Absorption Properties Of Graphene-dielectric Composite Materials

Due to its unique properties such as zero bandgap and adjustable Fermi level,graphene has attracted great interest for a wide range of applications such as photodetection,biosensing,and solar energy harvesting.As limited by its atomic layer thickness,graphene exhibits a universal light absorption of about 2.3%.This low absorbance presents a limitation for graphene to be applied in optoelecrronic devices.To enhance graphene's absorption,researchers have proposed to use plasmonic effect,resonant cavity,and evanescent wave excitation.In addition,some scholars have also explored wideband absorption of graphene by using overlapped multiple resonances in nano-structured graphene,which in reality are difficulty to make due to fabrication challenges.Current studies on broadband absorption of graphene are mostly pure theoretical.In this thesis,we propose to use quarter-wave cavities to engineer graphene absorption in terms of both efficiency and bandwidth.In three-layered structures made of Cu/SiO₂/graphene,graphene absorption of up to 40%is obtained in both theory and experiment.The absorption bandwidth is also varied between 10%and 52%of the central wave wavelength depending on the choosen SiO₂ thickness.The main contents and contributions of this thesis are summarized as below:?1?Theoretical models for multiplayered graphene composite materials are proposed by using transfer matrix method and finite difference time domain method.Our simulation results show that,in a quarter-wavelength cavity,graphene absorption can be enhanced for TE polarization,whose amplitude increases with the the incident angle.However,for TM polarization,graphene absorption decreases with the incident angle.Absorption frequencies of both TE and TM polarizations are blue-shifted as the incident angle increases.In addition,as the thickness of the dielectric layer increases,higher order cavity modes are excited and their bandwidthis gradually narrowed,resulting in decreasing bandwidth of graphene absorption.?2?The mechanism of engineering graphene absorption in quarter-wavelength cavity is illustrated.When the thickness of the dielectric layer is an odd number of quarter-wavelength,resonant condition occurs.Maximum field of the cavity modes lies on top of the dielectric layer,right in overlap with graphene,leading to enhanced graphene absorption.Also,as linewidth of the resonant modesis inversely proportional to the cavity length,by changing the dielectric layer thickness,it is possible to control the linewith of the cavity mode and bandwidth of the graphene absorption.?3?Multi-layered graphene samples are fabricated using standard thin film deposition techniques.Experimental results of graphene absorption are in good agreement with theoretical calculation.For TM polarization,a maximum graphene absorption of 40%is measured as the incident angle increases from 15°to 85°.Meanwhile as the thickness of SiO₂ increases from a quarter-wavelength to seven quarter-wavelengths,bandwidth of the graphene absorption is varied between 10%and52%of the central wavelength,which validates our proposed mechism of enginnering graphene absorption using quarter-wavelength cavities.The graphene-dilectric thin-film cavities demonstrated in this thesis are simple in structure.They are compatible with standard thin-film deposition techniques,and thus present an effective way of engineering graphene absorptions for applications in optoelectronic devices.