Glass with modified surface regions on opposing sides and methods and apparatuses for forming the same via electro-thermal poling

A method of forming a glass substrate with modified surface regions, comprising:

• providing a glass substrate having a concentration of alkali and opposing surfaces that include a first surface and a second surface;
• reducing the concentration of alkali in at least one first region extending for a first thickness from the first surface via thermal poling and, near-simultaneously, reducing the concentration of alkali in at least one second region extending for a second thickness from the second surface via thermal poling,
• wherein, after thermal poling, each of the at least one first region and the at least one second region has a substantially homogenous composition,
• wherein thermal poling comprises contacting a first electrode to the first surface, contacting a second electrode to the second surface, and applying an alternating electrical potential difference to the glass substrate such that the first and second electrodes are alternatingly positively-biased relative to the glass substrate to induce alkali depletion in the at least one first and second regions, respectively,
• wherein applying an alternating electrical potential difference to the glass substrate comprises using an alternating current (AC) waveform,
• wherein using the (AC) waveform includes increasing a total voltage to the first and second electrodes as a function of time, and
• wherein the AC waveform defines a duty cycle with a first half cycle in which the first electrode is positively biased and a second half-cycle in which the second electrode is positively biased, the duty cycle controlled to induce asymmetry between the first thickness and the second thickness.