Novel Integrated Optical Gradient Force Devices

Light is a kind of electromagnetic wave,which carries momentum and energy.It will give object force when the objects are irradiated.The optical force effect provides us with a new interaction mechanism between light and matter.However,the optical force is so weak that it is difficult to be observed when it acts on macroscopic objects.With the development of integrated photonics,photonic integrated devices provide an excellent platform for the study of the optical force effect.Integrated photonic devices have the advantages of small mass,small mode volume and high optical power density.Integrated nanowaveguides can produce the deformation of hundreds of nanometers when the optical gradient force is acted.Because the change of the effective refractive index caused by the deformation of the nanowaveguide is several orders of magnitude larger than that caused by the optical Kerr effect,it can cause huge optical phase shift.This provides a new scheme for the realization of tunable photonic devices.At present,the study on the optical force has made great progresses and has formed a new research field.This dissertation focuses on the research of optical gradient force effects in integrated photonic devices.We have theoretically studied the optical gradient force between the coupled graphene sheets and the optical gradient force in the PT-symmetric coupler.Experimental fabrication and test of integrated optomechanical devices and whispering-gallery-mode microcavities have also been studied.The detailed contents of this dissertation include the following parts:(1)Optical gradient force characteristics of the coupled monolayer graphene waveguides in the middle infrared band have been theoretically studied.The results show that,when the wavelength is 10 μm,due to the extremely strong field confinement capability,a huge optical force density can be obtained and the optical force between the graphene nanoribbons can be changed by adjusting the chemical potential.When the wavelength is 4.5 μm,there is a surface plasmon mode in graphene that cannot exist in the metal,that is,the TE surface plasmon mode.But this mode is almost cut-off,the optical force effect of this mode is negligible.A compact optical phase shifter in the mid-infrared band is proposed based on the characteristics of the optical force between coupled graphene sheets.The optical phase shift can be realized by changing the optical power and voltage.(2)Optical gradient force in a PT-symmetric coupler is studied theoretically.The evolution of the optical force density in the PT-symmetric coupler when the system transit from PT-symmetric region to broken-PT-symmetric region was studied.It shows that by using the optical gradient force we can achieve the transition of the system from the PT-symmetric region to the broken-PT-symmetric region.Finally,as long as the system is in the broken-PT-symmetric region and the waveguide length is much longer than the propagation length of the lossy mode,the total optical force acting on the waveguides decreases with the decrease of the waveguide spacing.All of these characteristics are quite different from those in traditional coupled waveguides.(3)We have explored the fabrication technology of integrated silicon optomechanical devices.The fabrication of silicon optical force devices involves many steps,especially the technology of HF etching.We find that in the case of the gap is 260 nm and the length of the suspended waveguide is 20 μm,the optical phase shift caused by the optical gradient force is very small.Moreover,the thermal effect in the suspended nanowaveguide is much stronger than that in the unsuspended nanowaveguide.In order to reduce the thermal effect and enhance the optical force effect,the key is to improve the HF etching process to further reduce the gap between the waveguide and the substrate.In addition,in order to improve the fabrication efficiency,the critical point dryer or HF vapor-phase etching technology could be considered.(4)Fabrication of the silica microsphere cavity and observation of the optomechanical self-induced oscillation effect in microsphere are carried out.The ultrahigh Q factor of the microsphere cavity makes the field enhancement factor very high and the life time of photons very long,which is important for achieving low-threshold cavity optomechanical devices.We focus on the fabrication of the microsphere cavity and the microfiber and how to realize the high-efficiency coupling between them.Finally,we realized the optomechanical self-induced oscillation in the microsphere cavity,which lays the foundation for the further study of cavity optomechanics.