (increase 2 times) [177], fibers ($2­3 times) [178,179,180] and glasses ($1­2 times) [102] and not only correlated to material phase transformation. Explanations for the increase in the w (3) were attempted [181], invoking local field corrections. In contrast, various reports also exist reporting that the w (3) does not change significantly in waveguides [113] and fibers [170] after poling. The w (3) is a measure of the polarizability and therefore an intrinsic property of the material. However, charging, ion exchange, and crystallization are reported consequences of poling. It is expected that when the amorphous material undergoes a phase change and crystallizes, the w (3) is also modified. Even in the absence of a structural change, it is known that charges and embedded ions modify the polarizability of the medium [182]. Changes of w (3) in the vicinity of the space charge layer are perhaps not surprising. On the other hand, the w (3) modification is likely to be restricted to the neighborhood of the space charge and therefore remain local. Variations of w (3) over a few microns may explain the discrepancy of results from various authors, which probe the effect with a light guiding core situated at various distances from the anode electrode [102]. 12.7.4 Increasing x (3) through Resonance and Doping Exciting results of a 25 pm/V nonlinearity induced in silica glass poled with silver-painted electrodes have been reported [109,183]. Frequency doubling measured in reflection used to characterize the poled samples showed that sur- face plasmon resonance (l p $ 0.4À0.5 mm) was responsible for the large enhancement of w (3) (2o, o, o). The induced electro-optic effect exploiting w (3) (0, o, o) unfortunately remains low. It has been impossible to achieve compara- ble w (2) values in transmission measurements through glass samples as in reflec- tion, and it is likely that the high value of measured w (2) is associated with a large and unavoidable absorption, which hinders the exploitation of the large w (2) in frequency doubling applications. The enhanced w (3) expected from doping fibers and planar waveguides with rare earths [184] has surprisingly not been exploited in poling. 12.7.5 Glasses Other Than Silica There are a large number of articles describing poling of glass systems other than silica with the objective of achieving a large w (2) . Some of these studies have been motivated by plain scientific curiosity and others by the concrete goal of increasing the induced nonlinearity, as the w (3) of most glasses is higher than in silica. Also, soft glasses such as soda-lime and borosilicate glass are consider- ably cheaper than pure silica and facilitate the combination of poling with the