Dual bound states in the continuum in metamaterials

Abstract: 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. It was first proposed in quantum mechanics, and extended to general wave phenomena such as acoustic, water, elastic, and electromagnetic waves. 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. Here, we demonstrate dual bound states in the continuum in a subwavelength planar metamaterial cavity that reveal symmetry-protected features excited by orthogonal polarizations. The spectral features of dual BICs are experimentally verified in the terahertz domain by breaking the C2 symmetry that invokes a leakage channel. The radiative Q factors tend to infinity at a discrete symmetry-restoring point and obey an inverse square dependence on the structural asymmetry. Metamaterials allow field confinement in extremely small mode volumes, thereby improving the rate of spontaneous emission in the cavity with a much larger Purcell factor along with high Q factor. The symmetry-protected BICs in metamaterials also possess the unique advantage of scalability at different wavelengths for potential applications in sensing, lasing, switching, and spectral filtering.