含石墨烯的金属-分布式布拉格反射镜-金属结构中光学Tamm态的研究

Optical Tamm States in Metal-Distributed Bragg Reflection-Metal Structure with Graphene

基于光学Tamm态(OTS)的局域场增强特性和石墨烯的电控特性,提出一种含石墨烯的金属-分布式布拉格反射镜-金属(M-DBR-M)复合结构。考虑石墨烯层对介质界面连续性的影响,基于修正的传输矩阵法建立完善的理论模型,并研究结构参数以及驱动电压对OTS特性的影响。结果表明,随着驱动电压的增大,两个OTS的本征波长均发生蓝移,当驱动电压大于石墨烯的突变电压时,本征波长发生蓝移且趋于稳定,说明可以通过电控石墨烯来实现OTS的本征波长在一定范围内动态可调;增大DBR周期数,两个OTS的本征波长分别发生红移和蓝移,说明吸收率对周期数的波动具有鲁棒性;入射端的金属层厚度在25~35 nm之间,能够实现完美吸收;出射端的金属层厚度则在70 nm处有吸收率的极大值。

Objective Optical Tamm state(OTS)is a new type of surface-localized state,which can be excited at the interfaces between metal and distributed Bragg reflection(M-DBR)or between two photonic crystals.The electric fields of OTSs are localized at the interfaces and intensity of the electric fields decays because it is away from the interfaces.Compared with the traditional surface states,OTS has a high-intensity local mode and perfect absorption of incident electromagnetic wave and has a narrower linewidth,which can be excited by transverse electric(TE)and transverse magnetic(TM)polarized waves at a wide-angle range incidence.Researchers have conducted several studies on OTS,but the structural parameters are fixed.Besides,they do not have the tunability and cannot expand the application field.Thus,the emergence of tunable structures is an inevitable tendency.Graphene is a new type of two-dimensional(2 D)carbon atom material with special optical and electrical properties,which can interact with incident electromagnetic waves.By changing the chemical potential(ν),the conductivity can be adjusted to achieve tunable absorption.In this study,we propose a metal-DBR-metal(M-DBR-M)structure with graphene based on the local characteristics of OTS and the electronic control characteristics of graphene.Tuning intrinsic wavelength by adjusting the driving voltage.The proposed structure can provide references for the research and design of photonic devices,such as sensors,absorbers,narrow-band selective filters,and optical detectors based on OTS.Methods In this paper,the dielectric constant of the metal layer is modeled using the Drude-Lorentz dispersion model.The influence of the graphene layer on dielectric interface continuity is considered.Thus,the modified transfer matrix method is used instead of the traditional transfer matrix method.The proposed structure is simulated using the COMSOL multiphysics software.The influence of material dispersion is neglected.We assume that the structure is in the air,and light incident into the structure from one side of the metal layer.The changes in the structure absorption spectra and intrinsic wavelength of OTS are compared and analyzed.The influence of the periodicity of DBR,the thickness of the metal layer,and driving voltage on the intrinsic wavelength,tunable range,and absorptivity of the structure are investigated.Results and Discussions There are two absorption peaks in the absorption spectrum of the M-DBR-M structure(Fig.2).The electric field is localized at the interface between the two M-DBR.The electric field intensity decreases with distance away from the interface(Fig.4).OTS 1 and OTS 2 correspond to absorption peaks 1 and 2,respectively.With the increase in driving voltage,the intrinsic wavelengths of the two OTSs in the M-DBR-M with graphene structure blue-shift until the driving voltage is greater than the abrupt change voltage of graphene,the intrinsic wavelength blue-shift tends to be stable(Fig.5).The maximum-tuning range is determined by the intrinsic wavelength shift corresponding to the abrupt voltage.Thus,the intrinsic wavelength of OTS can be dynamically adjusted in a certain range by electronically controlled graphene.With the increase in DBR periodicity,the intrinsic wavelength of OTS 1 is red-shift with the maximum amplitude of 102 nm,and OTS 2 is blue-shift with the maximum amplitude of 100 nm.After applying the abrupt voltage,the tunable range of OTS 1 and OTS 2 hardly changes with the DBR periodicity.The absorptivity of the structure changes little without voltage or the abrupt voltage is applied;thus,it is robust to the fluctuation of the DBR periodicity(Fig.7).With the increase in the thickness of M 1 and M 2,the intrinsic wavelengths of OTS 1 and OTS 2 are blue-shifted without voltage and with abrupt voltage.However,the tunable range almost do not change with the thickness of the metal layer.The absorptivity curve increases first and then decreases with the increase in M 1 thickness and increases with the increase in M 2 thickness(Figs.8 and 9).The metal layer M 1 thickness in the range of 25--30 nm,the absorptivity has the optimal value.However,the absorptivity has a maximum value when the thickness of M 2 is 70 nm.Conclusions Based on the local characteristics of OTS and the electrical control characteristics of graphene,a metal-DBR-metal structure with graphene is proposed and the theoretical model is improved.The proposed structure is simulated using COMSOL software.The results show that the intrinsic wavelength of OTS has a red-shift compared with the M-DBR-M structure.The driving voltage regulates the intrinsic wavelength continuously and dynamically within a certain range.The maximum-tunning range is determined by the abrupt voltage.With the increase in the DBR periodicity,the structure tunable range is 9--10 nm,and hardly alters with the DBR periodicity.As the thickness of the metal layers M 1 and M 2 increases,the tunable range hardly alters with the thickness variation of the metal layer.When the thickness of the M 1 is in the range of 25--30 nm,the absorptivity has an optimal value.The absorptivity curve increases with the thickness of M 2.The metal layer M 2 shall be thicker to improve the absorptivity of the structure.Our research shows that by introducing agraphene layer and rationally designing structural parameters,a tunable structure with high absorptivity and absorption peak position can be obtained.