# DC Voltage Induces Quadratic Optical Nonlinearity in Ion-Exchanged Glasses at Room Temperature
## 3 Results and Discussion
### The Effect of Annealing  
We investigated the thermal stability of the phenomenon under discussion. Evidently, heating a charged specimen should accelerate the decay of the induced nonlinearity. However, this does not necessarily diminish its ability to subsequently accumulate a charge. We chose the K\({}^{+}\) specimen exchanged for 2 h and annealed it for 1 h at several temperatures lying between 200 \({}^{\circ}\)C and 500 \({}^{\circ}\)C, with the latter temperature approaching the glass transition temperature, \(T_{\mathrm{g}}\), of our glass (-560 \({}^{\circ}\)C). After each annealing, we subjected the specimen to DC voltage and measured its gain in optical nonlinearity. The dependence of the SH signal generated by the charged specimen on the annealing temperature is presented in Figure 5.  
Figure 4: Temporal dependences of (**a**) the currents through the K\({}^{+}\)- and Ag\({}^{+}\)-enriched IE specimens after the application of 1300 V DC voltage; (**b**) the SH signal from these specimens during their charging and after turning off voltage; the dependence for the Ag\({}^{+}\) specimen is 2.5 times enhanced for visual comfort. The K\({}^{+}\) specimen was exchanged for 2 h under 365 \({}^{\circ}\)C and the Ag\({}^{+}\) specimen—for 20 min under 325 \({}^{\circ}\)C.  
Figure 5: Maximal SH signal from the K\({}^{+}\) specimens exchanged for 2 h, then annealed for 1 h, and then subjected to 1300 V DC for 30 min vs. temperature of annealing. The dependence normalized to the SH signal from the charged unannealed specimen. The dotted line is a guide for eyes. Subfigures: Maker fringes (the SH signal vs. the fundamental light incident angle) of the specimens for each anneal; the curves are plotted in the uniform scale. In Figure 5, we observe that the relatively low-temperature anneals (up to at least 350 \({}^{\circ}\)C) have virtually no effect on the ability of the specimen to accumulate charge and, therefore, form an inner electric field and generate the SH. This means that these temperatures are insufficient for a noticeable redistribution of the invasive K\({}^{+}\) ions in the glass and to level the gradient of the conductivity. It is somewhat intriguing that this applies to a temperature of 350 \({}^{\circ}\)C, which is close to the temperature of the initial IE processing (365 \({}^{\circ}\)C). We speculate that this might be due to the structural changes and evolution of the glass network in the course of IE [38; 39], in particular, because of the mechanical stresses caused by potassium ions, which are noticeably bigger than sodium ones. This can increase the activation energy for reverse ion shuffling. Note that since Na\({}^{+}\) ions are much faster than K\({}^{+}\) ions, even an inessential dilution of the K\({}^{+}\)-enriched layer with Na\({}^{+}\) ions would noticeably level the gradient of the conductivity and significantly reduce the ability of the specimens to accumulate the Maxwell-Wagner charge. However, for the 350 \({}^{\circ}\)C anneal, we observe none of this: the SH signal after charging is literally the same (within several percent of the instrumental error) as it was before all the anneals (see subfigures in Figure 5). On the other hand, the 425 \({}^{\circ}\)C anneal resulted in a noticeable decrease in the SH signal (about 50%), indicating partial ion shuffling. The 500 \({}^{\circ}\)C anneal almost demolished the effect of the charging. We believe that this is due to the significantly faster diffusion of alkalis and to a stronger dilution of the potassium-enriched layer with sodium at the temperature closer to the glass \(T_{g}\). A closer look at the charging process of the specimen after the high-temperature (425 and 500 \({}^{\circ}\)C) anneals showed that the effect did not completely disappear after the 500 \({}^{\circ}\)C anneal. The fact is that the SH signal evaluated via the Maker fringes (see the rightmost subfigure in Figure 5) is understated in this case because, as we demonstrate below, the specimen is partially discharging during the measurements.