Study Of Optical Modulators And Wavefront Controllers Based On Surface Plasmons With Metasurfaces

Optical devices with wide bandwidth,high transmission efficiency and high-density integration have always been a focus of research on optoelectronic integrated devices.Traditional optical devices can achieve a variety of functionalities in projection,imaging,detection,medicine,etc.However,due to their large sizes,it is not convenient for them to be densely integrated,or their application is limited in some cases owing to the optical diffraction limit.As a new kind of structures designed artificially,metasurfaces can control the phase/amplitude/polarization states of the electromagnetic field at the sub-wavelength scale,which not only show a strong ability to manipulate electromagnetic waves,but also have the advantage of being extremely compact.These characteristic makes it possible to realize high-density optical devices while achieving optical functionalities.Surface plasmon polaritons(SPPs)are evanescent waves bounded at a metal/dielectric interface,which can squeeze a light field into a deep subwavelength scale.Metasurfaces can greatly enhance the interaction between light and devices by controlling surface plasmons,and realize many attractive functions on chip.In this thesis,we have studied graphene optical modulators based on surface plasmons with metasurfaces,which show extremely high modulation depth over a wide bandwidth,as well as polarization-independent wavefront controllers based on surface plasmons with metasurfaces,which can realize Airy SPP,near-field focus and lossless SPP independent of polarization states.The main contents of research are as follows:(1)In the study of graphene optical modulators based on surface plasmons with metasurfaces,graphene has been explored to developing active optical devices such as photodetectors and sensors owing to its high electron mobility,wide operation bandwidth and tunability of conductivity.Plasmonic metasurfaces consist of gold rods array,which could support SPP with its main electric field component parallel with the graphene plane.By optimizing the geometric parameters of metasurfaces,the SPP field is enhanced,which greatly improves the interaction between electric field and graphene.Through tunning the optical property of graphene by gating,a high-modulation-depth modulator is obtained.The numerical results based on finite-difference time-domain method show that compared with previous graphene modulators,the modulator presents a significantly improved modulation depth(MD)over a wide spectral range,which has a MD of 4.66 d B/?m and an insertion loss of 1.4 d B/?m,while still having ease of fabrication.This design could inspire the construction of diverse nanophotonic devices,with simultaneous SPP field enhancement and SPP field being oriented in-plane.(2)In the study of polarization-independent wavefront controllers based on surface plasmons with metasurfaces,we propose an effective approach to polarization-independent control SPP's phase and amplitude.Plasmonic metasurfaces consist of nanoslits array on a thin metal film.By changing the rotation angles and positions of nanoslits as well as the number of nanoslit columns,wavefront manipulation and SPP field enhancement are implemented under incident beams of arbitrary polarization states.To validate our mechanism,we demonstrate polarization-independent generation of Airy plasmons,near-field focus and lossless SPP through both numerical simulation and experimental measurement.Both results show that the method based on plasmonic metasurfaces to manipulate SPP is effective.Our scheme provides extra dimensions to control the SPs regardless of polarization states,which can broaden the applications of nanophotonic designs for polarization-controlled plasmonic circuits and surface optical manipulation.