▁disp la ced ▁by ▁the ▁pol ing ▁field ▁from ▁the ▁an ode ▁side ▁of ▁the ▁sample . ▁This ▁creates ▁a ▁region ▁a ▁few ▁mic rom eters ▁thick ▁of ▁high ▁elect rical ▁resist ivity ▁de ple ted ▁of ▁positive ▁ ions ▁near ▁the ▁an od ic ▁surface . ▁The ▁de ple ted ▁region ▁is ▁neg atively ▁charged , ▁and ▁if ▁the ▁sample ▁is ▁co o led ▁to ▁room ▁temperature ▁when ▁the ▁pol ing ▁voltage ▁is ▁on , ▁the ▁distribution ▁of ▁electrons ▁becomes ▁fro zen . ▁After ▁pol ing , ▁positive ▁charge ▁attract ed ▁to ▁the ▁an od ic ▁surface ▁and ▁negative ▁charge ▁inside ▁the ▁glass ▁create ▁a ▁recorded ▁field ▁that ▁can ▁reach ▁ 1 0 9   V / m . ▁More ▁detailed ▁studies , ▁show ▁that ▁there ▁is ▁little ▁or ▁no ▁accum ulation ▁of ▁c ations ▁near ▁the ▁c ath ode ▁electro de , ▁and ▁that ▁the ▁layer ▁nearest ▁to ▁the ▁an ode ▁suff ers ▁partial ▁neutral ization ▁if ▁pol ing ▁pers ists ▁for ▁an ▁excess ively ▁long ▁time . ▁The ▁process ▁of ▁glass ▁pol ing ▁is ▁very ▁similar ▁to ▁the ▁one ▁used ▁for ▁An od ic ▁bond ing , ▁where ▁the ▁recorded ▁electric ▁field ▁b onds ▁the ▁glass ▁sample ▁to ▁the ▁an ode . ▁ ▁In ▁thermal ▁pol ing , ▁one ▁explo its ▁effects ▁of ▁non linear ▁opt ics ▁created ▁by ▁the ▁strong ▁recorded ▁field . ▁An ▁effective ▁second - order ▁optical ▁non - linear ity ▁ar ises ▁from ▁ χ ( 2 ) eff ▁~ ▁ 3 ▁ χ ( 3