Fig. 1. Schematic of microthermal poling sandwich and image of Ni mesh. (a) Schematic; (b) image of Ni mesh

Fig. 2. SHG patterns. (a) Microstructured SHG pattern of SLG sample poled at 240 ℃ and 1.8 kV in N₂ for 0.5 h; (b) microstructured SHG pattern of SLG cover glass; (c) overlapping the sample and the cover glass

Fig. 3. Variation of voltage and current with time and in situ DBD images of an anode sandwich overlapping an SLG sample, a mesh and an SLG cover glass recorded during the micro-thermal-poling process. (a) Voltage and current versus time; (b) onset of DBD; (c) first quadrate of DBD; (d) second quadrate of DBD; (e) center of DBD; (f) 10 min after saturation of DBD

Fig. 4. DBD mechanism diagram

Fig. 5. Raman spectra. (a) Raman spectra of three positions for SLG sample; (b) variation of relative intensity of O—O bond across a square

Fig. 6. Distribution of Na⁺ ions. (a)(b) Distribution of Na⁺ ions on sample surface and linear distribution of Na⁺ ions along dash line; (c)(d) distribution of Na⁺ ions on cover surface and linear distribution of Na⁺ ions along dash line