# Nanoscale mapping of optically inaccessible bound-states-in-the-continuum

## TL;DR

This paper uses a sub-1-nm electron beam to map and study optically hidden BIC resonances in silicon nanoantennas, revealing their potential for enhancing light emission.

## Contribution

The first experimental mapping of true BIC resonance using electron energy loss spectroscopy and demonstration of quasi-BIC's light emission enhancement.

## Key findings

- True BIC resonance near-field localization was mapped on silicon nanoantenna arrays using sub-1-nm electron beam.
- Quasi-BIC resonances enhance localized light emission via the Purcell effect by at least 60 times compared to unpatterned silicon.
- Coherent interaction length requires at least six neighboring antenna elements for quasi-BIC resonances.

## 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.

The optically inaccessible bound-states-in-the-continuum (BIC) is probed via sub-1-nm electron beam, where the cathodoluminescence emissions from Si nanoantenna arrays are able to reveal the coherent interaction length of quasi-BIC resonances.