# DC Voltage Induces Quadratic Optical Nonlinearity in Ion-Exchanged Glasses at Room Temperature
## 3 Results and Discussion  
### The EFISH Phenomenon in IE Glasses  
In order to establish the effect of DC voltage on the second harmonic generation (SHG) capabilities of IE glasses, we measured and compared the MF of the potassium-for-sodium \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Oxide** & **SiO2** & **Na2O** & **CaO** & **MgO** & **K2O** & **Al2O3** & **SO3** & **Fe2O3** \\ \hline Wt.\% & 72.20 & 14.30 & 6.40 & 4.30 & 1.20 & 1.20 & 0.30 & 0.03 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Composition (in wt.%) of soda–time glass used in the experiments [26]. IE specimen before and after the application of 1300 V DC voltage for 30 min at room temperature. The dependences for the specimen exchanged for 2 h are shown in Figure 1a (dashed line, left axis--before the exposure to DC; solid line, right axis--after the exposure to DC). They are normalized to the maximal SH signal from the IE specimen before the exposure to DC voltage. The fringes correspond to the typical interference between two SH signals generated by two opposite surfaces (or two thin subsurface nonlinear layers) of the specimen, while the bulk of the specimen is optically linear [33]. The maxima in Figure 1a correspond to the constructive interference of these signals, while the minima correspond to the destructive interference. Since the minima in Figure 1a are about zero, the magnitudes of the SH signals generated by the opposite sides of the specimen are about the same because of the equivalence of the sides, as expected. First, we emphasize that the MFs of the virgin glass slide almost coincide with the MFs of the IE slide presented in Figure 1a (dashed blue line, left axis). Thus, we do not demonstrate the MFs of the virgin glass slide. This means that the substitution of domestic sodium ions with invasive potassium ones in the subsurface layer of the glass does not provide any noticeable change in the SON of the specimen. Therefore, the origin of the SON in both virgin and IE glasses is the same, and it is a weak natural surface nonlinearity, which all structures, including isotropic ones, possess because of the break of the central symmetry at surfaces [34]. However, the application of DC voltage produces a dramatic effect. The IE specimen exposed to 1300 V DC for 30 min demonstrates more than a 100-fold increase in the SH output (see solid red line, right axis in Figure 1a). Note that the SON gained in this case is comparable with the SON that the same glasses demonstrate after thermo-electrical poling in an open anode configuration [35]. We attribute such behavior to the EFISH effect. The electric field inducing SHG in this case originates in the Maxwell-Wagner charge formed by the current flowing through the specimen. The charge accumulates at the fuzzy interface between the regions with different conductivities, namely the subsurface region enriched in relatively slow K\({}^{+}\) ions and the interior of the glass, where faster domestic Na\({}^{+}\) ions dominate. Moreover, the electric potential essentially drops in the less-conductive thin subsurface K\({}^{+}\)-enriched region, forming there a high electric field responsible for the SHG. Another important difference between the observed and the classical Maxwell-Wagner (and the accompanying EFISH) effects is that the observed effect persists and slowly relaxes after the DC voltage is turned off. However, this also takes time for the charge to be accumulated (30 min in our case). This is because the dynamics of the formation and relaxation of the Maxwell-Wagner charge depends on the conductivity of materials--lower conductivity corresponds to slower charge accumulation and dissolution. In our case, the conductivity  
Figure 1: (**a**) Maker fringes of the sodium-for-potassium IE specimen (\(t_{IE}\) = 2 h) before (dashed blue line, left axis) and right after (solid red line, right axis) the application of 1300 V DC voltage for 30 min at room temperature; the dependences are normalized to the maximal SH signal from the IE specimen before its exposure to DC.