Optical poling of silica fibers[1] allows for second-harmonic generation through the creation of a self-organized periodic distribution of charges at the core-cladding interface.
UV poling [2] received much attention because of the high non-linearity reported, but interest dwindled when various groups failed to reproduce the results.
Strong electric fields are created by thermal poling of silica,[3] subjecting the glass simultaneously to temperatures in the range of 280 °C and a few kilovolts bias for several minutes.
The depleted region is negatively charged, and if the sample is cooled to room temperature when the poling voltage is on, the distribution of electrons becomes frozen.
More detailed studies,[4][5] show that there is little or no accumulation of cations near the cathode electrode, and that the layer nearest to the anode suffers partial neutralization if poling persists for an excessively long time.