1.College of Information Engineering, Quzhou College of Technology, Quzhou 32400, China 2.College of Physics and Electronic Information, GanNan Normal University, Ganzhou 341000, China 3.Provincial and Ministerial Co-construction of Key Laboratory of Opto-electronics Technology, Beijing University of Technology, Beijing 100124, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 61575008, 61650404), the General Research Projects of Zhejiang Provincial Education Department, China (Grant Nos. Y201738091, Y201839950), the Jiangxi Natural Science Foundation, China (Grant No. 20171BAB202037), the Technology Project of Jiangxi Provincial Education Department, China (Grant No. GJJ170819), the Quzhou Science and Technology Project, China (Grant No. 2017G16), Intelligent Manufacturing Industry and Industrial Big Data Technology Application Innovation Team, China(Grant No. QZCX1801), and the Science and the Bidding for Gannan Normal University, China (Grant No. 16zb04).
Received Date:11 December 2018
Accepted Date:03 April 2019
Available Online:01 July 2019
Published Online:05 July 2019
Abstract:As an emerging new material, graphene has aroused the great research interest. How to improve its absorption efficiency is one of the hot research topics. However, currently most of the studies concentrate in THz band or middle-to-far-infrared region: the research in the visible and near-infrared regions is rare, which greatly limits the applications of graphene in opto-electric fields. In order to improve the absorption efficiency of single-layered graphene in visible and near-infrared band and realize multi-channel optical absorption enhancement, we propose a hybrid structure consisting of graphene-metal grating-dielectric layer-metal substrate. The proposed structure can realize three-channel light absorption enhancement at wavelengths λ1 = 0.553 μm, λ2 = 0.769 μm, and λ3 = 1.130 μm. The maximum absorption efficiency of graphene is 41%, which is 17.82 times that of single-layered graphene. The magnetic field distributions of the hybrid structure at three resonance wavelengths are calculated respectively. It can be found that for the resonance peak λ1, the energy of light field is distributed mainly on the surface of metal grating, which is the characteristic of surface plasmon polariton (SPP) resonance. Therefore, it can be judged that the enhancement of graphene absorption in this channel is due to the SPP resonance stimulated by metal grating. For the resonance peak λ2, the energy of the optical field is mainly confined into the metal grating groove, which is the remarkable resonance characteristic of the Fabry-Pérot (FP) cavity, it can be concluded that the enhancement of the optical absorption of graphene at the resonance peak λ2 is due to the resonance of the FP cavity. When the resonance peak is λ3, the energy of the light field mainly concentrates on the upper and lower edges of the metal grating and permeates into the SiO2 layer, and it can be observed that there are energy concentration points (reddish) at the left end and the right end of the metal grating edge, which is a typical magnetic polariton (MP) resonance feature. Therefore, the enhancement of absorption of graphene at the resonance peak λ3 is caused by the MP resonance induced by the metal grating. We also analyze the absorption characteristic (resonance wavelength and absorption efficiency) dependence on structure parameters by using the finite-difference time-domain (FDTD) simulation. Our study reveals that by increasing grating width, all the three resonance wavelengths are red-shifted, and the absorption efficiency at λ2 and λ3 are both enhanced whereas the absorption efficiency at λ1 almost keeps unchanged. By increasing dielectric layer thickness, λ2 will be red-shifted and λ3 will be blue-shifted, whereas the absorption efficiency at the three resonance wavelengths all remain constant. By increasing graphene chemical potential, none of the wavelengths of the three absorption peaks is shifted, and the absorption efficiency at λ3 decreases. According to our findings, we optimize structure parameters and achieve the light absorption efficiency larger than 97% at the three channels simultaneously, which can make metamaterial absorbers. Keywords:graphene/ metal grating/ absorption efficiency/ optical absorption enhancement channel
图6是介质层SiO2厚度d对混合结构吸收特性的模拟结果. 由图6可知混合结构共振峰λ1和λ2对d并不敏感, 当SiO2介质层厚度d从0.14 μm减少到0.1 μm过程中, 共振峰λ1相应的从0.566 μm蓝移到0.555 μm, 这是由混合在SPP共振中的FP腔共振引起. 而当d从0.1 μm减少到0.06 μm过程中, 共振峰λ1会从0.555 μm红移到0.573 μm, 这是由混合在SPP共振中的MPs共振引起[32, 33]. 图 6 SiO2层厚度d对混合结构吸特性的影响 Figure6. Influence of the thickness of the SiO2 layer on the absorption characteristics of the hybrid structure.
根据(5)式和(6)式可知, 石墨烯的吸收效率和εgimag有关, 式中E(x,y,z)是材料覆盖区域某点的电场强度, Einc是入射光电场强度, S是入射截面积(TM偏振光垂直入射), w(x,y,z)是在某一位置的能量散射密度. 将(5)式和(6)式与图8结合分析, 可解释图7混合结构吸收效率随着μc的变化趋势. 在波长λ3当μc < 0.5 eV时混合结构有高吸收效率, 是因为εgimag在μc < 0.5 eV范围内在8.2左右, 所以混合结构吸收效率随着μc增加变化较小. 但是当μc > 0.5 eV时, εgimag就突然下降到0, 所以混合结构的吸收效率就大幅下降, 最终与无石墨烯层时的吸收效率相当. 图 8 不同共振波长下化学势对石墨烯介电常数虚部的影响 Figure8. Influence of the chemical potential on the imaginary part of dielectric constant of graphene at different Resonance wavelength.
图 1 石墨烯-金属光栅-绝缘层-金属衬底混合结构
图 2 有和没有石墨烯层混合结构的吸收率
图 3 混合结构在共振波长处的磁场分布
图 4 光栅宽度对混合结构吸特性的影响
图 5 混合结构RLC等效电路
图 6 SiO2层厚度d对混合结构吸特性的影响
图 7 化学势对混合结构吸收特性的影响
图 8 不同共振波长下化学势对石墨烯介电常数虚部的影响
图 9 最优参数下混合结构的吸收效率