Design And Fabrication Of Two-dimensional Mechanical Resonators Based On Optical Reflectivity

Two-dimensional materials such as graphene and monolayer transition metal dichalcogenides(TMD)have extremely low mass,small thickness and high quality factor.For this reason,nano-mechanical resonators based on these two-dimensional membranes have received extensive attention.As these membranes can vibrate under small driving forces,so one important application of nano-mechanical resonators is sensitive force detection.The research of the optical reflectivity and structures of nano-mechanical resonators would promote the development of nanomechanical systems and their applications in optics and mechanics.Based on graphene,we design a new nano-mechanical system which combines the cavity with Bragg reflector,also,this system is intended to detect ultra-sensitive forces.The goal of this project is to detect small driving forces by optical methods,therefore,in order to improve the detection sensitivity,we analysis the work of Bragg reflector,the effect of the structure of nano-mechanical resonators on the optical reflectivity,then optimize the structure of the resonator based on the above analysis.The content of this subject includes not only the above theoretical research,but also the fabrications of nano-resonators based on theoretical optimization results of the resonator structure.Specifically,it includes:fabrication of electrodes,etching of cavities and the dry transfer of graphene.The prepared nano-resonators can be used not only to verify the previous theoretical results,but also to provide a way for the exploration of unknown physical phenomena.The main work of the thesis is as follows:1.Based on the analysis of common structures of optoelectronic nano-mechanical systems,this paper designs a nano-mechanical system,which mainly consisted of Bragg reflector and nano-resonator.The Bragg reflector is composed of monolayer titanium dioxide(TiO2)and monolayer silicon dioxide(SiO2),it is found that Bragg reflector can enhance the standing wave of electric field.The nano-resonator takes the structure of Fabray-Peort optical cavity which can enhance the optical reflectivity,and its structure is consisted of vacuum-suspended two-dimensional membrane-vacuum-silicon substrate.By analysis,we find that if the two-dimensional membrane is located between the node and the antinode of the standing wave,the intensity of reflected lights will be changed smoothly with the change of membrane's position Z,so the membrane's position in the cavity would affect the optical reflectivity of devices.2.This work uses the transfer matrix to simulate the distance between membrane and substrate under the normal incidence and oblique incidence.The main work and conclusions include:1)For normal incidence,calculate the optical responsivity as a function of the thickness of SiO2,and the distance between membrane and SiO2.Results show that,for graphene,the best distance between membrane and SiO2 is about 80 nm.2)For oblique incidence,when the incident light is s-polarized and p-polarized,calculate the reflectance as a function of incident angles and the distance between membrane and the reflective substrate.As a result,the tilted angle has no significant effect on the reflectance,but s-polarization is more affected by the incident angle than the p-polarization.3.Based on optimized parameters of the nano-resonator's structure obtained by simulations under the normal incidence,the device is fabricated on the silicon chip by nanotechnologies.The main fabrications include:firstly,exposing the resonator's pattern by Electron Beam Lithography;then,coating electrodes with metal by Electron Beam Evaporation,and then etching cavities by Reactive Ion Beam Etching.After the whole fabrications,take the atomic force microscope to characterize the flatness of chips.Finally,obtain graphene by mechanical stripping method,and transfer graphene to the cavity by dry transfer method,thereby complete whole fabrications of the nano-resonator.