| Graphene has attracted wide attention since its discovery.Graphene has been used in the preparation of supercapacitors,ultrafast transistors,ultra-fast photon detectors,ultra-fast mode locked lasers and other devices.There are also broad applications in field effect transistors,photovoltaic cells,liquid crystal display and other fields.However,graphene has the following shortcomings in the field of optical applications:the thickness of graphene is extremely thin and is not easily recognized.In the visible and near-infrared wavelengths,the graphene carrier occurs between bands to the conduction band,and the absorption of light is mainly determined by interband transition.The absorption rate measured by the experiment is only 2.3%.These shortcomings limit the application of graphene in the field of optoelectronics.Therefore,it is of practical significance to study the enhancement of optical absorption of graphene.In recent years,there have been many studies reported to improve the absorptivity of graphene.However,some methods have complicated structures or the absorption rate cannot achieve wide band and wide angle absorption enhancement.Therefore,this dissertation focuses on the optical absorption enhancement of graphene by the waveguide-grating resonance structure,and the absorption mechanism of each reinforced graphene is deeply studied.The main contents include the following points:1.A structure based on a guided mode resonance Brewster filter is proposed to achieve enhanced light absorption of monolayer graphene.The structure has a grating layer and a substrate.When the structure is not added to the graphene layer,a mutat reflection peak is generated due to the guided mode resonance effect of the structure.After the incident light is illuminated into the graphene layer,the interaction of the light will be enhanced at the resonance wavelength,which makes the electric field in the cross section and the absorption rate increase.As a result,the electric field at the cross-section increases and the absorption rate increases.Using the equivalent medium theory and the rigorous coupled-wave analysis method,the absorption spectrum of the structure can be calculated.It is found that a very narrow absorption peak appears at the 632.8 nm wavelength when the electromagnetic wave is incident at Brewster angle,and the highest absorption rate is increased to 55%.2.Introduce the "three-mode absorber" theoretical model,By using a grating layer and the medium layer to recombine a three layer waveguide structure of the grating layer or the medium layer,the enhancement of the absorption of graphene to light is achieved.Among them,the three layer waveguide structure can be considered as two lossless resonators.The graphene layer can be considered as a lossy resonator.When the leakage rate of the whole structure is equal to the loss rate,namely the "critical coupling"condition,there will be 100%absorption.According to the absorption mechanism of the "three mode absorber",two different structures are designed,and the related studies are carried out for each structure.The absorption spectra are calculated by the rigorous coupled wave analysis method.The two structures will be absorbed nearly 100%near the 1680 nm and 980 nm wavelengths,respectively.3.Considering that the designed structure needs to be applied to the actual photovoltaic device,a new type of graphene-dielectric-metal(GIM)structure which is more easily extended to large area production is proposed.The GIM structure makes the graphene layer and the bottom metal layer form a FP resonance cavity,which can improve the optical absorption of graphene in the resonant cavity with the light interact with graphene many times in the resonant cavity.In the calculation of the structure,it is found that two 100%absorption peaks will appear near the range of 500 nm and 800 nm when the graphene layer is 10 layers.And more absorption peaks will be produced by adjusting the thickness of the medium layer. |
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