Control Of Bound States In The Continuum In Graphene-assisted Gratings

The bound state in the continuum(BIC)is a phenomenon in which the perfect confinement of light can still be achieved insidethe light cone,and BIC is also described as resonance state with features of zero width and zero leakage.Because of the quality factor of BIC state can reach infinite,so it has important applications in constructing lasers,sensors and filters.It is well known that the conductivity of graphene can be modulated by adjusting its Fermi level and carrier mobility.In this dissertation,we propose to use such strategy to control the bound state in the continuum,and further to realize a light modulator.The main work is exhibited as follows:According to the property that the conductivity of graphene can be modulated by electrostatic bias,a graphene grating structure with a substrate is designed in this dissertation.Under the illumination of TM polarized light,the excited surface plasmon wave has very strong localization to light.In addition,changing Fermi energy level of graphene will affect the resonant frequency of transmission peak,and changing carrier mobility will affect the amplitude of resonance peak.Because the diffraction grating with subwavelength has special diffraction characteristics,the single-grating and symmetric dual-grating structure with a period of micronmeters are designed in this dissertation.We use light of different polarization in the oblique incidence to these two kinds of structure,and use Rigorous Coupled Wave Analysis(RCWA)method to calculate the grating's reflectivity,we found that the optical bound state in the continuum can occur under the specific incident light wave vector and grating spacing.At the same time,the results show that the different polarization states affect the pitch of the double grating array supporting the BIC.Base on the symmetric dual-grating structure which is capable of realizing the optical bound state in the continuum,two layers of graphene are laid symmetrically on the gratings,and found that under the light incidencewith a specific frequency and angle,the bound state in the continuum can also be realized.Since the conductivity of graphene can be modulated,so the optical bound state in the continuum can be regulated by changing the Fermi energy level and carrier mobility of graphene.At the same time,the results show that the decrease in the spacing of graphene grating array causes the blue shift of the transmission peak of the excited graphene surface plasma wave.Combining the excellent properties of graphene and sub-wavelength diffraction gratings,we designed double-layer graphene grating structures to realize the adjustment of the optical bound state in the continuum,which can provides a new method for further studying the bound state in the continuum,and it has potential application value in constructing lasers and modulator.