# DC Voltage Induces Quadratic Optical Nonlinearity in Ion-Exchanged Glasses at Room Temperature
## 4 Conclusions  
Thus, we have demonstrated that applying DC voltage at room temperature to ion-exchanged glass leads to the significant second optical harmonic generation of laser radiation in the subsurface region of the glass. This phenomenon arises from the break of the central symmetry of the glass by the electric field generated by the Maxwell-Wagner charge accumulated in the region with the gradient of the glass conductivity, in which ions with lower mobility are introduced. This second harmonic signal from the tested potassium ion-exchanged soda-time glass is comparable with the signal obtained from the same kind of glass after thermal poling in an open anode configuration. The second harmonic output grows approximately linearly with the duration of the ion exchange, which correlates with the proposed simple model. The second harmonic signal obtained from the glass subjected to potassium-for-sodium ion exchange noticeably exceeds the signal from the glass subjected to silver-for-sodium ion exchange because the higher ionic mobility of silver results in a lower contrast in conductivities between the interior of the glass and the ion-exchanged region. For the same reason, the formation and relaxation times of the Maxwell-Wagner charge and the second-order optical nonlinearity induced by the electric field of this charge are longer in the case of potassium ion exchange.  
The phenomenon under discussion turned out to be relatively resistant to thermal annealing. After one hour of annealing at 350 \({}^{\circ}\)C (the temperature approaching that of the initial IE) and below, the effect of the SON acquisition under DC voltage did not suffer the slightest recession. We associate this with changes in the glass structure induced by potassium ions that penetrated it. However, the higher-temperature anneal resulted in a noticeable degradation of the effect, although its dynamics (time for the second-order optical nonlinearity to be developed under the voltage or time of its degradation after the DC voltage was turned off) also became faster.  
Finally, the Maxwell-Wagner effect, which we registered in the glass with the gradient of the charge carrier mobility provided using ion-exchange processing, can be used both to characterize graded conductivity regions and to provide and control the second-order optical nonlinearity of transparent isotropic media.  
Conceptualization, S.S. and A.L.; data curation, D.T.; formal analysis, S.S.; funding acquisition, A.L.; investigation, G.K.; methodology, G.K.; project administration, A.L.; supervision, D.T. and A.L.; validation, S.S. and G.K.; visualization, S.S.; writing--original draft, S.S.; writing--review and editing, D.T. and A.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Ministry of Science and Higher Education of the Russian Federation, project number FSRM-2023-0009.  
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.  
The authors declare no conflicts of interest.