Design And Fabrication Of A Novel Au/WS₂/TiO₂ Hybrid Waveguide Circulator Device

Two-dimensional transition metal dichalcogenides(TMDs)have excellent optoelectronic properties that bulk materials lack,which are popular topics for advanced on-chip integrated optoelectronic devices.The electronic transition of the monolayer TMDs with direct band gap occurs at the degenerate±K points located at the edge of the two-dimensional hexagonal Brillouin zone.Due to the break of the spatial inversion symmetry,+K and-K points are two different valleys which have the opposite berry curvatures corresponding to the opposite orbital angular momentum.It makes the TMDs monolayer perfect object for selective excitation rules of valleys.The strong spin-orbit coupling effect locks the electronic spin state of the monolayer TMDs with corresponding valley index,thus forming the valley selection excitation rule of the transition.The efficiency of valley selection at room temperature is quiet low.There is still a lack of effective research methods to improve the efficiency of valley selection of two-dimensional TMD materials.Surface plasmon polaritons(SPPs)have the advantages of sub-wavelength limitation in z space and local field enhancement effects,and are excellent carriers for photon manipulation and integration at the micro-nano scale.Through the spin angular momentum locking of the plasmon mode,the valley information of two-dimensional materials can be transformed into directional photon transmission and emission process under the corresponding system,and the corresponding valley index of the TMD materials can be read out.It is difficult to effectively combine SPPs with the valley index reading out process of TMD materials.Based on the current research situations,this thesis mainly conducts the following research work:(1)With reference to the system that puts the single silver nanowire on the tungsten disulfide(WS₂)flake,we come up with a circulator device with the effect of valley polarization selection based on a hybrid plasmon waveguide structure.This waveguide structure supports the optical waveguide mode of the transverse optical spin angular momentum that can read the valley index of monolayer WS₂.We determined that titanium dioxide(TiO₂)was used as the waveguide material,SiO₂ functioned as a gap,and gold as the metal medium through FDTD solutions simulation,and the corresponding specific parameters were determined.(2)Based on the nowadays micro-nano fabrication technologies,we use electron beam evaporation technology,dry transfer methods of two-dimensional materials,electron beam lithography technology and inductively coupled plasma etching technology to fabricate the sample.we adapt two proposals which are double-layer lift-off process for deposition and the etching methods of fabrication of dielectric structure by mask protection.Both these two processes can realize the nano-scale fabrication of the TiO₂ structure.Through a series of characterization of the structures processed by these two proposals,we found that the waveguide structure fabricated by etching is much closer to the initial designed size,which can reduce the distortion of the designed pattern caused by the electron beam scattering,and achieve a higher machining precision.(3)Under the self-built optical microscopy test system,we use left-handed circular polarized light with a wavelength of 532 nm as the pump source,and the excited fluorescence of monolayer WS₂ with different circular polarization states will carry the corresponding valley index through the designed waveguide of circulator and finally emits at the designed port of the waveguide.The experiment verified that we can read the valley index of the monolayer WS₂ via the hybrid plasmon waveguide structure.