Figure 3

(a) The horizontal electric field and (b) horizontal electric field gradient distribution of the plane between the tip and substrate, where d = 1 nm, ϕ = 20°. (c) The ratio of (a) over (b). (d) and (e) The I1, I2 and I1/I2 is plotted as a function of the lateral displacement which can reveal the spatial resolution of I1, I2 by full width at half maximum (FWHM). (f) I1/I2 is plotted as a function of the lateral displacement which describes the ratio of electric field to its gradient. The electric gradient is in unit of au. The excitation wavelength is 632.8 nm.