Abstract
Bound-states-in-the-continuum (BIC) is an emerging concept in nanophotonics with potential impact in applications, such as hyperspectral imaging, mirror-less lasing, and nonlinear harmonic generation. As true BIC modes are non-radiative, they cannot be excited by using propagating light to investigate their optical characteristics. In this paper, for the 1st time, we map out the strong near-field localization of the true BIC resonance on arrays of silicon nanoantennas, via electron energy loss spectroscopy with a sub-1-nm electron beam. By systematically breaking the designed antenna symmetry, emissive quasi-BIC resonances become visible. This gives a unique experimental tool to determine the coherent interaction length, which we show to require at least six neighboring antenna elements. More importantly, we demonstrate that quasi-BIC resonances are able to enhance localized light emission via the Purcell effect by at least 60 times, as compared to unpatterned silicon. This work is expected to enable practical applications of designed, ultra-compact BIC antennas such as for the controlled, localized excitation of quantum emitters.
Highlights
Bound-states-in-the-continuum (BIC) is a fascinating concept that has its origins in quantum mechanics in 19291
We provide experimental and theoretical evidence of true BIC resonances that are locally excited by a nanometer electron beam in the scanning transmission electron microscope (STEM) and directly probed using energy loss spectroscopy (EELS) and CL spectroscopy
When this focused electron beam is positioned near a nanostructure, a fraction of the energy of the electron beam will be used to polarize the free and bound electrons within the material that start to oscillate according to the modes supported by the nanostructures
Summary
Bound-states-in-the-continuum (BIC) is a fascinating concept that has its origins in quantum mechanics in 19291. Optical modes with energies higher than the potential wells, i.e., in the freely propagating continuum, can still be localized and bound in space when these wells are appropriately designed[1]. This BIC concept was first demonstrated experimentally in semiconductor heterostructures in 19922, based on earlier theoretical predictions[3,4]. As the (quasi-) BIC modes are supported by nanoantennas, controlling their geometry can achieve unidirectional scattering[27], sensitive hyperspectral
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