1.Modern College of Humanities and Sciences of Shanxi Normal University, Linfen 041000, China 2.College of Physics and Information Engineering, Shanxi Normal University, Linfen 041004, China 3.Binjiang College, Nanjing University of Information Science & Technology, Wuxi 214105, China
Abstract:The plasmons in graphene have the superior properties to metal surface plasmons, such as high field confinement, low Ohmic loss and long wave propagation, highly tunable via electrostatic. More importantly, the frequency of plasmons ranges from terahertz to infrared which indicates that graphene is an ideal candidate for terahertz plamsonics. On the other hand, the strong coupling between incident photons and plasmons in graphene can lead the optical absorption to be enhanced. However, it is difficult for light to couple directly with plasmons in graphene, for the momentum of incident photons cannot match the plasmons in graphene. A metal grating can be used to compensate for the momentum of photons so that it can match that of plasmons in graphene. In this work, we theoretically investigate the effect of plasmons on the terahertz optical absorption of graphene with grating based on finite difference time domain. A great enhancement of electric field component of light field can be obtained near the gold grating strip in the sheet of graphene. Thus, the photons, of which the momentum is compensated for by the grating, can strongly couple with plasmons in graphene. An obviously decrease of the transmission of the graphene structure can be seen at the resonant frequency. The transmission peak corresponds to the resonant frequency spliting into two peaks due to the fact that two plasmon polariton modes are formed by the coupling of photons and palsmons. So we also study the plasmon polariton modes made by coupling photon with palsmon based on the many-body self-consistent method. Two plasmon polariton modes are obtained and an obviously splitting at the resonant frequency can be seen due to the coupling between photons and plasmons. The work conduces to deepening the understanding of the photoelectric properties of graphene and the terahertz plasmonics based on graphene. Keywords:graphene/ plasmon/ optical absorption/ plasmon polariton
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2.1.理论模型
图1显示的等离激元装置是由施加了光栅的石墨烯层和光学谐振腔两个基本部分组成. 在石墨烯和光栅之间存在一层厚度为20 nm的Al2O3薄膜, 作为上门电压的介电层. 石墨烯下面是一层厚度为300 nm的SiO2层并且放置在厚度为200 μm的Si基底上. 利用光在Al2O3薄膜上界面以及Si下界面的反射作用, 在虚线框内的Al2O3/SiO2/Si结构可以充当光学谐振腔. 而石墨烯作为等离激元的载体. 光学谐振腔内的光子在光学谐振腔中经过多次反射形成驻波, 对于垂直入射的光场, 腔内光子的模式可以由${f_n} = {\omega _n}/2{\text{π}} = nc/(2\sqrt {{\varepsilon _{\rm{s}}}} L)$表示, 这里${\omega _n}$为腔模光子的角频率, $n = 1, 2, 3,\cdots$是不同谐振腔光子模式的量子数, c是真空中的光速, L是谐振腔的长度, ${\varepsilon _{\rm{s}}}$是谐振腔中介质的介电常数, 由于Al2O3和SiO2层的厚度远小于THz光的波长以及Si层的厚度, 所以我们取Si的介电常数${\varepsilon _{\rm{s}}} = 11.9$. 图 1 基于石墨烯的装置示意图(从上至下依次是金光栅/Al2O3薄膜/石墨烯/SiO2/Si; d和w分别代表光栅的周期和光栅条的宽度; 虚线框内的结构可以充当谐振腔的作用; L是谐振腔的厚度) Figure1. Schematic illustration of the device based on graphene. From top to bottom, there are the gold grating layer, Al2O3 dielectric medium, graphene sheet, and SiO2/Si layer. Here, d and w are respectively the period and the width of the gold strips. The structure sketched by the dotted line can be served as cavity and L is the cavity length.