Time evolution of depletion region in poled silica

The electric field recorded in silica samples thermally poled during various intervals was measured by etching. The field is nearly uniform and increases to a maximum (∼3.6×108 V/m) in few minutes of poling and subsequently decreases slowly toward a steady level (∼1.4×108 V/m). The depletion layer becomes neutral even before the field reaches steady state. An upper limit of (0.7±0.2) μm was found for the thickness of the negatively charged edge of the depletion region. The value of χ(3) is the same for all poling times.