Triple Friedrich–Wintgen bound states in the continuum in asymmetric all–silicon photonic crystal slabs

In this research, we introduce a technique for achieving F–W BICs in terahertz all–silicon photonic crystals (PhC), which also enables Electromagnetically induced transparency (EIT). By employing a symmetric PhC featuring an array of cylindrical holes, we can create a symmetry–protected bound state in the continuum (SP BIC) supported by an electric dipole. When the C_2v symmetry is broken, the BIC evolves into a quasi–SP BIC. By adjusting the asymmetry parameters, quasi–BIC couples to guided mode resonance (GMR) via destructive interference and transforms into F–W BIC. As these parameters are varied, the two modes undergo multiple transitions between strong and weak coupling, resulting in the emergence of three F–W BICs. Additionally, we show that at the third F–W BIC, a slight deviation from the BIC conditions triggers EIT. We have successfully realized three F–W BICs in terahertz PhC and observed a distinctive mode recovery phenomenon in the mode coupling region by adjusting the asymmetry parameter α. At the third BIC deviation point, we achieved EIT primarily influenced by electric quadrupoles (EQ) and found that altering the rectangular hole width L and plate thickness t can independently control EIT.