Fig. 4: Characterizing the coherent interaction length of the quasi-BIC resonance in a Si nanoantenna array with a tilt angle θ of 70°.

a STEM HAADF image, showing the corner of a fabricated Si nanoantenna array with 130 × 130 elements. The white dotted rectangle highlights the area used for the CL mapping experiment and the colored dots denote the measurement positions for investigating the CL spectrum evolution. b CL intensity map at 722 nm, the quasi-BIC resonance wavelength; note the lower emission closer to the array edge. The wavelength bandwidth for the CL integration is 17 nm. c CL spectra from the “tip” position and “middle” position of one selected antenna as highlighted by the white dashed line in (b). The 722 nm emission at the “tip” position is strongly enhanced due to the quasi-BIC resonance. The CL spectrum as measured from un-patterned 90-nm-thick Si film is plotted in gray for benchmarking. d Evolution of the measured CL emission spectra when going from the array edge towards the array center. It shows that the quasi-BIC resonance requires the presence of neighboring array antennas. e, f CL maps at the Mie resonance wavelengths of the individual antennas (e) 522 nm and (f) 614 nm; note the uniform emission all the way to the array edge. g Simulations of the CL spectrum evolution