Figure 2: Experimental observation of AE compared with ordinary three identical waveguides.

(a,b) Far-field and NSOM measurements in three identical waveguides configuration. Owing to the coupling between the waveguides, light appears in all waveguides along the propagation, regardless if the input of the light is injected to (a) the outer or (b) the middle waveguide. (c,d) Experimental observation of AE in an AE configuration with a wider middle waveguide. We observe the ‘2+1’ decomposition of three-coupled modes into (c) an effective two-coupled mode system of the outer waveguides and (d) a dark middle waveguide. (c) Light is coupled to an outer waveguide in an AE configuration. Far-field image shows that only the outer waveguides emit light at the output. The near-field image confirms the absence of light in the middle waveguide along the entire propagation. A full NSOM scan showing a complete inversion can be viewed in Supplementary Fig. 8. (d) Light is coupled to the middle waveguide in an AE configuration. Only the middle waveguide emits light at the output without coupling into nearby outer waveguides. The near-field image shows that no light leaks out from the middle waveguide. Atomic force microscopy images (inset) show each of the three waveguides with the inputs and the outputs. The scanning electron microscopy images (right column) show the cross-section of the fabricated waveguides for the AE configuration and for the identical waveguides configuration. We used a CW laser with λ=1,310 nm, the width of the outer waveguides was fixed to W₁=W₃=220 nm, where for the AE configuration W₂=275 nm, and for the control waveguide configuration W₂=220 nm. The height of the waveguides is 340 nm and the gap between the outer waveguides kept at 720 nm for both cases. The propagation length is 300 μm for all configurations. Colour bars for the NSOM measurements are for the intensity and are in arbitrary units.