Size-dependence study of SHG originating from gold and ITO. (a) SHG contributed by the gold (red dot) and ITO (blue dot) as a function as size parameter T where other geometric parameters change proportionally. Where the dashed cyan line represents a quadratic function with a coefficient of 6.4 × 10⁻¹⁵. (b)–(d) SHG spectrum below/at/above the critical point. (b) T = 10 nm, D = 2 nm and d = 0.1 nm; (c) T = 100 nm, D = 20 nm and d = 1 nm; (d) T = 200 nm, D = 40 nm and d = 2 nm.