Metamaterial Electrodes Of Graphene Longwave Infrared Photodetector

Graphene is a two-dimensional material with the thickness of single atom,which has unique properties of ultrahigh carrier mobility,extremely broad absorption spectrum and tunable chemical potential.As one of the most popular materials,graphene is potential to be applied for various optoelectronics and microelectronics,such as absorber,sensor,detector,modulator,field effect transistor,and so on.Metamaterial is a kind of artificial composite structure,which has the electromagnetic properties that the natural material does not possess.The combination of graphene and metamaterial can improve the performance of optoelectronic devices.For example,the resonance frequency of a graphene-based metamaterial absorber is easy to be changed,while the responsivity of a graphene photodetector can be improved by employing metamaterial structures due to surface plasmonics enhancing optical absorption.On the basis of the two cases above,in this thesis we carried out the following simulation designs and fundamental experimental studies of graphene long wave infrared(LWIR)photodetector and absorber.1.Having the structure of broadband metamaterial absorber which consists of three narrowband absorbers,a broadband graphene photodetector was proposed.Absorption bandwidth of the broadband absorber is increased by three times that of each narrowband absorber.Electromagnetic wave of the free space is concentrated to gaps among the cut wires,where the absorption of graphene is greatly enhanced with the broader bandwidth as a result of the ultrahigh electric field intensity inside the gaps.Thus the device can be expected to achieve broadband detection with high responsivity.In addition,the metallic antennas in the metamaterial structure can be used as electrodes to collect the photogenerated carriers more effectively,resulting in a further improvement of the responsivity of the photodetector as well as a shorter response time.2.Based on the structure of electromagnetically induced transparency metamaterial,a polarization-independent graphene photodetector was proposed.Incident electromagnetic radiation in different polarizations can be confined into the same gap of a metallic unit cell in period,leading to enhancement of the absorption of graphene.Therefore,the device is expected to achieve polarization-independent detection with high responsivity.The metallic cells can also be used as electrodes to further improve the responsivity and shorten the response time of the device,as described in the last paragraph.3.A tunable dual-band and wide-angle perfect absorber consisted of graphene and VO2 was presented.By tuning the chemical potential of graphene ribbons and the insulator-to-metal transition of VO2,a wide tunable range was obtained in the LWIR spectral range.Moreover,the two absorption peaks,including location and intensity,can be tuned by either electrical,thermal,or their combination.Besides,the absorption intensity of our device suffers little attenuation over wide angle range of incidence.