| Metasurfaces based on bound state in the continuum(BIC)are functional devices with infinitely high Q factors.They are particularly attractive to the terahertz community because it may essentially solve the deficiencies in terahertz filters,sensors,lasers and nonlinear sources.However,most BIC metasurfaces are limited to specified incident angles that seriously dim their application prospects.Here,BIC with incident-angle robustness is obtained by applying dual-period dielectric grating.Through the coupling of two quasi-BIC resonances,slow-light effect based on electromagnetic induced transparency is also realized.The details can be concluded as follows:1.Theory and experiment technology related to dual-period dielectric grating are introduced.For theory: Guided-mode resonance and coupled-mode theory are introduced to predict the frequency of BIC and analyze the coupling between modes.For experiments: The preparation process of sample based on deep silicon etching is introduced,as well as the principle and structure of terahertz time-domain spectrum.Automatic mosaic algorithm is applied to acquire spectrum with a higher resolution and an optical path with a single lens is used to improve collimation.2.It is testified that multiple BIC based on guided-mode resonance is supported by dual-period dielectric grating.They can be classified into two types based on different asymmetry axis.The leakage of such two kinds can be manipulated separately when the symmetry of the grating is tuned in different ways,and thus forms different quasi-BIC resonance.It is proved theoretically,within the framework of Bloch Theorem,that BICs in dual-period dielectric grating have infinite Q factors under arbitrary incident angle.This robustness is also demonstrated in experiments by varying incident angle.3.Tuning geometric asymmetry is an efficient way to manipulate the leakage and coupling of these BICs,which can mimic the spectrum of electromagnetic induced transparency(EIT)effect.The coupling between bright and dark mode is analyzed based on coupled-mode theory,and the linewidth of the transparent window is manipulated via changing structural parameter.A slow light effect with a measured group delay of 117 ps was achieved.The incidence-insensitive BICs proposed here may greatly extend the application scenarios of BIC effect.And the quasi-BIC with high Q factors and the outstanding slow light effect based on the coupling of quasi-BIC resonance will promote the development of filters,sensors,dispersive devices and nonlinear devices in terahertz band with high performance. |
