Spontaneous emission

Physics

Symmetry (geometry)

Mathematics

Asymmetry

Laser

Photonic crystal

Resonator

Plasmon

Optics

Metamaterial

Electromagnetically induced transparency

Fano resonance

Q factor

Photonics

Geometry

Lasing threshold

Quantum mechanics

Advanced Optical Materials

Bound state in the continuum (BIC) is a mathematical concept with an infinite radiative quality factor ( Q ) that exists only in an ideal infinite array of resonators. In photonics, it is essential to achieve high Q resonances for enhanced light‐mater interactions that could enable low‐threshold lasers, ultrasensitive sensors, and optical tweezers. Hence, it is important to explore BICs in different photonic systems including subwavelength metamaterials where symmetry‐protected dual BICs exist. The spectral features of dual BICs are experimentally verified in the terahertz domain by breaking the C 2 symmetry that invokes a leakage channel in the form of weakly radiating Fano resonance and electromagnetically induced transparency. The radiative Q factors tend to infinity at discrete symmetry‐restoring points and obey an inverse square dependence on the structural asymmetry. BICs in metamaterials allow extreme field confinement with small mode volumes, thereby improving the rate of spontaneous emission in the cavity with much larger Purcell factor. In addition, the topological nature enables a robust existence of BICs with a vector beam profile that is ideal for lasing. The symmetry‐protected BICs in metamaterials also possess a unique advantage of scalability at different wavelengths for potential applications in sensing, lasing, switching, and spectral filtering.

Symmetry‐Protected Dual Bound States in the Continuum in Metamaterials