Visualization of the nonlinear optical space-charge region of bulk thermally poled fused-silica glass

We etched thermally poled fused-silica coverslips in 49% HF for 30 s transverse to the poling direction to reveal structural details of the nonlinear region. A peaked ridge below the anode surface, corresponding to a slower etch rate than that of the bulk SiO(2) , was located approximately 5 microm below the anode surface for a poling time of 30 s. The ridge moved deeper into the glass logarithmically with poling time. This trend is qualitatively consistent with a recent model for the formation of the space-charge region that includes injection of hydrogen ions at the anode surface.