  In this paper, we present a novel universal coupled theory for metamaterial
Bound states in the continuum (BIC) or quasi-Bound states in the continuum
(quasi-BIC) which provides ultra-high Q resonance for metamaterial devices. Our
theory analytically calculates the coupling of two bright modes with phase. Our
method has much more accuracy for ultra-strong coupling comparing with the
previous theory (the coupling of one bright mode and one dark mode). Therefore,
our theory is much more suitable for BIC or quasi-BIC and we can accurately
predict the transmission spectrum of metamaterial BIC or quasi-BIC for the
first time.
