Nonreciprocity Based On Cavity Optomechanical Systems

When light hits the surface of an object,it transfers momentum to the object.The light pressure is generated because of the momentum conservation.Then the concept of radiation pressure is introduced.In the end of 1960s,in order to understand the principle of a gravitational wave observatory,Braginsky built a model that monochromatic light trapped in a high-finesse Fabry-Perot cavity exerts radiation pressure on the massive end mirrors,coupling their oscillatory motion to the light,which started the research of radiation pressure on the massive objects.And it led a new field,cavity optome-chanics.Light circles in the cavity many times,which can enhance radiation pressure.Braginsky and coworkers recognized that radiation pressure can change the dynamics of the mechanical degree of freedom,effectively adding an optically induced viscous damping to the mirror motion.This process is called dynamical backaction.It can be used to amplify or cool the motion of the mirror,akin to laser cooling.Based on the platform of cavity optomechanics,we can study quantum ground state cooling,quan-tum state control,precision magnetic field,mass sensing,optomechanically induced storage,gravitational wave detection and so on.Non-reciprocal devices are important basic components in the field of classical and quantum information processing.Nonreciprocal devices are the devices that break the symmetry of forward and backward paths of wave propagation from sound to light.Non-reciprocal devices can avoid interference and protect the light source,and expand the information capacity of optical paths,such as isolator,and circulator in front of the laser.Traditional optical isolators are generally based on magneto-optical effect and quite large for requiring external magnetic field.However,the integration of such devices is challenging due to the high material loss and large device size.Therefore,it is very meaningful to realize non-magnetic non-reciprocal devices by other methods.The effective modulation of the medium in time and space to achieve non-reciprocity can be realized through a variety of methods,such as acousto-optic modulation,nonlinear optical effect,cavity optomechanics and so on.Here,based on cavity optomechanics system inwhispering gallery mode microcav-ity,we study the optomechanically induced non-reciprocity.The main contents include:1.Background introduction of whispering gallery modes microcavity and cav-ity optomechanicsFirstly,we introduce the theoretical basis of whispering gallery modes microcav-ity,including optical mode and mechanical mode supported by microcavity,coupling input-output mechanism,optomechanical interaction.The optical mode and mechani-cal mode both with high Q are very important to enhance the interaction between photon and phonon.We proposed BIC mechanism to transform the(1,2,1)mechanical mode into a bound state,which theoretically eliminated the clamping loss and improves the mechanical Q value.It is used to design the size of microcavity and connecting stem.Meanwhile,the preparation of microspheres and fiber taper,as well as the measure-ment of optical and mechanical modes in the whispering gallery modes microcavity are introduced.Finally,we introduce the technique of optomechanically transient measure-ment.These are the basis for our later study of nonreciprocity in cavity optomechanical systems.2.Reconfigurable optomechanical circulator and directional amplifierUsing two nondegenerate optical traveling wave modes with the opposite momen-tums,which are clockwise(CW)and counterclockwise(CCW)optical cavity modes,under the condition of momentum matching,only when the driving light and the signal light are coupled to the same optical mode,the driving light can stimulate the coherent conversion of signal photons and phonons,thus resulting in the non-reciprocity of light propagation.Based on this,we realized the non-reciprocal phenomenon of optome-chanically induced transparency and amplification by the unidirectional driving light,and demonstrated the non-reciprocal phase shift of up to 40 degrees,which was the ba-sis of realizing optical isolator and circulator.The non-reciprocity mechanism studied in this experiment is universal and can be extended to any traveling wave mode systems with mechanical mode,integrated optical chips and even single-photon isolator.3.Synthetic magnitism in cavity optomechanical systemIn the cavity optomechanical systems,backscattering can cause the coupling be-tween the optical CW and CCW modes,and the non-reciprocity caused by traveling waves can be reduced.To solve this problem,in the system of CW and CCW optical modes and mechanical mode,the phase related CW and CCW control lights were used to generate synthetic magnetic flux and achieve non-reciprocity in cavity optomechan-ical system.By observing the photon transmission spectrum of the cavity mode,we could observe the non-reciprocal phononemenon experimentally.4.Tunable Raman laser in the microcavityThe dynamic evolution of Raman gain based on thermal effect in microcavity was studied.Meanwhile,we also studied continuously tuable Raman laser with a wide range.The tuning range was up to 132GHz with resolution of 85MHz.In addition,the frequency tuning of optical mode and mechanical mode were studied in the mi-crobottle cavity,the tuning range of optical mode reached a free spectral region,and the tuning range of mechanical mode reached more than 1MHz,which was expected to increase the scope of work for non-reciprocal devices in cavity optomechanical system.