Figure 3

Near field spectra calculated 15 nm in front of the PC for two different BNA-on-tip: in (a) the BNA resonates meaning that its metallic arms are along the y-direction. In addition, the tip radius is fixed to R = 250 nm in order to minimize the blueshift that directly depends on the tip size. Consequently, blueshift only appears for a small distance d = 30 nm while the BNA resonance is responsible for the redshift because a part of the light penetrates inside the gap leading to a larger mode volume. In (b) the radius of the tip is increased to R = 500 nm to enhance the mode squeezing and the BNA is rotated by 90° in order to inhibit its resonance. In this case, the BNA-on-tip almost plays the role of a metallic rod as in ref. 2 leading only to a blueshift whatever the value of the distance d is as experimentally demonstrated in¹⁵.