Graphene Absorption Enhancement Based On Electromagnetic Surface Wave

The two-dimensional(2D)material graphene has attracted considerable attention in photonic,optoelectronic,and related fields due to its unique optoelectronic benefits.However,the poor absorption of monolayer graphene(~ 2.3%)is one of the key limitations for high performance.Most of light-trapping strategies can effectively improve optoelectronic performance but also rely on the highly nanostructured configurations,including plasmonics and metamaterials by nanopatterned metallic systems or microcavity composed by tens of dielectric layers.In this study,focusing on the low absorptivity problem in monolayer graphene,we proposed planar system to improve its absorption,by modifying photonic surface wave to achieve perfect absorption of graphene,and explored its application in sensing field.The core research contents of this thesis are listed as follows:Firstly,transfer matrix method and admittance loci which used in this study were introduced,specifically including electromagnetic field calculation,admittance loci calculation,admittance trajectories equation,equivalent phase thickness contours equation,reflection phase contours equation and isoreflectance contours.According to the boundary condition of magnetic field,graphene is regarded as an ideal 2D material(without thickness)and introduced into the transfer matrix calculation.Then,the conditions for ESW excitation and the band structure of photonic crystal(PC)were solved.Finally,based on the presented research findings,we explored the Bloch surface wave(BSW)excitation in semi-infinite one dimensional(1D)PC and the origin of ESW relying on the admittance loci and virtual cavity model,which established the theoretical foundation for follow-up study.Secondly,coupled with the strongly localized BSW field,a monolayer graphene shows an absorption ~ 100% at the designed infrared band(1310 nm,which can be tuned readily).The proposed system has a simple structure for fabrication without the parasitic absorption from metal in plasmonic system.Due to the intrinsic similarity of 1D PC and antireflection coating system,the admittance loci in thin film optics were used for design.Enlightened by two admittance matching conditions,we depicted forward/backward admittance loci and derived the admittance matching condition for BSW-based graphene perfect absorber(BSGPA).A convenient photonic design for rapidly realizing the perfect absorption in graphene was presented.The underlying physics and the photonic controllability of the BSW for achieving the perfect absorption in monolayer graphene were further discussed by analyzing admittance loci and barrier model.With detailedly addressing the excitation condition of BSW,it was found that the perfect absorber could also be realized based on more generalized surface waves from aperiodic structure.Considering the extremely high electromagnetic enhancement by BSW and the advantages of graphene in biosensing,we explored the sensing application of B-SGPA,and investigated its wavelength and angular sensing performance.With the first-order perturbation theory,we showed that the influences of incident condition,structural parameters and absorption loss on the electric field distribution and the sensing performance of graphene-based BSW refractive index sensor(G-BSW RI sensor).The study shows that the wavelength sensitivity and figure of merit of the optimized system can reach 7023 nm/RIU and 196.44,respectively.In addition,the absorption loss has a great influence on the lineshape of reflection spectrum and sensing performance of BSW device.The BSW device consisted by low absorption loss material has higher electric field enhancement,but it may also increase the pairs of PC and complex the production process.In this study,we proposed 7-layer dielectric planar system to achieve perfect absorption of graphene at infrared band.It is believed that the thin-film and purely dielectric surface system provides two-dimensional devices a promising opportunity for low-cost and highperformance applications.On the other hand,comparing with the Au-BSW device,G-BSW RI sensor has stronger electric field enhancement and higher sensing performance.By adjusting the absorption loss,the strong electric field confinement/enhancement of BSW devices can be realized.We believe that it can provide opportunities for BSW applications in nonlinear enhancement,fluorescence enhancement,sensing field,etc.