Nonreciprocal Transmission And Asymmetric Fast-slow Light Effect Based On PT-symmetric Optomechanical Systems

Cavity optomechanics is the field of studying the interaction between optical cavity and mechanical oscillators.In recent decades,cavity optomechanics has achieved great success in exploring macroscopic quantum effects and studying the interaction between electromagnetic radiation and micromechanical motion.As an important platform for observing macroscopic quantum phenomena and verifying the basic laws of quantum mechanics,cavity optomechanical systems can produce many interesting quantum effects,such as optomechanically induced transparency(OMIT),optical non-reciprocity,quantum entanglement,photon blockade,and mechanical ground-state cooling.Among them,similar to electromagneticlly induced transparency,OMIT is caused by the radiation pressure coupling between the optical mode and the mechanical mode.The anti-Stokes sideband can generate the quantum interference with the photonic transition path of the same frequency,thus,the absorption of the weak probe field can be suppressed,which results in OMIT.In addition,since the optomechanical interaction is a kind of nonlinear coupling,optomechanical systems can induce not only strong nonlinear effects,but also many novel physical phenomena.The research on these phenomena can be applied in many fields,such as the optical information storage,fast and slow light effects,and optical switching scheme.With the development of the nanomachining technology,the mass of the mechanical oscillator in the optomechanical system is gradually decreasing,so that the spatial scale of the optomechanical system is decreased and then various optical microcavities with high precision appear.Therefore,the optomechanical system can also realize the precise measurement of some weak signals,such as the weak force and weak displacement.In this thesis,we improve the traditional optomechanical system with the introduction of a parity-time(PT)-symmetric mechanical structure,mainly to study the optical response of the weak probe field and related dynamic modulation in the system.We examine the nonreciprocal transmission behavior in the optomechanical system,in which two passive cavity modes are coupled with each other and also with two PT-symmetric mechanical modes.Two cavities,one of which is probed by a weak field,are driven by two strong control fields,respectively.With the active-passive mechanical-resonator scheme,such a closed-loop four-mode system can show an amplification behavior of the probe field with three transmission windows based on OMIT.Due to the breaking of the time-reversal symmetry corresponding to the relative phase between two control fields,the amplified nonreciprocal transmission can be realized in the middle OMIT window and its direction can be controlled via the phase modulation.In addition,the system can also show asymmetric group velocities of light propagation,i.e.,the tunable asymmetric fast-slow light effects,for example,slow lights along a direction and fast lights along the other one.It is of interest that the dynamic asymmetric fast-to-slow light conversion can be realized periodically by phase modulation.Such a system of three OMIT windows,acting as the schemes of the directional amplifier and all-optical switch of the direction and velocity of light,may provide underlying applications in the photonic network and information communicating process involving multi-signal transmission.