Figure 5

(a) Resonance wavelengths (RW) calculated in the case of a resonant BNA-on-tip placed above the CL7 center in position A (see white arrow in Fig. 2d) as a function of the tip-to-CL7 distance d. The RW shift is both positive and negative with respect to d demonstrating the double-way tunability. The case of a pure dielectric tip (similar to that of ref. 16) is also presented in dashed-dotted-dotted magenta line. (b) same as (a) in the case of non-resonant BNA. Electric effect is very weak and only blueshifts are obtained. The case of a pure metallic tip (as in ref. 2) is presented for comparison. (c) same as (a,b) but for tip apex positioned at the vertical of points B and C (see Fig. 2c,d). Note that the B position corresponds to a maximum of the magnetic field vertical component and a zero transversal electric field while all these three components vanish at position C. (d) Q-factors of the thirteen studied configurations corresponding to different apex radii (from R = 180 nm to R = 500 nm) and BNA state (on or off resonance) including uncoated dielectric tip (DT) and metallic tip (MT) as a function of the BNA-to-CL7 distance d. The red dashed horizontal lines at the top of the figure corresponds to the case of the unperturbed CL7 cavity (Q_CL7 = 1590).