Unveiling Symmetry Protection of Bound States in the Continuum with Terahertz Near-field Imaging

TL;DR

This paper demonstrates a novel terahertz near-field imaging technique that directly visualizes bound states in the continuum (BICs), revealing their symmetry protection and robustness, which advances photonic cavity engineering.

Contribution

It introduces a double THz near-field method to map BICs and shows that mirror symmetry is not essential, only $c$-rotation symmetry is needed for BIC formation.

Findings

First direct electromagnetic field mapping of BICs

BICs form with anti-symmetric phases in metasurfaces

Mirror symmetry is not necessary for BICs, only $c$-rotation symmetry

Abstract

Bound states in the continuum (BICs) represent a new paradigm in photonics due to the full suppression of radiation losses. However, this suppression has also hampered their direct observation. By using a double terahertz (THz) near-field technique that allows the local excitation and detection of the THz amplitude, we are able to map for the first time the electromagnetic field of BICs over extended areas, unveiling the field-symmetry protection that suppresses far-field radiation. This investigation, done for metasurfaces of dimer rod resonators, reveals the in-plane extension and formation of BICs with anti-symmetric phases, in agreement with coupled-dipole calculations. By displacing the rods, we demonstrate that mirror symmetry is not a necessary condition for BIC formation. Only $\pi$-rotation symmetry is required, making BICs exceptionally robust to structural changes. This work makes the local field of BICs experimentally accessible, which is crucial for the engineering of cavities with infinite lifetimes.